Fwd: FW: [Central-Electronics] CE 20A Phase shift question

[Jim Candela]

Ron,

  Hats off to you for remembering how to use Lissajous
Figures! I forgot about that stuff, and you are
correct that two signals 90 degrees apart fed into
scope in X:Y mode should make a nice circle. I did set
up my 20a a few times with the scope, and with
perfectly compensated 10X probes (OK, best I could do)
I was able to get pretty close to correct setup by
varying the two audio pots within the 20a. Figuring
out 90 degrees phase shift was pretty tough, but 90
+/-5 was doable. Heck next time I'll try Lissajous
figures! Setting the RF phase shifter with a scope
(looking for 90 degrees shift) is more challenging,
and a 100 Mhz high quality scope with two matching
probes helps. Still, doing it the CE way is best as
described in the original alignment instructions.

   The fuzz in your circle was probably the 9000 Khz
from the crystal oscillator. Those bypass capacitors
C11, and C12 don't filter out the 9 MC very well, and
they do it differently which is I think due to
different circuit layout between the two audio
channels. I bet the circle intensity modulation maxima
and minima you saw was due to power supply ripple
beating with the tone injected from your audio
generator. 

I do use the RCA jack in the back for a scope input
(Trapezoid pattern on AM), and the 9 Mc signal is
pretty high. I filter it out with a simple R-C of 1.2K
series, and 2700 pf shunt at the jack.

   So over what range of frequencies can you maintain
a circle? My experience is that the 20a phase shifter
does a decent job from 300-3000 hz, and beyond that
the sideband rejection falters. There is a low pass
filter in the audio (RC at gain control input), but
the drop off is not all that much up to 5 Kc. I chose
to fiddle with the phase shifter so it behaves better
up to 5 Kc. What I did was to bridge R15 (400 ohms)
with 1.8K, and then make a series R-C consisting of
390K and 180pf. The RC goes between PS-1 pins 2 and 9.
After the mod, go back and set up the pots again, and
see if things are close to 90 degrees phase shifted up
to 5 kc.

I just got off 20 meters USB with my 20A.....

Regards,
Jim
WD5JKO

> 
> -----Original Message-----
> From: central-electronics-bounces at mailman.qth.net
>
[mailto:central-electronics-bounces at mailman.qth.net]On
> Behalf Of K3PID
> Sent: Sunday, January 29, 2006 11:12 AM
> To: central-electronics at mailman.qth.net
> Subject: [Central-Electronics] CE 20A Phase shift
> question
> 
> 
> Setup:
> 
> CE 20A to dummy load
> 1 KHz audio input tone
> scope accross the output of the phase shift network
> ( 90 deg shift)
> 
> Observation:
> 
> The X-Y scope display shows a very nice circle which
> varies in diameter with
> the amplitude of the input and also varies in
> diameter (although less
> variation) with the input frequency. The inside of
> the circle always looks
> to be a well defined circle but the thickness of the
> line has some variation
> around the circumference.  Thin and sharp at 6:00
> thru 10:00 and a little
> thicker from 12:00 to 2:00.
> 
> The two signals look great in the time domain just
> like two sine waves out
> of phase.
> 
> Question:
> 
> Is there something going on that needs correction?
> 
> K3PID
> Ron H.
> 
> 
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