Orban OPTIMOD-AM 9300, Руководство по эксплуатации

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2-30
INSTALLATION ORBAN MODEL 9300
not be produced by adjusting the
LF FREQ
control, transmitter low-
frequency response is inadequate and there is too much low-frequency
rolloff.
Because equalization occurs below the audible frequency range, a trans-
mitter that cannot be fully equalized can cost up to 4dB average modula-
tion even though audible frequency response measures essentially flat.
This problem cannot be corrected without modifying the transmitter. In
many cases, such modification is easy: it merely requires bypassing the
highpass filter(s) in the input stage of the transmitter. It also may require
replacing coupling capacitors with capacitors of a larger value. In other
cases, fundamental inadequacies in the input, interstage transformers (if
used), and/or modulation transformers (if used) are the cause. See the
discussion on page 1-13.
Unless the transmitter is of a relatively modern solid-state design, being
unable to equalize it fully is a good reason to replace it with an up-to-
date solid-state design using a switching or digital modulator. In most
cases, this purchase will pay for itself in reduced power bills and the new
transmitter will sound far better on the air.
K) Turn off the transmitter and allow it to cool down for several minutes.
13. Equalize transmitter high-frequency response.
A) Set the 9300's square wave controls to produce a 1 kHz square wave at 30%
modulation:
a) Navigate to
S
ETUP
> T
EST .
b) If necessary, set the
M
ODE to
S
QUARE .
c) Set
S
QUARE
F
REQ
to
1000 H
Z
.
Note: Because the 9300 is digital, its square wave generator cannot pro-
duce any harmonics higher than 16 kHz (one-half of its 32 kHz sampling
frequency). To prevent visible ringing of the square wave due to this
sharp cutoff of its higher harmonics, we have applied an internal digital
filter to the output of the 9300's square wave generator. This filter
rounds off the edges and prevents significant ringing. You may want to
look directly with the scope at the unequalized output of the 9300 to get
a feel for what this waveform looks like before it is applied to your
transmitter.
B) Make sure that the oscilloscope is synchronized to the square wave.
C) Turn on the carrier. Observe the RF envelope at the common point with a DC-
coupled oscilloscope and trim the
S
QUARE
L
VL
control (if necessary) to produce
30% modulation.
D)
Navigate to
S
ETUP
> M
ODIFY
TX P
ST
> TX1/D
AY
.
E) If necessary, press
N
EXT until you see the screen containing the four transmit-
ter equalizer controls.
To avoid overheating the transmitter, perform steps (C) through (G) quickly.