Optimod-AM 9400 V1.2 Operating Manual - Orban
Optimod-AM 9400 V1.2 Operating Manual - Orban
Optimod-AM 9400 V1.2 Operating Manual - Orban
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OPTIMOD-<strong>AM</strong> DIGITAL INTRODUCTION 1-15<br />
limits on occupied bandwidth specified by the governing authority and will greatly<br />
degrade the spectral control provided by OPTIMOD-<strong>AM</strong>.<br />
To achieve the full performance capability built into OPTIMOD-<strong>AM</strong>, any filters in the<br />
transmitter must be bypassed. This is essential! OPTIMOD-<strong>AM</strong> contains low-pass and<br />
high-pass filters that are fully capable of protecting the transmitter and controlling<br />
occupied bandwidth. Because of their location within OPTIMOD-<strong>AM</strong>, the internal<br />
filters do not introduce spurious modulation peaks.<br />
Any built-in peak clippers in the transmitter should be defeated. OPTIMOD-<strong>AM</strong> contains<br />
a clipping system that is fully capable of controlling transmitter modulation<br />
without introducing out-of-band energy. If the drive level to the transmitter is even<br />
slightly excessive, the transmitter clipper will be driven hard enough to create excessive<br />
spurious spectrum. Defeating any clippers in the transmitter prevents this possibility.<br />
This problem will be even worse if OPTIMOD-<strong>AM</strong>'s transmitter equalizer is in use.<br />
OPTIMOD-<strong>AM</strong>'s output level will frequently exceed 100% modulation because it is<br />
pre-distorted to complement the transmitter's pulse response. The transmitter's<br />
built-in safety clipper will surely clip this pre-distorted waveform.<br />
Power Supplies<br />
An <strong>AM</strong> transmitter is required to provide 150% of equivalent unmodulated carrier<br />
power when it is modulating 100%. High-voltage power supplies are subject to two<br />
major problems: sag and resonance.<br />
Sag is a result of inadequate steady-state regulation. It causes the conventional carrier<br />
shift that is seen on a modulation monitor. Good transmitter engineering practice<br />
usually limits this shift to -5% (which corresponds to about 0.5dB not a highly<br />
significant loudness loss).<br />
A more serious problem is dynamic carrier shift, or bounce. This has been known to<br />
cause up to 3dB loudness loss. Resonances in the power supply's LC filter network<br />
usually cause it. Any LC network has a resonant frequency. In order to achieve reasonable<br />
efficiency, the power supply filter network must be under-damped. Therefore,<br />
high modulation excites this resonance, which can cause overmodulation on<br />
the low-voltage peaks of the resonance.<br />
Curing bounce is not at all straightforward because of the requirement that the<br />
power supply filter smooth the DC sufficiently to achieve low hum. One approach<br />
that has been employed is use of a 12-phase power supply. Upon rectification, the<br />
ripple component of the DC is down about -40dB without filtering. A singlecapacitor<br />
filter can thus be used, eliminating the filter inductor as a potential source<br />
of resonance with the capacitor.<br />
Other sources of resonance include the modulation reactor and modulation transformer<br />
in conventional plate-modulated transmitters. Such transmitters will not<br />
greatly benefit from a 12-phase power supply.