The ME-525A/USM makes possible fast, accurate measurements of amplitude modulation. Optimum accuracy is
achieved by using an input signal level between 100 and 1000 millivolts rms (lowest residuals), and' minimum
measurement bandwidth consistent with the modulation frequency used. True rms measurement of the recovered
audio output at the connector on the front panel of the ME-525A/USM will improve measurement accuracy in the
presence of noise and/or distortion.
High Percentage AM Measurements
Difficulties may be experienced when attempting to measure AM modulations approaching 100%. The ME-
525A/USM automatically searches for, and locks onto the highest carrier level between 10 MHz and 1.2 GHz. If
this carrier is amplitude-modulated the carrier tends to disappear in the troughs of modulation as the percentage of
AM approaches 100%. This causes the ME-525A/USM to lose lock and start a search for a higher level carrier.
The point at which frequency lock is lost is a function of carrier frequency and level. Worst case occurs at low
frequencies at low levels where lock can be lost at approximately 93% of AM. The situation improves to
approximately 98% at high carrier frequencies and levels.
TYPICAL AUDIO RESPONSE