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


Bob Bruhns bbruhns at erols.com
Wed Jan 11 21:42:46 EST 2006


Thanks, Larry, I'll be interested in that.

I understand that one large AM network - Clear Channel,
I believe - is cutting their AM audio bandwidth to 5
KHz to make distant AM reception better (less splash
from adjacent channels).

I remember back in the late 60s, a little 250 watt
daytime station on 540 KHz in Islip, Long Island ran
some sort of clipping to sound louder.  I don't know
about splatter, but it really didn't sound good that
way.  It was pretty loud, though.

But in amateur operation, some gentle curvature can
curb the peaks that would get sharply snipped off by
overmodulation, without a lot of splatter.  Overall, it
would probably reduce the general splatter level
somewhat.  Also it would be useful to have some kind of
diode and resistor to catch peaks that do overmodulate,
and keep them from making a voltage spike that could
blow the modulation transformer.  But if the circuit is
being used to achieve high audio levels, then a low
pass filter ought to follow the diodes.  And the
transient performance of the filter must be such that
overshoot is minimized, because overshoot would
overmodulate and splatter too.  Generally a
non-overshoot filter gives a soft cutoff rather than a
sharp cutoff, unless it is complex and high order.

I use low level asymmetrical clipping, and I can filter
that with a 3.5 KHz low-pass filter in crowded
conditions when I push the clipper hard.  I
accidentally found that a side chain servo loop could
have its time constant aligned with the modulator, and
produce very good clipping control, so that's how I do
it.  The clipper is really a limited amount of
extremely fast peak limiting compression, with a time
constant around a millisecond or so, riding on the
slower 0.2 second time constant of the peak limiter,
with a slower time constant coupled on a resistive
divider for some slow average compression action as
well.  using a moderately complex RC network as a gain
control loop filter.  Because of this time constant,
sibilents are treated with a more peak limiting action
so there is less intermodulation, and lower frequency
stuff is softly clipped.  I shorten the time constant
and increase the audio drive to push this harder,
accepting some distortion for punch.  When I push it
like that, I use the low-pass filter, which is just a
second-order Sallen-Key.  I got a comment on how narrow
the signal was, and yet it sounded clear because of
upper midrange boost.  The millisecond range time
constant of the servo-clipper avoids a lot of high
frequency harmonic generation before the audio hits the
filter.

 Bacon, WA3WDR


----- Original Message ----- 
From: "Larry Will" <lhwill at verizon.net>
To: "Discussion of AM Radio" <amradio at mailman.qth.net>
Sent: Wednesday, January 11, 2006 7:07 PM
Subject: Re: [AMRadio] Negative Loading circuits -
good, bad, or ?


