LMX2485/LMX2485E
50 MHz - 3.0 GHz High Performance Delta-Sigma Low
Power Dual PLLatinum™ Frequency Synthesizers with
800 MHz Integer PLL
General Description
The LMX2485 is a low power, high performance delta-sigma
fractional-N PLL with an auxiliary integer-N PLL. The device
is fabricated using National Semiconductor’s advanced process.
With delta-sigma architecture, fractional spurs at lower offset
frequencies are pushed to higher frequencies outside the
loop bandwidth. The ability to push close in spur and phase
noise energy to higher frequencies is a direct function of the
modulator order. Unlike analog compensation, the digital
feedback technique used in the LMX2485 is highly resistant
to changes in temperature and variations in wafer processing. The LMX2485 delta-sigma modulator is programmable
up to fourth order, which allows the designer to select the
optimum modulator order to fit the phase noise, spur, and
lock time requirements of the system.
Serial data for programming the LMX2485 is transferred via
a three line high speed (20 MHz) MICROWIRE interface.
The LMX2485 offers fine frequency resolution, low spurs,
fast programming speed, and a single word write to change
the frequency. This makes it ideal for direct digital modulation applications, where the N counter is directly modulated
with information. The LMX2485 is available in a 24 lead
4.0 X 4.0 X 0.8 mm LLP package.
Applications
n Cellular phones and base stations
CDMA, WCDMA, GSM/GPRS, TDMA, EDGE, PDC
n Direct digital modulation applications
n Satellite and cable TV tuners
n WLAN Standards
Features
Quadruple Modulus Prescalers for Lower Divide Ratios
n RF PLL: 8/9/12/13 or 16/17/20/21
n IF PLL: 8/9 or 16/17
Advanced Delta Sigma Fractional Compensation
n 12 bit or 22 bit selectable fractional modulus
n Up to 4th order programmable delta-sigma modulator
Features for Improved Lock Times and Programming
n Fastlock / Cycle slip reduction
n Integrated time-out counter
n Single word write to change frequencies with Fastlock
Wide Operating Range
n LMX2485 RF PLL: 500 MHz to 3.0 GHz
n LMX2485E RF PLL: 50 MHz to 3.0 GHz
Useful Features
n Digital lock detect output
n Hardware and software power-down control
n On-chip crystal reference frequency doubler.
n RF phase comparison frequency up to 50 MHz
n 2.5 to 3.6 volt operation with ICC = 5.0 mA at 3.0 V
Functional Block Diagram
20087701
PLLatinum™ is a trademark of National Semiconductor Corporation.
© 2006 National Semiconductor Corporation
DS200877
www.national.com
LMX2485/LMX2485E High Performance Delta-Sigma Low Power Dual PLLatinum Frequency
Synthesizer
October 2006
LMX2485/LMX2485E
Connection Diagram
Top View
24-Pin LLP (SQ)
20087722
Pin Descriptions
Pin #
Pin
Name
I/O
Pin Description
0
GND
-
Ground Substrate. This is on the bottom of the package and must be grounded.
1
CPoutRF
O
RF PLL charge pump output.
2
GND
-
RF PLL analog ground.
3
VddRF1
-
RF PLL analog power supply.
4
FinRF
I
RF PLL high frequency input pin.
5
FinRF*
I
RF PLL complementary high frequency input pin. Shunt to ground with a 100 pF
capacitor.
6
LE
I
MICROWIRE Load Enable. High impedance CMOS input. Data stored in the shift
registers is loaded into the internal latches when LE goes HIGH
7
DATA
I
MICROWIRE Data. High impedance binary serial data input.
8
CLK
I
MICROWIRE Clock. High impedance CMOS Clock input. Data for the various counters is
clocked into the 24 bit shift register on the rising edge
Power supply for RF PLL digital circuitry.
9
VddRF2
-
10
CE
I
Chip Enable control pin. Must be pulled high for normal operation.
11
VddRF5
I
Power supply for RF PLL circuitry.
12
Ftest/LD
O
Test frequency output / Lock Detect.
13
FinIF
I
IF PLL high frequency input pin.
14
VddIF1
-
IF PLL analog power supply.
15
GND
-
IF PLL digital ground.
16
CPoutIF
O
IF PLL charge pump output
17
VddIF2
-
IF PLL power supply.
18
OSCout
O
Buffered output of the OSCin signal.
19
ENOSC
I
Oscillator enable. When this is set to high, the OSCout pin is enabled regardless of the
state of other pins or register bits.
20
OSCin
I
Input for TCXO signal.
21
NC
I
This pin must be left open.
22
VddRF3
-
Power supply for RF PLL digital circuitry.
23
FLoutRF
O
RF PLL Fastlock Output. Also functions as Programmable TRI-STATE CMOS output.
24
VddRF4
-
RF PLL analog power supply.
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2
Parameter
Value
Symbol
Power Supply Voltage
Min
Typ
Units
Max
VCC
-0.3
4.25
Voltage on any pin with GND = 0V
Vi
-0.3
VCC+0.3
V
Storage Temperature Range
Ts
-65
+150
˚C
Lead Temperature (Solder 4 sec.)
TL
+260
˚C
V
Recommended Operating Conditions
Parameter
Value
Symbol
Power Supply Voltage (Note 1)
Operating Temperature
Units
Min
Typ
Max
VCC
2.5
3.0
3.6
V
TA
-40
25
+85
˚C
Note 1: “Absolute Maximum Ratings” indicate limits beyond which damage to the device may occur. "Recommended Operating Conditions" indicate conditions for
which the device is intended to be functional, but do not guarantee specific performance limits. For guaranteed specifications and test conditions, see the Electrical
Characteristics. The guaranteed specifications apply only for the test conditions listed. The voltage at all the power supply pins of VddRF1, VddRF2, VddRF3,
VddRF4, VddRF5, VddIF1 and VddIF2 must be the same. VCC will be used to refer to the voltage at these pins and ICC will be used to refer to the sum of all currents
through all these power pins.
Note 2: This Device is a high performance RF integrated circuit with an ESD rating < 2 kV and is ESD sensitive. Handling and assembly of this device should only
be done at ESD-free workstations.
Electrical Characteristics
Symbol
(VCC = 3.0V; -40˚C ≤ TA ≤ +85˚C unless otherwise specified)
Parameter
Conditions
Value
Min
Typ
Max
Units
Icc PARAMETERS
ICCRF
Power Supply Current,
RF Synthesizer
IF PLL OFF
RF PLL ON
Charge Pump TRI-STATE
3.3
mA
ICCIF
Power Supply Current,
IF Synthesizer
IF PLL ON
RF PLL OFF
Charge Pump TRI-STATE
1.7
mA
ICCTOTAL
Power Supply Current,
Entire Synthesizer
IF PLL ON
RF PLL ON
Charge Pump TRI-STATE
5.0
mA
ICCPD
Power Down Current
CE = ENOSC = 0V
CLK, DATA, LE = 0V
1
10
µA
RF SYNTHESIZER PARAMETERS
fFinRF
Operating
Frequency
(Note 3)
LMX2485
LMX2485E
pFinRF
Input Sensitivity
fCOMP
Phase Detector
Frequency
(Note 4)
ICPoutRFSRCE
RF Charge Pump
Source Current
(Note 5)
RF_P = 8
500
2000
RF_P = 16
500
3000
RF_P = 8
50
2000
RF_P = 16
50
3000
500 - 3000 MHz
-15
0
50 - 500 MHz (LMX2485E only)
-5
50
MHz
dBm
MHz
RF_CPG = 0
VCPoutRF = VCC/2
95
µA
RF_CPG = 1
VCPoutRF = VCC/2
190
µA
...
µA
1520
µA
...
RF_CPG = 15
VCPoutRF = VCC/2
3
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LMX2485/LMX2485E
Absolute Maximum Ratings (Notes 1, 2)
LMX2485/LMX2485E
Electrical Characteristics (VCC = 3.0V; -40˚C ≤ TA ≤ +85˚C unless otherwise specified)
Symbol
Parameter
Conditions
(Continued)
Value
Min
Typ
Max
Units
RF SYNTHESIZER PARAMETERS
ICPoutRFSINK
RF Charge Pump Sink
Current
(Note 5)
RF_CPG = 0
VCPoutRF = VCC/2
-95
µA
RF_CPG = 1
VCPoutRF = VCC/2
-190
µA
...
µA
-1520
µA
...
