[AMRadio] negative cycle loading

Donald Chester k4kyv at hotmail.com
Mon Apr 5 12:01:38 EDT 2004

Sounds like the old Ultramodulation circuit, that first appeared in QST in 
1956.  I tried it years ago (using 866A's and 200-watt power resistors) but 
about the only thing it increased on the receiving end was distortion.  
Listeners told me the audio didn't sound any louder or more intelligible, 
but had a raspy quality.  The DC plate meter kicked upwards on modulation 
peaks, and the rf ammeter kicked higher than with normal modulation, but 
much of the kick is due to rectified audio being added to the DC from the 
power supply.  Audio power is too expensive and difficult to generate, to 
waste in a power resistor, or convert to DC to boost carrier output on 
modulation peaks.  There are other, more economical means of generating 
"controlled carrier" (which I don't recommend in any case).

I would recommend a good broadcast type peak limiter with separately 
adjustable positive and negative peaks, or a clipper circuit adjusted to 
shave off only the infrequent, abnormally high amplitude negative peaks.  Be 
sure that everything that follows the clipper or limiter has excellent low 
frequency response.

If you are looking for extended positive peaks, the best way to achieve that 
is to take advantage of the natural asymmetry of the human voice.  Use a 
good microphone, the best audio transformers you can find, and design the 
audio chain to have a flat frequency response at least one octave above and 
below the high and low frequency extremes you actually intend to transmit. 
Make sure distortion in the audio chain (both phase and intermod) are as low 
as you can make them.  Use a low-level audio filter or equaliser near the 
mic preamp for any frequency response shaping.  Low-value coupling 
capacitors and other response-shaping gizmos inserted throughout the audio 
chain will introduce phase shift distortion that will destroy the 
asymmetrical peaks.


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