Special Considerations for Analog Demod

Analog demodulation requires certain conditions and has certain constraints, as described in the following paragraphs:

• The VSA 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.

• Analog demodulation requires a positive start frequency.

• 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.

• The resulting time record displays amplitude versus time (AM Amplitude Modulation - CW modulation using amplitude variation in proportion to the amplitude of the modulating signal. Usually taken as DSB-LC for commercial broadcast transmissions and DSB-SC for multiplexed systems. demodulation), frequency versus time (FM Frequency Modulation demodulation), and phase versus time (PM demodulation).

• Only one demodulator per channel can be selected at a time.

• The span must be set to include the carrier and all of its sidebands. Setting a span which is too narrow will cause distortion.

• The bandwidth of the demodulator is approximately equal to the span.

• When using Analog Demodulation, features such as gating, band power markers, and averaging work as described for vector measurements. When demodulating, the entire time record is demodulated.