TM 11-6625-3059-10
PRELIMINARY INFORMATION
FM Measurements
FM measurements for modulating frequencies from 30 Hz to 100 kHz and deviations up to 300 kHz peak-to peak
can be made. To achieve maximum accuracy, the signal level applied to the RF input should be greater than 100
millivolts. This reduces residual FM within the ME-525A/USM to a minimum value. To further reduce residuals,
the minimum measurement bandwidth consistent with the modulation frequency should be used. For instance, for
measurements at a 1-kHz modulation rate, the 300 Hz 3 kHz bandwidth should be used. Because the audio
detectors are true peak responding, the residual noise is added directly to the recovered signal being measured.
True rms measurements of the recovered audio signal available at the front panel connector will provide a more
precise indication of modulation in the presence of noise. Because of the low residual FM characteristics of the
ME-525A/USM, direct measurements of residual FM are possible. To obtain a meaningful indication of the
combined noise of the M E-525A/USM and the device under test, true rms measurement is suggested. Accurate
measurements of the ME-525A/USM noise characteristics are made by driving the RF input with a crystal source at
the desired RF frequency and a level exceeding 100 millivolts rms. Note that the LED display will respond to the
highest noise peaks.
Typical Residual Response
2-13
