[Boatanchors] Re-Thread: Programmable Oscillators as Crystals

[David Stinson]

( Reposted as requested.  Please skip if you have this already)

Re: Using Programmable Oscillators as Crystals

Here is an "executive summary" of the thread:

2-10-13
This brings me to a project which is still in the "noodling" stage.
"New" crystals can be ordered, but the price is prohibitive-
$40 to $50 for one!
But there is an alternative- custom frequency clock oscillators.
You can order these programmed for any frequency you like
for just a few dollars each.
Voltage derived from the filaments will power them.
May or may not need a little "buffer" stage after the
output. This would be easy to build and install
right at the crystal socket.
.............................
Here is a page from Digikey. Lowest-price for through-hole
oscillator, programmed to order by them is $4.10 in quantites of 1.
http://tinyurl.com/b6kxgeo
I haven't ordered any yet but will soon.
I'd go with the 5-Volt type for the bigger rail-to-rail swing.
Some even have "Tri-state" control pins so switching is easy.
....................
Here is a typical oscillator's datasheet:
http://pdf1.alldatasheet.com/datasheet-pdf/view/161044/EPSON/SG-8002DB.html
It is programmable in 0.1 KC steps up to 125 MC
(Digikey will program it for you.  They require you to
state the freq in the notes at the bottom of the order form
as "x.xxxx MHz" since they don't know KC).
.....................

 Here is some data from a guy that has made some measurements.
That is not to say that yours may be different since the actual programming
frequency and other factors determine the actual results. It is still worth
while trying
one in your application and see how it works out. Mine was to for qrp tx
and rx.
Tx is less of a problem as long as the spurious responses are low enough.
Rx applications may be different. In my case up to 7 MHz seemed to be OK.
I was also looking into using one for a plasma geneator at 13.56 Mhz.
http://lowfer.us/k0lr/epson/epson.htm
73
Bill wa4lav
.........................

2-13
I have several WWII sets that need crystals to operate.
Finding "old stock" can take a very long time and purchasing
them "new" from the few remaining suppliers is prohibitively
expensive. So.... alternatives.

I'm an advocate of leaving WWII equipment "operating as designed,"
meaning that limited changes that are 100% reversable which
allow the unit to live are OK. So I've often thought of
"Borg Implants" like my simple converter to get them going.

My rare SCR-274N VHF set needs crystals, especially
for the BC-942 ( ARC-5 R-28 ) receiver.
Programmable oscillators in both TTL and CMOS format
set for any frequency one would wish are cheap
and plentiful. So I decided to try them out.

The BC-942 / R-28 uses a 12SH7 as an Electron
Coupled Oscillator / Quadrupler. When using a
regular crystal, I scoped 3 V P-T-P at the cathode.

I had a couple of older can-type TTL output
clock oscillator units for 4.000 MC,
which puts the set on 102.9 mHz.
I powered the oscillator with a old, weak 6 Volt battery.
Under load, the battery sourced 5.3 Volts for Vdd.

Driving the osc. straight into the stage (with a blocking cap)
did not develop enough drive voltage, so I wound a little
4:1 UNUN and this developed 2 V P-T-P at the cathode,
which appears to be just enough because the set
came right up and played. I think a 9:1 UNUN will do better
(it didn't).

Next, I ordered four Epson programmable oscillators
from Digikey: 4.652, 4.829, 5.722 and 5.818 mHz
There are probably better ones out there,
but I picked this one because it was a through-hole
package and *cheap* ;-)

http://www.doveonline.com/pdfs/SG8002DB.pdf

I ordered them pre-programmed from Digikey and they arrived
in just a couple of days. Less than $18 for four channels-
less than half the price of a single "new" crystal.

Of course, the first thing I did was hook the blasted
battery up *backwards.*
A little smoke sword shot out of the middle of the chip
and it disemboweled itself......... "YAAAAAA!"
So much for the very active 118.55 MC
DFW airport departure freq.....

When I quit banging my head on the floor,
I connected a survivor correctly and tried again.
The output of this chip would not work into an UNUN,
but for a wonder, it drove the 12SH7 directly (with the
DC blocking cap, of course) and developed the
2 Volts drive needed. I'm listening to the Unicoms
on 122.8 right now. If the "jitter" is causing a problem,
I can't hear it. Sounds like airplanes to me ;-)

This CMOS chip outputs about supply voltage - .4V and it
can be Vdd up to 7 volts, which would give me more drive.
I'll build a little regulator to supply all four boards
with 6.75 V and that should do it.
Here's a sketch of what's going on:

http://home.netcom.com/~arc5/ItWorks2.jpg
........................
2-19
So here are some "shade-tree" conclusions:

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

The LO oscillators [ Fxtal MC = (Fop - 6.9 MC) / 24 ]
need a little 4:1 voltage-boosting UNUN
to deliver enough drive to the R-28.
I got mine all pre-made and ready to go from
Tanner's surplus house in Carrolton.
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 and they worked.

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 a bit of jitter "phase noise"
in the receiver, raising the noise floor noticably.
The LO oscillators did not do so. I don't know "why not."

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

73 DE Dave AB5S