RF_CPG = 15
VCPoutRF = VCC/2
ICPoutRFTRI
RF Charge Pump
TRI-STATE Current
Magnitude
Magnitude of RF CP
| ICPoutRF%MIS | Sink vs. CP Source
Mismatch
0.5 ≤ VCPoutRF ≤ VCC -0.5
2
10
nA
VCPoutRF = VCC/2 RF_CPG > 2
TA = 25˚C
RF_CPG ≤ 2
3
10
%
3
13
%
2
8
%
0.5 ≤ VCPoutRF ≤ VCC -0.5
TA = 25˚C
| ICPoutRF%V |
Magnitude of RF CP
Current vs. CP Voltage
| ICPoutRF%T |
Magnitude of RF CP
VCPoutRF = VCC/2
Current vs. Temperature
4
%
IF SYNTHESIZER PARAMETERS
fFinIF
Operating Frequency
75
800
MHz
pFinIF
IF Input Sensitivity
-10
5
dBm
fCOMP
Phase Detector
Frequency
10
MHz
ICPoutIFSRCE
IF Charge Pump Source
VCPoutIF = VCC/2
Current
3.5
mA
ICPoutIFSINK
IF Charge Pump Sink
Current
VCPoutIF = VCC/2
-3.5
mA
ICPoutIFTRI
IF Charge Pump
TRI-STATE Current
Magnitude
0.5 ≤ VCPoutIF ≤ VCC RF -0.5
| ICPoutIF%MIS |
Magnitude of IF CP Sink VCPoutIF = VCC/2
vs. CP Source Mismatch TA = 25˚C
| ICPoutIF%V |
Magnitude of IF CP
Current vs. CP Voltage
| ICPoutIF%TEMP
Magnitude of IF CP
VCPoutIF = VCC/2
Current vs. Temperature
0.5 ≤ VCPoutIF ≤ VCC -0.5
TA = 25˚C
2
10
nA
1
8
%
4
10
%
4
%
OSCILLATOR PARAMETERS
fOSCin
Oscillator Operating
Frequency
vOSCin
Oscillator Input
Sensitivity
IOSCin
Oscillator Input Current
OSC2X = 0
5
110
MHz
OSC2X = 1
5
20
MHz
0.5
VCC
VP-P
-100
100
µA
SPURS
Spurs in band
www.national.com
(Note 6)
-55
4
dBc
Symbol
Parameter
Conditions
(Continued)
Value
Min
Typ
Max
Units
PHASE NOISE
LF1HzRF
RF_CPG = 0
-202
RF_CPG = 1
RF Synthesizer
Normalized Phase Noise RF_CPG = 3
Contribution (Note 7)
RF_CPG = 7
-202
-206
RF_CPG = 15
LF1HzIF
dBc/Hz
-208
-210
IF Synthesizer
Normalized Phase Noise
Contribution
-209
dBc/Hz
DIGITAL INTERFACE (DATA, CLK, LE, ENOSC, CE, Ftest/LD, FLoutRF)
VIH
High-Level Input Voltage
VIL
Low-Level Input Voltage
0.4
V
IIH
High-Level Input Current VIH = VCC
-1.0
1.0
µA
IIL
Low-Level Input Current
VIL = 0 V
-1.0
1.0
µA
VOH
High-Level Output
Voltage
IOH = -500 µA
VOL
Low-Level Output
Voltage
IOL = 500 µA
1.6
VCC
VCC-0.4
V
V
0.4
V
MICROWIRE INTERFACE TIMING
tCS
Data to Clock Set Up
Time
See MICROWIRE Input Timing
25
ns
tCH
Data to Clock Hold Time See MICROWIRE Input Timing
8
ns
tCWH
Clock Pulse Width High
See MICROWIRE Input Timing
25
ns
tCWL
Clock Pulse Width Low
See MICROWIRE Input Timing
25
ns
tES
Clock to Load Enable
Set Up Time
See MICROWIRE Input Timing
25
ns
tEW
Load Enable Pulse
Width
See MICROWIRE Input Timing
25
ns
Note 3: A slew rate of at least 100 V/uS is recommended for frequencies below 500 MHz for optimal performance.
Note 4: For Phase Detector Frequencies above 20 MHz, Cycle Slip Reduction (CSR) may be required. Legal divide ratios are also required.
Note 5: Refer to table in Section 2.4.2 RF_CPG -- RF PLL Charge Pump Gain for complete listing of charge pump currents.
Note 6: In order to measure the in-band spur, the fractional word is chosen such that when reduced to lowest terms, the fractional numerator is one. The spur offset
frequency is chosen to be the comparison frequency divided by the reduced fractional denominator. The loop bandwidth must be sufficiently wide to negate the
impact of the loop filter. Measurement conditions are: Spur Offset Frequency = 10 kHz, Loop Bandwidth = 100 kHz, Fraction = 1/2000, Comparison Frequency =
20 MHz, RF_CPG = 7, DITH = 0, and a 4th Order Modulator (FM = 0). These are relatively consistent over tuning range.
Note 7: Normalized Phase Noise Contribution is defined as: LN(f) = L(f) – 20log(N) – 10log(fCOMP) where L(f) is defined as the single side band phase noise
measured at an offset frequency, f, in a 1 Hz Bandwidth. The offset frequency, f, must be chosen sufficiently smaller than the PLL loop bandwidth, yet large enough
to avoid substantial phase noise contribution from the reference source. Measurement conditions are: Offset Frequency = 11 kHz, Loop Bandwidth = 100 kHz for
RF_CPG = 7, Fraction = 1/2000, Comparison Frequency = 20 MHz, FM = 0, DITH = 0.
MICROWIRE INPUT TIMING DIAGRAM
20087775
5
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LMX2485/LMX2485E
Electrical Characteristics (VCC = 3.0V; -40˚C ≤ TA ≤ +85˚C unless otherwise specified)
LMX2485/LMX2485E
Typical Performance Characteristics : Sensitivity
(Note 8)
RF PLL Fin Sensitivity
TA = 25˚C, RF_P = 16
20087745
RF PLL Fin Sensitivity
VCC = 3.0 V, RF_P = 16
20087746
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6
(Note 8)
LMX2485/LMX2485E
Typical Performance Characteristics : Sensitivity
(Continued)
IF PLL Fin Sensitivity
TA = 25˚C, IF_P = 16
20087747
IF PLL Fin Sensitivity
VCC = 3.0 V, IF_P = 16
20087748
7
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LMX2485/LMX2485E
Typical Performance Characteristics : Sensitivity
(Note 8)
(Continued)
OSCin Sensitivity
TA = 25˚C, OSC_2X = 0
20087749
OSCin Sensitivity
VCC = 3.0 V, OSC_2X = 0
20087756
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8
(Note 8)
LMX2485/LMX2485E
Typical Performance Characteristics : Sensitivity
(Continued)
OSCin Sensitivity
TA = 25˚C, OSC_2X = 1
20087773
OSCin Sensitivity
VCC = 3.0 V, OSC_2X = 1
20087774
9
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LMX2485/LMX2485E
Typical Performance Characteristic : FinRF Input Impedance
(Note 8)
20087768
FinRF Input Impedance
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Frequency (MHz)
Real (Ohms)
Imaginary (Ohms)
50
670
-276
100
531
-247
200
452
-209
300
408
-212
400
373
-222
500
337
-231
600
302
-237
700
270
-239
800
241
-236
900
215
-231
1000
192
-221
1100
172
-218
1200
154
-209
1300
139
-200
1400
127
-192
1500
114
-184
1600
104
-175
1700
96
-168
1800
88
-160
1900
80
-153
2000
74
-147
2200
64
-134
2400
56
-123
2600
50
-113
2800
45
-103
3000
39
-94
3200
37
-86
3400
33
-78
3600
30
-72
3800
28
-69
4000
26
-66
10
LMX2485/LMX2485E
Typical Performance Characteristic : FinIF Input Impedance
(Note 8)
20087754
FinIF Input Impedance
Frequency (MHz)
Real (Ohms)
Imaginary (Ohms)
50
583
-286
75
530
-256
100
499
-241
200
426
-209
300
384
-209
400
347
-219
500
310
-224
600
276
-228
700
244
-228
800
216
-223
900
192
-218
1000
173
-208
11
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LMX2485/LMX2485E
Typical Performance Characteristic : OSCin Input Impedance
(Note 8)
20087755
Frequency (MHz)
Powered Up
Powered Down
Real
Imaginary
Magnitude
Real
Imaginary
Magnitude
5
1730
-3779
4157
392
-8137
8146
10
846
-2236
2391
155
-4487
4490
20
466
-1196
1284
107
-2215
2217
30
351
-863
932
166
-1495
-1504
40
316
-672
742
182
-1144
1158
50
278
-566
631
155
-912
925
60
261
-481
547
153
-758
774
70
252
-425
494
154
-652
669
80
239
-388
456
147
-576
595
90
234
-358
428
145
-518
538
100
230
-337
407
140
-471
492
110
225
-321
392
138
-436
458
120
219
-309
379
133
-402
123
130
214
-295
364
133
-374
397
140
208
-285
353
132
-349
373
150
207
-279
348
133
-329
355
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12
LMX2485/LMX2485E
Typical Performance Characteristics : Currents
(Note 8)
Power Supply Current
CE = High
20087759
Power Supply Current
CE = LOW
20087761
13
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LMX2485/LMX2485E
Typical Performance Characteristics : Currents
(Note 8)
(Continued)
RF PLL Charge Pump Current
VCC = 3.0 Volts
20087767
IF PLL Charge Pump Current
VCC = 3.0 Volts
20087765
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14
(Note 8)
LMX2485/LMX2485E
Typical Performance Characteristics : Currents
(Continued)
Charge Pump Leakage
RF PLL
VCC = 3.0 Volts
20087764
Charge Pump Leakage
IF PLL
VCC = 3.0 Volts
20087763
Note 8: Typical performance characteristics do not imply any sort of guarantee. Guaranteed specifications are in the electrical characteristics section.
