[AMRadio] RE: negative cycle loading


John Coleman, ARS WA5BXO wa5bxo at pctechref.com
Tue Apr 6 21:58:57 EDT 2004


Bret,	
	What Dennis is saying about the clipping is correct.  It would
seem that it is all a matter of degree.  That is, less abruptness, in
the clipping or limiting, means less higher order harmonics distortion
and vise versa.  There or other things that may cause splatter during
high modulation.  I have taken the liberty of making a list of check
points that I use.  You may be familiar with these, but here they are
for the record any way.  I have also used shaping circuits in the low
level preamps of the modulation system.  High capacitance Varicap diodes
can do some really neat stuff in high Z low level circuits.

Check points:

1. The Class C circuit that is being modulated may go into spurious
oscillations at the positive peaks of the modulation and this may not
occur until a certain voltage at a certain audio freq appears on the
plate.  It is more likely to happen if a circuits positive peaks or
allowed to go higher than normal.  The gain of the tube begins to
increase a lot on positive modulation peaks.

	 Cures 
		1. Make sure that the tube is capacitively neutralized
well, and that there is no inductive feed back path.

		2. Make sure that you have enough RF grid drive for the
high positive peaks.  As you raise and lower the plate voltage the plate
current should follow linearly and so should the output RF.  (no change
in efficiency).

		3. On tetrode tubes the screen should be modulated as
well as the plate circuit but when the plate reaches double the
quiescent positive voltage, the screen should only have reached 150% of
its quiescent voltage.  The same is true for the negative going audio.
When the plate voltage has reached 0 volts on the negative audio peak,
the screen should only have gone down to about 1/2 of its nominal value.
The positive peaks here are the ones that will cause the tubes gain to
increase causing any RF feedback to break into oscillation.  The
incorrect negative peak ratios may cause the tube to shut of quicker
than normal.  The exact correct ratio of plate modulation to  screen
modulation may vary from one circuit to another.

		4.  Reducing the load on a tank circuit so as to draw
less plate current may cause the Q of the tank circuit to be increased
and this will increase the possibility of spurious oscillations if any
feed back can be found.  The better way to reduce power is to lower the
plate voltage (and screen supply if a tetrode) keeping the Q of the tank
where it should be.

	2. I have heard that it is possible that some of the splatter
from over modulation (carrier pinch off) occurs when the plate current
shuts off and the tank circuit is left to ring.  I can see this to some
extent and more so in high Q tanks but I would suspect this to be at a
very low level only.  Others may have more info on this? I think the
majority of the splatter is audio harmonic distortion imposed on the
modulated envelope.  One thing is for sure.  It is a good idea to
provide a load for the modulation circuit after the final has reached
below the zero plate voltage point on the negative peaks of the audio
cycle.  This can be done with a single HV diode in series with a
resistor of the same resistance as Ep/Ip.  This series network is place
from the plate modulation point to ground so that the diode is reverse
biased except when the voltage at the plate goes negative.  This will do
nothing for the ratio of positive to negative peaks of RF but it will
help provide a load for the modulator during this period and keep the
modulation XFMR from acting like a spark coil.

Good luck to you on your experiments. And keep doing them, that's where
everybody learns, especial when you share the experience.

73, John, WA5BXO



		

-----Original Message-----
From: amradio-admin at mailman.qth.net
[mailto:amradio-admin at mailman.qth.net] On Behalf Of W7QHO at aol.com
Sent: Tuesday, April 06, 2004 4:17 PM
To: amradio at mailman.qth.net; brett.gazdzinski at mci.com
Subject: Re: [AMRadio] RE: negative cycle loading

Brett,

Are you using the three diode circuit at   <
http://www.amwindow.org/tech/htm/3diodeka.htm>?
If so, this thing will clip all negative peaks that cross the "keep
alive" 
level.   How abruptly would depend on the internal impedance (i.e.,
stiffness) 
of the keep-alive supply but would give essentially the same effect as
clipping 
in the PA by crossing the zero line. This is where your splatter is
coming 
from.   Give the circuit at
<http://www.amwindow.org/tech/htm/3diode.htm> a try. 
  I've had good results with this setup adjusted to give up to 200% in
the 
positive direction without overmodulating the negative side.   For the
resistor, 
I've found a value around the load impedance presented by the PA works
well. 
You must use a scope to adjust this scheme.

Good luck.

Dennis D. W7QHO
Glendale, CA


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