Demodulating at 10.7 MHz IF with the SA605

AN1996
Demodulating at 10.7 MHz IF with the SA605
Rev. 2 — 28 August 2014
Application note
Document information
Info
Content
Keywords
RSSI extender circuit, RSSI dynamic range, SAW filter, quadrature tank, S
curve, tapped-C transform matching network.
Abstract
This application note discusses RF circuit techniques and principles that
will enhance stable receiver operation. Consideration is given to PCB
layout, special circuits, such as, the RSSI extender, and passive
component selection. Performance data is provided for specific
applications at 240 MHz and 45 MHz RF inputs.
AN1996
NXP Semiconductors
Demodulating at 10.7 MHz IF with the SA605
Revision history
Rev
Date
Description
2
20140828
Application note; second release
Modifications:
1
19971023
•
The format of this application note has been redesigned to comply with the new identity
guidelines of NXP Semiconductors.
•
Legal texts have been adapted to the new company name where appropriate.
Application note; intial release
Contact information
For more information, please visit: http://www.nxp.com
For sales office addresses, please send an email to: salesaddresses@nxp.com
AN1996
Application note
All information provided in this document is subject to legal disclaimers.
Rev. 2 — 28 August 2014
© NXP Semiconductors N.V. 2014. All rights reserved.
2 of 25
AN1996
NXP Semiconductors
Demodulating at 10.7 MHz IF with the SA605
1. Introduction
The need for high-speed communications is increasing in the market place. To meet these
needs, high-performance receivers must demodulate at higher IF frequencies to
accommodate for the wider deviations in FM systems.
The standard 455 kHz IF frequency, which is easier to work with, and thus more forgiving
in production, no longer satisfies the high-speed communication market. The next higher
standard IF frequency is 10.7 MHz. This frequency offers more potential bandwidth than
455 kHz, allowing for faster communications.
Since the wavelength at 10.7 MHz is much smaller than 455 kHz, the demand for a good
RF layout and good RF techniques increases. These demands aid in preventing
regeneration from occurring in the IF section of the receiver. This application note will
discuss some of the RF techniques used to obtain a stable receiver and reveal the
excellent performance achieved in the lab.
1.1 Background
If a designer is working with the SA605 for the first time, it is highly recommended that
AN1994 (Ref. 2) and AN1995 (Ref. 3) be read. These two application notes discuss the
SA605 in great detail and provide a good starting point in designing with the chip.
Before starting a design, it is also important to choose the correct part. NXP offers an
extensive receiver line to meet the growing demands of the wireless market. Table 1
displays the different types of receivers and their key features. With the aid of this chart,
a designer will get a good idea for choosing a chip that best fits their design needs.
If low-voltage receiver parts are required in a design, a designer can choose between
SA606 or SA636. These low-voltage receivers are designed to operate at 3 V while still
providing high performance to meet the specifications for cellular radio. All of these parts
can operate with an IF frequency as high as 2 MHz. However, the SA636 can operate with
a standard IF frequency of 10.7 MHz and also provide fast RSSI speed. Additionally the
SA636 has a Power-down mode to conserve battery power.
AN1996
Application note
All information provided in this document is subject to legal disclaimers.
Rev. 2 — 28 August 2014
© NXP Semiconductors N.V. 2014. All rights reserved.
3 of 25
xxxxxxxxxxxxxxxxxxxxx xxxxxxxxxxxxxxxxxxxxxxxxxx xxxxxxx x x x xxxxxxxxxxxxxxxxxxxxxxxxxxxxxx xxxxxxxxxxxxxxxxxxx xx xx xxxxx
xxxxxxxxxxxxxxxxxxxxxxxxxxx xxxxxxxxxxxxxxxxxxx xxxxxx xxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxx xxxxxxxxxxxx x x
xxxxxxxxxxxxxxxxxxxxx xxxxxxxxxxxxxxxxxxxxxxxxxxxxxx xxxxx xxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxx xxxxxxxx
xxxxxxxxxxxxxxxxxxxxxxxxx xxxxxxxxxxxxxxxxxxxx xxx
NXP Semiconductors
AN1996
Application note
Table 1.
FM/IF family overview
Specification
SA602A
SA604A
SA605
SA606
SA636
VCC
4.5 V to 8 V
4.5 V to 8 V
4.5 V to 8 V
2.7 V to 7 V
2.7 V to 5.5 V
ICC
2.4 mA at 6 V
3.3 mA at 6 V
5.7 mA at 6 V
3.5 mA at 3 V
6.5 mA at 3 V
Number of pins
8
16
20
20
20
Packages
SA602AD/01: SO8
SA604AD/01: SO16
SA605D/01: SO20
SA606DK/01: SSOP20
SA636BS: HVQFN20
SA605DK/01: SSOP20
SA606DK/02: SSOP20
SA636DK/01: SSOP20
SA606DK/03: SSOP20
12 dB SINAD
(RF = 45 MHz; IF = 455 kHz);
1 kHz tone; 8 kHz deviation
120 dBm / 0.22 V
120 dBm / 0.22 V
120 dBm / 0.22 V
117 dBm / 0.31 V
112 dBm / 0.54 V
(RF = 240 MHz;
IF = 10.7 MHz)
Process ft
8 GHz
8 GHz
8 GHz
8 GHz
8 GHz
For lower-cost version and
less performance
SA612A
SA614A
SA615
SA616
-
•
•
Features
•
Audio and data pins
IF bandwidth of
25 MHz
No external
matching required
for standard
455 kHz IF filter
•
•
•
•
•
Audio and data pins
IF bandwidth of
25 MHz
No external
matching required
for standard
455 kHz IF filter
•
•
•
Audio and data pins
IF bandwidth of
25 MHz
No external
matching required
for standard
455 kHz IF filter
•
•
•
•
•
•
Low-voltage
Internal RSSI and
audio op amps
No external
matching required
for standard
455 kHz IF filter
IF bandwidth of
2 MHz
•
•
Power-down mode
Low-voltage
Fast RSSI time
IF bandwidth of
25 MHz
Internal RSSI and
audio op amps
No external
matching required
for standard
10.7 MHz IF filter
RSSI output section
90 dB
90 dB
90 dB
90 dB
90 dB
Accuracy
1.5 dB
1.5 dB
1.5 dB
1.5 dB
1.5 dB
-
1.4 s
-
-
-
-
21.3 s
-
-
-
Rise time[1]
-
1.5 s
-
-
1.2 s
Fall time[1]
-
19.4 s
-
-
2 s
455 kHz IF
Rise time[1]
Fall
time[1]
10.7 MHz IF
AN1996
4 of 25
© NXP Semiconductors N.V. 2014. All rights reserved.
