CN0340: True RMS RF Detector with 95 dB Detection Range (Rev. 0) PDF

Circuit Note
CN-0340
Circuits from the Lab™ reference circuits are engineered and
tested for quick and easy system integration to help solve today’s
analog, mixed-signal, and RF design challenges. For more
information and/or support, visit www.analog.com/CN0340.
Devices Connected/Referenced
AD8368
800 MHz, Linear-in-dB VGA with AGC Detector
ADL5902
50 MHz to 9 GHz 65 dB TruPwr Detector
True RMS RF Detector with 95 dB Detection Range
EVALUATION AND DESIGN SUPPORT
Design and Integration Files
Schematics, Layout Files, Bill of Materials
Circuit Evaluation Boards
AD8368 Customer Evaluation Board (AD8368-EVALZ)
ADL5902 Customer Evaluation Board (ADL5902-EVALZ)
EPCOS B5070 SAW Filter Evaluation Board or Equivalent
+5V
+5V
+5V
C11
1nF
C12
1nF
VPSO VPSO
10
ICOM
OCOM
GAIN
OCOM
9
CIN
10nF
ICOM
ICOM
ICOM
22
C13
0.1µF
23
AD8368
6
1
7
–2dB –4dB
18
GND
LS1
68nH
COUT
10nF
3
–36dB
4
REF
14
50Ω
DECL
ATTENUATOR LADDER
–
15
X2
HPFL
CASE GND
C23
10nF
DECL
DECL
2
DETO
5
167MHz
CASE GND
+5V
4
2
GND
OUTPUT GND
CASE GND
OUTPUT
LS4
100nH
3
GND
LP3
120nH
R3
60.4Ω
C7
0.1µF
C5
100pF
VPOS
VPOS
10
3
TEMPERATURE
SENSOR
ADL5902
8
7
INLO
6
5
LP2
68nH
C4
100pF
INHI
B5070
(EPCOS)
11
C20
1nF
DETI
C3
0.1µF
C10
100pF
10
12
CASE GND
7
8
1
C2
5.6pF
+
20
INPUT GND
C4
1nF
DECL
GND
9
INPUT
8
19
17
+5V
FIXED-GAIN OUTPUT 24 ENBL
AMPLIFIER BUFFER
OUTP
GAIN INTERPOLATOR
0dB
L1
10nH
13
12
gm STAGES
INPT
C10
VPSI VPSI 1nF
VPSI VPSI VPSI
11
16
R3
215Ω
RFIN
MODE
21
14
TEMP
VSET
IDET
X2
15
C12
100pF
LINEAR-IN-dB VGA
(NEGATIVE SLOPE)
X2
ITGT
NC 2
NC 16
G=5
BIAS AND POWERDOWN CONTROL
6
26pF
R12
301Ω
12
11
VREF
R9
3.09kΩ
VTGT
COMM
R11
2kΩ
C9
10µF
4
9
R10
3.74kΩ
VREF
(BLACK)
VRMS
CLPF
5
NC 13
TADJ/PWDN
R1
3.83kΩ
R15
1.5kΩ
VREF
2.3V
1
VOUT
COMM
VTGT
(BLACK)
11953-001
C15
0.1µF
C14
0.1µF
Figure 1. 95 dB RMS Responding RF Detector
Rev. 0
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CN-0340
Circuit Note
CIRCUIT DESCRIPTION
The 65 dB range of the ADL5902 linear-in-dB true rmsresponding RF detector can be extended using a stand-alone
variable gain amplifier (VGA). The gain control input of the
VGA is derived directly from the ADL5902 VOUT pin. This
extends the dynamic range by the gain control range of the
VGA (in practice the achieved range extension is slightly less).
When this VGA also provides a linear-in-dB (exponential) gain
control function, the overall measurement remains linearly
scaled in decibels. The VGA gain must decrease with an
increase in its gain bias in the same way as the ADL5902. All of
these conditions are met by the AD8368. Figure 1 shows the
circuit schematic.
The ADL5902 rms computation circuit uses a VGA leveling
architecture. An internal linear-in-dB VGA with a negative gain
control slope drives the input of a low-range rms detector. The
output level of this detector is compared to the output of second
setpoint detector using a current balancing architecture; one
detector sources current and the other sinks current. If the
output levels of the two detectors are not equal, the residual
current charges or discharges a capacitor (capacitance is equal
to the parallel combination of the internal 26 pF capacitance
and the external capacitance on Pin 6 (CLPF) of the ADL5902.