15
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LMX2485/LMX2485E
Bench Test Setups
20087769
Charge Pump Current Measurement Procedure
The above block diagram shows the test procedure for testing the RF and IF charge pumps. These tests include absolute current
level, mismatch, and leakage measurement. In order to measure the charge pump currents, a signal is applied to the high
frequency input pins. The reason for this is to guarantee that the phase detector gets enough transitions in order to be able to
change states. If no signal is applied, it is possible that the charge pump current reading will be low due to the fact that the duty
cycle is not 100%. The OSCin Pin is tied to the supply. The charge pump currents can be measured by simply programming the
phase detector to the necessary polarity. For instance, in order to measure the RF charge pump, a 10 MHz signal is applied to
the FinRF pin. The source current can be measured by setting the RF PLL phase detector to a positive polarity, and the sink
current can be measured by setting the phase detector to a negative polarity. The IF PLL currents can be measured in a similar
way. Note that the magnitude of the RF PLL charge pump current is controlled by the RF_CPG bit. Once the charge pump
currents are known, the mismatch can be calculated as well. In order to measure leakage, the charge pump is set to a TRI-STATE
mode by enabling the RF_CPT and IF_CPT bits. The table below shows a summary of the various charge pump tests.
Current Test
RF_CPG
RF_CPP
RF_CPT
IF_CPP
IF_CPT
RF Source
0 to 15
0
0
X
X
RF Sink
0 to 15
1
0
X
X
RF TRI-STATE
X
X
1
X
X
IF Source
X
X
X
0
0
IF Sink
X
X
X
1
0
IF TRI-STATE
X
X
X
X
1
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16
LMX2485/LMX2485E
Bench Test Setups
(Continued)
Charge Pump Current Specification Definitions
20087750
Sink Current at VCPout = Vcc - ∆V
Sink Current at VCPout = Vcc/2
Sink Current at VCPout = ∆V
Source Current at VCPout = Vcc - ∆V
Source Current at VCPout = Vcc/2
I6 = Charge Pump Source Current at VCPout = ∆V
∆V = Voltage offset from the positive and negative supply rails. Defined to be 0.5 volts for this part.
vCPout refers to either VCPoutRF or VCPoutIF
I1
I2
I3
I4
I5
=
=
=
=
=
Charge
Charge
Charge
Charge
Charge
Pump
Pump
Pump
Pump
Pump
ICPout refers to either ICPoutRF or ICPoutIF
Charge Pump Output Current Magnitude Variation vs. Charge Pump Output Voltage
20087751
Charge Pump Sink Current vs. Charge Pump Output Source Current Mismatch
20087752
Charge Pump Output Current Magnitude Variation vs. Temperature
20087753
17
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LMX2485/LMX2485E
Bench Test Setups
(Continued)
20087770
Frequency Input Pin
DC Blocking Capacitor
Corresponding Counter
Default Counter Value
MUX Value
OSCin
1000 pF
RF_R / 2
50
14
FinRF
100 pF// 1000 pF
RF_N / 2
502 + 2097150 / 4194301
15
FinIF
100 pF
IF_N / 2
534
13
OSCin
1000 pF
IF_R / 2
50
12
Sensitivity Measurement Procedure
Sensitivity is defined as the power level limits beyond which the output of the counter being tested is off by 1 Hz or more of its
expected value. It is typically measured over frequency, voltage, and temperature. In order to test sensitivity, the MUX[3:0] word
is programmed to the appropriate value. The counter value is then programmed to a fixed value and a frequency counter is set
to monitor the frequency of this pin. The expected frequency at the Ftest/LD pin should be the signal generator frequency divided
by twice the corresponding counter value. The factor of two comes in because the LMX2485 has a flip-flop which divides this
frequency by two to make the duty cycle 50% in order to make it easier to read with the frequency counter. The frequency counter
input impedance should be set to high impedance. In order to perform the measurement, the temperature, frequency, and voltage
is set to a fixed value and the power level of the signal is varied. Note that the power level at the part is assumed to be 4 dB less
than the signal generator power level. This accounts for 1 dB for cable losses and 3 dB for the pad. The power level range where
the frequency is correct at the Ftest/LD pin to within 1 Hz accuracy is recorded for the sensitivity limits. The temperature,
frequency, and voltage can be varied in order to produce a family of sensitivity curves. Since this is an open-loop test, the charge
pump is set to TRI-STATE and the unused side of the PLL (RF or IF) is powered down when not being tested. For this part, there
are actually four frequency input pins, although there is only one frequency test pin (Ftest/LD). The conditions specific to each pin
are shown in above table.
Note that for the RF N counter, a fourth order fractional modulator is used in 22-bit mode with a fraction of 2097150 / 4194301
is used. The reason for this long fraction is to test the RF N counter and supporting fractional circuitry as completely as possible.
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18
LMX2485/LMX2485E
Bench Test Setups
(Continued)
20087771
Input Impedance Measurement Procedure
The above block diagram shows the test setup used for measuring the input impedance for the LMX2485. The DC blocking
capacitor used between the input SMA connector and the pin being measured must be changed to a zero Ohm resistor. This
procedure applies to the FinRF, FinIF, and OSCin pins. The basic test procedure is to calibrate the network analyzer, ensure that
the part is powered up, and then measure the input impedance. The network analyzer can be calibrated by using either calibration
standards or by soldering resistors directly to the evaluation board. An open can be implemented by putting no resistor, a short
can be implemented by soldering a zero ohm resistor as close as possible to the pin being measured, and a short can be
implemented by soldering two 100 ohm resistors in parallel as close as possible to the pin being measured. Calibration is done
with the PLL removed from the PCB. This requires the use of a clamp down fixture that may not always be generally available.
If no clamp down fixture is available, then this procedure can be done by calibrating up to the point where the DC blocking
capacitor usually is, and then implementing port extensions with the network analyzer. Zero ohm resistor is added back for the
actual measurement. Once the setup is calibrated, it is necessary to ensure that the PLL is powered up. This can be done by
toggling the power down bits (RF_PD and IF_PD) and observing that the current consumption indeed increases when the bit is
disabled. Sometimes it may be necessary to apply a signal to the OSCin pin in order to program the part. If this is necessary,
disconnect the signal once it is established that the part is powered up. It is useful to know the input impedance of the PLL for
the purposes of debugging RF problems and designing matching networks. Another use of knowing this parameter is make the
trace width on the PCB such that the input impedance of this trace matches the real part of the input impedance of the PLL
frequency of operation. In general, it is good practice to keep trace lengths short and make designs that are relatively resistant
to variations in the input impedance of the PLL.
19
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LMX2485/LMX2485E
be rather involved, but there are many simulation tools and
references available, such as the one given at the end of the
functional description block.
Functional Description (Note 9)
1.0 GENERAL
The LMX2485 consists of integrated N counters, R counters,
and charge pumps. The TCXO, VCO and loop filter are
supplied external to the chip. The various blocks are described below.
1.5 N COUNTERS AND HIGH FREQUENCY INPUT PINS
The N counter divides the VCO frequency down to the
comparison frequency. Because prescalers are used, there
are limitations on how small the N value can be. The N
counters are discussed in greater depth in the programming
section. Since the input pins to these counters ( FinRF and
FinIF ) are high frequency, layout considerations are important.
High Frequency Input Pins, FinRF and FinIF
1.1 TCXO, OSCILLATOR BUFFER, AND R COUNTER
The oscillator buffer must be driven single-ended by a signal
source, such as a TCXO. The OSCout pin is included to
provide a buffered output of this input signal and is active
when the OSC_OUT bit is set to one. The ENOSC pin can
be also pulled high to ensure that the OSCout pin is active,
regardless of the status of the registers in the LMX2485.
