[AMRadio] Negative Loading circuits - good, bad, or ?


Donald Chester k4kyv at hotmail.com
Fri Jan 13 16:14:29 EST 2006


>From: Brett gazdzinski <Brett.gazdzinski at mci.com>
>
> > I've had good results with the "three diode" circuit too.   Earliest
> > reference to the scheme I can find is in QST, October 1956
> > using 866 rectifiers.
> > Covered again in ER #3, July 1989, this time with solid state
> > diodes.
>
>I did not say I had good luck with the circuit, although it looks
>like it works on the scope, and on the mod monitor, it results
>in a very wide signal. Not much point in running it if you go
>50Kc wide...


I tried the 3-diode neg cycle loading circuit, aka "ultramodulation"  years 
ago.  My experience was that I never got a signal report saying the audio 
sounded louder or that it was more readable than straight audio, but nearly 
all reports said the signal was "fuzzy" or had more distortion.

Think about how the circuit works.  What you are really doing is introducing 
a certain amount of controlled carrier, and introducing even-harmonic 
distortion to the audio.  The cortrolled carrier comes from the increase in 
average DC plate voltage produced by the rectified audio.  The even harmonic 
distortion comes from the asymmetrical clipping of the audio waveform, in 
which everything below the baseline is attenuated, but everything above 
baseline is unattenuated.  The abrupt change in slope at the zero point 
produces many higher order harmonics.  These are relatively low in 
amplitude, but if the signal is strong they will be heard.

When you apply modulation, the plate current rises, and the plate glows 
brighter, impressive but that doesn't mean the signal is any stronger or has 
any more talk power.  That additional rf output is almost entirely carrier 
power due to the upward carrier shift, and spurious sideband products.

Besides, audio power is too expensive to generate, and then rectify and use 
to heat up a large wirewound power resistor.

A method that I found to be much more effective was symmetrical high level 
speech clipping.  I used a variac on the modulator power supply, and 
installed a low pass pi-section audio filter between the modulation 
transformer and rf final.  I would deliberately drive the modulator tubes to 
saturation (way past flat-topping region) and adjusted the modulator plate 
voltage with the variac until the negative modulation peaks were just a 
little under 100%.  To avoid a mushy sound and problems with canting or 
tilting of the clipped waveforms, I cut back substantially the low frequency 
response below about 200~.

The signal wasn't broadcast quality or even hi-fi, but sounded at least as 
good as a stock DX-100 or Viking I or II, but the audio was much louder and 
carried a punch that would often rival or exceed that of a SSB signal 
running about the same power and signal strength.

But there was one serious flaw.  The circuit ate modulation transformers.  I 
must have gone through half a dozen kilowatt size and larger modulation 
transformers in less than six months.  I even blew one that was rated at 
2000 watts of audio, that I purchased brand new out of box! It was intended 
as a replacement modulation transformer for a 4 kw ship-to-shore 
radiotelephone transmitter.

I finally figured out what was happening to all the modulation transformers. 
  The low pass filter, near the knee of the curve at the cutoff frequency, 
presents the modulation transformer with a highly reactive load.  Modulation 
transformers are designed to work into purely resistive loads.  One strong 
audio frequency component right at the critical frequency and zorch! smoke!

I abandoned the high level clipping and haven't blown a modulation 
transformer since.

A better way to achieve the high positive peaks, which produces no audible 
distortion, is to take advantage of the natural asymmetry of the human 
voice.  It requires a good microphone, a low distortion audio chain from mic 
preamp to mod xfmr, a flat frequency response at least one octave above and 
below the highest and lowest audio frequencies you intend to transmit, and a 
modulator capable of substantially higher than normal peak power output.  
The ultramodulation scheme requires the high peak power output capability as 
well.  By properly phasing the polarity of the audio input to the modulator, 
most voices will produce from 150% to as much as 200% positive peaks with no 
rectifiers in the circuit at all.

Of course, the extreme positive peaks themselves produce little "talk power" 
and add little loudness to the signal, even though they look impressive on 
the oscilloscope screen.  The gain in loudness comes from being able to 
modulate closer to 100% in the negative direction before flat-topping in the 
positive direction, so that the overall result is a cleaner, louder signal 
with less distortion.  And you don't need to attenuate the low frequencies 
quite so much because phase shift in the rounded unclipped audio waveform 
has less detrimental effect.

Don K4KYV

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