[time-nuts] is there a cheap and simple way to measure OCXOs?

Bob Camp lists at rtty.us
Wed May 30 16:33:46 UTC 2012


For phase noise of this class OCXO, the "easy way" is to hook up an RPD-1
running into an OP-37. Set up the op amp for about 20 db gain in a
non-inverting configuration. RF isolation amps should not be needed in this

Run a second op amp with variable gain, and a pot for offset off of the
OP-37 output. Use it into the EFC of one of the OCXO's to hold them in
quadrature (mixer output at roughly zero volts). An OP-27 will do, but is
not really needed. You will need to play a little with gain (low gain) and
roll off (cutoff below say 10 Hz) to get things stable and to keep it from
impacting the phase noise.  

If you have a good audio spectrum analyzer, feed the rig into that. Measure
the noise and do the math. If you are running a sound card as your FFT
analyzer, you likely will need some more gain between the OP-37 and the
card. Just how much depends on the card you have. It's likely in the
vicinity of 40 db or so. 

Calibration is done by breaking the loop and looking at the beat note. Since
it's a nice 360 degrees per cycle, you can use a scope (or what ever) to
come up with the phase slope of the mixer. With high enough supply voltage
on the OP-37 it should feed the scope without clipping. If not, drop the
gain a bit below 20 db. 

There's a little more to it, but not much. You need an LC filter between the
mixer and the OP-37. It needs to terminate the mixer in 500 ohms at RF and
5K ohms at DC. It needs to nuke the OCXO output and it's second harmonic
pretty well. It needs to be reasonably non-peaking over the audio band you
are looking at. Three coils, three caps, two resistors normally do the job.
A little playing with LT-Spice will convince you that the parts in your junk
box are "good enough". 

There's maybe $50 in parts including the PC board if you have an empty junk


-----Original Message-----
From: time-nuts-bounces at febo.com [mailto:time-nuts-bounces at febo.com] On
Behalf Of Attila Kinali
Sent: Wednesday, May 30, 2012 11:00 AM
To: Discussion of precise time and frequency measurement
Subject: Re: [time-nuts] is there a cheap and simple way to measure OCXOs?

On Wed, 30 May 2012 07:18:12 -0700
Jim Lux <jimlux at earthlink.net> wrote:

> Got a mixer around, so you can beat them against each other (or against 
> your 5680)?

Not yet. I'm currently thinking about geting an mixer from minicircuits
for the ZX05-1L[1] and a low pass filter BLP-1.9[2].

But what then? The frequency offset is low enough that it is still
problematic to measure with standard measurements instruments.
And the phase noise.. out of question.

			Attila Kinali

The trouble with you, Shev, is you don't say anything until you've saved
up a whole truckload of damned heavy brick arguments and then you dump
them all out and never look at the bleeding body mangled beneath the heap
		-- Tirin, The Dispossessed, U. Le Guin

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