It is generally recommended that the VCO output go through
a resistive pad and then through a DC blocking capacitor
before it gets to these high frequency input pins. If the trace
length is sufficiently short ( < 1/10th of a wavelength ), then
the pad may not be necessary, but a series resistor of about
39 ohms is still recommended to isolate the PLL from the
VCO. The DC blocking capacitor should be chosen at least
to be 27 pF, depending on frequency. It may turn out that the
frequency is above the self-resonant frequency of the capacitor, but since the input impedance of the PLL tends to be
capacitive, it actually is a benefit to exceed the tune frequency. The pad and the DC blocking capacitor should be
placed as close to the PLL as possible
Complementary High Frequency Pin, FinRF*
These inputs may be used to drive the PLL differentially, but
it is very common to drive the PLL in a single ended fashion.
A shunt capacitor should be placed at the FinRF* pin. The
value of this capacitor should be chosen such that the impedance, including the ESR of the capacitor, is as close to
an AC short as possible at the operating frequency of the
PLL. 100 pF is a typical value, depending on frequency.
The R counter divides this TXCO frequency down to the
comparison frequency.
1.2 PHASE DETECTOR
The maximum phase detector operating frequency for the IF
PLL is straightforward, but it is a little more involved for the
RF PLL since it is fractional. The maximum phase detector
frequency for the LMX2485 RF PLL is 50 MHz. However, this
is not possible in all circumstances due to illegal divide ratios
of the N counter. The crystal reference frequency also limits
the phase detector frequency, although the doubler helps
with this limitation. There are trade-offs in choosing the
phase detector frequency. If this frequency is run higher,
then phase noise will be lower, but lock time may be increased due to cycle slipping and the capacitors in the loop
filter may become rather large.
1.3 CHARGE PUMP
For the majority of the time, the charge pump output is high
impedance, and the only current through this pin is the
Tri-State leakage. However, it does put out fast correction
pulses that have a width that is proportional to the phase
error presented at the phase detector.
The charge pump converts the phase error presented at the
phase detector into a correction current. The magnitude of
this current is theoretically constant, but the duty cycle is
proportional to the phase error. For the IF PLL, this current is
not programmable, but for the RF PLL it is programmable in
16 steps. Also, the RF PLL allows for a higher charge pump
current to be used when the PLL is locking in order to reduce
the lock time.
1.6 POWER PINS, POWER DOWN, AND POWER UP
MODES
It is recommended that all of the power pins be filtered with
a series 18 ohm resistor and then placing two capacitors
shunt to ground, thus creating a low pass filter. Although it
makes sense to use large capacitor values in theory, the
ESR ( Equivalent Series Resistance ) is greater for larger
capacitors. For optimal filtering minimize the sum of the ESR
and theoretical impedance of the capacitor. It is therefore
recommended to provide two capacitors of very different
sizes for the best filtering. 1 µF and 100 pF are typical
values. The small capacitor should be placed as close as
possible to the pin.
The power down state of the LMX2485 is controlled by many
factors. The one factor that overrides all other factors is the
CE pin. If this pin is low, the part will be powered down.
Asserting a high logic level on this pin is necessary to power
up the chip, however, there are other bits in the programming
registers that can override this and put the PLL back in a
power down state. Provided that the voltage on the CE pin is
high, programming the RF_PD and IF_PD bits to zero guarantees that the part will be powered up. Programming either
one of these bits to one will power down the appropriate
section of the synthesizer, provided that the ATPU bit does
not override this.
1.4 LOOP FILTER
The loop filter design can be rather involved. In addition to
the regular constraints and design parameters, delta-sigma
PLLs have the additional constraint that the order of the loop
filter should be one greater than the order of the delta sigma
modulator. This rule of thumb comes from the requirement
that the loop filter must roll off the delta sigma noise at 20
dB/decade faster than it rises. However, since the noise can
not have infinite power, it must eventually roll off. If the loop
bandwidth is narrow, this requirement may not be necessary.
For the purposes of discussion in this datasheet, the pole of
the loop filter at 0 Hz is not counted. So a second order filter
has 3 components, a 3rd order loop filter has 5 components,
and the 4th order loop filter has 7 components. Although a
5th order loop filter is theoretically necessary for use with a
4th order modulator, typically a 4th order filter is used in this
case. The loop filter design, especially for higher orders can
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20
LMX2485/LMX2485E
Functional Description (Note 9)
(Continued)
CE Pin
RF_PD
ATPU
Bit Enabled +
Write to RF
N Counter
Low
X
X
Powered Down
(Asynchronous)
High
X
Yes
Powered Up
High
0
No
Powered Up
High
1
No
Powered Down
( Asynchronous )
PLL State
1.7 DIGITAL LOCK DETECT OPERATION
The RF PLL digital lock detect circuitry compares the difference between the phase of the inputs of the phase detector
to a RC generated delay of e. To indicate a locked state
(Lock = HIGH) the phase error must be less than the e RC
delay for 5 consecutive reference cycles. Once in lock (Lock
= HIGH), the RC delay is changed to approximately δ. To
indicate an out of lock state (Lock = LOW), the phase error
must become greater δ. The values of e and δ are dependent
on which PLL is used and are shown in the table below:
PLL
e
δ
RF
10 ns
20 ns
IF
15 ns
30 ns
When the PLL is in the power down mode and the Ftest/LD
pin is programmed for the lock detect function, it is forced
LOW. The accuracy of this circuit degrades at higher comparison frequencies. To compensate for this, the DIV4 word
should be set to one if the comparison frequency exceeds 20
MHz. The function of this word is to divide the comparison
frequency presented to the lock detect circuit by 4. Note that
if the MUX[3:0] word is set such as to view lock detect for
both PLLs, an unlocked (LOW) condition is shown whenever
either one of the PLLs is determined to be out of lock.
20087704
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LMX2485/LMX2485E
Functional Description (Note 9)
nally intended only for second order filters, so when implementing it with higher order filters, the third and fourth poles
can not be too close in, or it will not be possible to keep the
loop filter well optimized when the higher charge pump current and Fastlock resistor are engaged.
(Continued)
1.8 CYCLE SLIP REDUCTION AND FASTLOCK
The LMX2485 offers both cycle slip reduction (CSR) and
Fastlock with timeout counter support. This means that it
requires no additional programming overhead to use them. It
is generally recommended that the charge pump current in
the steady state be 8X or less in order to use cycle slip
reduction, and 4X or less in steady state in order to use
Fastlock. The next step is to decide between using Fastlock
or CSR. This determination can be made based on the ratio
of the comparison frequency ( fCOMP ) to loop bandwidth
( BW ).
Comparison
Frequency
( fCOMP )
Fastlock
fCOMP ≤ 1.25 MHz
Noticeable better
than CSR
1.25 MHz <
fCOMP ≤ 2 MHz
Marginally better
than CSR
fCOMP > 2 MHz
Same or worse
than CSR
1.8.1 Using Cycle Slip Reduction (CSR) to Avoid Cycle
Slipping
Once it is decided that CSR is to be used, the cycle slip
reduction factor needs to be chosen. The available factors
are 1/2, 1/4, and 1/16. In order to preserve the same loop
characteristics, it is recommended that the following constraint be satisfied: (Fastlock Charge Pump Current) /
(Steady State Charge Pump Current) = CSR
In order to satisfy this constraint, the maximum charge pump
current in steady state is 8X for a CSR of 1/2, 4X for a CSR
of 1/4, and 1X for a CSR of 1/16. Because the PLL phase
noise is better for higher charge pump currents, it makes
sense to choose CSR only as large as necessary to prevent
cycle slipping. Choosing it larger than this will not improve
lock time, and will result in worse phase noise.
Consider an example where the desired loop bandwidth in
steady state is 100 kHz and the comparison frequency is 20
MHz. This yields a ratio of 200. Cycle slipping may be
present, but would not be too severe if it was there. If a CSR
factor of 1/2 is used, this would reduce the ratio to 100 during
frequency acquisition, which is probably sufficient. A charge
pump current of 8X could be used in steady state, and a
factor of 16X could be used during frequency acquisition.
This yields a ratio of 1/2, which is equal to the CSR factor
and this satisfies the above constraint. In this circumstance,
it could also be decided to just use 16X charge pump current
all the time, since it would probably have better phase noise,
and the degradation in lock time would not be too severe.
Cycle Slip
Reduction
( CSR )
Likely to provide
a benefit,
provided that
fCOMP > 100 X
BW
Cycle Slip Reduction (CSR)
Cycle slip reduction works by reducing the comparison frequency during frequency acquisition while keeping the same
loop bandwidth, thereby reducing the ratio of the comparison
frequency to the loop bandwidth. In cases where the ratio of
the comparison frequency exceeds about 100 times the loop
bandwidth, cycle slipping can occur and significantly degrade lock times. The greater this ratio, the greater the
benefit of CSR. This is typically the case of high comparison
frequencies. In circumstances where there is not a problem
with cycle slipping, CSR provides no benefit. There is a glitch
when CSR is disengaged, but since CSR should be disengaged long before the PLL is actually in lock, this glitch is not
an issue. A good rule of thumb for CSR disengagement is to
do this at the peak time of the transient response. Because
this time is typically much sooner than Fastlock should be
disengaged, it does not make sense to use CSR and Fastlock in combination.
