Special Considerations for Analog Demodulation
Analog demodulation requires certain conditions and has certain constraints. For
this reason some default conditions for analog demodulation differ from other
instrument modes.
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The analyzer automatically selects zoom mode if you select analog
demodulation. Analog demodulation requires zoom mode in order to identify
the carrier. The center frequency should be set as close to the carrier
frequency as possible. The center frequency is programmed into the digital
local oscillator, and represents the initial estimate of the carrier frequency.
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Analog demodulation requires a positive start frequency.
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The visible demodulated span contains only half the number of points that
were contained in the original signal—the complex waveform is transformed
into a real waveform which cuts spectral information in half. Analog
demodulation operates only on double sideband signals. Therefore, the
information in one sideband is redundant and the analyzer mixes the signal
down to baseband (zero Hertz start), eliminates the carrier, and eliminates the
lower sideband.
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Selecting analog demodulation automatically enables time corrections. Time
corrections correct for amplitude and phase distortion caused by the anti-alias
and digital filters prior to demodulation. If you choose not to use time
correction, you must turn it off before selecting a demodulation type (AM, FM
or PM). The next section discusses the role of time correction in demodulated
measurements in more detail.
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The time record is altered by analog demodulation resulting in time domain
displays of amplitude versus time (AM demodulation), frequency versus time
(FM demodulation), and phase versus time (PM demodulation).
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The demodulator alters the time record, and the time record is the basis for
all other measured results. Therefore, all other measured results are computed
using the demodulated waveform. For example, the spectrum is the spectrum
of the modulating signal.
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Only one demodulator per channel can be selected at a time.
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The span must be set to include the carrier and all of its sidebands. Setting a
span which is too narrow will cause distortion.
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The bandwidth of the demodulator is approximately equal to the span.
Analog Demodulation Concepts
20 - 4
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