Orban OPTIMOD-AM 9300, Operating Manual

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OPTIMOD-AM DIGITAL INTRODUCTION
1-13
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
limits on occupied bandwidth specified by the governing authority and will greatly
degrade the spectral control provided by OPTIMOD-AM.
To achieve the full performance capability built into OPTIMOD-AM, any filters in the
transmitter must be bypassed. This is essential! OPTIMOD-AM contains low-pass and
high-pass filters that are fully capable of protecting the transmitter and controlling
occupied bandwidth. Because of their location within OPTIMOD-AM, the internal
filters do not introduce spurious modulation peaks.
Any built-in peak clippers in the transmitter should be defeated. OPTIMOD-AM con-
tains a clipping system that is fully capable of controlling transmitter modulation
without introducing out-of-band energy. If the drive level to the transmitter is even
slightly excessive, the transmitter clipper will be driven hard enough to create exces-
sive spurious spectrum. Defeating any clippers in the transmitter prevents this possi-
bility.
This problem will be even worse if OPTIMOD-AM's transmitter equalizer is in use.
OPTIMOD-AM's output level will frequently exceed 100% modulation because it is
pre-distorted to complement the transmitter's pulse response. The transmitter's
built-in safety clipper will surely clip this pre-distorted waveform.
Power Supplies
An AM transmitter is required to provide 150% of equivalent unmodulated carrier
power when it is modulating 100%. High-voltage power supplies are subject to two
major problems: sag and resonance.