CN-0187 Datasheet, PDF (4/7 Pages) Analog Devices – Crest Factor, Peak, and RMS RF Power Measurement Circuit Optimized for High Speed, Low Power, and Single 3.3 V Supply

CN-0187
Circuit Note
PULSED RFIN
400mV rms RF INPUT
250mV rms
160mV rms
70mV rms
VRMS
1ms/DIV
Figure 7. Output Response to Various RF Input Pulse Levels, Supply 3 V,
900 MHz Frequency, Square-Domain Filter Open, Output Filter 0.1 μF
with Parallel 1 kΩ
The RMS and PEAK outputs of the ADL5502 pass through
unity gain buffers that drive cross-coupled stages for converting
the single-ended outputs to differential signals. The internal
+2.5 V reference of the AD7266 (via the DCAPA and DCAPB pins)
passes through another unity gain buffer and a voltage divider.
This sets the common-mode voltage of the network to +1.25 V.
The AD7266 achieves simultaneous samples of the RMS and
PEAK outputs and transfers the data within a 1 µs response
time. The data is provided on a single serial data line. Because
slope and intercept vary from device to device, board-level
calibration must be performed to achieve high accuracy. In
general, calibration is performed by applying two input power
levels to the ADL5502 and measuring the corresponding output
voltages. The calibration points are generally chosen to be
within the linear operating range of the device. The best-fit line
is characterized by calculating the conversion gain (or slope)
and intercept using the following equations:
Gain = (VVRMS2 − VVRMS1)/(VIN2 − VIN1)
(1)
Intercept = VVRMS1 − (Gain × VIN1)
(2)
where:
VIN is the rms input voltage to RFIN.
VVRMS is the voltage output at VRMS.
Once gain and intercept are calculated, an equation can be
written that allows calculation of an (unknown) input power
based on the measured output voltage.
VIN = (VVRMS − Intercept)/Gain
(3)
For an ideal (known) input power, the law conformance error of
the measured data can be calculated as
ERROR (dB)
=
20
×
log


V_{VRMS, MEASURED} – Intercept
Gain × V_{IN, IDEAL}


(4)
Figure 8 and Figure 9 show plots of the VRMS and PEAK error
at 25°C, the temperature at which the ADL5502 is calibrated.
Note that the error is not zero; this is because the ADL5502
does not perfectly follow the ideal linear equation, even within
its operating region. The error at the calibration points is,
however, equal to zero by definition.
3
2
1
0
450MHz
–1
900MHz
1900MHz
2350MHz
–2
2600MHz
–3
–25 –20 –15 –10 –5
0
5
10
15
INPUT (dBm)
Figure 8. Measured VRMS Linearity Error vs. Input Level, 450 MHz, 900 MHz,
1900 MHz, 2350 MHz, 2600 MHz, Supply +3.3 V
3
2
1
0
450MHz
–1
900MHz
1900MHz
2350MHz
–2
2600MHz
–3
–25 –20 –15 –10 –5
0
5
10
15
INPUT (dBm)
Figure 9. Measured PEAK Linearity Error vs. Input Level, 450 MHz, 900 MHz,
1900 MHz, 2350 MHz, 2600 MHz, Supply +3.3 V
When the characteristics (slope and intercept) of the VRMS and
PEAK outputs are known, the calibration for the CF calculation
is complete. A three-stage process must be taken to measure
and calculate the crest factor of any waveform. First, the
unknown signal must be applied to the RF input, and the
corresponding VRMS level is measured. This level is indicated
in Figure 10 as V . VRMS-UNKNOWN The RF input, VIN, is calculated
using VVRMS-UNKNOWN and Equation 3.
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