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RTU-292 Operations Manual
6.5.2.2 Alternate Hybrid Test Procedure
This alternate test procedure uses a more easily obtainable audio signal generator in place of
the noise generator. While this procedure cannot give a complete picture of the unit’s
performance in actual use, it will allow a determination of whether or not the RTU-292 is
operating properly.
1. Disconnect the RTU-292 from all other equipment and connect the test equipment as shown
in Figure 6.1, except use an audio signal generator in place of the noise generator.
2. . Set the PSTN levels for -9 dBm using Option SW2-1, -2, and -3.
3. Start with the following pushbuttons OFF: TEL LINE/PHONE, TEL LINE/RADIO, and
TEL VOX. The positions of the other buttons are unimportant. Place the RTU-292 in set-up
mode by setting SW2-7 ON and then turning on the power. (See paragraph 2.10.1 for further
details about set-up mode.)
4. Adjust the TX Level Adjust, R133, so that the audio voltmeter reads -6dBm (0.387V rms).
This sets the transmit signal output path in the RTU-292 for unity gain.
5. Set the output amplitude of the audio signal generator to about 0dBm (0.775V rms). Adjust
the RX Level Adjust so that the PEAK LED just comes on. This will set the average noise
level into the RTU-292’s hybrid to approximately 0dBm.
6. Remove the unit from Set-up Mode by turning the RTU-292 Main Power OFF, setting SW2-
7 OFF, and then turning the main power back ON.
7. Set the audio generator frequency to about 1 kHz.
8. Push the TEL LINE/RADIO button to start the adaptation cycle.
9. When the initial adaptation cycle is complete, the tone output can be read on the audio
voltmeter. The voltage should be typically less than -50dBm (2.5mV). The trans-hybrid loss is
the difference between this reading and the approximate 0dBm tone level.
NOTE
The reading should be made as soon as the adaptation cycle is complete. The
RTU-292 continuously adapts itself to the conditions to which it is exposed. With
the single tone input presented by this test, the unit will eventually adapt to reject
only this single tone instead of the intended broad range of frequencies. This may
take several seconds to several minutes, and an erroneous reading will result when
this happens. The reading will always be correct (and an accurate depiction of
operation under normal conditions) immediately following the initial adaptation
cycle, since the unit has adapted broad band using its own noise source, and has
not yet had time to adapt to the single tone only.
10. To measure the unit at a different frequency, change the audio generator frequency and
repeat steps 8 and 9.
JPS Communications, Inc.
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