Orban Optimod-AM 9400, Operating Manual, Page: 36 / 295

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INTRODUCTION ORBAN MODEL 9400
station is highly dependent upon the high-power sections of the transmitter, and
upon the behavior of the antenna system.
The extremely high average power and the pre-emphasized high-frequency compo-
nent of audio processed by OPTIMOD-AM put great demands upon the performance
of the transmitter and antenna system. While improved results can be expected
from most plants, outstanding results can only be achieved by plants having trans-
mitters that can accurately reproduce OPTIMOD-AM's output without changing the
shape of the waveform, and having wideband, symmetrical antenna arrays.
Any AGCs, compressors, limiters, and clippers that follow OPTIMOD-AM in the circuit
should be bypassed. OPTIMOD-AM provides all of these functions itself.
Bypassing the Transmitter's Internal Filters and Clippers
Some AM transmitters, especially those supplied to stations outside of North or
South America, contain built-in filters and clippers after their audio inputs. The fil-
ters may have various purposes: A low-pass filter is often included to ensure that the
transmitter's output spectrum adheres to the occupied bandwidth specifications of
the governing authority. A high-pass filter may be present to protect the transmitter
from damage. Safety clippers are often present to prevent the modulator from be-
ing over-driven.
As discussed in earlier sections, accurate reproduction of OPTIMOD-AM's output re-
quires that the deviation from linear phase must be less than 10 degrees, 30-9500Hz.
Frequency response must be less than 3dB down at 0.15Hz, and less than 0.1dB
down at 9.5 kHz.
The highly processed output of OPTIMOD-AM is carefully band-limited and peak-
controlled. This output will often contain waveforms with flattops like square
waves. If the transmitter has constant group delay above 30Hz, these difficult wave-
forms will be transmitted intact and peak modulation will be accurately controlled.
However, if low-frequency response is more than 3dB down at 0.15Hz, as would be
true if a high-pass filter is present, the group delay above 30Hz will not be constant.
For example, a typical 50Hz high-pass filter introduces significant non-constant
group delay to 500Hz — ten times the cutoff frequency. This non-constant group de-
lay will tilt the flattops produced by OPTIMOD-AM. The tilt increases the peak level
of the audio waveform, but not the average level. This will force you to decrease
the average modulation to prevent the spurious peaks from overmodulating.
Similarly, a typical EBU 4.5 kHz filter will introduce significant non-constant group
delay down to 1 kHz about one-fourth the cutoff frequency. This will cause over-
shoot in the highly processed waveforms produced by OPTIMOD-AM. The overshoot
increases the peak level of the audio waveform, but not the average level. This will
force you to decrease average modulation even more.
Alternatively, if you do not decrease the average modulation to accommodate the
spurious peaks introduced by the filters, the transmitter’s safety clipper will clip the
peaks. This will introduce out-of-band energy that will almost certainly violate the