Dynamic range
Demodulating at 10.7 MHz IF with the SA605
Rev. 2 — 28 August 2014
All information provided in this document is subject to legal disclaimers.
1 kHz tone;
70 kHz deviation
xxxxxxxxxxxxxxxxxxxxx xxxxxxxxxxxxxxxxxxxxxxxxxx xxxxxxx x x x xxxxxxxxxxxxxxxxxxxxxxxxxxxxxx xxxxxxxxxxxxxxxxxxx xx xx xxxxx
xxxxxxxxxxxxxxxxxxxxxxxxxxx xxxxxxxxxxxxxxxxxxx xxxxxx xxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxx xxxxxxxxxxxx x x
xxxxxxxxxxxxxxxxxxxxx xxxxxxxxxxxxxxxxxxxxxxxxxxxxxx xxxxx xxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxx xxxxxxxx
xxxxxxxxxxxxxxxxxxxxxxxxx xxxxxxxxxxxxxxxxxxxx xxx
FM/IF family overview …continued
Specification
SA602A
SA604A
SA605
SA606
SA636
17 dB
-
13 dB
17 dB
13 dB
3rd-order intercept point (input) 13 dB
f1 = 45 MHz; f2 = 45.06 MHz
-
10 dBm
9 dBm
11 dBm
(f1 = 240.05 MHz;
f2 = 240.35 MHz)
NXP Semiconductors
AN1996
Application note
Table 1.
Mixer
Max. conversion power gain
(RF = 45 MHz; IF = 455 kHz)
5 dB
-
5 dB
6.2 dB
11 dB at 240 MHz
1.5 k
-
4.7 k
8 k
4.7 k
3.5 pF
3 pF
3.5 pF at 240 MHz
Output resistance
1.5 k
-
1.5 k
1.5 k
330 
Total IF gain
-
100 dB
100 dB
100 dB
96 dB (includes 6 dB
pad)
Total IF bandwidth
-
25 MHz
25 MHz
2 MHz
25 MHz
Input impedance
-
1.6 k
1.6 k
1.5 k
330 
Output impedance
-
1.0 k
1.0 k
330 
330 
Gain
-
40 dB
40 dB
44 dB
44 dB
Bandwidth
-
41 MHz
41 MHz
5.5 MHz
40 MHz
-
1.6 k
1.6 k
1.5 k
330 
3 pF
IF Section
IF amplifier
IF limiter
Input impedance
-
330 
330 
330 
330 
Gain
-
60 dB
60 dB
58 dB
58 dB
Bandwidth
-
28 MHz
28 MHz
4.5 MHz
28 MHz
Output
No IF filters in the circuit.
AN1996
5 of 25
© NXP Semiconductors N.V. 2014. All rights reserved.
[1]
impedance[1]
Demodulating at 10.7 MHz IF with the SA605
Rev. 2 — 28 August 2014
All information provided in this document is subject to legal disclaimers.
Noise Figure at 45 MHz
RF input resistance and
capacitance at 45 MHz
AN1996
NXP Semiconductors
Demodulating at 10.7 MHz IF with the SA605
1.2 Objective
The objective of this application note is to show that the SA605 can perform well at an IF
frequency of 10.7 MHz. Since most NXP Semiconductors receiver demoboards are
characterized at RF = 45 MHz/IF = 455 kHz, we decided to continue to characterize at
this frequency. This way we could compare how much degradation (for different IFs) there
was with a RF = 45 MHz/IF = 455 kHz versus RF = 45 MHz/ IF = 10.7 MHz. As we will
discuss later, there was minimal degradation in performance.
We also tested at RF = 240 MHz/IF = 10.7 MHz. The 240 MHz RF is sometimes referred
to as the first IF for double conversion receivers. Testing the board at RF = 83.16 MHz
(which is also a common first IF for analog cellular radio) and IF = 10.7 MHz was not done
because the conversion gain and noise figure does not change that much compared to
45 MHz input. Therefore, we can expect the same type of performance at 83.16 MHz.
The RF = 240 MHz/IF = 10.7 MHz demoboard is expected to perform less than the
RF = 45 MHz/IF = 10.7 MHz demoboard because the mixer conversion gain decreases
while the noise figure increases. These two parameters will decrease the performance of
the receiver as the RF frequency increases.
For systems requiring low voltage operation, IF = 10.7 MHz and fast RSSI speed, the
SA636 will be the correct choice.
2. Board setup and performance graphs
Figure 1 and Figure 2 show the SA605 schematics for the 240 MHz and 45 MHz boards,
respectively.
Table 2 lists the basic functions of each external component for both Figure 1 and
Figure 2.
AN1996
Application note
All information provided in this document is subject to legal disclaimers.
Rev. 2 — 28 August 2014
© NXP Semiconductors N.V. 2014. All rights reserved.
6 of 25
AN1996
NXP Semiconductors
Demodulating at 10.7 MHz IF with the SA605
D1
R18
J1
RF IN
56k
C2
10pF
C3
330pF
C7
1-5pF
C1
1-5pF
L5
8.2μH
L1
56nH
MXR OUT
RF IN
C6
15pF
L2
82nH
C8
330pF
C10
10μF
VCC
C26
47pF
C9
.1μF
BYPASS
DECOUP
OSC OUT
OSC IN
MUTE IN
VCC
RSSI OUT
AUDIO OUT
DATA OUT
QUAD IN
IF IN
DECOUP
R9
100k
C16
39pF
C32
220pF
IF OUT
GND
LIM IN
DECOUP
DECOUP
LIM OUT
C31
220pF
TO
RSSI
(PIN 7)
C21
0.1μF
C17
C25
0.1μF
0.1μF
C22
0.1μF
L3
8.2μH
C18
0.1μF
R10
100k
C30
220pF
8.2μH
C19
47pF
C20
47pF
C14
1nF
C13
150pF
R20
5.1k
Q1
BC857B
IFT1
5.6μH
AUDIO
R19
100k
C29
47pF
L4
C15
1pF
RSSI
C12
.01μF
C27 C28
47pF 47pF
C23
0.1μF
U1 SA605D
C11
.1μF
BAS16
3
2
C24
1nF
J2
LO
VCC
FLT1
1
FLT1
1
3
2
R17
220
R16
R11
100k
120
DATA
an1996_1
Fig 1.
