CN-0187 Datasheet, PDF (3/7 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
Typical measured performance characteristics of the circuit are
presented in Figure 2 through Figure 5.
10
1
450MHz
900MHz
0.1
1900MHz
2350MHz
2600MHz
0.01
–25 –20 –15 –10 –5
0
INPUT (dBm)
5
10
15
Figure 2. Measured VRMS Output vs. Input Level (Log Scale), 450 MHz,
900 MHz, 1900 MHz, 2350 MHz, 2600 MHz, Supply +3.3 V
2.0
1.8
1.6
1.4
1.2
1.0
450MHz
0.8
900MHz
1900MHz
0.6
2350MHz
2600MHz
0.4
0.2
0
0
0.2
0.4
0.6
0.8
1.0
INPUT (V rms)
Figure 3. Measured VRMS Output vs. Input Level (Linear Scale), 450 MHz,
900 MHz, 1900 MHz, 2350 MHz, 2600 MHz, Supply +3.3 V
10
1
450MHz
900MHz
0.1
1900MHz
2350MHz
2600MHz
0.01
–25 –20 –15 –10 –5
0
INPUT (dBm)
5
10
15
Figure 4. Measured PEAK Output vs. Input Level (Log Scale), 450 MHz,
900 MHz, 1900 MHz, 2350 MHz, 2600 MHz, Supply +3.3 V
CN-0187
2.0
1.8
1.6
1.4
1.2
1.0
0.8
0.6
450MHz
900MHz
0.4
1900MHz
2350MHz
0.2
2600MHz
0
0
0.2
0.4
0.6
0.8
1.0
INPUT (V rms)
Figure 5. Measured PEAK Output vs. Input Level (Linear Scale), 450 MHz,
900 MHz, 1900 MHz, 2350 MHz, 2600 MHz, Supply +3.3 V
The turn-on time and pulse response is strongly influenced by
the size of the square-domain filter (CFLTR) and output shunt
capacitor connected to the VRMS output. Figure 6 (taken from
the ADL5502 data sheet) shows a plot of the output response to
an RF pulse on the RFIN pin, with a 0.1 μF output filter
capacitor and no square-domain filter capacitor (CFLTR). The
falling edge is particularly dependent on the output shunt
capacitance.
PULSED RFIN
400mV rms RF INPUT
250mV rms
160mV rms
70mV rms
VRMS
1ms/DIV
Figure 6. Output Response to Various RF Input Pulse Levels, Supply3 V,
900 MHz Frequency, Square-Domain Filter Open, Output Filter 0.1 μF
To improve the falling edge of the enable and pulse responses,
a resistor can be placed in parallel with the output shunt
capacitor. The added resistance helps to discharge the output
filter capacitor. Although this method reduces the power-off
time, the added load resistor also attenuates the output (see the
Output Drive Capability and Buffering section of the ADL5502
data sheet). Figure 7 (taken from the ADL5502 data sheet)
shows the improvement obtained by adding a parallel 1 kΩ
resistor.
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