Orban OPTIMOD-AM 9300, Operating Manual, Page: 75 / 242

OPTIMOD-AM DIGITAL INSTALLATION
2-29
D) Turn on the carrier.
E) Observe the RF envelope at the common point with a DC-coupled oscilloscope
and advance the
S
QUARE
M
OD
control to produce 30% modulation.
F)
Navigate to
S
ETUP
> M
ODIFY
S
YS
P
ST
> TX1/D
AY
.
G) If necessary, press
N
EXT until you see the screen containing the four transmit-
ter equalizer controls.
H) Review the RF envelope display.
Many transmitters (particularly older designs) will produce an RF enve-
lope resembling
Figure 2-8 . If the oscilloscope display looks like this, con-
tinue to step (I).
If the oscilloscope display looks like Figure 2-9 , low frequency equaliza-
tion is unnecessary. Skip to step 13 on page 2-30.
I) Set the LF
GAIN
to 10.0 dB.
Setting the LF GAIN control to maximum low-frequency boost ensures re-
sponse that is closest to true DC-coupling, optimizing square wave re-
sponse.
Depending on the transmitter, this large amount of boost at sub-audible
frequencies might cause bounce and/or distortion on heavy bass tran-
sients in music. In step 14 on page 2-32, you will be instructed to turn the
LF GAIN
control down until these problems are no longer observed. This
will make the measured square wave response poorer. However, engi-
neering realities force a compromise between best small signal (i.e.,
square wave) response and best large signal (i.e., bounce and distortion)
performance. This compromise is best made by careful experimentation
with program material to find the setting of the LF GAIN control that
gives the highest average modulation without audible distortion.
J) Adjust the
LF FREQ
to make the square wave as flat as possible.
Work quickly to avoid overheating the transmitter. Figure 2-9 shows the
result of a successful adjustment. If a display like that in Figure 2-9 could
Figure 2-8: Unequalized RF envelope
(showing tilt)
Figure 2-9: RF envelope requiring
no tilt equalization