SA605 schematic: RF = 240 MHz, LO = 229.3 MHz, IF = 10.7 MHz
AN1996
Application note
All information provided in this document is subject to legal disclaimers.
Rev. 2 — 28 August 2014
© NXP Semiconductors N.V. 2014. All rights reserved.
7 of 25
AN1996
NXP Semiconductors
Demodulating at 10.7 MHz IF with the SA605
D1
R18
J1
RF IN
56k
C2
120pF
C3
330pF
C1
5-30pF
L5
8.2 μH
L1
620nH
C7
5-30pF
RF IN
C6
150pF
L2
390nH
C8
330pF
C10
10μF
V CC
C26
47pF
C9
.1 μF
MXR OUT
BYPASS
DECOUP
OSC OUT
OSC IN
MUTE IN
V CC
RSSI OUT
AUDIO OUT
DATA OUT
QUAD IN
IF IN
DECOUP
C16
39pF
IF OUT
GND
LIM IN
DECOUP
DECOUP
LIM OUT
8.2 μH
C12
.01 μF
C17
0.1 μF
C22
0.1 μF
L3
8.2 μH
C19
47pF
FLT1
1
C20
47pF
C14
1nF
3
2
R17
220
R16
R11
100k
C13
150pF
TO
RSSI
(PIN 7)
C25
C18
0.1 μF
R10
100k
C31
220pF
C21
0.1 μF
IFT1
5.6 μH
AUDIO
C32
220pF
C30
220pF
L4
C23
0.1 μF
C15
1pF
RSSI
C29
47pF
R20
10k
Q1
BC857B
0.1 μF
R9
100k
R19
100k
2
U1 SA605D
C11
.1 μF
BAS16
3
C27 C28
47pF 47pF
C24
1nF
J2
LO
V CC
FLT1
1
120
DATA
Fig 2.
an1996_2
SA605 schematic: RF = 45 MHz, LO = 55.7 MHz, IF = 10.7 MHz
Table 2.
AN1996
Application note
SO layout schematic component list
Component
Description
U1
SA605D/01
FLT1
10.7 MHz ceramic filter Murata SFE10.7MA5-A (280 kHz bandwidth)[1]
FLT2
10.7 MHz ceramic filter Murata SFE10.7MA5-A (280 kHz bandwidth)[1]
C1
part of the tapped-C network to match the front-end mixer
C2
part of the tapped-C network to match the front-end mixer
C3
used as an AC short to Pin 2 and to provide a DC block for L1, which prevents
the upsetting of the DC biasing on Pin 1
C6
part of the tapped-C network to match the LO input
C7
part of the tapped-C network to match the LO input
C8
DC blocking capacitor
C9
supply bypassing
C10
supply bypassing (this value can be reduced if the SA605 is used with a battery)
C11
used as a filter, cap value can be adjusted when higher RSSI speed is preferred
over lower RSSI ripple
C12
used as a filter
C13
used as a filter
C14
used to AC ground the quad tank
C15
used to provide the 90 phase shift to the phase detector
C16
quad tank component to resonant at 10.7 MHz with IFT1 and C15
All information provided in this document is subject to legal disclaimers.
Rev. 2 — 28 August 2014
© NXP Semiconductors N.V. 2014. All rights reserved.
8 of 25
AN1996
NXP Semiconductors
Demodulating at 10.7 MHz IF with the SA605
Table 2.
SO layout schematic component list …continued
Component
Description
C17
IF limiter decoupling capacitor
C18
DC block for L3 which prevents the upsetting of the DC biasing on Pin 14
C19
part of the tapped-C network for FLT2
C20
part of the tapped-C network for FLT2
C21
IF amplifier decoupling capacitor
C22
DC blocking capacitor
C23
IF amplifier decoupling capacitor and DC block for L3 which prevents the
upsetting of the DC biasing on Pin 14
C24
provides DC block for L5 which prevents the upsetting of the DC biasing on
Pin 20
C25
IF limiter decoupling capacitor
C26
part of the tapped-C network for FLT1
C27
part of the tapped-C network for FLT1
C28
part of the tapped-C network for FLT1
C29
part of the tapped-C network for FLT1
R9
used to convert the current into the RSSI voltage
R10
converts the audio current to a voltage
R11
converts the data current to a voltage
R16
used to kill some of the IF signal for stability purposes
R17
used in conjunction with R16 for a matching network for FLT2
L1
part of the tapped-C network to match the front-end mixer
L2
part of the tapped-C network to match the front-end mixer
L3
part of the tapped-C network to match the input of FLT2
L4
part of the tapped-C network to match the input of FLT1
L5
part of the tapped-C network to match the input of FLT1
RSSI extender circuit
R18
provides bias regulation, the gain will stay constant over varying VCC
R19
for biasing, buffer RF DC voltage
R20
provides the DC bias, RSSI gain (when R20 increases, RSSI gain decreases)
C30
DC blocking capacitor which connects the ceramic filter’s output to the PNP
transistor’s input
C31
decoupling capacitor, and should be removed for measuring RSSI systems
speed
C32
peak detector charge capacitor
D1
diode to stabilize the bias current
Q1
NXP BC857B PNP transistor
IFT1
part of the quad tank circuit
[1]
AN1996
Application note
If a designer wants to use different IF bandwidth filters than the ones used in this application note, the quad
tank’s S-curve may need to be adjusted to accommodate the new bandwidth.
All information provided in this document is subject to legal disclaimers.