> Bacon,
>
>
> I'll pull out a copy of the NRSC spec on the  AM
B'CST audio shelving
> filter when I get a chance and pass along some
details.
>
>
>
> Larry
>
> At 11:16 AM 1/11/2006, you wrote:
> >Any distortion of the modulating waveform causes
> >harmonic distortion and therefore splatter.  The
> >sharper a waveform discontinuity is, the more
> >high-level harmonic energy it contains, and that
> >harmonic energy becomes splatter on the air.  This
> >is why a low-pass filter is used in speech
> >clipping systems.  But the sharp clipping caused
> >by overmodulation can not be filtered at the audio
> >level.
> >
> >The extreme sharpness of clipping resulting from
> >overmodulation is the reason that overmodulation
> >causes so much splatter.  The idea of the diode
> >loading system is to produce a softer clipping
> >that produces much less splatter than raw
> >overmodulation.  Additional diodes and resistors
> >are often added to provide protective loading for
> >the modulator on negative peaks that would have
> >been unloaded in simple diode systems or with no
> >diodes at all.  This protective loading reduces
> >voltage spikes that can destroy the modulation
> >transformer.
> >
> >Some distortion is still produced with the diode
> >loading system, and therefore some splatter will
> >result.  But unless there is some other problem,
> >the splatter is much less severe than raw
> >overmodulation, and the high frequency products
> >caused by this action can be filtered at the audio
> >level.  You can add a high level splatter filter,
> >although that will limit your high frequency
> >response.  You can have a few filters or a few
> >filter settings, like 10 KHz for clear conditions,
> >6 KHz for intermediate conditions, and 3.5 KHz for
> >crowded conditions.
> >
> >Some technical problems can cause extra splatter.
> >If the modulator is marginally stable, it is
> >possible that the dynamic change in loading
> >resulting from the diode action can cause
> >triggered parasitics at specific points on the
> >audio waveform.  This can result significant
> >splatter, and it might have a distinctive resonant
> >sound, which you would hear as resonances or
> >concentrated spectral points in the splatter on a
> >sideband receiver tuned some distance from the
> >carrier.  Negative feedback can cause problems if
> >gain and phase margins are exceeded, which often
> >happens at frequency extremes, and this can result
> >in triggered parasitics even if diode loading is
> >not used.  If there is a modulator stability issue
> >at high audio frequencies, it can cause splatter.
> >
> >Excessive modulation of the screen grid of a
> >plate-modulated tetrode or pentode modulated stage
> >can cause sharp distortion too.  You can see a
> >kink at about 85% negative when this occurs, and
> >that means you have a waveform discontinuity that
> >can cause significant splatter that can not be
> >filtered at the audio level.  The typical circuit
> >of the screen dropping resistor going to modulated
> >B+ should be modified for better linearity.  A
> >good job can be done by simply connecting the top
> >of the screen dropping resistor to unmodulated B+
> >and allowing the screen grid to self-modulate, but
> >the best arrangement uses a resistive divider
> >supplying the screen from modulated B+ and
> >unmodulated B+.  Screen-choke systems are
> >self-modulating and do not exhibit that
> >distortion.
> >
> >Class C Optimization for Ultra Low Distortion
> >http://www.amfone.net/AMPX/71.htm
> >
> >The self-modulated screen grid
> >http://www.amfone.net/AMPX/98.htm
> >
> >Remember also that your receiver can be listening
> >to a really strong signal while you are testing in
> >the shack.  The splatter that you hear on that
> >strong signal in the shack may be mild on a normal
> >signal on the air, and the receiver can be adding
> >to the problem is it is overloading.
> >
> >   Bacon, WA3WDR
> >
> >
> >----- Original Message -----
> >From: "Brett gazdzinski"
> ><Brett.gazdzinski at mci.com>
> >To: "'Discussion of AM Radio'"
> ><amradio at mailman.qth.net>
> >Sent: Wednesday, January 11, 2006 8:22 AM
> >Subject: RE: [AMRadio] Negative Loading circuits -
> >good, bad, or ?
> >
> >
> > > Dennis,
> > >
> > >
> > > > 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...
> > >
> > >
> > >
> > >
> > > > Don't believe the type of diodes used would
> >have any
> > > > significant effect on
> > > > this.   Splatter is generated in the PA tank
> >circuit when
> > > > plate voltage is
> > > > suddenly cut off   on the audio negative half
> >cycle.   Same
> > > > splatter would be
> > > > produced if the PA was being fed straight off
> >the secondary
> > > > of the mod transformer.
> > >
> > > The 3 diode circuit is supposed to prevent the
> >plate voltage
> > > from going to zero. I use variacs on the power
> >supply so I can set
> > > the point at which the circuit starts working,
> >and no matter if I
> > > set it to 95, 90, or 85% I get splatter if the
> >audio would exceed
> > > 100% negative, so the circuit seems to do no
> >good.
> > >
> > > It LOOKS like it works, I get current in the
> >negative cycle loading
> > > circuit, mod monitor shows the limiting working,
> >scope looks ok,
> > > the signal just gets REAL wide, no matter what
> >rig I run.
> > >
> > > Everyone would assume it works, I did, till I
> >got reports I was real wide
> > > and dug out the spectrum analyzer...
> > >
> > > Perhaps the very high frequency stuff gets past
> >the circuit?
> > > I should run some tests...
> > >
> > >
> > > Brett
> > > N2DTS
> > >
> > > >
> > > > Dennis D. W7QHO
> > > > Glendale, CA
> > >
> > >
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> > >
> >
>
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