— —
20
9. Adjust R65 (between IC21 and IC22) for a right-channel null.
NOTE:
The third filter null is at approximately 34.5kHz. Rather than
to match
frequency
response between channels at this point, the third
filter adjustment is used to help match
phase
response in the next
procedure.
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
DAVID-II
will reflect the
inaccuracy.
1. Connect the
COMPOSITE OUTPUT
of the
DAVID-II
to the input of
the Spectrum Analyzer (or directly to the Composite Input of the
Mod-Monitor).
2. Turn the output of the Audio Generator completely off.
3. With the
DAVID-II
MODE
switch in
STEREO
, and
PILOT
switch
ON
, adjust either the front-panel
OUTPUT LEVEL
control (or the
Spectrum Analyzer or Mod-Monitor input gain attenuator) for a
19kHz pilot reading of –20dB (10% injection).
4. Adjust R20 (between IC12 and IC17) to null the 38kHz second-
harmonic-distortion component of the stereo pilot. The Spectrum
Analyzer should display the 38kHz component at the noise floor;
the Mod-Monitor should indicate a 38kHz “residual” reading at the
bottom of its most sensitive scale (–70dB or better).
5. Next, re-apply the Audio Generator test signal to the
LEFT
and
RIGHT
PROGRAM LINE INPUT
of the
DAVID-II
as described in Step
3 of the Low-Pass Filter Matching section of this Procedure. The
Audio Generator frequency should be set to 1kHz, and the level set
the same as before. If in doubt about the level, check for 6-volt p-p
signals on the right-hand ends of R22 and R27.
6. Monitor the “sub channel.” This is the 38kHz double-sideband,
suppressed-carrier component on the Spectrum Analyzer (or an
“L–R” Mod-Monitor reading).
7. Carefully adjust the front-panel
INPUT GAIN L
control for a null in
the sub channel.
8. Adjust R157 (to the left of IC35) 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 7 and 8 until no
further improvement in the sub-channel null can be achieved.
9. Increase the Audio Generator frequency to 10kHz. Adjust R111
(between IC24 and IC25) for the best null in the sub channel at
10kHz.
10. Repeat Steps 7, 8 (at 1kHz) and 9 (at 10kHz) until the sub-channel
null at both frequencies is at maximum. You may wish to “split the
difference” to balance crosstalk measurements, sacrificing a null at