Rev. 2 — 28 August 2014
© NXP Semiconductors N.V. 2014. All rights reserved.
9 of 25
AN1996
NXP Semiconductors
Demodulating at 10.7 MHz IF with the SA605
There are minor differences between Figure 1 and Figure 2. The RF and LO tapped-C
component values are changed to accommodate for the different RF and LO test
frequencies (RF = 240 MHz and 45 MHz and LO = 229.3 MHz and 55.7 MHz). The other
difference is the value of R20. This resistor value was changed to optimize the RSSI
curve’s linearity (see RSSI extender section in this application note for further details).
The recommended SA605 layout is shown in Figure 3. This layout can be integrated with
other systems.
5.2cm
SA605
4.8cm
T O P S IL K S C R E E N
T O P V IE W
B O T T O M V IE W
an1996_3
Fig 3.
SA605 SO demoboard layouts (not actual size)
Figure 4 and Figure 5 show the performance graphs for the SA605 at 240 MHz and
45 MHz RF inputs.
AN1996
Application note
All information provided in this document is subject to legal disclaimers.
Rev. 2 — 28 August 2014
© NXP Semiconductors N.V. 2014. All rights reserved.
10 of 25
AN1996
NXP Semiconductors
Demodulating at 10.7 MHz IF with the SA605
10.00
0.00
AUDIO
--10.00
--20.00
--30.00
THD + N(dB)
--40.00
--50.00
AM REJ (dB)
--60.00
NOISE
--70.00
--80.00
--125
--115
--105
--95
--85
--75
--65
--55
--45
--35
--25
--15
--5
RF LEVEL (dBm)
5.0
4.5
4.0
3.5
3.0
RSSI (V)
2.5
2.0
1.5
1.0
0.5
0.0
--130
--120
--110
--100
--90
--80
--70
--60
RF IN (dBm)
--50
--40
--30
--20
--10
0
10
an1996_4
Fig 4.
SA605 SO performance graphs at 240 MHz
10.00
0.00
AUDIO
--10.00
--20.00
--30.00
THD + N(dB)
--40.00
--50.00
AM REJ (dB)
--60.00
NOISE
--70.00
--80.00
--125
--115
--105
--95
--85
--75
--65
--55
--45
--35
--25
--30
--20
--15
--5
RF LEVEL (dBm)
5.0
4.5
4.0
3.5
RSSI (V)
3.0
2.5
2.0
1.5
1.0
0.5
0.0
--130
--120
--110
--100
--90
--80
--70
--60
--50
RF IN (dBm)
--40
--10
0
10
an1996_5
Fig 5.
AN1996
Application note
SA605 SO performance graphs at 45 MHz
All information provided in this document is subject to legal disclaimers.
Rev. 2 — 28 August 2014
© NXP Semiconductors N.V. 2014. All rights reserved.
11 of 25
AN1996
NXP Semiconductors
Demodulating at 10.7 MHz IF with the SA605
3. RF input
The SA605 board is set up to receive an RF input of 240 MHz (see Figure 1). This is
achieved by implementing a tapped-C network. The deviation should be set to 70 kHz to
achieve 110 dBm to 112 dBm for 12 dB SINAD. However, the deviation can be
increased to 100 kHz, depending on the bandwidth of the IF filter and the Q of the
quad tank.
Because we wanted to test the board at 45 MHz, we changed the values of the tapped-C
network for the RF and LO ports (see Figure 2). We found that a 116 dBm to 118 dBm
for 12 dB SINAD could be achieved. With these results, we were pretty close to
achieving performance similar to our standard 455 kHz IF board.
A designer can also make similar RF and LO component changes if he/she needs to
evaluate the board at a different RF frequency. It should be noted that if a designer
purchases a stuffed SA605 demoboard from NXP Semiconductors, its setup will be
for an RF input frequency of 240 MHz. AN1994 will aid the designer in calculating the
tapped-C values for other desired frequencies, while AN1995 will be of value for making
S11 bench measurements. Just remember that the input impedance will differ for different
RF frequencies.
4. LO input
The LO frequency should be 229.3 MHz for the RF = 240 MHz demoboard and have a
drive level of 10 dBm to 0 dBm (this also applies for the RF = 45 MHz and
LO = 55.7 MHz). The drive level is important to achieve maximum conversion gain. The
LO input also has a matched tapped-C network for efficiency purposes which makes for
good RF practices.
If a designer wanted to change the matching network to inject a different LO frequency,
he/she could follow the steps in AN1994 and assume that the input impedance is around
10 k for low frequency inputs. The main goal is to get maximum voltage transfer from the
signal generator to the inductor.
An external oscillator circuit was used to provide greater flexibility in choosing different RF
and LO frequencies; however, an on-board oscillator can be used with the SA605. New
high frequency fundamental crystals, now entering the market, can also be used for high
LO frequency requirements. Most receiver systems, however, will use a synthesizer to
drive the LO port.
5. 10.7 MHz ceramic filters
The input and output impedance of the 10.7 MHz ceramic IF filters are 330 . The SA605
input and output impedances are roughly 1.5 k. Therefore, a matching circuit had to be
implemented to obtain maximum voltage transfer. Tapped-C networks were used to match
the filters input and output impedance. But in this case, we decided to go with non-tuning
elements to reduce set-up time. Figure 6 shows the values chosen for the network.
Although our total deviation is 140 kHz, we used 280 kHz IF bandwidth filters to maximize
for fast RSSI speed. The SINAD performance difference between using 180 kHz
bandwidth filter versus 280 kHz band shaping filter was insignificant.
AN1996
Application note
All information provided in this document is subject to legal disclaimers.
Rev. 2 — 28 August 2014
© NXP Semiconductors N.V. 2014. All rights reserved.
