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c)
The PREEMPHASIS trimmers are used to adjust the entire OPTIMOD-TV for
best
conformance to the standard FM preemphasis. Connect a precision 75us ( or
50us) deemphasis network to the TEST JACK corresponding to the channel which
you
are aligning. Connect the output of the deemphasis network to the input of
the 5L4N spectrum analyzer.
7.50K I%
VV\t
---
•
o
INPUT
OUTPUT
C1011.1P
I %
o
o
Fig. E-1: PRECISION 75us DEEMPHASIS NETWORK
(NOTE: To preserve accuracy, load must be greater than lmeg.)
d)
Set
the
5L4N
for
a 0-20kHz sweep ( 2kHz/div). Set
its input
sensitivity
to
-10dBV
in
dB
mode.
Set
the
vertical
sensitivity
to
2dB/division,
and
set
the
output
level
of the tracking generator to obtain
an on- screen trace. ( You may
have
to
readjust
the
OPTIMOD-TV
INPUT
ATTEN
if
gain
is
insufficient.)
e)
You are now sweeping the entire OPTIMOD-TV system, including the complex
filters in Cards # 8 and # 9. Adjust R618(652) to achieve maximally flat response,
similar
to
Fig.
E-2.
The
response
should
be + 0.75dB
or
better,
50-15,000Hz.
[NOTE:
Misadjustrnent
of
the Hilbert- Transform
Clipper
circuitry
on
Cards # 0
and # 1 can affect the response below approximately 3kHz. Therefore, adjustment
of
R618(652) should be made primarily on the basis of the flatness of response
from 3-15kHz.]
Fig. E-2: OVERALL DEEMPHASIZED RESPONSE
f)
Now
turn
R626(660) (FET
BIAS)
slowly
counterclockwise
until
the
swept
response just begins to roll off. Back off until
no rolloff is observed, and go a
little further for safety.
g)
Repeat steps ( d) through ( f) for the right channel.
h)
Connect the oscillator to the OPTIMOD-TV left (right) input.
i)
Observe
the
output
of
IC605A(611A)
with
the
distortion
meter.
Set
the
oscillator frequency to 10kHz, and the oscillator level to produce + 10dBm at the
output of IC605A(611A).
j)
Turn
R678 ( HF LIMITING--front-panel
control) fully CW. Turn the LIMITER
PROOF/OPERATE switch ( on Card # 6) to OPERATE. The 10kHz level should go
down
to approximately + 6dBm. Now
adjust R631(665) (DIST NULL) to minimize
THD. Bear in mind that you are observing a preemphasized signal, and that THD
will be
even lower after deemphasis. Even without deemphasis, THD is typically
less than 0.1%.
E-5
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