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Model 8901B
Service
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PI0 A3 AUDIO 0E.EMPHASIS AN0 OUTPUT ASSEMBLY
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SOHzHPF
From
Audio Fillsn
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OUTPUT
AMPLIFIER
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I N V F R T l N G l
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FM DE.EMPHASISANO
AMPLIFIER
U14A
3
kHz
LPF
0
MOO NETWORKS
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Figure
8B-1. Example Showing High-Pass Filter Switching
Notice that the display no longer shows a measurement result. No annunciators are lighted (except
those related to HP-IB, if the Special Function is entered via HP-IB), and only the SPCL key
is lighted. If any key other than a number key, S (Shift) key, or the LCL key is pressed, the
instrument hardware will revert back to the measurement mode it was in before the Direct Control
Special Function was entered. Thus, in this example, unless the 50 Hz High-Pass Filter had been
previously selected with the front-panel key, it would be removed from the audio path, when
any other key is pressed. (However, note that there are some Service Special Functions that will
maintain the requested configurations even if another key is pressed.)
Table 8B-2 indicates that 0.14A will also select the 50 Hz High-Pass Filter. Any Direct Control
Special Function of the form 0.14d also controls the pre-display filter on/off switches U14A and
U14B. For pre-display on (0.142), U14A is closed. For pre-display off (0.14A), U14B is closed. As
it turns out, 0.14d codes other than those shown in the table will also affect the high-pass filters.
For example, 0.147 will close U12A, U12B, and U12C, simultaneously (with U14A also closed).
This fact is ascertained from the service sheet schematic.
Example
2
A second example from the A3 assembly illustrates data readback when using the Direct Control
Special Function. One of the means of detecting an overrange of the audio circuits is by the
Audio Overvoltage Detector. The detector is on the audio input line before any active (and hence,
distortable) filters (see Figure 8B-2). The audio input line is the same as in the previous example.
The Audio Overvoltage Detector senses the peak signal level on the line and
U9
compares it
against a reference. If the detected level rises above the reference, the output of U9 goes low and
resets flip-flop U19D. Other flip-flops (not shown) are also reset and open the audio path ahead of
the detector (without intervention of the Controller). U21D and U21C, when enabled, invert the
output of U19D twice. The output of U21C is across the least-significant bit of the readback data
line of the Instrument Bus. In the normal measurement cycle, the Controller reads the status of
the Audio Overvoltage Detector (by enabling U21D and reading the output of U21C) and displays
an error if U9 has tripped.
Special Functions
8B-3
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