12 of 25
AN1996
NXP Semiconductors
Demodulating at 10.7 MHz IF with the SA605
FROM
MIXER
(PIN 20)
C26
47pF
L5
8.2 μH
10.7MHz
CERAMIC
FILTER
C 27
47pF
C24
1nF
C29
47pF
FLT1
Z = 330 Ω
C 28
47pF
L4
8.2 μH
TO
IF AMP
INPUT
(PIN18)
C23
0.1 μF
Z = 330 Ω
T A P P E D -C N E T W O R K
T A P P E D -C N E T W O R K
IF LIMITER IN
(PIN14)
FLT2
R17
220 Ω
FROM IF AMP OUT
(PIN 16)
C22
0.1 μF
R16
120 Ω
10.7MHz
CERAMIC
FILTER
C19
47pF
C20
47pF
L3
8.2 μH
C23
.1 μF
an1996_6
Fig 6.
AN1996
Application note
Matching configuration for FLT1 and FLT2
All information provided in this document is subject to legal disclaimers.
Rev. 2 — 28 August 2014
© NXP Semiconductors N.V. 2014. All rights reserved.
13 of 25
AN1996
NXP Semiconductors
Demodulating at 10.7 MHz IF with the SA605
6. Stabilizing the IF section from regeneration
Because the gain in the IF section is 100 dB and the wavelength for 10.7 MHz is small,
the hardest design phase of this project was to stabilize the IF section.
The steps below show the methods used to obtain a stable layout.
1. The total IF section (IF amp and limiter) gain is 100 dB, which makes it difficult to
stabilize the chip at 10.7 MHz. Therefore, a 120  resistor (R16 of Figure 1) was used
to kill some of the IF gain to obtain a stable system.
Remark: Expect AM rejection performance to degrade as you decrease the IF gain
externally.
2. Since the tapped-C inductors for FLT1 and FLT2 are not shielded, it is important not to
place them too close to one another. Magnetic coupling will occur and may increase
the probability of regeneration.
3. It was also found that if the IF limiter bypass capacitors do not have the same physical
ground, the stability worsens. Referring to Figure 1, the IF limiter bypass capacitors
(C17, C25) are connected to assure a common ground.
4. The positioning of ground feedthroughs are vital. A designer should put feedthroughs
near the IF bypass capacitors ground points. In addition, feedthroughs are needed
underneath the chip. Other strategic locations are important for feedthroughs where
insufficient grounding occurs.
5. Shielding should be used after the best possible stability is achieved. The SA605
demoboard is stable, so shielding was not used. However, if put into a bigger system,
shielding should be used to keep out unwanted RF frequencies. As a special note, if a
good shield is used, it can increase the R16 resistor value such that there is less IF
gain to kill to achieve stability. This means the RSSI dynamic range is improved. So if
a designer does not want to implement the RSSI extender circuit, but is still
concerned with SINAD and RSSI range, he/she can experiment with R16 and
shielding because there is a correlation between them (see RSSI extender section in
this application note for more information). In addition, AM rejection performance will
improve due to the greater availability of the total IF gain.
The key to stabilizing the IF section is to kill the gain. This was done with a resistor (R16 in
Figure 6) to ground. All the other methods mentioned above are secondary compared to
this step. Lowering the value of this resistor reduces the gain and the increasing resistor
value kills less gain. For our particular layout, 120  was chosen to obtain a stable board,
but we were careful not to kill too much gain. One of the downfalls of killing too much gain
is that the SINAD reading will become worse and the RSSI dynamic range is reduced.
7. RSSI dynamic range
There are two main factors which determine the RSSI dynamic range:
• How stable is the board?
• How much gain is killed externally?
AN1996
Application note
All information provided in this document is subject to legal disclaimers.
Rev. 2 — 28 August 2014
© NXP Semiconductors N.V. 2014. All rights reserved.
14 of 25
AN1996
NXP Semiconductors
Demodulating at 10.7 MHz IF with the SA605
If the board is unstable, a high RSSI voltage reading will occur at the bottom end of the
curve. If too much gain is taken away, the upper half of the curve is flattened. Thus the
dynamic range can be affected. Figure 7 shows how the linear range can be decreased
under the conditions mentioned above.
STRONG
RSSI (V)
IF GAIN
DECREASING
STABILITY
DECREASING
RSSI DYNAMIC
RANGE
RSSI DYNAMIC RANGE
WEAK
WEAK
RF INPUT (dBm)
STRONG
an1996_7
Fig 7.
RSSI dynamic range
It is important to choose the appropriate resistor to kill enough gain to get stability but not
too much gain to affect the upper RSSI curve dynamic range. Because we had to kill
some IF gain to achieve good board stability and good SINAD readings, our RSSI overall
dynamic range was reduced on the upper end of the curve.
Because SINAD and the RSSI dynamic range are two important parameters for most of
our customers, we decided to add an ‘RSSI extender’ modification to the board to get the
best of both worlds. Together with the RSSI external modification and the ‘stability
resistor’, we can now achieve excellent SINAD readings and maintain a wide RSSI
dynamic range.
8. RSSI extender circuit
The RSSI extender circuit increases the upper dynamic range roughly about 20 dB to
30 dB for the 240 MHz demoboard. The SA605 demoboard has 90 dB to 100 dB of linear
dynamic range when the RSSI modification is used.
Referring to Figure 8, one can see that one transistor is used with a few external
components. The IF input signal to the PNP transistor is tapped after the ceramic filter to
ensure a clean IF signal. The circuit then senses the strength of the signal and converts it
to current, which is then summed together with the RSSI output of the chip.
AN1996
Application note
All information provided in this document is subject to legal disclaimers.
Rev. 2 — 28 August 2014
© NXP Semiconductors N.V. 2014. All rights reserved.
15 of 25
AN1996
NXP Semiconductors
Demodulating at 10.7 MHz IF with the SA605
R18
56kΩ
VCC
D1
BAS16
R20
5.1kΩ for 240MHz
10kΩ for 45MHz
R19
100kΩ
INPUT
(FROM FLT1 OUTPUT)
C30
220pF
Q1
BC857B
C32
220pF
C31
220pF
TO RSSI
(PIN 7)
an1996_8
R18 — provides bias regulation, the gain will stay constant over varying VCC
R19 — for biasing, buffer RF DC voltage
R20 — provides the DC bias, RSSI gain (when R20 increases, RSSI gain decreases)
C30 — DC blocking capacitor that connects the ceramic filter’s output to the PNP transistor’s input
C31 — decoupling capacitor
C32 — peak detector charge capacitor
D1 — diode to prevent improper current flow
Q1 — NXP BC857B PNP transistor
Fig 8.
