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Theory of Operation
3–20
AM700 Audio Measurement Set Service Manual
The digital audio board has circuitry that supports the following interface
measurements. These are the qualities of the serial data waveform itself and not
the coded audio data.
H Amplitude. A DC level from the AGC circuit provides a measure of the
input signal level.
H Cable loss. A DC level from the auto equalization circuit provides a measure
of the signal degradation (rolloff) caused by the transmission path.
H Clock frequency and relative phase. A digital counter cycles through a series
of measurements. These measurements provide the numbers for the Main
Sample Frequency, the Reference Sample Frequency, and the Phase
Difference (Main versus Ref) readouts in the Eye Diagram display.
H Eye diagram. Data for the eye diagram is generated by an equivalent-time
sampling circuit. The eye diagram display will not appear if the eye/jitter
phase-lock-loop circuit is not locked to a valid input signal (such as when the
signal is too noisy, has too much jitter, or is outside the sample frequency
range). The output of the sampler is sent to the DSP by the Sample
Transmitter. The DSP plots the eye diagram and derives the Peak-to-Peak
Jitter readout in the Eye Diagram display.
H Jitter spectrum. Data for the jitter spectrum display is generated by real-time
sampling of eye/jitter phase-lock-loop phase error signal. The output of the
sampler is sent to the DSP by the Sample Transmitter. The DSP performs a
FFT on the data to produce the jitter spectrum display.
H LF Rejection 120 Hz/1200 Hz. The low frequency rejection of the eye/jitter
phase-lock-loop circuit is selectable between 120 Hz and 1200 Hz. For the
Eye diagram jitter components below the selected cutoff frequency are
reduced. In Jitter Spectrum display, this filtering effect is compensated out
by software to maintain the jitter spectrum flatness.
H X1/X8 Jitter Gain. With a low-jitter signal, the amplifier gain for the jitter
spectrum display may be switched from unity to X8 gain to increase the jitter
amplitude for better digitization thereby decreasing the noise floor. The X8
gain is compensated out by the software to maintain constant reference level
in the jitter spectrum display.
H Equalization Bypass. In Jitter Spectrum mode only, the auto-equalization
circuitry is bypassed. This provides a truer representation of the effects of
transmission path induced jitter on an unequalized receiver.
Reference Receiver. The Reference Receiver, also shown as part of Figure 3–7,
decodes the reference input digital audio signal and supplies timing signals for
relative timing measurements. The Reference Receiver depends on the following
circuitry for proper operation.
Digital Interface Tester
Measurement Functions