[Amps] RE: 4CX250B Screen Supplies

[Radio WC6W]

On Wed, 28 Jul 2004 03:24:07 -0700 R.Measures <r at somis.org> writes:
> 
> On Jul 27, 2004, at 1:17 AM, peter.chadwick at Zarlink.Com wrote:
> >
> > Some professional tests use a very narrow spacing between tones - as 
> > low as 30Hz - to simulate the syllabic nature of voice, and to
exercise the 
> > power supply regulation. Another approach, which, if I remember
correctly, 
> > has been advocated here by Tom, W8JI, is to use a three tone test
with two 
> > of the tones very closely spaced. As Steve, G8GSQ says, the screen
current
> > varies at a frequency dependent upon the tone spacing.
> >
> > Because of the peak to average nature of a voice signal, it isn't
easy 
> > to relate the performance on a two tone test with that under voice 
> > conditions.  The advantage of a two tone test is that it does (or
should!) give a 
> > number that can be reproduced in testing by other people.
> 
> Peter --  Should, yes.  With 30Hz or so tone spacing, it's going to 
> take an incredibly capable spectrum analyzer to separate and measure 
> the 3rd and 5th order products.  If the thing we are concerned about is

> on-the-air rotten splatter from voice modulation, why not measure 
> splatter from voice modulation by listening to the adjacent channels 
> with another receiver?   Sure, a precise ± 0.1db measurement is not 
> going to happen, but, ± a couple of db accuracy is good enough in 
> this case.  //  cheers

Hi Rich,
   There is a technique in which any old spectrum analyzer should
suffice.

   Place directional couplers in both the input line (-30dB sample) and
the output line (-40dB sample) which is preferably terminated in a dummy
load for testing.

   Then with a suitable attenuator and phase shift (a variable length
line would be nice) connected to the output of one of the couplers
combine this signal with the other coupler using a 3dB combiner.   The
phase shift need only be a few degrees for a GG amp.   It will need to be
near 180° for grid driven amps.

   If the attenuation & phase are properly adjusted, the original signal
(two tone, three tone, voice or whatever) will be cancelled (by 40dB or
thereabouts...) at the output of the combiner and only distortion
products will remain for display.

   A stable screen supply is a prerequisite for using this technique
since if the screen V is wandering the amplifier gain will also be
varying.

73 & Good morning,
   Marv  WC6W  










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