External RSSI extender circuit
The PNP transistor stage has to be biased as a class B amplifier. The circuit provides two
functions. It is a DC amplifier and an RF detector. The gain of the RSSI extender can be
controlled by R20 and R9 (Gain = R9 / R20). Adjusting R20 is preferable because it
controls the upper half of the RSSI curve, whereas adjusting R9 shifts the whole RSSI
curve.
If a different RF frequency is supplied to the mixer input, it is important to set the external
RSSI gain accordingly. When the RF input was changed from 240 MHz to 45 MHz, the
conversion gain of the mixer increased. Therefore, the earlier gain settings for the RSSI
extender was too much. A lower gain setting had to be implemented such that a smoother
transition would occur.
9. Quad tank
The quad tank is tuned for 10.7 MHz ( f = 1  2 LC ). Figure 1 shows the values used
(C14,C15, C16, IFT1) and Figure 9 shows the S-curve. The linear portion of the S-curve is
roughly 200 kHz. Therefore, it is a good circuit for a total deviation of 140 kHz. It is
possible to deviate at 200 kHz, but this does not leave much room for part tolerances.
AN1996
Application note
All information provided in this document is subject to legal disclaimers.
Rev. 2 — 28 August 2014
© NXP Semiconductors N.V. 2014. All rights reserved.
16 of 25
AN1996
NXP Semiconductors
Demodulating at 10.7 MHz IF with the SA605
4
AUDIO (V)
3
2
1
0
10.0
10.5 10.7
11.0
11.5
IF IN (MHz)
an1996_9
Fig 9.
10.7 MHz quad tank S-curve
If more deviation is needed, a designer can lower the S-curve with a parallel resistor
connected to the quadrature tank. A designer should play with different value resistors
and plot the S-curve to pick the best value for the design. To key in on the resistor value
with minimum effort, a designer can put a potentiometer in parallel with the quad tank and
tune it for best distortion. Then the designer can use fixed value resistors that are close to
the potentiometer’s value.
Fixed quad tank component values can be used to eliminate tuning, but a designer must
allow for part tolerances and temperature considerations. For better performance over
temperature, a resonator/discriminator can be used. Thus, no tuning is required for the
quad tank section, which will save on production costs.
10. RSSI system speed
The RSSI rise and fall times are important in applications that use pulsed RF in their
design. The way we define the speed is how fast the RSSI voltage can travel up and down
the RSSI curve. Figure 10 shows a representation of this. Five different pulsed RF levels
were tested to get a good representation of the RSSI speed. One can predict that the
stronger the pulsed signal, the higher the RSSI voltage and the longer it will take for the
fall time to occur. Generally speaking, the rise time is determined by how long it takes to
charge up an internal capacitor. The fall time depends on how long it takes to discharge
this capacitor.
AN1996
Application note
All information provided in this document is subject to legal disclaimers.
Rev. 2 — 28 August 2014
© NXP Semiconductors N.V. 2014. All rights reserved.
17 of 25
AN1996
NXP Semiconductors
RSSI (V)
Demodulating at 10.7 MHz IF with the SA605
tR
tF
- 80
- 60
- 40
- 20
0
RF LEVEL (dBm)
an1996_10
Fig 10. RSSI curve with pulsed RF levels
It is also important to understand that there are two types of RSSI speeds. The first type is
the RSSI chip speed and the second is the RSSI system speed. The RSSI chip speed
will be faster than the system speed. The bandwidth of the external filters and other
external parts can slow down the RSSI system speed dramatically.
Figure 11 shows the bench setup for the RSSI system speed measurements. The pulsed
RF was set for 10 kHz and the RSSI output was monitored with a digital oscilloscope.
Figure 12 shows how the rise and fall times were measured on the oscilloscope.
PHILIPS SIGNAL GENERATOR
10kHz SQ. WAVE
HP8640B SIG GEN
“RF SOURCE”
HP8640B SIG GEN
“LO SOURCE”
HP PRINTER
RF LEVEL: --20dBm
FREQ: 240MHz
DEV: ±70kHz
MOD: Pulsed
HP 54503 DIGITAL O’SCOPE
PULSED
RF
RF LEVEL: --10dBm
FREQ: 229.3MHz
DEV: Off
RF IN
HP6216A
RSSI
OUT
POWER SUPPLY
LO IN
VCC
DUT: 605 or 625
an1996_11
Fig 11. RSSI speed setup
AN1996
Application note
All information provided in this document is subject to legal disclaimers.
Rev. 2 — 28 August 2014
© NXP Semiconductors N.V. 2014. All rights reserved.
18 of 25
AN1996
NXP Semiconductors
Demodulating at 10.7 MHz IF with the SA605
AMPLITUDE (V)
RF PULSED INPUT
TO THE 625
625 RSSI OUTPUT
RESPONSE
tR
tF
TIME ( μs)
an1996_12
Fig 12. Oscilloscope display of RSSI system rise and fall times
The modifications done on the SA605 board are shown in Figure 13. The RSSI caps C11
and C31 were eliminated, and the RSSI resistor values were changed. We wanted to see
how much time was saved by using a smaller RSSI resistor value.
C31
FLT1
FLT2
SA605
RSSI
1
RF INPUT
@ 240MHz:
0dBm
--20dBm
--40dBm
--60dBm
--80dBm
7
R9 = 10kΩ,
C11
51kΩ,
or 100k Ω
an1996_13
Fig 13. SA605 RSSI test circuit configuration
The RSSI system speed for the 240 MHz SA605 demo board is shown in Figure 14.
Again, the only modification was that the RSSI caps (C11 and C31) were taken out and
the RSSI resistor value (R9) was varied. For different RF levels, the speed seems to vary
slightly, but this is expected. The higher the RSSI voltage, the longer it will take to come
back down the RSSI curve for the fall time.
AN1996
Application note
All information provided in this document is subject to legal disclaimers.
Rev. 2 — 28 August 2014
© NXP Semiconductors N.V. 2014. All rights reserved.
