OPTIMOD-FM OPERATION
4-9
It is assumed that the technician is thoroughly familiar with the operation of this equip-
ment.
This procedure is useful for detecting and diagnosing problems with the 8400's perform-
ance. It includes checks of frequency response, noise and distortion performance, and
output level capability.
This performance audit assesses the performance of the analog-to-digital and digital-to-
analog converters and verifies that the digital signal processing section (DSP) is passing
signal correctly. Ordinarily, there is a high probability that the DSP is performing the dy-
namic signal processing correctly. There is therefore no need to measure such things as
attack and release times—these are defined by software and will automatically be correct
if the DSP is otherwise operating normally.
It is often more convenient to make measurements on the bench away from high RF
fields which could affect results. In a high RF field it is, for example, very difficult to
accurately measure the very low THD produced by a properly operating 8400 at most
frequencies. However, in an emergency situation (and is there any other kind?), it is usu-
ally possible to detect many of the more severe faults that could develop in the 8400 cir-
cuitry even in high-RF environments.
See the assembly drawings in Section 6 for component locations. Be sure to turn the
power off before removing or installing circuit boards.
Follow these instructions in order without skipping steps.
Note: To obtain an unbalanced output, jumper pin 1 (ground) to pin 3, and measure be-
tween pin 1 (ground) and pin 2 (hot).
Note: All analog output measurements are taken with a 620Ω ±5% resistor tied between
pin 2 and 3 of the XLR connector.
1. Prepare the unit.
A) Set the GND LIFT switch to the earth ground symbol setting (left position) to con-
nect chassis ground to circuit ground.
B) Use the front panel controls to set the 8400's software controls to their default set-
tings, as follows:
a) From the main menu, choose Input/Output. Using the Locate joystick, navigate
in turn to each of four
Input/Output screens and write down the settings so you
can restore them after testing.
b) Navigate to the Input/Output 1 screen. Set controls as in the table below:
Set Input to analog
Analog Ref. Level +4.0 dBu
Clip Level +20.0 dBu
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