Fastlock
Fastlock works by increasing the loop bandwidth only during
frequency acquisition. In circumstances where the comparison frequency is less than or equal to 2 MHz, Fastlock may
provide a benefit beyond what CSR can offer. Since Fastlock
also reduces the ratio of the comparison frequency to the
loop bandwidth, it may provide a significant benefit in cases
where the comparison frequency is above 2 MHz. However,
CSR can usually provide an equal or larger benefit in these
cases, and can be implemented without using an additional
resistor. The reason for this restriction on frequency is that
Fastlock has a glitch when it is disengaged. As the time of
engagement for Fastlock decreases and becomes on the
order of the fast lock time, this glitch grows and limits the
benefits of Fastlock. This effect becomes worse at higher
comparison frequencies. There is always the option of reducing the comparison frequency at the expense of phase
noise in order to satisfy this constraint on comparison frequency. Despite this glitch, there is still a net improvement in
lock time using Fastlock in these circumstances. When using
Fastlock, it is also recommended that the steady state
charge pump state be 4X or less. Also, Fastlock was origiwww.national.com
1.8.2 Using Fastlock to Improve Lock Times
20087740
Once it is decided that Fastlock is to be used, the loop
bandwidth multiplier, K, is needed in order to determine the
theoretical impact of Fastlock on the loop bandwidth and the
resistor value, R2p, that is switched in parallel during Fastlock. This ratio is calculated as follows: K = ( Fastlock
Charge Pump Current ) / ( Steady State Charge Pump
Current )
K
Loop Bandwidth
R2p Value
Lock Time
1
1.00 X
Open
100 %
2
1.41 X
R2/0.41
71 %
3
1.73 X
R2/0.73
58%
4
2.00 X
R2
50%
8
2.83 X
R2/1.83
35%
9
3.00 X
R2/2
33%
16
4.00 X
R2/3
25%
The above table shows how to calculate the Fastlock resistor
and theoretical lock time improvement, once the ratio , K, is
22
usually one-half or one-fourth. There are trade-offs between
fractional spurs, sub-fractional spurs, and phase noise. The
rules of thumb presented in this section are just that. There
will be exceptions. The bits that impact the fractional spurs
are FM and DITH, and these bits should be set in this order.
(Continued)
known. This all assumes a second order filter (not counting
the pole at 0 Hz). However, it is generally recommended that
the loop filter order be one greater than the order of the delta
sigma modulator, which means that a second order filter is
never recommended. In this case, the value for R2p is
typically about 80% of what it would be for a second order
filter. Because the Fastlock disengagement glitch gets larger
and it is harder to keep the loop filter optimized as the K
value becomes larger, designing for the largest possible
value for K usually, but not always yields the best improvement in lock time. To get a more accurate estimate requires
more simulation tools, or trial and error.
The first step to do is choose FM, for the delta sigma
modulator order. It is recommended to start with FM = 3 for
a third order modulator and use strong dithering. In general,
there is a trade-off between primary and sub-fractional
spurs. Choosing the highest order modulator (FM = 0 for 4th
order) typically provides the best primary fractional spurs,
but the worst sub-fractional spurs. Choosing the lowest
modulator order (FM = 2 for 2nd order), typically gives the
worst primary fractional spurs, but the best sub-fractional
spurs. Choosing FM = 3, for a 3rd order modulator is a
compromise.
The second step is to choose DITH, for dithering. Dithering
has a very small impact on primary fractional spurs, but a
much larger impact on sub-fractional spurs. The only problem is that it can add a few dB of phase noise, or even more
if the loop bandwidth is very wide. Disabling dithering (DITH
= 0), provides the best phase noise, but the sub-fractional
spurs are worst (except when the fractional numerator is 0,
and in this case, they are the best). Choosing strong dithering (DITH = 2) significantly reduces sub-fractional spurs, if
not eliminating them completely, but adds the most phase
noise. Weak dithering (DITH = 1) is a compromise.
The third step is to tinker with the fractional word. Although
1/10 and 400/4000 are mathematically the same, expressing
fractions with much larger fractional numerators often improve the fractional spurs. Increasing the fractional denominator only improves spurs to a point. A good practical limit
could be to keep the fractional denominator as large as
possible, but not to exceed 4095, so it is not necessary to
use the extended fractional numerator or denominator.
This steps can be done in different orders and it might take
a few iterations to find the optimum performance. Special
considerations should be taken for lower frequencies that
are below about 100 MHz. In addition squaring up the wave,
it is often helpful to use lowest terms fractions instead of
highest terms fractions. Also, dithering may turn out to not be
so useful. All the things are to introduce a methodical way of
thinking about optimizing spurs, not an exact method. There
will be exceptions to all these rules.
1.8.3 Capacitor Dielectric Considerations for Lock
Time
The LMX2485 has a high fractional modulus and high
charge pump gain for the lowest possible phase noise. One
consideration is that the reduced N value and higher charge
pump may cause the capacitors in the loop filter to become
larger in value. For larger capacitor values, it is common to
have a trade-off between capacitor dielectric quality and
physical size. Using film capacitors or NPO/COG capacitors
yields the best possible lock times, where as using X7R or
Z5R capacitors can increase lock time by 0 – 500%. However, it is a general tendency that designs that use a higher
compare frequency tend to be less sensitive to the effects of
capacitor dielectrics. Although the use of lesser quality dielectric capacitors may be unavoidable in many circumstances, allowing a larger footprint for the loop filter capacitors, using a lower charge pump current, and reducing the
fractional modulus are all ways to reduce capacitor values.
Capacitor dielectrics have very little impact on phase noise
and spurs.
1.9 FRACTIONAL SPUR AND PHASE NOISE
CONTROLS
Control of the fractional spurs is more of an art than an exact
science. The first differentiation that needs to be made is
between primary fractional and sub-fractional spurs. The
primary fractional spurs are those that occur at increments of
the channel spacing only. The sub-fractional spurs are those
that occur at a smaller resolution than the channel spacing,
Note 9: For more information concerning delta-sigma PLLs, loop filter design, cycle slip reduction, Fastlock, and many other topics, visit wireless.national.com. Here
there is the EasyPLL simulation tool and an online reference called "PLL Performance, Simulation, and Design", by Dean Banerjee.
23
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LMX2485/LMX2485E
Functional Description (Note 9)
LMX2485/LMX2485E
Programming Description
2.0 GENERAL PROGRAMMING INFORMATION
The 24-bit data registers are loaded through a MICROWIRE Interface. These data registers are used to program the R counter,
the N counter, and the internal mode control latches. The data format of a typical 24-bit data register is shown below. The control
bits CTL [3:0] decode the register address. On the rising edge of LE, data stored in the shift register is loaded into one of the
appropriate latches (selected by address bits). Data is shifted in MSB first. Note that it is best to program the N counter last, since
doing so initializes the digital lock detector and Fastlock circuitry. Note that initialize means it resets the counters, but it does NOT
program values into these registers. The exception is when 22-bit is not being used. In this case, it is not necessary to program
the R7 register.
MSB
LSB
DATA [21:0]
CTL [3:0]
23
4
3
2
1
0
2.0.1 Register Location Truth Table
The control bits CTL [3:0] decode the internal register address. The table below shows how the control bits are mapped to the
target control register.
C3
C2
C1
C0
DATA Location
x
x
x
0
R0
0
0
1
1
R1
0
1
0
1
R2
0
1
1
1
R3
1
0
0
1
R4
1
0
1
1
R5
1
1
0
1
R6
1
1
1
1
R7
2.0.2 Control Register Content Map
Because the LMX2485 registers are complicated, they are organized into two groups, basic and advanced. The first four registers
are basic registers that contain critical information necessary for the PLL to achieve lock. The last 5 registers are for features that
optimize spur, phase noise, and lock time performance. The next page shows these registers.
www.national.com
24
(Continued)
22
0
0
0
0
0
0
R7
0
RF_FD[21:12]
0
RF_CPF[3:0]
1
CSR[1:0]
0
0
0
0
RF_CPG[3:0]
17
16
15
14
13
12
11
10
0
1
1
0
IF_N[18:0]
0
9
0
1
IF_R[11:0]
1
RF_FD[11:0]
16
0
DITH
[1:0]
0
15
14
13
12
11
10
9
8
0
0
FM
[1:0]
DIV4
0
0
OSC
_2X
1
OSC
_OUT
RF_
CPP
1
8
6
5
0
0
0
RF_FN[11:0]
7
0
0
1
6
IF_RST
0
4
5
IF_CPT
MUX
[3:0]
RF_RST
RF_FN[11:0]
7
RF_FN[21:12]
IF_P
IF_R[11:0]
RF_TOC[13:0]
IF_
CPP
IF_N[18:0]
RF_FD[11:0]
DATA[19:0] ( Except for the RF_N Register, which is [22:0] )
RF_CPG[3:0]
R6
ATPU
ACCESS[3:0]
R5
R4
R3
IF_PD
18
R2
19
RF_R[5:0]
0
20
RF_N[10:0]
21
RF_PD
RF_P
17
Complete Register Map The complete register map shows all the functionality of all registers, including the last five.
