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

Larry Will lhwill at verizon.net
Wed Jan 11 19:07:27 EST 2006


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.


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
>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
>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
>Class C Optimization for Ultra Low Distortion
>The self-modulated screen grid
>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
> >
> > 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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