The integration increases or decreases the value of VOUT. With
VOUT tied directly the gain control input of the ADL5902
VGA, this increases or decreases the VGA gain until the output
levels from the two detectors are equal. When this point is
reached, VOUT and the gain of the VGA settles. Because the
ADL5902 VGA has a linear-in-dB transfer function, the
resulting VOUT voltage is proportional to the log of the rms
value of the input signal.
While the nominal output voltage range of the ADL5902 rms
detector is 0 V to 3.5 V, the AD8368 VGA requires a control
voltage range from 0 V to 1 V to fully exercise its 34 dB gain
control range. Therefore, the fed-back voltage from VOUT must
be scaled down by a factor of 3.5. This is easily implemented
using a resistor divider (R1 and R15 in Figure 1).
Figure 2 shows the resulting transfer function when the input
power is swept at 167 MHz. Optimum linearity is achieved using a
4-point calibration with calibration points at +15 dBm, −15 dBm,
−55 dB, and −70 dBm. A 2-point calibration can also be used,
but results in degraded linearity across the input power range.
Note that the AD8368 on-board RF detector and automatic level
control (ALC) function are not used in this circuit. Therefore, the
DETI and DETO pins on the AD8368 can be left open.
1.0
5
0.9
4
3
0.8
+85°C
+25°C
−40°C
0.7
Rev. 0 | Page 2 of 6
2
0.6
1
0.5
0
0.4
–1
0.3
–2
0.2
–3
0.1
0
–80
ERROR (dB)
The detection range of the ADL5902 rms detector is 65 dB and
is extended to 95 dB by the addition of the linear-in-dB AD8368
VGA. The ADL5902 TADJ function is used to provide temperature
stability for the complete circuit. A SAW filter is placed between
the VGA to reduce noise and increase sensitivity. This also
reduces the frequency range of the circuit to the pass-band
range of the SAW filter.
Adding additional variable gain to the signal path extends the
detection range of the circuit. The fed-back VOUT signal now
drives both the ADL5902 VGA gain control input along with
the gain control input of the AD8368 VGA. The AD8368
MODE pin must be tied low to make the gain control slope
negative. Because the AD8368 VGA provides both gain and
attenuation (GMAX = 22 dB GMIN = −12 dB), this extends the
detection range at both the top and bottom end of the ADL5902
nominal range. However, to optimize the range extension, the
voltage driving the gain control pin of the AD8368 must be
correctly scaled.
–4
–70
–60
–50
–40 –30 –20
RFIN (dBm)
–10
0
10
–5
20
Figure 2. Transfer Function of 95 dB RMS Responding RF Detector,
Measured at 167 MHz
11953-002
The circuit in Figure 1 is a true rms responding power detector
using a variable gain amplifier (VGA) and an rms-responding
power detector to provide an extremely wide detection range of
approximately 95 dB. RMS detectors are useful in many
applications such as receivers and transmitters where accurate
measurement of signal power is required. Because the circuit
measures rms power, it is suitable for use in systems with
diverse or varying crest factors. Examples of such systems
include GSM/EDGE, CDMA, WCDMA, TD-SCDMA and LTE
based wireless base stations along with any system that uses
QAM modulation.
The detection range of the ADL5902 is primarily determined by
the gain control range of its internal VGA. As the input signal
decreases in size, VGA control voltage decrease until the VGA
reaches maximum gain. For large and increasing input signals,
the VGA gain control voltage increases (thereby decreasing the
VGA gain) until minimum gain is reached.
VRMS (V)
CIRCUIT FUNCTION AND BENEFITS
Circuit Note
CN-0340
RF Input Power Sensitivity
With the dramatic narrowing of the noise bandwidth along with
the 7.3 dB insertion loss of the filter, the integrated output noise
of the VGA/SAW combination drops dramatically to −77 dBm,
well below the input sensitivity of the ADL5902 rms detector.
This ensures that the circuit is not noise limited when the VGA
is at its maximum gain.
11953-003
To achieve the excellent sensitivity shown in Figure 2, a narrowband filter must be placed between the VGA and the detector as
shown in Figure 1. Without a filter, the broadband output noise
of the AD8368 swamps the low-end sensitivity of the ADL5902.
Figure 3 shows a screenshot of the output noise calculation of
the AD8368 performed in ADIsimRF. The AD8368 VGA is at
maximum gain when the smallest input signals are present. With a
3 dB bandwidth of 800 MHz and assuming a first order roll-off,
an equivalent noise bandwidth of 1272 MHz (that is, 800 MHz
times 1.57) was used to calculate the output noise power from
the VGA. This results in an output noise power level of
approximately −51 dBm, which is almost 10 dB above the
nominal input sensitivity of the ADL5902. Therefore, some
filtering is essential to maximize low-end sensitivity.
