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6–46
AM700 Audio Measurement Set Service Manual
Table 6–11: Digital audio circuit board testing (cont.)
Test name Test description
Test group 3 Receiver tests, Generator tests which require the receiver.
3.3 input_level_linearity This test checks the accuracy of the input level measurement circuit described in 3.2
above for all external inputs, and at several amplitude levels.
Same description as 3.2 above. These considerations apply to 3.2, too: Since the
measurement involves a peak detector, this test is sensitive to overshoot on the signal
being measured. This test is also sensitive to square-wave symmetry of the signal. Since
all inputs are ac-coupled, an a-symmetry in time (duty factor other than 50%) will cause
the peak value of the waveform to change.
3.4 agc_response This test connects the generator internally to the receiver and steps the generator level
back and forth between two levels as the operator observes the acg loop’s transient
response.
This time constant is determined by all the gain setting parts in the agc loop: R259,
R260, R268, R257, and C161.
3.5 opto_level This test measures the receiver signal level from the optical input by measuring the agc
control voltage. The operator connects the optical output to the optical input.
The optical input to the agc loop is selected by U109 under control of the OPTO_EN
signal from the serial control chain. The agc loop operates as described in
input_level_cal, 3.2, above. OPTO_IN is the signal from the optical receiver on the rear
panel board. The optical receiver’s ttl output is either high or low depending on whether
the light level crosses a threshold. This means that this test should always measure the
same amplitude regardless of source intensity or cable length, as long as the light level
crosses the receiver threshold.
On the generator side, the rear panel optical transmitter is driven by the OPTO_OUT
signal from U108 pin 39.
3.6 adj_ej_pll This test internally connects the generator to the receiver so the operator can adjust the
center frequency of the eye sampler pll. The pll consists of programmable synthesizer
U122, phase error amp U113A and B, resonant tank CR24/L16, and oscillator IC U123.
The oscillator’s 12.288 MHz ecl level output is fed back to U122 pin 4.
The pll locks to the INMCK input at U122 pin 1. This is 12.288 MHz ttl from the main
decoder, U86 pin 19. U86 pin 9 requires an AES signal, which comes from U129 pin 3.
U129 selects EYE_SOURCE, U129 pin 2, which comes from TP3.