CN-0187 Datasheet, PDF (5/7 Pages) Analog Devices – Crest Factor, Peak, and RMS RF Power Measurement Circuit Optimized for High Speed, Low Power, and Single 3.3 V Supply

Circuit Note
3
V_PEAK-UNKNOWN
V_VRMS-UNKNOWN
V_PEAK-CW ²
0
PEAK OF
UNKNOWN WAVEFORM
VRMS OF
UNKNOWN WAVEFORM
(RESULT INDEPENDENT
OF WAVEFORM)
PEAK OF
CW, CF = 0dB
1
V_IN INPUT (V rms)
Figure 10. Procedure for Crest Factor Calculation
Next, the CW reference level of PEAK, VPEAK-CW, is calculated
using VIN (that is, the output voltage that would be seen if the
incoming waveform was a CW signal).
VPEAK-CW = (VIN GainPEAK) + InterceptPEAK (5)
Finally, the actual level of PEAK, V , PEAK-UNKNOWN is measured and
the CF can be calculated as
CF = 20 log10 (VPEAK-UNKNOWN /VPEAK-CW)
(6)
where VPEAK-CW is used as a reference point to compare
V . PEAK-UNKNOWN If both VPEAK values are equal, then the CF is 0 dB,
as shown in Figure 11 with the CW signal (taken from the
ADL5502 data sheet). Across the dynamic range, the calculated
CF hovers about the 0 dB line. Likewise, for complex waveforms
of 3 dB, 6 dB, and 9 dB CFs, the calculations accurately hover
about the corresponding CF levels.
10
8-TONE WAVEFORM, 9dB CF
9
8
7
4-TONE WAVEFORM, 6dB CF
6
5
4
2-TONE WAVEFORM, 3dB CF
3
2
1
CW, 0dB CF
0
–1
–25 –20 –15 –10 –5
0
5
INPUT (dBm)
10
15
Figure 11. Reported Crest Factor of Various Waveforms
CN-0187
3.0
2.5
2.0
1.5
450MHz
1.0
900MHz
1900MHz
0.5
2350MHz
2600MHz
0
–0.5
–1.0
–25
–15
–5
5
15
INPUT (dBm)
Figure 12. Measured Crest Factor of CW Signals vs. Input Level, 450 MHz,
900 MHz, 1900 MHz, 2350 MHz, 2600 MHz, Supply +3.3 V
The performance of this or any high speed circuit is highly
dependent on proper PCB layout. This includes, but is not
limited to, power supply bypassing, controlled impedance lines
(where required), component placement, signal routing, and
power and ground planes. (See MT-031 Tutorial, MT-101 Tutorial,
and article, A Practical Guide to High-Speed Printed-Circuit-
Board Layout, for more detailed information regarding PCB
layout.)
A complete design support package for this circuit note can be
found at http://www.analog.com/CN0187-DesignSupport.
COMMON VARIATIONS
For applications that require less RF detection range, the
AD8363 rms detector can be used. The AD8363 has a detection
range of 50 dB and operates at frequencies up to 6 GHz. For
non-rms detection applications, the AD8317/AD8318/AD8319
or ADL5513 can be used. These devices offer varying detection
ranges and have varying input frequency ranges up to 10 GHz
(see CN-0150 for more details).
CIRCUIT EVALUATION AND TEST
This circuit uses the EVAL-CN0187-SDPZ circuit board and the
EVAL-SDP-CB1Z System Demonstration Platform (SDP)
evaluation board. The two boards have 120-pin mating
connectors, allowing for the quick setup and evaluation of the
circuit’s performance. The EVAL-CN0187-SDPZ board contains
the circuit to be evaluated, as described in this note, and the
SDP evaluation board is used with the CN0187 evaluation
software to capture the data from the EVAL-CN0187-SDPZ
circuit board.
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