19 of 25
AN1996
NXP Semiconductors
Demodulating at 10.7 MHz IF with the SA605
40
C11 and C31 REMOVED FROM CIRCUIT
RSSI = 10k Ω RISE
RSSI = 10k Ω FALL
RSSI = 51k Ω RISE
30
TIME (microseconds)
RSSI = 51k Ω FALL
RSSI = 100k Ω RISE
RSSI = 100k Ω FALL
20
10
0
0
- 20
- 40
RF INPUT (dBm)
- 60
- 80
an1996_14
Fig 14. RSSI systems rise and fall time with different RSSI resistor values
Looking more closely at Figure 14, one can note that the 0 dBm input level has a faster fall
time than the 20 dBm level. This occurs because of the limited dynamic range of the test
equipment. The equipment does not have sufficient on/off range, so at 0 dBm the ‘off’
mode is actually still on. Therefore, you do not get a true reading.
At 0 dBm the RSSI voltage is lower than 20 dBm. The reason why this happens is
because the RSSI linearity range stops at 10 dBm. When the RF input drive is too high
(for example, 0 dBm), the mixer conversion gain decreases, which causes the RSSI
voltage to drop.
AN1996
Application note
All information provided in this document is subject to legal disclaimers.
Rev. 2 — 28 August 2014
© NXP Semiconductors N.V. 2014. All rights reserved.
20 of 25
AN1996
NXP Semiconductors
Demodulating at 10.7 MHz IF with the SA605
11. Questions and answers
Question:
What should the audio level at Pin 8 be?
Answer:
The audio level is at 580 mV (peak-to-peak) looking directly at the audio
output pin and does not include a C-message filter. However, the audio
output level will depend on two factors: the ‘Q’ of the quadrature tank and the
deviation used. The higher the quad tank’s ‘Q’, the larger the audio level.
Additionally, the more deviation applied, the larger the audio output. But the
audio output will be limited to a certain point.
Question:
Am I required to use the 10 F supply capacitor?
Answer:
No, a smaller value can be used. The 10 F capacitor is a suggested value
for evaluation purposes. Most of the time a power supply is used to evaluate
our demo boards. If the supply is noisy, it will degrade the receiver
performance. We have found that a lower value capacitor can be used when
the receiver is powered by a battery. But it is probably safer to stay at a
reasonable capacitor size.
Question:
Can I use different IF filters for my required bandwidth specifications?
Answer:
Yes, you can order different IF filters with different bandwidths. Some of the
standard manufacturers have 180 kHz, 230 kHz, and 280 kHz bandwidths for
10.7 MHz ceramic filters. Just be sure that the quad tank ‘S-curve’ is linear
for your required bandwidth. The SA605 demoboard has a 200 kHz linearity
for the quad tank. So 70 kHz deviation is perfect.
We have also found that even though the IF filter’s bandwidth might be more
than our requirements, it does not really degrade overall receiver
performance. But to follow good engineering practices, a designer should
order filters that are closest to their requirements. Going with wider
bandwidth filters will give you better RSSI system speed.
Question:
I want to use part of your demo board for my digital receiver project. Can you
recommend a good 10.7 MHz filter with accurate 10.7 MHz center frequency
which can provide minimum phase delay?
Answer:
At the present time, I only know of one manufacturer that is working on a filter
to meet digital receiver requirements. Murata has a surface mount 10.7 MHz
filter. The number is FX-6502 (SFECA 10.7). It was specifically designed for
Japanese digital cordless phones. You can adapt these filters to our SA605
demoboard.
We also used these filters in our layout and got similar SINAD and RSSI
system speed performance compared to the standard 10.7 MHz filters
(280 kHz bandwidth). I believe the difference between the filters will be
apparent for digital demodulation schemes.
AN1996
Application note
All information provided in this document is subject to legal disclaimers.
Rev. 2 — 28 August 2014
© NXP Semiconductors N.V. 2014. All rights reserved.
21 of 25
AN1996
NXP Semiconductors
Demodulating at 10.7 MHz IF with the SA605
Question:
Why does the AM rejection performance look better on the SA605, 455 kHz
IF board than the SA605 10.7 MHz IF demoboard?
Answer:
For the 455 kHz IF demoboard there is more IF gain available compared to
the 10.7 MHz IF board. Recall that for the 10.7 MHz IF board, some of the IF
gain was killed externally for stability reasons. Since the IF gain helps
improve AM rejection performance, by killing IF gain, AM rejection is
decreased.
Question:
The SA605 10.7 MHz IF demoboard is made for the SO package. Can I use
your SSOP package and expect the same level of performance?
Answer:
We have not done a SSOP layout yet. But if the same techniques are used, I
am sure the SSOP package will work. The SA636DK demoboard is available
in SSOP package.
Question:
I tried to duplicate your RSSI system reading measurements using your
demoboard and I get slower times. What am I doing wrong?
Answer:
The RSSI system speed measurements are very tricky. Make sure your cable
lengths are not too long. I have found that when making microsecond
measurements, lab setup is of utmost importance. Also, make sure the RSSI
caps (C11 and C31) are removed from the circuit.
Also be sure that the bandwidth of your IF filters is not slowing down the
RSSI system speed (Cf: section on RSSI system speed).
Question:
I am going to use your design in my NTT cordless digital phone. Can you
recommend a 240.05 MHz filter?
Answer:
Murata SF2055A 240.050 MHz SAW filter is a filter you can use for your
application.
12. Abbreviations
Table 3.
AN1996
Application note
Abbreviations
Acronym
Description
AM
Amplitude Modulation
FM
Frequency Modulation
IF
Intermediate Frequency
LC
inductor-capacitor network
LO
Local Oscillator
NTT
Nippon Telegraph and Telephone
PNP
bipolar transistor with P-type emitter and collector and an N-type base
RF
Radio Frequency
RSSI
Received Signal Strength Indicator
SINAD
Signal-to-Noise And Distortion ratio
All information provided in this document is subject to legal disclaimers.
Rev. 2 — 28 August 2014
© NXP Semiconductors N.V. 2014. All rights reserved.
