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


Bob Bruhns bbruhns at erols.com
Wed Jan 11 11:16:13 EST 2006


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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