Figure 4 shows the same noise calculation when an EPCOS
B5070 SAW filter is added to the circuit with a center frequency
of 167 MHz. In this calculation, the analysis bandwidth has been
reduced to be equal to the bandwidth of the SAW filter (18 MHz).
11953-004
Figure 3. ADIsimRF Calculation of Output Noise of the AD8368 VGA at Maximum Gain
Figure 4. ADIsimRF Calculation of Output Noise of the AD8368 VGA with 167 MHz SAW Filter with 18 MHz Bandwidth
Rev. 0 | Page 3 of 6
CN-0340
Circuit Note
COMMON VARIATIONS
The circuit can be modified to accommodate a different center
frequency, bandwidth, and filter insertion loss. As already noted,
using the B5070 filter, the input noise to the ADL5902 is −77 dBm
when the AD8368 VGA is a maximum gain. Increasing the
bandwidth of the filter will increase the noise level. The output
noise level of the VGA should ideally be kept below the input
sensitivity level of the detector (approximately −60 dBm).
The center frequency of the filter can also be increased or
decreased. Increases in the center frequency are ultimately limited
by degradation in the linearity and gain control range of the
AD8368 VGA, which has a 3 dB corner frequency of 800 MHz. A
SAW filter with a lower center frequency can also be chosen, but
low frequency operation is limited by the ADL5902, which operates
at frequencies down to 50 MHz. This frequency limit is driven by
internal ac-coupling in the ADL5902.
A discrete LC filter could be used as an alternative to the SAW
filter. Consideration should be given to the bandwidth and
insertion loss of the filter.
It is also possible to operate the circuit without a filter. However
as already noted, this will significantly limit the low end sensitivity.
Figure 5 shows a plot of output voltage vs. input power where
no filter is used. The long non-linear arc in the output voltage
and error plot at low input power levels indicates the decreasing
input signal is being swamped by the noise of the VGA.
The matching network at the input of the AD8368 VGA (L1,
R3) is not a narrowband matching circuit. Therefore if an
6
1.1
5
1.0
4
0.9
3
0.8
2
0.7
1
0.6
0
0.5
–1
0.4
–2
0.3
–3
0.2
–4
0.1
–5
0
–80
ERROR (dB)
1.2
–6
–60
–40
–20
0
20
RFIN (dBm)
11953-005
The ADL5902 incorporates a temperature compensation
function. By setting the voltage on the TADJ pin (Pin 1), the
detector’s intercept temperature stability at a particular operating
frequency can be optimized. In the range extension circuit
shown in Figure 1, any temperature variations in the gain of the
VGA will degrade the overall drift of the circuit one-for-one
(i.e. a 1 dB drift vs. temperature in the gain of the VGA will
degrade the overall temperature stability by 1 dB). In the case of
the AD8368 VGA, Figure 5 in the AD8368 datasheet shows that
the gain drifts vs. temperature by approximately ±0.7 dB. Note
that the intercept of the VGA transfer function drifts (that is,
gain drift error is constant at all gains). Therefore, the detector
and the VGA have similar temperature drift signatures. By
adjusting the voltage on the ADL5902 TADJ pin, the combined
temperature drift of the detector and VGA can be compensated.
For the 167 MHz operating frequency, it was experimentally
determined that a TADJ voltage of 0.2 V provided the optimum
temperature compensation that is achieved in Figure 2.
operating frequency other than 167 MHz is chosen, it should
remain in place.
VRMS (V)
Temperature Stability
Figure 5. Transfer Function of the Circuit with the SAW Filter Removed
As an alternative to the ADL5902, the ADL5906 linear-in-dB
rms detector could also be used. However, the temperature
compensation function in this device compensates the temperature
drift of the detector’s SLOPE (the ADL5902 TADJ function
compensates for INTERCEPT drift). Since the temperature drift
of the AD8368 VGA is primarily INTERCEPT-based, the
ADL5906 temperature compensation function does not reduce
the contribution of the VGA to the overall temperature drift.
The operating frequency range of the circuit can be expanded
using a broadband front-end mixer and frequency agile PLL
synthesizer. In this case, the frequency to be measured is mixed
down to the center frequency of the SAW filter. The operating
frequency range of such a circuit would be limited only by the
frequency ranges of the mixer and PLL synthesizer.
Circuit Note CN-0178 describes how the output of the ADL5902
can be interfaced to the 12-bit precision AD7466 ADC.
