Operating instructions

OPTIMOD-AM DIGITAL INSTALLATION
2-31
11. Test the equipment downstream from OPTIMOD-AM.
Test the RF envelope at the transmitter’s output to determine if it exhibits tilt,
overshoot, or ringing. If you observe these problems, you can often adequately
equalize it them with the 9400’s transmitter equalizer, whose settings are de-
termined by the on-air Transmission Preset.
Dealing with tilt and overshoot may seem fussy, but every dB of tilt or overshoot
is a dB of loudness lost!
Use the 9400's built-in square wave generator to make this test:
A) Navigate to S
ETUP > TEST.
B) Set the T
ONE MOD to 0%.
C) Set the M
ODE to SQUARE.
D) You may now turn the final amplifier on. Observe the RF envelope at the
common point with a DC-coupled oscilloscope and advance the T
ONE MOD
control until you can easily see the shape of the square wave.
Sweep the T
ONE FREQ control from 125 to 1000 Hz and observe the shape
of the square wave as you do so. If you are driving more than one trans-
mitter and/or your antenna load changes between day and night, test all
combinations that you will be using.
If the square wave is free from tilt and ringing at all frequencies in the
sweep, you do not need to set up the transmitter equalizer in steps 12
through (13.H)a) below. Otherwise, you must do so to achieve the high-
est loudness and coverage that your facility can produce.
If you observe problems with some combinations
of transmitter and load
but not others, record which combinations cause problems. You will only
need to set up set up the Transmitter Equalizer for these combinations.
You will dedicate one Transmission Preset for each problematic combina-
tion so that each combination can be equalized independently.
Figure 2-9 on page 2-33 shows tilt and Figure 2-11 on page 2-35 shows
ringing.
Caution: To avoid damaging the transmitter, do not exceed 50% modulation
with square waves.
Important: Do not place additional clipping devices after OPTIMOD-AM! The
additional distortion introduced by these devices will totally nullify the advan-
tages of OPTIMOD-AM's distortion-canceling clipper and will cause the out-of-
band energy induced by clipping to violate FCC or ITU-R standards.
12. Equalize the transmitter’s low frequency square wave response.
[Skip the Transmitter Equalizer adjustment steps [(steps 12 though (13.H)a)] if
the RF envelope square wave test you preformed in step 11 above showed no
sign of tilt, overshoot, or ringing.]