The Importance Of Carrier Identification; Auto Carrier With Am Demodulation; Auto Carrier With Pm Demodulation; Auto Carrier With Fm Demodulation - Agilent Technologies 89410A Operator's Manual

Analog Demodulation Concepts

The Importance of Carrier Identification

Accurate angle demodulation depends on precisely identifying the carrier
frequency. The demodulation algorithm for each demodulation type (AM, FM and
PM) assumes that the carrier is at the center frequency. The auto carrier feature
examines phase ramping to estimate the frequency error between the carrier and
the analyzer's local oscillator. Auto carrier implementation varies depending on the
selected demodulator.

Auto carrier with AM demodulation

AM demodulation is the same with or without auto carrier.

Auto carrier with PM demodulation

The analyzer removes frequency error and phase offset between its center
frequency and the detected carrier frequency. The carrier estimate is based on data
within a single time record and is updated on a record-by-record basis. This
method of carrier identification does not involve carrier locking with a
phase-locked loop.

Auto carrier with FM demodulation

FM is obtained by differentiating PM, so the auto carrier calculation, in removing
the phase ramp, eliminates frequency offsets in FM as well.

Special considerations for auto carrier use

Turning off auto carrier may yield the results you want if:
l
You want to evaluate frequency or phase variations above or below the
carrier frequency. Examples of this situation would occur in examining a
hopping carrier, transients, or oscillations varying around 0 degrees. In these
cases you must turn off auto carrier to make the instability visible.
l
Your signal has a suppressed carrier.
l
You have locked the analyzer and device-under-test to a common reference
and carefully set the center frequency.
l
Your signal's sidelobes are truncated.
l
Your signal contains low frequency modulation (fewer than 5 cycles over the
time record) or transients.
l
Your carrier is not stationary. In this case auto carrier may cause phase
wrapping.
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