Complete schematics, layout files, and bill of materials for CN-0340
can be found in the CN-0340 Design Support Package
(www.analog.com/CN0340-DesignSupport )
CIRCUIT EVALUATION AND TEST
The circuit can be easily built up using standard evaluation boards
with some slight modifications and adjustments of jumper
settings. Fully populated evaluation boards for the AD8368 and
the ADL5902 are available from Analog Devices (ADL5902EVALZ, AD8368-EVALZ). A fully populated evaluation board
for the B5070 SAW filter can be obtained from EPCOS. This
board includes the four required matching components (LS1,
LP2, LS3, LP4 shown in Figure 1). A functional diagram of the
test setup is shown in Figure 6.
Rev. 0 | Page 4 of 6
Circuit Note
CN-0340
AGILENT E3631A
POWER SUPPLY
5.000
0.350A
6V
+
±25V
–
+ COM –
+5V
GND
VPOS
RF SIGNAL GENERATOR
AGILENT E8648C
RF
OUT
GND
AD8368
EVALUATION BOARD
(AD8368-EVALZ)
B5070
EVALUATION BOARD
(EPCOS)
INPUT
OUTPUT
SW1 (MODE) = LOW
GAIN
GND
VPOS
INHI
VOUT
ADL5902
EVALUATION BOARD
(ADL5902-EVAL-Z)
0.901V
11953-006
AGILENT 34401A
MULTIMETER
Figure 6. Measurement Setup
The primary measurement equipment that was used to evaluate
the circuit was an RF signal generator operating at 167 MHz
(for example, Agilent 8648C, Rohde & Schwarz SMT03, SMIQ or
equivalent), a 5 V power supply (Agilent E3631A or equivalent),
and a digital voltmeter (for example, Agilent 34401A or equivalent).
Equipment Needed
The following equipment (or equivalent) is required to make
the measurements described in this circuit note:
•
•
•
•
•
•
AD8368 evaluation board (AD8368-EVALZ)
ADL5902 evaluation board (ADL5902-EVALZ)
(modified as described below)
SAW filter on evaluation board (EPCOS B5070, 167
MHz or equivalent)
RF Signal Generator: Agilent 8648C, Rohde &
Schwarz SMT03 or SMIQ
5 V, 400 mA Power Supply: Agilent E3631A
Multimeter: Agilent 34401A
Setup and Test
Since all three evaluation boards have 50 Ω interfaces, they can
be connected directly using barrel SMA connectors. The
connection from the output of the ADL5902 detector back to
the gain control input of the AD8368 VGA can be conveniently
implemented with an SMA cable or with clip leads since it is a
low speed signal. The resistor divider that is required to scale
down the ADL5902 detector’s output voltage can be implemented
by placing surface mount resistors on the R1 (3.83 kΩ) and R15
(1.5 kΩ) pads on the ADL5902 evaluation board. The TADJ
voltage that optimizes the temperature stability of the circuit at
167 MHz can be set by the R9/R12 resistor divider, which is
derived from the 2.3 V on-chip voltage reference. To set the
TADJ voltage to the recommended 0.2 V level, change R9 to
3.09 kΩ (R12 keeps its existing value of 301 Ω).
For precision RF detector power sweeps, it is normally
recommended that the source power to the detector be measured
using an RF power meter (for example, signal from the signal
generator is split with half going to the detector and half going
to the RF power meter). However, in this case, covering the
95 dB detection range with an RF power meter was quite
difficult. Therefore, the signal generator output power display
was used as the source power reading. It is therefore advisable
to choose an RF signal generator whose output power level
display is known to be accurate, particularly at low and high
power levels.
Rev. 0 | Page 5 of 6
CN-0340
Circuit Note
LEARN MORE
CN-0340 Design Support Package:
http://www.analog.com/CN0340-DesignSupport
ADIsimRF Design Tool
MT-073 Tutorial, High Speed Variable Gain Amplifiers (VGAs),
Analog Devices.
MT-101 Tutorial, Decoupling Techniques, Analog Devices.
CN-0150 Circuit Note, Software Calibrated 1 MHz to 8 GHz
70 dB RF Power Measurement System, Analog Devices.
CN-0178 Circuit Note, Software Calibrated 50 MHz to 9 GHz
RF Power Measurement System, Analog Devices.
AN-1040 Application Note, RF Power Calibration Improves
Performance of Wireless Transmitters, Analog Devices.
B5070 Datasheet (EPCOS)
Data Sheets and Evaluation Boards
ADL5902 Data Sheet and Evaluation Board
AD8368 Data Sheet and Evaluation Board
REVISION HISTORY
11/13—Revision 0: Initial Version
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CN11953-0-11/13(0)
Rev. 0 | Page 6 of 6
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