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The World Leader in High-Performance Signal Processing Solutions
ADI 2007
Obscurities & Applications of
RF Power Detectors
Obscurities & Applications of
RF Power DetectorsCarlos Calvo, Applications Engineer
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Why measure RF/IF power?
Set mobile’s power level (RSSI measurement in BTS receiver)Signal Leveling in receivers (high precision generally not required, usually done at IF)Prevent interference with other systems and other users in same cell (mobile handset).Improve mobile talk time (operate at low end of permissible range, reduce SAR).Improve network robustness (operate at high end of permissible range). Thermal Dimensioning (mostly HPA)
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TX/RXSWITCH
IQ DEMOD
VCO-Synth-DDS
IQ MOD
VGA
LOG / RMSCONTROL
RSSI/Phase Det
AMP
AMP
LNA
AMP
VGA
MIXER
MIXER
ADC
ADC
DAC
DACPA/DRIVER
Typical RF Signal Chain
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Typical Detector Applications
PA
Detector ADC
PA
Detector
ADC
ADCDetector
LO2
TX power controlTx Power Measurement
ADC
DetectorLO2
∫Vagc
Vgain for RSSI
Received Power Measurement Received Power Control
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RF Power DetectorsCritical Specifications
Linearity and Temperature Stability of OutputDynamic RangePulse Response Variations due to Power Supply and Frequency ChangesEase of Use and CalibrationChange in response vs. signal crest factorSize and overall Component Count
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ADI 2007
RF Power Measurement TechniquesRF Power Measurement Techniques
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Power Measurement TechniquesDiode Detection
6 8Ω
D1
100pF R1
R F I N V o u t
Source: “A Supressed Harmonic Power Detector for Dual Band Phones” Alan Rixon and Raymond Waugh “Applied Microwave and Wireless”, November 1999
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Transfer Function of Diode DetectorVout vs. Pinput
0.00001
0.0001
0.001
0.01
0.1
1
10
-25 -20 -15 -10 -5 0 5 10 15 20 25
Pinput-dBm
Vout
-Vol
ts
-25 deg C 25 deg c +85 deg C -30 DEG C -40 DEG C
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Diode Detector with Temperature Compensation
68Ω
D1
100pF
R1R F I N V o u t
R2
D2
Source: “A Supressed Harmonic Power Detector for Dual Band Phones” Alan Rixon and Raymond Waugh “Applied Microwave and Wireless”, November 1999
10
-3-2.5
-2-1.5
-1-0.5
00.5
11.5
22.5
3
-20 -10 0 10 20
INPUT (dBm)
ERR
OR
(dB
) .
0.01
0.1
1
10
VOU
T (V
)-40 degC
+85 degC
+25 degC
Transfer Function of Temperature Compensated Diode Detector
•Excellent temperature stability at high power•Limited Dynamic Range and poor low end temp. stability•High Resolution ADC required for low end power measurement•Lots of patented techniques which probably improve this performance
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ADI 2007
Logarithmic AmplifiersLogarithmic Amplifiers
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Log Amp Block Diagram
Signal propagates through gain chain until it limitsDetectors full-wave rectify the signal at the output of each stageOutputs of detectors are summed and low-pass filtered
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Log Amp Transfer Function in Time Domain
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Log Amp Transfer Function - Slope and Intercept
INPUT AM PLITUDE – dBm
2.0
–70
1.81.61.4
1.21.00.80.60.40.20
–60 –50 –40 –30 –20 –10 0 10
V S = +5 V5
4321
0–1–2–3–4–5
ERRO
R – dB+25 degC
+85 degC
–40 degCV ou
t - Volts
–80–90–100
Slope = (VO2 - VO1) / (PI2 - PI1)
Intercept = PI1 - VO1 / Slope
Vout = Slope · (Pin - Intercept)
Pin = (Vout / Slope) + Intercept
Intercept
Slope
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RF Power Detector Calibration
VOUT1
VOUT2
PIN1PIN2
VOUTIDEAL = SLOPE x (PIN - INTERCEPT)
SLOPE = (VOUT1-VOUT2)/(PIN1-PIN2)
INTERCEPT = PIN1-(VOUT1/SLOPE)
Error (dB) = (VOUT-VOUTIDEAL)/SLOPE
INTERCEPT
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±1 dB Dynamic Range
55 dB Dynamic Range
Temperature Drift can reduce Dynamic Range
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Detector Calibration Procedure
