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S:\Hp8960\E1962B CDMA 2000\Pi Release\Reference Guide\Chapters\cdma2000_meas_fm_desc.fm
Frequency Modulation (FM) Measurement Description
Frequency Modulation (FM) Measurement Description
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 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.
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.
Refer to IS-137, 3.3.1, for more information on measuring analog modulation characteristics.
Triggering
Triggering is always immediate for this measurement. Trigger Delay and other trigger sources are not used.
Considerations When Making Manual FM Measurements
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.
Distortion State and Distortion Fundamental Frequency Operation
The Distortion Fundamental Frequency value specifies the demodulated audio frequency to use when making
a distortion measurement, and must be entered to make a valid measurement. However, specifying the
fundamental frequency does not affect the measurement state. The Distortion State must be turned On
separately to make the measurement.
Modulation Frequency State
The modulation frequency measurement is enabled and disabled using the Mod Freq State setting. The
modulation frequency measurement is made after any filtering is applied, and can be used to make
measurements such as SAT frequency.
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