22 of 25
AN1996
NXP Semiconductors
Demodulating at 10.7 MHz IF with the SA605
13. References
AN1996
Application note
[1]
SA605, “High performance low power mixer FM IF system” —
Product data sheet; NXP Semiconductors;
www.nxp.com/documents/data_sheet/SA605.pdf
[2]
AN1994, “Reviewing key areas when designing with the SA605” —
Application note; NXP Semiconductors;
www.nxp.com/documents/application_note/AN1994.pdf
[3]
AN1995, “Evaluating the SA605 SO and SSOP demoboard” — Application note;
NXP Semiconductors; www.nxp.com/documents/application_note/AN1995.pdf
All information provided in this document is subject to legal disclaimers.
Rev. 2 — 28 August 2014
© NXP Semiconductors N.V. 2014. All rights reserved.
23 of 25
AN1996
NXP Semiconductors
Demodulating at 10.7 MHz IF with the SA605
14. Legal information
14.1 Definitions
Draft — The document is a draft version only. The content is still under
internal review and subject to formal approval, which may result in
modifications or additions. NXP Semiconductors does not give any
representations or warranties as to the accuracy or completeness of
information included herein and shall have no liability for the consequences of
use of such information.
14.2 Disclaimers
Limited warranty and liability — Information in this document is believed to
be accurate and reliable. However, NXP Semiconductors does not give any
representations or warranties, expressed or implied, as to the accuracy or
completeness of such information and shall have no liability for the
consequences of use of such information. NXP Semiconductors takes no
responsibility for the content in this document if provided by an information
source outside of NXP Semiconductors.
In no event shall NXP Semiconductors be liable for any indirect, incidental,
punitive, special or consequential damages (including - without limitation - lost
profits, lost savings, business interruption, costs related to the removal or
replacement of any products or rework charges) whether or not such
damages are based on tort (including negligence), warranty, breach of
contract or any other legal theory.
Notwithstanding any damages that customer might incur for any reason
whatsoever, NXP Semiconductors’ aggregate and cumulative liability towards
customer for the products described herein shall be limited in accordance
with the Terms and conditions of commercial sale of NXP Semiconductors.
Right to make changes — NXP Semiconductors reserves the right to make
changes to information published in this document, including without
limitation specifications and product descriptions, at any time and without
notice. This document supersedes and replaces all information supplied prior
to the publication hereof.
Suitability for use — NXP Semiconductors products are not designed,
authorized or warranted to be suitable for use in life support, life-critical or
safety-critical systems or equipment, nor in applications where failure or
malfunction of an NXP Semiconductors product can reasonably be expected
AN1996
Application note
to result in personal injury, death or severe property or environmental
damage. NXP Semiconductors and its suppliers accept no liability for
inclusion and/or use of NXP Semiconductors products in such equipment or
applications and therefore such inclusion and/or use is at the customer’s own
risk.
Applications — Applications that are described herein for any of these
products are for illustrative purposes only. NXP Semiconductors makes no
representation or warranty that such applications will be suitable for the
specified use without further testing or modification.
Customers are responsible for the design and operation of their applications
and products using NXP Semiconductors products, and NXP Semiconductors
accepts no liability for any assistance with applications or customer product
design. It is customer’s sole responsibility to determine whether the NXP
Semiconductors product is suitable and fit for the customer’s applications and
products planned, as well as for the planned application and use of
customer’s third party customer(s). Customers should provide appropriate
design and operating safeguards to minimize the risks associated with their
applications and products.
NXP Semiconductors does not accept any liability related to any default,
damage, costs or problem which is based on any weakness or default in the
customer’s applications or products, or the application or use by customer’s
third party customer(s). Customer is responsible for doing all necessary
testing for the customer’s applications and products using NXP
Semiconductors products in order to avoid a default of the applications and
the products or of the application or use by customer’s third party
customer(s). NXP does not accept any liability in this respect.
Export control — This document as well as the item(s) described herein
may be subject to export control regulations. Export might require a prior
authorization from competent authorities.
Translations — A non-English (translated) version of a document is for
reference only. The English version shall prevail in case of any discrepancy
between the translated and English versions.
14.3 Trademarks
Notice: All referenced brands, product names, service names and trademarks
are the property of their respective owners.
All information provided in this document is subject to legal disclaimers.
Rev. 2 — 28 August 2014
© NXP Semiconductors N.V. 2014. All rights reserved.
24 of 25
AN1996
NXP Semiconductors
Demodulating at 10.7 MHz IF with the SA605
15. Contents
1
1.1
1.2
2
3
4
5
6
7
8
9
10
11
12
13
14
14.1
14.2
14.3
15
Introduction . . . . . . . . . . . . . . . . . . . . . . . . . . . . 3
Background . . . . . . . . . . . . . . . . . . . . . . . . . . . 3
Objective. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 6
Board setup and performance graphs. . . . . . . 6
RF input . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 12
LO input . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 12
10.7 MHz ceramic filters . . . . . . . . . . . . . . . . . 12
Stabilizing the IF section from regeneration . 14
RSSI dynamic range . . . . . . . . . . . . . . . . . . . . 14
RSSI extender circuit. . . . . . . . . . . . . . . . . . . . 15
Quad tank . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 16
RSSI system speed . . . . . . . . . . . . . . . . . . . . . 17
Questions and answers . . . . . . . . . . . . . . . . . 21
Abbreviations . . . . . . . . . . . . . . . . . . . . . . . . . . 22
References . . . . . . . . . . . . . . . . . . . . . . . . . . . . 23
Legal information. . . . . . . . . . . . . . . . . . . . . . . 24
Definitions . . . . . . . . . . . . . . . . . . . . . . . . . . . . 24
Disclaimers . . . . . . . . . . . . . . . . . . . . . . . . . . . 24
Trademarks. . . . . . . . . . . . . . . . . . . . . . . . . . . 24
Contents . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 25
Please be aware that important notices concerning this document and the product(s)
described herein, have been included in section ‘Legal information’.
© NXP Semiconductors N.V. 2014.
All rights reserved.
For more information, please visit: http://www.nxp.com
For sales office addresses, please send an email to: salesaddresses@nxp.com
Date of release: 28 August 2014
Document identifier: AN1996