[time-nuts] Designing and building an OCXO and GPSDO

Philip Pemberton lists at philpem.me.uk
Mon Aug 11 16:25:29 EDT 2008

David C. Partridge wrote:
 > Stupid question - why build your own OCXO when you can buy a pretty good
 > Oscilloquartz OCXO from eBay item number 300247357254 for almost a song?

I can think of a couple of reasons:

1) If it breaks, I can rip it open, poke and probe it, figure out what's wrong
and then hopefully fix it. I've got loads of T&M gear that sadly never sees
the use it should -- a very nice Tek TDS2024B DSO that's been used maybe a
dozen times, a HP 1651B logic analyser that's been used about as much (but
bought second-hand with severe screen burn), the list goes on.

2) It's nice to know how things work - I have a severe case of
blackboxophobia. I hate little black boxes that do magic things, and have zero
parts availability. This has gotten to the point where I've built a model
logic analyser to learn how they work, and most of my complex signal
generators are homebrew too. I've got a nice DDS signal generator that I'm
working on at the moment, which should by virtue of the ADI DDS chip be able
to do AM, FM, BPSK, QPSK, (maybe) 8PSK and a few other modulation modes. I'm
building it for... testing a Radio 4 timecode receiver. Bonus points if I can
make it produce DVB-S compliant streams at 70MHz IF and upconvert to 1.x GHz
L-band to test satellite receivers. That's quite far down on the projects list
though :)

3) I try and avoid ebay where possible. I've had pretty good luck with UK and
US sellers, but the few I've dealt with from other countries have been
horrendous, especially those in the vicinity of China and Hong Kong. Six
months for one parcel to arrive...

Anyone here listen to the "SolderSmoke" podcast? #72 pretty much sums it up. I
like to know what things do, and I can't stand to buy something if I can build
it from parts I've got in my junk box. In my weird world, if I've already
bought the parts, then effectively my cost is zero.. I've got the bits, I
might as well use them.

Plus I was actually looking at new OCXOs -- at the time I decided to do this,
there weren't any on eBay. The lowest quote I got was £132 +VAT (rounded up
from the quote I just dug out...) for a 12V, fairly low-tech OCXO. The shiny
new high-tech one with the TTL status output and other stuff was about £240
and some change.

In effect, it's cost me £37 for the crystals, and ~£10 for a dozen each of the
temperature sensors and voltage references. I'm still below half the cost of a
brand-new commercial OCXO.

 > Yes I know - it's fun!

And it's fun. That too. :)

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