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Author: John Innes
Digital AM? Don’t Throw Away That Ampliphase Transmitter!
Perhaps you want to dip a toe in the water when AM digital tests start, probably with a DRM exciter, and are wondering about transmitter conversions. If you have an RCA Ampliphase transmitter, maybe in use as a standby or just gathering dust, consider the following:
Rule of thumb: if a transmitter can perform well in AM Stereo, it should pass digital AM. The Ampliphase transmitters made good test figures on C-QUAM using the identical exciter settings to the internal test transmitter in the exciter.
There is no PWM frequency to produce aliasing artifacts, and no modulator filter to limit audio bandwidth.
A standard Ampliphase transmitter was 3 dB down at 26 kHz. Its audio bandwidth was so wide that it produced "monkey chatter" in adjacent channels from audio noise above 10 kHz in the feed from the studio, until the introduction of NRSC-compliant audio limiters.
Delay is identical in the audio and the RF path, with a group delay equalizer incorporated in the original design to ensure this.
DRM requires that the audio response extend down to DC. Purists in the USA years ago used to make the simple modification needed to DC couple the audio input. DC coupling the drive regulator will need a little more work, but can be done.
Setup adjustments can be optimized to produce good plate efficiency at average powers that are a small fraction of peak power.
Of course, the old transmitter still runs up a considerable power bill. When solid state transmitters are available that are designed from scratch for DRM, they will be more economical than the Ampliphase, be more stable in adjustment and need less maintenance, and no doubt produce better test figures. When digital AM becomes a regular, revenue producing service, it will be time to buy a transmitter for it, and we will be able to help you with that purchase. But until then, consider a new use for the old transmitter.
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