Factory Calibration: Using a precise power source, measure output voltage from the detector with two known input powers at top and bottom of desired input rangePerform calibration measurements only at room temperatureCalculate SLOPE and INTERCEPT and store in non-volatile memoryWhen equipment is in operation measure detector output voltage using ADCCalculate power using “Pin = (Vout/Slope) + Intercept”No temperature compensation necessary
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Adjust Calibration Points for optimal accuracy over a narrow range
Calibrate for highest accuracy at max RF power and degraded accuracy at lower powers
VOUT1
VOUT2
PIN1PIN2
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Temperature drift vs. Output Voltage at 25ºC
Calibration eliminates error due to non-linearity at 25 ºC
0.2
0.4
0.6
0.8
1
1.2
1.4
1.6
1.8
2
2.2
-65 -60 -55 -50 -45 -40 -35 -30 -25 -20 -15 -10 -5 0 5Pin-dBm
Vout
- Vo
lts
-2.5
-2
-1.5
-1
-0.5
0
0.5
1
1.5
2
2.5
Erro
r - d
B
Vout +25 degCVout -40 degCVout +85 degCError +25 degCError -40 degCError +85 degC
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Temperature drift vs. Output Voltage at 25ºC
Removes error due to non-linearity at 25ºCProvides larger dynamic range and improved accuracyMethod however does not account for non-linearity in the transfer function at room temperatureFor practical implementation, calibration measurements must be taken at multiple input powers (multi-point calibration vs. 2-point calibration)
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Log Amp Detectors vs. Diode Detectors
Log Amps have a higher dynamic range (40 dB or greater vs. 20-30 dB for a diode detector)Log Amps provide good temperature stability over a wide dynamic range.Diode detectors only provide good temperature stability at max input power (typically +15 dBm)
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Log Amp Pulse Response Time10ns Response Time (10% - 90%)
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2nd Generation Log Amp Detectors
Part No.RFFreq
(MHz)
DynamicRang
e(dB)
TempDrift(dB)
ResponseTime (ns) Package Comments
AD8302 dc to 2700 60 ±1 60 14-lead TSSOP
Dual gain & phase detector
AD8306 5 to 400 100 ±1 73 16-lead SOP Military specified part available
AD8307 dc to 500 92 ±1 400 8-lead SOIC/DIP -
AD8309 5 to 500 100 ±1 67 16-lead TSSOP
Amplitude and limiter outputs
AD8310 dc to 440 95 ±1 15 8-lead MSOP Low cost
AD8313 100 to 2500 70 ±1.25 40 8-lead
MSOP -
AD8314 100 to 2700 45 ±1 70
8-lead MSOP/C
SP
Small package, lower power
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3rd Generation Log Amp Detectors
Part No.RFFreq
(MHz)
DynamicRange(dB)
TempDrift(dB)
ResponseTime(ns)
Package Comments
AD8317 1 to 10000 50 ±0.5 88-Lead3x2 mm
CSP
Smaller package, Lower cost version of AD8318
AD8318 1 to 8000 60 ±0.5 1016-Lead4x4 mm
CSP
50 ohm drive,Integrated Temp
Sensor
AD8319 1 to 10000 40 ±0.5 88-Lead3x2 mm
CSP
Reduced dynamic range and lower cost version of AD8317
ADL5519 1 to 10000 50 ±0.5 <10 24-LeadLFCSP Dual Log Detector
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AD8318: Highest Performance Log Amp
KEY SPECIFICATIONSBandwidth 1MHz to 8GhzStability over temperature: ±0.5 dBPulse response time 10 nsPackage: 4mm×4mm, 16-pin LFCSP
KEY SPECIFICATIONSKEY SPECIFICATIONSBandwidth 1MHz to 8GhzStability over temperature: ±0.5 dBPulse response time 10 nsPackage: 4mm×4mm, 16-pin LFCSP
FEATURESIntegrated temperature sensorLow noise measurement/controller output VOUTPower-down feature: <1.5 mW at 5 VFabricated using high speed SiGe process
FEATURESFEATURESIntegrated temperature sensorLow noise measurement/controller output VOUTPower-down feature: <1.5 mW at 5 VFabricated using high speed SiGe process
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Log Amps - Summary
Provide power detection over large dynamic range (up to 100 dB)Operation from DC to 10 GHzWith 2-Point Calibration, measurement accuracy of << ±1 dB is achievable.Devices are generally configured to provide a broadband 50 Ω matchPulse Response times of <10 ns are achievable.Power consumption varies from 5 mA to 70 mA
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ADI 2007
RMS-RespondingRF Detectors
RMS-RespondingRF Detectors
