— —
22
3. When the Audio Generator has been fine-tuned to null the right
channel, adjust R116 (next to IC31) for a null in the left channel
filter. Both channels should null to the same relative amplitude.
4. Increase the Audio Generator frequency to the second filter null,
approximately 21.8kHz. Again, fine-tune the Audio Generator
frequency for the deepest null in the right channel signal.
5. Adjust R122 (next to IC33) for a left-channel null.
6. Increase the Audio Generator frequency to the third filter null,
approximately 34.6kHz. Once again, fine-tune the Audio Generator
frequency for the deepest null in the right channel signal.
7. Adjust R141 (next to IC37) for a left-channel null.
Crosstalk Trim
If your station Mod-Monitor includes a true “precision” FM-stereo
demod (crosstalk/separation resolution of 70dB or better), it may be
used in lieu of a Spectrum Analyzer for this part of the Calibration
Procedure. If a Mod-Monitor of
insufficient precision
is used, final
Calibration (and performance!) of the 708 will reflect the inaccuracy.
1. Connect the
COMPOSITE OUTPUT
of the 708 to the input of the
Spectrum Analyzer (or directly to the Composite Input of the Mod-
Monitor).
2. Next, re-apply the Audio Generator test signal to the left and right
PROGRAM LINE INPUTS
of the 708 as described under the Input
Level Set section at the beginning of this Procedure. The Audio
Generator frequency should be 400Hz, but the level should be
reduced for a peak-to-peak waveform amplitude of 6 volts on the
right-hand ends of R128 and R135.
3. Monitor the “sub channel.” This is the 38kHz double-sideband,
suppressed-carrier component on the Spectrum Analyzer (or an
“L–R” Mod-Monitor reading).
4. Carefully adjust the front-panel
INPUT GAIN L
control for a null in
the sub channel.
5. Adjust R156 (to the left of IC41) to further null the sub channel.
NOTE:
Because the effect of the two preceding adjustments will
appear to interact, it will be necessary to repeat Steps 4 and 5 until no
further improvement in the sub-channel null can be achieved.
6. Increase the Audio Generator frequency to 15kHz. Adjust R141 (to
the left of IC37) for the best null in the sub channel at 15kHz.
7. Repeat Steps 4, 5 (at 400Hz) and 6 (at 15kHz) until the sub-channel
null at both frequencies is at maximum.
Because the Inovonics 708 utilizes digital techniques (instead of analog
multiplication) to generate the subcarrier, crosstalk is symmetrical;
that is, sub-to-main crosstalk will be identical to the main-to-sub which
was optimized in the foregoing Procedure.