PRELIMINARY INFORMATION

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