0
R1
23
1
R0
REGISTER
0
R4
0
0001
R3
RF_R[5:0]
IF_PD
RF_P
18
DATA[19:0] ( Except for the RF_N Register, which is [22:0] )
19
RF_PD
20
R2
21
R1
22
RF_N[10:0]
23
R0
REGISTER
RF_CPT
4
1
0
0
0
C3
3
3
1
1
1
1
0
0
0
C3
0
1
1
0
C2
2
2
1
1
0
0
1
1
0
C2
0
1
0
1
C1
1
1
1
0
1
0
1
0
1
C1
1
1
1
1
1
1
1
0
C0
0
1
1
1
1
0
C0
0
Quick Start Register Map Although it is highly recommended that the user eventually take advantage of all the modes of the LMX2485, the quick start register map is shown in order
for the user to get the part up and running quickly using only those bits critical for basic functionality. The following default conditions for this programming state are a third order
delta-sigma modulator in 12-bit mode with no dithering and no Fastlock.
Programming Description
LMX2485/LMX2485E
25
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LMX2485/LMX2485E
Programming Description
(Continued)
2.1 R0 REGISTER
Note that this register has only one control bit, so the N counter value to be changed with a single write statement to the PLL.
REGISTER
23
22
21
20
19
18
17
16
15
14
13
12
11
10
9
8
7
6
5
4
3
2
1
DATA[22:0]
R0
0
C0
RF_N[10:0]
RF_FN[11:0]
0
2.1.1 RF_FN[11:0] -- Fractional Numerator for RF PLL
Refer to section 2.6.1 for a more detailed description of this control word.
2.1.2 RF_N[10:0] -- RF N Counter Value
The RF N counter contains an 8/9/12/13 and a 16/17/20/21 prescaler. The N counter value can be calculated as follows:
N = RF_P·RF_C + 4·RF_B + RF_A
RF_C ≥Max{RF_A, RF_B} , for N-2FM-1 ... N+2FM is a necessary condition. This rule is slightly modified in the case where the
RF_B counter has an unused bit, where this extra bit is used by the delta-sigma modulator for the purposes of modulation.
Consult the tables below for valid operating ranges for each prescaler.
Operation with the 8/9/12/13 Prescaler (RF_P=0)
RF_N
RF_N [10:0]
RF_C [6:0]
< 25
RF_B [1:0]
RF_A [1:0]
N values less than 25 are prohibited.
25-26
Possible only with a second order delta-sigma engine.
27-30
Possible only with a second or third order delta-sigma engine.
31
0
0
0
0
0
1
1
0
1
1
...
.
.
.
.
.
.
.
0
.
.
.
1023
1
1
1
1
1
1
1
0
1
1
1
> 1023
1
N values above 1023 are prohibited.
Operation with the 16/17/20/21 Prescaler (RF_P=1)
RF_N [10:0]
RF_N
RF_C [6:0]
RF_B [1:0]
< 49
N values less than 49 are prohibited.
49-50
Possible only with a second order delta-sigma engine.
51-54
Possible with a second or third order delta-sigma engine.
RF_A [1:0]
55
0
0
0
0
0
1
1
0
1
1
...
.
.
.
.
.
.
.
.
.
.
.
2039
1
1
1
1
1
1
1
0
1
1
1
2040-2043
Possible with a second or third order delta-sigma engine.
2044-2045
Possible only with a second order delta-sigma engine.
> 2045
N values greater than 2045 are prohibited.
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26
1
(Continued)
2.2 R1 REGISTER
REGISTER
23
22
R1
RF_PD
RF_P
21
20
19
18
17
16
15
14
13
12
11
10
9
8
7
6
5
DATA[19:0]
RF_R[5:0]
RF_FD[11:0]
4
3
2
1
0
C3
C2
C1
C0
0
0
1
1
2.2.1 RF_FD[11:0] -- RF PLL Fractional Denominator
The function of these bits are described in section 2.6.2.
2.2.2 RF_R [5:0] -- RF R Divider Value
The RF R Counter value is determined by this control word. Note that this counter does allow values down to one.
R Value
RF_R[5:0]
1
0
0
0
0
0
...
.
.
.
.
.
1
.
63
1
1
1
1
1
1
2.2.3 RF_P -- RF Prescaler bit
The prescaler used is determined by this bit.
RF_P
Prescaler
Maximum Frequency
0
8/9/12/13
2000 MHz
1
16/17/20/21
3000 MHz
2.2.4 RF_PD -- RF Power Down Control Bit
When this bit is set to 0, the RF PLL operates normally. When it is set to one, the RF PLL is powered down and the RF Charge
pump is set to a TRI-STATE mode. The CE pin and ATPU bit also control power down functions, and will override the RF_PD bit.
The order of precedence is as follows. First, if the CE pin is LOW, then the PLL will be powered down. Provided this is not the
case, the PLL will be powered up if the ATPU bit says to do so, regardless of the state of the RF_PD bit. After the CE pin and the
ATPU bit are considered, then the RF_PD bit then takes control of the power down function for the RF PLL.
27
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LMX2485/LMX2485E
Programming Description
LMX2485/LMX2485E
Programming Description
(Continued)
2.3 R2 REGISTER
REGISTER
23
R2
IF_PD
22
21
20
19
18
17
16
15
14
13
12
11
10
9
8
7
6
5
4
DATA[19:0]
IF_N[18:0]
3
2
1
0
C3
C2
C1
C0
0
1
0
1
2.3.1 IF_N[18:0] -- IF N Divider Value
IF_N Counter Programming with the 8/9 Prescaler (IF_P=0)
IF_N[18:0]
N Value
IF_B
IF_A
≤23
N values less than or equal to 23 are prohibited because IF_B ≥ 3 is required.
24-55
Legal divide ratios in this range are:
24-27, 32-36, 40-45, 48-54
56
0
0
0
0
0
0
0
0
0
0
0
0
1
1
1
0
0
0
0
57
0
0
0
0
0
0
0
0
0
0
0
0
1
1
1
0
0
0
1
...
.
.
.
.
.
.
.
.
.
.
.
.
.
.
.
.
.
.
.
262143
1
1
1
1
1
1
1
1
1
1
1
1
1
1
1
0
1
1
1
Operation with the 16/17 Prescaler (IF_P=1)
N Value
IF_B
IF_A
≤47
N values less than or equal to 47 are prohibited because IF_B ≥ 3 is required.
48-239
Legal divide ratios in this range are:
48-51, 64-68, 80-85, 96-102, 112-119, 128-136, 144-153, 160-170, 176-187, 192-204, 208-221, 224-238
240
0
0
0
0
0
0
0
0
0
0
0
1
1
1
1
0
0
0
0
241
0
0
0
0
0
0
0
0
0
0
0
1
1
1
1
0
0
0
1
...
.
.
.
.
.
.
.
.
.
.
.
.
.
.
.
.
.
.
.
524287
1
1
1
1
1
1
1
1
1
1
1
1
1
1
1
1
1
1
1
2.3.4 IF_PD -- IF Power Down Bit
When this bit is set to 0, the IF PLL operates normally. When it is set to 1, the IF PLL powers down and the output of the IF PLL
charge pump is set to a TRI-STATE mode. If the ATPU bit is set and register R0 is written to, the IF_PD will be reset to 0 and the
IF PLL will be powered up. If the CE pin is held low, the IF PLL will be powered down, overriding the IF_PD bit.
www.national.com
28
(Continued)
2.4 R3 REGISTER
REGISTER
23
22
21
20
19
18
17
16
15
14
13
12
11
10
9
8
7
6
5
4
DATA[19:0]
R3
ACCESS[3:0]
RF_CPG[3:0]
3
2
1
0
C3
C2
C1
C0
0
1
1
1
IF_R[11:0]
2.4.1 IF_R[11:0] -- IF R Divider Value
For the IF R divider, the R value is determined by the IF_R[11:0] bits in the R3 register. The minimum value for IF_R is 3.
R Value
IF_R[11:0]
3
0
0
0
0
0
0
0
0
0
0
1
...
.
.
.
.
.
.
.
.
.
.
.
1
.
4095
1
1
1
1
1
1
1
1
1
1
1
1
2.4.2 RF_CPG -- RF PLL Charge Pump Gain
This is used to control the magnitude of the RF PLL charge pump in steady state operation.
