Question on negative peak limiting

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w4bfs:
this is a good discussion ... rather than hijack this one believe I'll start a new one about transmitt audio...73 ....John

10:38 am  ...just got off the air in a qso with Bill, W8VYZ in snowy Astabula ... we discussed his 3 diode circuit ... he verified that he was using an 836 in series with a load resistor of 4 - 200 Watt units that Bill said got 'very warm' during use ... he didn't specify their value ... Bill mentioned a 80% negative mod limit ... also said he was using 10 - 1kV piv diodes in a string for each of the other diode functions ... Bill kept referencing the CQ and QST magazine articles in the 1950's and reported that the QST article was "absoutely correct' .... this was an interesting qso and Bill is wanting to assert am operation on 7290 kHz ...73 ... John

steve_qix:
It's really ok to overmodulate in the positive direction and assuming the waveform is not modified in any way (distorted/clipped/etc.), doing so will produce no extra harmonics or splatter than if the transmitter were modulated
less than 100% (positive) - again, assuming no distortion positive or negative and all other things being equal.

Most naturally produced waveforms (musical instruments, human voices, etc.) are naturally asymmetrical.  My voice waveform certainly is - by quite a large amount.

As long as nothing distorts the waveform  AND if the waveform is fed to the transmitter in such a way as to allow higher voltage peaks to modulate the transmitter in the positive direction (correct phase), all will be OK, and since there is no distortion, no splatter will be produced.  Only overmodulation in the negative direction produces splatter (assuming no other distortion or limitations in the transmitter).

Regards,

Steve

Steve - WB3HUZ:
Quote

Most naturally produced waveforms (musical instruments, human voices, etc.) are naturally asymmetrical.  My voice waveform certainly is - by quite a large amount.


Indeed. The first attachment is a scope shot of my voice while I was saying the universal AM test phrase (uuuureeeiiiii). You can see it is asymmetrical at almost a 2:1 ratio.

Steve - WB3HUZ:
If that happens, just flip the phase/polarity. Simple really.

See below.

WB6VHE:
Since nobody appears to have mentioned it, I refer those interested to
an article in Electric Radio, No. 154, March 2002, by Tom Bonomo, K6AD.
Title of article: Modulation Transformer Protection for AM Transmitters.
I have been using this circuit for over a year and it works great!  Prior to this
I have blown two (expensive!) mod transformers when the RF amp load went
away (tank cap flashover).  While the idea of the article is to protect your mod iron,
the  circuit also functions as a neg peak limiter.  I can shout into the mike (not that I want to)
now, and it is not possible to overmodulate on the negative peaks.
The audio both looks (on a scope and looking at the sig on a spec analyzer)
and sounds great.  The resistors hardly get warm.  I use this with my
homebrew 813 rig, 811A's for modulators, roughly 400 Watts input to the rf amp.

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