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

[K3PID]

Jim, Thanks for the tips!!! I had completely overlooked the 9 MHz getting
into the audio. I added both of your mods!

I could only hold the circle from about 250 Hz to just under 2500 Hz, after
the mod, the circle remains pretty true ( your eyes start to play tricks
after you stare at it for awhile) through just over 4000 Hz ( maybe higher
:>) ).  I can suppress the carrier to about -72db and I can get the unwanted
sideband a little lower.  I just finished converting a BC-458 to the
multiband VFO and while I haven't tried it on 15 or 10 yet it seems to be
working fine on 160 ( my favorite band) and 80, 40 & 20.

I haven't had this much fun since I first got my ticket in 1960......

Next... feed the 20A into the T-368 for some real punch!!


73 & Tnx
K3PID
Ron H.




----- Original Message ----- 
From: "Jim Candela" <jcandela at prodigy.net>
To: <central-electronics at mailman.qth.net>
Sent: Sunday, January 29, 2006 12:44 PM
Subject: Fwd: FW: [Central-Electronics] CE 20A Phase shift question


>
>
> 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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