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Making Measurements
Measuring Amplifiers
Measuring Gain Compression Gain compression occurs when the input power of an amplifier is increased to a level that
reduces the gain of the amplifier and causes a nonlinear increase in output power. The
point at which the gain is reduced by 1 dB is called the 1 dB compression point. The gain
compression will vary with frequency, so it is neces sary t o fi nd the wo rst- c ase point of gain
compression in the frequency band.
Once that point is identified, you can perform a power sweep of that CW frequency to
measure the input power at which the 1 dB compression occurs and the absolute power out
(in dBm) at compression. The following steps provide detailed instruction on how to apply
various features of the analyzer to accomplish these measurements.
NOTE In a compression measurement it is necessary to know the RF input or output
power at a certain level of gain compression. Therefore, both gain and
absolute power level need to be accura tely characterized. Uncertainty in a
gain compression measurement is typically less than 0.05 dB. Also, each
input channel of the analyzer is calibrated to display absolute power
(typically within +0.5 dBm up to 3 GHz, and +1 dB up to 6 GHz). This can be
improved by calibrating the power meter. Refer to "Power Meter
Measurement Calibration" on page 6-33 for information on calibrating the
power meter.
Figure 1 -43 Diagram of Gain Compression
1. Set up the stimulus and response parameters for your amplifier under test. To reduce
the effect of noise on the trace, press:
or on ET models:
2. Perform the desired error correction procedure. Refer to Chapter 6, "Calibrating for
Increased Measurement Accuracy" for instructions on how to make a measurement
correction.
3. Connect the amplifier under test.
Avg
IF BW
1000 x1
Chan 1 Meas
Trans:FWD S21 ( B/R)
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