[Amps] RE: 4CX250B Screen Supplies

[Gary Schafer]

If you want to prove that a two tone signal really DOES vary the screen 
current (as well as plate current) try this: for your two tone generator 
use two tunable audio oscillators. Set one for 1 khz. Tune the other one 
close to 1 khz also. As the second one approaches the frequency of the 
first you will see the plate and screen meters start to dance. You will 
be seeing the difference frequency of the two tones. This simulates low 
voice modulation frequencies. If they are very close in frequency the 
difference will be only a few cycles.

The same thing happens when the difference frequency is higher like 1 or 
more khz. Only then the meters can't follow and it appears as though the 
current is steady when in fact it is varying at the audio rate of the 
difference frequency.

Your scope connected to the RF output of the amp will also show the 
variation of the output at that same audio rate with the two tones.

Two tone modulation does the same thing as voice as far as modulating 
the transmitter. But with two tones things are easier to measure because 
everything appears to stand still.

In order to pick apart the higher order products on a spectrum analyzer 
the tones must be far enough apart to be able to discern the difference 
between the tone frequencies and the 3rd, 5th order products due to the 
bandwidth limitation of the analyzer. Closer together tone frequencies 
work the dynamic capabilities of the amp harder and probably produce a 
more realistic response but then you can't measure it because the 
analyzer can't tell the difference between the fundamental and IM products.

The differences that Rich talks about using voice for IM performance is 
that it is tougher on the dynamics of the transmitter. More things show 
up when it can't handle the dynamics as well. But the principles are the 
same. It is harder to make exact measurements with voice as things don't 
stand still like they do with two tones.

Multiple tones could also be used rather than just two tones but that 
gets tough to unscramble as many more products are produced.

73
Gary  K4FMX


Will Matney wrote:
> Bill,
> The only problem is that the test tone signal is a constant peak to peak 
> voltage no matter what the frequency. On the human voice, the peak to 
> peak voltage would vary thus varying the current. Now if the test tones 
> voltage would be made to vary up and down, it may be made to be close 
> but most test equipment I've seen use a constant voltage output. The 
> peak to peak voltage changes with the volume in a voice modulated signal 
> too. Needless to say, when using the voice, a mere varying of the mic to 
> and from the mouth would change the voltage along with the differences 
> in the modulated signal of the voice itself. All this being said, the 
> signal voltage and current would swing widely as compared to a two tone 
> test. This can be verified by watching the peak to peak voltage on a 
> scope using both voice and the test tone. The regulator must be able to 
> deal with this as you know. Adding capacitance will help the response 
> also as you mentioned.
> 
> Will Matney
> 
> 
> "I disagree. In the two tone situation the screen current is fluctuating 
> at the difference frequency of the two frequencies. And if you should 
> chose the frequency difference to be the lowest audio frequency 
> modulation used (300 Hz usually) you will take in account the bypass 
> capacitance reactance. The IMD could be improved simply by having a very 
> large AF bypass capacitor having a very low reactance at a few hundred Hz.
> 
> 73
> Bill wa4lav"
> 
> 
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