How is a frequency modulation measurement made?
The Analog Frequency Modulation measurement measures the frequency deviation and, if enabled, the total harmonic distortion and modulation frequency, for frequency modulated analog signals (such as AMPS transmitters). Measurements are performed after any filtering, de-emphasis, and expansion are performed (selected by the user in the setup parameters).
The FM deviation and distortion measurements are not affected by a static (constant) frequency error on the input signal as long as the input signal is within 2.5 kHz of the expected frequency.
This measurement is not used for digital signals.
Operating Consideration
The distortion value is not returned if the Distortion State is set to OFF. The modulation frequency value is not returned if the Modulation Frequency State is set to OFF.
Triggering is always immediate for this measurement. Trigger Delay and other trigger sources are not used.
When manually controlling the test set from the front panel, all measurements default to using continuous triggering. If you make digital measurements on a DTC using RF Rise or Protocol triggering, and then hand off to an AVC and select an analog measurement, the analog measurement may not trigger and display a result. This is because the digital measurement is still waiting for a burst transmission to trigger it (which it won't get from an analog signal) and therefore the measurements "freeze". To prevent this from happening when making analog measurements, either turn off the digital measurements or set their trigger source to Immediate.
top
of page