RF_CPG
Charge Pump State
Typical RF Charge Pump Current at 3 Volts (µA)
0
1X
95
1
2X
190
2
3X
285
3
4X
380
4
5X
475
5
6X
570
6
7X
665
7
8X
760
8
9X
855
9
10X
950
10
11X
1045
11
12X
1140
12
13X
1235
13
14X
1330
14
15X
1425
15
16X
1520
2.4.3 ACCESS -- Register Access word
It is mandatory that the first 5 registers R0-R4 be programmed. The programming of registers R5-R7 is optional. The
ACCESS[3:0] bits determine which additional registers need to be programmed. Any one of these registers can be individually
programmed. According to the table below, when the state of a register is in default mode, all the bits in that register are forced
to a default state and it is not necessary to program this register. When the register is programmable, it needs to be programmed
through the MICROWIRE. Using this register access technique, the programming required is reduced up to 37%.
ACCESS Bit
Register Location
Register Controlled
ACCESS[0]
R3[20]
Must be set to 1
ACCESS[1]
R3[21]
R5
ACCESS[2]
R3[22]
R6
ACCESS[3]
R3[23]
R7
The default conditions the registers is shown below:
Register
23
22
21
20
19
18
17
16
15
14
13
12
11
10
9
8
7
6
5
4
Data[19:0]
R4
3
2
1
0
C3
C2
C1
C0
R4 Must be programmed manually.
R5
0
0
0
0
0
0
0
0
0
0
0
0
0
0
0
0
0
0
0
0
1
0
1
1
R6
0
0
0
0
0
0
0
0
0
0
0
0
0
0
0
0
0
0
0
0
1
1
0
1
R7
0
0
0
0
0
0
0
0
0
0
0
0
1
0
0
1
0
0
0
0
1
1
1
1
29
www.national.com
LMX2485/LMX2485E
Programming Description
LMX2485/LMX2485E
Programming Description
(Continued)
This corresponds to the following bit settings.
Register
R4
R5
R6
R7
www.national.com
Bit Location
Bit Name
Bit Description
Bit Value
Bit State
R4[23]
ATPU
Autopowerup
0
Disabled
R4[17:16]
DITH
Dithering
2
Strong
3rd Order
R4[15:14]
FM
Modulation Order
3
R4[12]
OSC_2X
Oscillator Doubler
0
Disabled
R4[11]
OSC_OUT
OSCout Pin Enable
0
Disabled
R4[10]
IF_CPP
IF Charge Pump Polarity
1
Positive
R4[9]
RF_CPP
RF Charge Pump Polarity
1
Positive
R4[8]
IF_P
IF PLL Prescaler
1
16/17
R4[7:4]
MUX
Ftest/LD Output
0
Disabled
R5[23:14]
RF_FD[21:12]
Extended Fractional Denominator
0
Disabled
R5[13:4]
RF_FN[21:12]
Extended Fractional Numerator
0
Disabled
R6[23:22]
CSR
Cycle Slip Reduction
0
Disabled
R6[21:18]
RF_CPF
Fastlock Charge Pump Current
0
Disabled
R6[17:4]
RF_TOC
RF Timeout Counter
0
Disabled
R7[13]
DIV4
Lock Detect Adjustment
0
Disabled (Fcomp ≤ 20 MHz)
R7[7]
IF_RST
IF PLL Counter Reset
0
Disabled
R7[6]
RF_RST
RF PLL Counter Reset
0
Disabled
R7[5]
IF_CPT
IF PLL Tri-State
0
Disabled
R7[4]
RF_CPT
RF PLL Tri-State
0
Disabled
30
(Continued)
2.5 R4 REGISTER
This register controls the conditions for the RF PLL in Fastlock.
REGISTER
23
22 21 20 19 18 17 16 15 14 13
12
11
10
9
8
7 6 5 4
DATA[19:0]
R4
ATPU
0
1
0
0
DITH
[1:0]
0
FM
[1:0]
0
3
2
1
0
C3 C2 C1 C0
OSC_ OSC_ IF_ RF_
IF_P
2X
OUT CPP CPP
MUX
[3:0]
1
0
0
1
2.5.1 MUX[3:0] Frequency Out & Lock Detect MUX
These bits determine the output state of the Ftest/LD pin.
MUX[3:0]
Output Type
Output Description
0
0
0
0
High Impedance
Disabled
0
0
0
1
Push-Pull
General purpose output, Logical “High” State
0
0
1
0
Push-Pull
General purpose output, Logical “Low” State
0
0
1
1
Push-Pull
RF & IF Digital Lock Detect
0
1
0
0
Push-Pull
RF Digital Lock Detect
0
1
0
1
Push-Pull
IF Digital Lock Detect
0
1
1
0
Open Drain
RF & IF Analog Lock Detect
0
1
1
1
Open Drain
RF Analog Lock Detect
1
0
0
0
Open Drain
IF Analog Lock Detect
1
0
0
1
Push-Pull
RF & IF Analog Lock Detect
1
0
1
0
Push-Pull
RF Analog Lock Detect
1
0
1
1
Push-Pull
IF Analog Lock Detect
1
1
0
0
Push-Pull
IF R Divider divided by 2
1
1
0
1
Push-Pull
IF N Divider divided by 2
1
1
1
0
Push-Pull
RF R Divider divided by 2
1
1
1
1
Push-Pull
RF N Divider divided by 2
2.5.2 IF_P -- IF Prescaler
When this bit is set to 0, the 8/9 prescaler is used. Otherwise the 16/17 prescaler is used.
IF_P
IF Prescaler
Maximum Frequency
0
8/9
800 MHz
1
16/17
800 MHz
2.5.3 RF_CPP -- RF PLL Charge Pump Polarity
RF_CPP
RF Charge Pump Polarity
0
Negative
1
Positive (Default)
2.5.4 IF_CPP -- IF PLL Charge Pump Polarity
For a positive phase detector polarity, which is normally the case, set this bit to 1. Otherwise set this bit for a negative phase
detector polarity.
IF_CPP
IF Charge Pump Polarity
0
Negative
1
Positive
2.5.5 OSC_OUT Oscillator Output Buffer Enable
OSC_OUT
OSCout Pin
0
Disabled (High Impedance)
1
Buffered output of OSCin pin
31
www.national.com
LMX2485/LMX2485E
Programming Description
LMX2485/LMX2485E
Programming Description
(Continued)
2.5.6 OSC2X -- Oscillator Doubler Enable
When this bit is set to 0, the oscillator doubler is disabled and the TCXO frequency presented to the IF R and RF R counters is
equal to that of the input frequency of the OSCin pin. When this bit is set to 1, the TCXO frequency presented to the RF R counter
is doubled. Phase noise added by the doubler is negligible.
OSC2X
Frequency Presented to RF R Counter
Frequency Presented to IF R Counter
0
fOSCin
fOSCin
1
2 x fOSCin
2.5.7 FM[1:0] -- Fractional Mode
Determines the order of the delta-sigma modulator. Higher order delta-sigma modulators reduce the spur levels closer to the
carrier by pushing this noise to higher frequency offsets from the carrier. In general, the order of the loop filter should be at least
one greater than the order of the delta-sigma modulator in order to allow for sufficient roll-off.
FM
Function
0
Fractional PLL mode with a 4th order delta-sigma modulator
1
Disable the delta-sigma modulator. Recommended for test use only.
2
Fractional PLL mode with a 2nd order delta-sigma modulator
3
Fractional PLL mode with a 3rd order delta-sigma modulator
2.5.8 DITH[1:0] -- Dithering Control
Dithering is a technique used to spread out the spur energy. Enabling dithering can reduce the main fractional spurs, but can also
give rise to a family of smaller spurs. Whether dithering helps or hurts is application specific. Enabling the dithering may also
increase the phase noise. In most cases where the fractional numerator is zero, dithering usually degrades performance.
Dithering tends to be most beneficial in applications where there is insufficient filtering of the spurs. This often occurs when the
loop bandwidth is very wide or a higher order delta-sigma modulator is used. Dithering tends not to impact the main fractional
spurs much, but has a much larger impact on the sub-fractional spurs. If it is decided that dithering will be used, best results will
be obtained when the fractional denominator is at least 1000.
DITH
Dithering Mode Used
0
Disabled
1
Weak Dithering
2
Strong Dithering
3
Reserved
2.5.9 ATPU -- PLL Automatic Power Up
When this bit is set to 1, both the RF and IF PLL power up when the R0 register is written to. When the R0 register is written to,
the PD_RF and PD_IF bits are changed to 0 in the PLL registers. The exception to this case is when the CE pin is low. In this
case, the ATPU function is disabled.
www.national.com
32
(Continued)
2.6 R5 REGISTER
REGISTER
23
22
21
20
19
18
17
16
15
14
13
12
11
10
9
8
7
6
5
4
DATA[19:0]
R5
RF_FD[21:12]
RF_FN[21:12]
3
2
1
0
C3
C2
C1
C0
1
0
1
1
2.6.1 Fractional Numerator Determination { RF_FN[21:12], RF_FN[11:0], ACCESS[1] }
In the case that the ACCESS[1] bit is 0, then the part operates in 12-bit fractional mode, and the RF_FN2[21:12] bits become do
not care bits. When the ACCESS[1] bit is set to 1, the part operates in 22-bit mode and the fractional numerator is expanded from
12 to 22-bits.