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Difficult Measurements: Complex Waveforms
IS-95 Reverse Link
IS-95 Forward Link (8Ch)
W-CDMA Forward Link, 4 Channels
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Response of a Successive Detection Log Amp to Varying Signals with Various Crest Factors
0
0.2
0.4
0.6
0.8
1
1.2
-45 -35 -25 -15 -5
INPUT (dBm)
VOU
T (V
)
-3.0
-2.0
-1.0
0.0
1.0
2.0
3.0
ERR
OR
(dB
)
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RMS-Responding RF Detector
RFIN
BUFFER
VPOS
VRMS
COMM
ERRORAMP
x2
x2i
i
TRANS-CONDUCTANCECELLS
ADL5501
100Ω
INTERNAL FILTERCAPACITOR
FLTR
BAND-GAPREFERENCE ENBL
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RMS Detector Waveform Independence
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Transfer Function and Temperature Drift RMS-To-DC Converter
Output Voltage increases exponentially as input increases in dB (i.e. response is linear in V/V, not logarithmicDevice achieves best temperature stability at max power (desirable for most applications)
-3
-2
-1
0
1
2
3
-25 -15 -5 5 15
INPUT (dBm)
ERR
OR
(dB
)
0.1
1
10
OU
TPU
T (V
)
+85°C
-40°C
+25°C
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ADI 2007
High Dynamic Range RMS Detection
High Dynamic Range RMS Detection
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60 dB TruPwr ™ RMS Detector
BIAS
x2
VOUT
VSET
PWDNCOMM
VREF
AD8362
INHI
INLO
VTGT
VPOS
CLPF
CHPF
x2
ACOM
DECL
Waveform and Modulation IndependentLinear-in-dB outputWaveform and Modulation IndependentLinear-in-dB output
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Response of AD8362 RMS Detector to CW, QPSK and QAM Signals
@1.9 GHz, @1.9 GHz, VtgtVtgt = 0.625 V = 0.625 V
0
0.5
1
1.5
2
2.5
3
3.5
4
-70 -60 -50 -40 -30 -20 -10 0 10 20
Pin (dBm)
Vout
(V)
-4
-3
-2
-1
0
1
2
3
4
Erro
r (d
B)
Vout CWVout QPSKVout 64QAMVout 256QAMVout WCDMA TM1-64Error CWError QPSK 4dB CFError 64QAM 7.7dB CFError 256QAM 8.2dB CFError WCDMA TM1-64 10.6dB CF
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TruPwr™ RMS Detectors- Modulation Independent RF Measurements
Part#RF Freq
(MHz)
Dynamic Range (dB)
Temp Stability
(dB)
VoltageSupply
(V)
Supply Current
(mA) Package
AD8361 2500
ADL5501 4000 30 ±0.1 2.7 to 5.5 1.0 SC-70
2700
2700
30 ±0.25 2.7 to 5.5 1.1
6-Lead SOT-23, 8-Lead uSOIC
AD8362 60 ±1 4.5 to 5.5 2016-Lead
SOP
AD8364(Dual Channel) 60 ±0.5 4.5 to 5.5 72
32-Lead LFCSP
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AD8362 TruPwr ™ RMS Detector
FEATURESTrue RMS responding power detectorWaveform and Modulation IndependentLinear-in-dB output
FEATURESFEATURESTrue RMS responding power detectorWaveform and Modulation IndependentLinear-in-dB output
KEY SPECIFICATIONSDynamic Range: >60dBTemperature Stability: +/-1dBFrequency Range: LF to 2.7GHzPackage: 16 Lead TSSOP
KEY SPECIFICATIONSKEY SPECIFICATIONSDynamic Range: >60dBTemperature Stability: +/-1dBFrequency Range: LF to 2.7GHzPackage: 16 Lead TSSOP
38
ADI 2007
Controlling AGC Loops with RF Detectors
Controlling AGC Loops with RF Detectors
39
A Typical AGC Loop
VGA
Detector
Vref(e.g. 1V)
Vin Vout
Vin (ac)
Vout (dc)
Vcontrol
Gain
I
RdV/dt = I/CC
Detector measures output power from a variable gain amplifier orpower amplifierMeasured result is compared to a setpoint valueError amplifier/Integrator adjusts gain so that output power corresponds to setpointIntegrator capacitor/resistor set response time of loopMany of ADI’s detectors have an integrated “Controller Mode”
40
A Practical AGC Loop using a Log Amp
Setpoint is applied to Detector VSET inputVout varies up or down to balance loopUse to set output to a fixed value (fixed VSET, variable input power) or to vary output power (variable VSET, fixed or variable input power)Set response time of loop by varying Cflt
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Controlling Gain with a Dual RMS Detector
Dual RMS Detector can also operate in Controller ModeDetector measures and controls VGA in an analog loopDetector tries to balance input power at its two RF inputs Gain setpoint is controlled by difference in external attenuators
42
Gain vs. Input Power for Analog Gain Control Loop
Gain varies by only +/-0.25 over a 60 dB input rangeExcellent stability over temperature
