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RTU-292 Operations Manual
6.5.2 HYBRID BALANCE MEASUREMENT
The test procedure outlined below allows measurement of the hybrid balance, or trans-hybrid
loss, attainable with the RTU-292. This characteristic determines how much radio receiver
input signal will leak into the transmitter output of the RTU-292. In the test set-up, the noise
generator simulates the radio receiver, the audio voltmeter simulates the transmitter and the
resistor simulates the telephone line. If the hybrid balance were perfect, the trans-hybrid loss
would be and none of the receiver signal would leak into the transmitter (the audio voltmeter in
this set-up would measure nothing except residual noise.)
The RTU-292 trans-hybrid loss specification is at least 30dB when measured with a noise
source with an output bandwidth that matches the RTU-292’s bandwidth. This means that the
noise output measured by the audio voltmeter should be at least 30dB below the noise input to
the RTU-292’s hybrid. The test is performed with a 600 Ohm resistor simulating the phone
line in order to have a repeatable standard impedance on which to base the specification. In
actual use, the hybrid balance of the RTU-292 is excellent into impedances other than 600 Ohm
(such as the complex impedances of the telephone lines) and is far better than any conventional
hybrid under these conditions.
The procedure for this test using a noise generator is listed in paragraph 6.5.2.1. If a noise
generator is not available, the alternate test method that employs an audio signal generator (see
paragraph 6.5.2.2) may be used. Figure 6.1 shows equipment interconnections necessary to
perform this test.
NOTE
The accuracy of the trans-hybrid loss measurement depends on the bandwidth of
the noise generator used to perform the test. It is possible, however, to achieve an
evaluation of the unit to within a few dB tolerance without a commercial noise
generator by using the audio output of an SSB or AM receiver that has no antenna
connected. This type of receiver with a 2.5 to 3.5 kHz SSB IF bandwidth makes
an ideal noise generator for this measurement because the output noise is band
limited by the receiver’s IF filters. (FM receivers, however, have too wide a noise
bandwidth and will produce erroneous results using this procedure.) The
receiver’s noise output should be between -20dBm and 0dBm so that the RTU-
292 can be adjusted for the correct hybrid input level.
JPS Communications, Inc.
6-20