Fractional
RF_FN[21:12]
Numerator
( These bits only apply in 22- bit mode)
RF_FN[11:0]
0
In 12- bit mode, these are do not care.
In 22- bit mode, for N < 4096,
these bits should be all set to 0.
1
...
4095
0
0
0
0
0
0
0
0
0
0
0
0
0
0
0
0
0
0
0
0
0
0
0
1
.
.
.
.
.
.
.
.
.
.
.
.
1
1
1
1
1
1
1
1
1
1
1
1
4096
0
0
0
0
0
0
0
0
0
1
0
0
0
0
0
0
0
0
0
0
0
0
...
.
.
.
.
.
.
.
.
.
.
.
.
.
.
.
.
.
.
.
.
.
.
4194303
1
1
1
1
1
1
1
1
1
1
1
1
1
1
1
1
1
1
1
1
1
1
2.6.2 Fractional Denominator Determination { RF_FD[21:12], RF_FD[11:0], ACCESS[1]}
In the case that the ACCESS[1] bit is 0, then the part is operates in the 12-bit fractional mode, and the RF_FD[21:12] bits become
do not care bits. When the ACCESS[1] is set to 1, the part operates in 22-bit mode and the fractional denominator is expanded
from 12 to 22-bits.
Fractional
RF_FD[21:12]
Denominator
( These bits only apply in 22- bit mode)
0
In 12- bit mode, these are do not care.
In 22- bit mode, for N < 4096,
these bits should be all set to 0.
1
...
RF_FD[11:0]
4095
4096
0
0
0
0
0
0
0
0
0
1
0
0
0
0
0
0
0
0
0
0
0
0
0
0
0
0
0
0
0
0
0
0
0
1
.
.
.
.
.
.
.
.
.
.
.
.
1
1
1
1
1
1
1
1
1
1
1
1
0
0
0
0
0
0
0
0
0
0
0
0
...
.
.
.
.
.
.
.
.
.
.
.
.
.
.
.
.
.
.
.
.
.
.
4194303
1
1
1
1
1
1
1
1
1
1
1
1
1
1
1
1
1
1
1
1
1
1
33
www.national.com
LMX2485/LMX2485E
Programming Description
LMX2485/LMX2485E
Programming Description
(Continued)
2.7 R6 REGISTER
REGISTER
23
22
21
20
19
18
17
16
15
14
13
12
11
10
9
8
7
DATA[19:0]
R6
CSR[1:0]
RF_CPF[3:0]
RF_TOC[13:0]
6
5
4
3
2
1
0
C3
C2
C1
C0
1
1
0
1
2.7.1 RF_TOC -- RF Time Out Counter and Control for FLoutRF Pin
The RF_TOC[13:0] word controls the operation of the RF Fastlock circuitry as well as the function of the FLoutRF output pin.
When this word is set to a value between 0 and 3, the RF Fastlock circuitry is disabled and the FLoutRF pin operates as a general
purpose CMOS TRI-STATE I/O. When RF_TOC is set to a value between 4 and 16383, the RF Fastlock mode is enabled and
the FLoutRF pin is utilized as the RF Fastlock output pin. The value programmed into the RF_TOC[13:0] word represents two
times the number of phase detector comparison cycles the RF synthesizer will spend in the Fastlock state.
RF_TOC
Fastlock Mode
Fastlock Period [CP events]
FLoutRF Pin Functionality
0
Disabled
N/A
High Impedance
1
Manual
N/A
Logic “0” State.
Forces all Fastlock conditions
2
Disabled
N/A
Logic “0” State
3
Disabled
N/A
Logic “1” State
4
Enabled
4X2 = 8
Fastlock
5
Enabled
5X2 = 10
Fastlock
…
Enabled
…
Fastlock
16383
Enabled
16383X2 = 32766
Fastlock
2.7.2 RF_CPF -- RF PLL Fastlock Charge Pump Current
Specify the charge pump current for the Fastlock operation mode for the RF PLL. Note that the Fastlock charge pump current,
steady state current, and CSR control are all interrelated.
RF_CPF
RF Charge Pump State
Typical RF Charge Pump Current at 3 Volts (µA)
0
1X
95
1
2X
190
2
3X
285
3
4X
380
4
5X
475
5
6X
570
6
7X
665
7
8X
760
8
9X
855
9
10X
950
10
11X
1045
11
12X
1140
12
13X
1235
13
14X
1330
14
15X
1425
15
16X
1520
www.national.com
34
(Continued)
2.7.3 CSR[1:0] -- RF Cycle Slip Reduction
CSR controls the operation of the Cycle Slip Reduction Circuit. This circuit can be used to reduce the occurrence of phase
detector cycle slips. Note that the Fastlock charge pump current, steady state current, and CSR control are all interrelated. Refer
to section 1.8 for information on how to use this.
CSR
CSR State
0
Disabled
Sample Rate Reduction Factor
1
1
Enabled
1/2
2
Enabled
1/4
3
Enabled
1/16
35
www.national.com
LMX2485/LMX2485E
Programming Description
LMX2485/LMX2485E
Programming Description
(Continued)
2.8 R7 REGISTER
REGISTER 23 22 21 20 19 18 17 16 15 14
13
12 11 10 9 8
7
6
5
4
Data[19:0]
R7
0
0
0
0
0
0
0
0
0
0 DIV4 0
1
3
2
1
0
C3 C2 C1 C0
0 0 1 IF_RST RF_RST IF_CPT RF_CPT 1
1
1
1
2.8.1 DIV4 -- RF Digital Lock Detect Divide By 4
Because the digital lock detect function is based on a phase error, it becomes more difficult to detect a locked condition for larger
comparison frequencies. When this bit is enabled, it subdivides the RF PLL comparison frequency (it does not apply to the IF
comparison frequency) presented to the digital lock detect circuitry by 4. This enables this circuitry to work at higher comparison
frequencies. It is recommended that this bit be enabled whenever the comparison frequency exceeds 20 MHz and RF digital lock
detect is being used.
2.8.2 IF_RST -- IF PLL Counter Reset
When this bit is enabled, the IF PLL N and R counters are reset, and the charge pump is put in a Tri-State condition. This feature
should be disabled for normal operation. Note that a counter reset is applied whenever the chip is powered up via software or CE
pin.
IF_RST
IF PLL N and R Counters
IF PLL Charge Pump
0 (Default)
Normal Operation
Normal Operation
1
Counter Reset
Tri-State
2.8.3 RF_RST -- RF PLL Counter Reset
When this bit is enabled, the RF PLL N and R counters are reset and the charge pump is put in a Tri-State condition. This feature
should be disabled for normal operation. This feature should be disabled for normal operation. Note that a counter reset is applied
whenever the chip is powered up via software or CE pin.
RF_RST
RF PLL N and R Counters
RF PLL Charge Pump
0 (Default)
Normal Operation
Normal Operation
1
Counter Reset
Tri-State
2.8.4 RF_TRI -- RF Charge Pump Tri-State
When this bit is enabled, the RF PLL charge pump is put in a Tri-State condition, but the counters are not reset. This feature is
typically disabled for normal operation.
RF_TRI
RF PLL N and R Counters
RF PLL Charge Pump
0 (Default)
Normal Operation
Normal Operation
1
Normal Operation
Tri-State
2.8.5 IF_TRI -- IF Charge Pump Tri-State
When this bit is enabled, the IF PLL charge pump is put in a Tri-State condition, but the counters are not reset. This feature is
typically disabled for normal operation.
IF_TRI
IF PLL N and R Counters
IF PLL Charge Pump
0 (Default)
Normal Operation
Normal Operation
1
Normal Operation
Tri-State
www.national.com
36
inches (millimeters) unless otherwise noted
Plastic Quad LLP (SQ), Bottom View
Order Number LMX2485SQ or LMX2485ESQ for 1000 Unit Reel
Order Number LMX2485SQX or LMX2485ESQX for 4500 Unit Reel
NS Package Number SQA24A
National does not assume any responsibility for use of any circuitry described, no circuit patent licenses are implied and National reserves
the right at any time without notice to change said circuitry and specifications.
For the most current product information visit us at www.national.com.
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LMX2485/LMX2485E High Performance Delta-Sigma Low Power Dual PLLatinum Frequency
Synthesizer
Physical Dimensions