[ARC5] Programmable Oscillators for Boatanchor RX - Final

[David Stinson]

Programmable Oscillators for Boatanchor RX - Final

Well, the BC-942 (R-28/ARC-5) VHF receiver 
is buttoned-up and doing well.  
Listened to some "ultra-light" guys enjoying an early-morning 
flight and gabbing on 122.8 MC Unicom.

Now, I don't "calculate" project designs- I ain't that smart.
I bludgeon them into submission.
You "smart people" can tell me the "whys."
So here are some "shade-tree" conclusions.

http://arc5@home.netcom.com/www/ItWorks2.jpg

I used two configurations- "LO" oscillators on the same freq
as an original crystal would have used and "HI" oscillators,
set at the *4 freq of the first multiplier in the LO chain.
Built them on little perf-boards mounted to the bases
from defunct crystals. 

 Made a 6-volt power buss from one of those
 little DC-DC "buck" converters:
http://www.ebay.com/itm/261053380853
Tucked it under the Audio section and ran the buss
line through the channel relay mount and one of the 
holes in the center of the crystal sockets.
Because the little oscillators provide a constant DC load,
I could have used a simple voltage-dropping 
resistor.  390 Ohms from the 24-volt fil line worked
well.  But we're talking about a big resistor 
generating a lot of heat and this rig gets hot enough.
One of the channels ( 118.55 MC ) does have a 
light "converter whistle" in the background, but 
I can live with that.  Just don't mount the thing
right under the IF strip.

The LO oscillators [  Fxtal MC = (Fop - 6.9 MC) / 24  ]
need a little 4:1 voltage-boosting UNUN
to deliver enough drive.   I got mine all pre-made and 
ready to go from Tanner's surplus house.  
He's got about a bizillion for, I think, 25 cents each.
I also tried a 9:1, but that didn't do any better than 4:1.
The two capacitors are there for DC blocking.
Values not critical, but .01 uFd is "too much."
Loads the things down a little.
I settled on .004 uFd because I had a bunch of 
small ones on hand.
Yes, I know the things are speced to drive 15 pFd load.
I did try a simple series cap feed so the chip would 
see 15 pFd, but this didn't develop enough drive at
the multiplier grid to work.
I don't know "why;"  that's what I have you 
Smart People for ;-)
Compared the LO Oscillator to a crystal and found 
little difference in performance between them.

The HI Oscillators [  Fxtal MC = (Fop - 6.9 MC) / 6  ]
did not require the UNUN.  2 volts of drive appear 
to be enough when using this freq.

The HI oscillators do indeed introduce jitter "phase noise" 
in the receiver,  raising the noise floor noticably.
The LO oscillators did not do so. I don't know "why not."
But this concern with the HI oscillators is offset 
by these considerations:

First- What is the goal?
Attempting to do "weak signal" work with an 
R-28 AN/ARC-5 VHF receiver would be foolishness.
The "goal" is to get this WWII rig operable and 
demonstratable, and (if lucky), to make a few
very local AM contacts.  Thus, a higher-than-spec
noise floor isn't an issue.

Second:  
Cost of a single new crystal:  about $45.
Cost of a single Prog Osc. substitute: about $6.
Case closed ;-)

Some notes on BC-942 / R-28 ARC-5 receiver:
Even carefully following the manual alignment procedure
and with decent test equipment, getting the front-end to
"track" and provide good sensitivity across the entire range
is difficult at best.  I spent as much time as I could afford on
it and finally accepted that the sensitivity was going to fall-off
considerably above 140 MC.  I'm not working DX with it
so this was acceptable.
Sensitivity in the Aircraft band is only a couple of dB below
that of a modern scanner.  Not bad.  

Wide-Band FM in the broadcast band sounds pretty good 
if you're lucky enough to have a crystal at just the right point 
on the slope.   But not so with Narrow-Band FM.
If you want to try "slope detecting" a 2-meter repeater, 
you will need a very strong signal and you've got to get
30-50 KC off center to demodulate it at all.
You cannot demodulate even a smidge of an on-freq FM signal.

Many different crystal "multiplication factors:'  x12, x18 etc.
will work with the LO chain.  Try what ya got.

Next - Will it work with the BC-950 ( T-23 ARC-5 )
 VHF transmitter?

73 DE Dave AB5S