[time-nuts] 10 MHz Oscillator comparison part II

Bob Camp lists at rtty.us
Wed Jan 12 18:24:06 UTC 2011


If you have a random frequency like 7.352 MHz that neither divides or
multiplies to 10 MHz harmonic or sub harmonic, you can indeed mix the signal
to 10 MHz.

If you do so, you will need to filter the outputs, since the mixing spurs
will mess up the input to the multiplier.

If the generator you use for the mixing has more noise or jitter than the
sources, that noise is likely to de-correlate unless the chains are
absolutely identical. Since they multiply to two different frequencies, they
really can't be identical. Net result is your measurement is messed up by
the noise of the generator. 

Once you add multiple mixers and filters in, along with a very low noise
generator, the error multiplier doesn't make a lot of sense. Much easier to
take the same generator and just mix down to an audio beat note. Feed that
into the counter and go from there. 10 MHz to 10 Hz gives you a 1x10^6
multiplication by mixing down. 


-----Original Message-----
From: time-nuts-bounces at febo.com [mailto:time-nuts-bounces at febo.com] On
Behalf Of Perry Sandeen
Sent: Tuesday, January 11, 2011 6:39 PM
To: time-nuts-request at febo.com
Subject: [time-nuts] 10 MHz Oscillator comparison part II


Wrote: However the exercise is probably pointless as the frequency
difference between the 2 signals as seen at the output of the cascaded
divide and mix chains are reduced by this scheme.

Umm.  I guess I didn't express my thoughts effectively enough.

The idea is to subtract 9 MHz in each DBM then take the 1 MHz error signal
and use it to phase lock  the next PLL at 10 MHz.  Thus the error is
multiplied by 10 in each section.  So after four sections the error is
multiplied by a factor of 10,000.

What I forgot to mention in my original post is the use of a decade counter
(1/2 of a 74HC390) from the output of each PLL to one input of its phase
detector so it would lock on the 1 MHz signal from the previous stage.
Maybe this is what caused confusion,  If so, I apologize.  If I've still
missed something please correct me.

This math is the same as multiplying 10 MHz to 10 GHz.  What this method
avoids is using very tricky-to-use frequencies.  And I suspect much cheaper.

Wrote:< You can see the Tracor frequency error meter, used the same mixer
method you describe but using 9 and 10 MHz frequency to mix together and a
decade frequency multiplier. The limit of the system is the phase noise of
the system and sources. The Tracor use an optional Xtal filter to limit the
noise. The filter can be inserted for high multiplication rate or for noisy

< See tracor schematic

Thanks.  I knew my method wasn't original, I was trying just update it and
make it both simpler and cheaper with newer chips

Wrote: <You need to multiply the input signals to a nominal 100 MHz then
subtract 90 MHz using a mixer and repeat the process.

This may be true but I don't understand why.  I think my math process is the
same as yours. I may need more "Edjurcation".

Wrote: The normal term for a gizmo that multiplies and then mixes down is an
error multiplier.

A. Right

Also thanks to Urlich who sent me a PDF copy of the Quartzlock (UK) Limited

Wrote:< Using the CD4046 (or HC4046) as the phase locked oscillator would
probably be counterproductive as its phase noise is very high (its in effect
an RC oscillator with an effective Q of around1/4)

A. I don't know.  Maybe there is a better IC choice for the PLL. I guess
building one would prove its feasibility.

An additional thought came to me that might make the whole process easier.

ASS-U_ME we have a "gold standard" 10 MHz signal from a GPS, Rb, or cesium
beam.  We input it into our frequency counter's external reference input.

We take the to-be-measured 10 MHz source and put it into the external input
of a synthesized signal generator.  Set the signal generator frequency
output to 100 Mhz or 200 Mhz or whatever are your highest limits are of the
generator/counter setup and count the difference using and extended start
and stop signal for the counter.

If one has a HP 3336A/B (referenced to our "standard") it can give up to a 1
micro-Hz clock signal. Or one can chain together a bunch of 74HC390 decade

Wrote:< DMTD is the obvious way to do this, but let's go the old fashion way

A. I don't know what you mean by DMDT but am willing to try to learn.

Wrote:< Going down, how low do you want to go? 100 KHz will give you 100X.

A. I thought when dividing down one was also dividing the error. 

Wrote:<Whatever way you do it, you are still stuck with the same old
problem. The input needs to be on a very specific frequency. With some
designs it can be a sub multiple of that frequency. With other designs it
can be a multiple.

A.  Not Necessarily.  If one puts a DBM in each channel which is referenced
to a common oscillator.  Granted one may need to add a post mixer filter.
But this is a diversion from the original issue.

Wrote:< Either way, 7.352 MHz is going to give you trouble. That's pretty
much why the error multiplier boxes all went into storage in the 1970's...

A. I have no idea what 7.352 MHz is or why it will cause trouble.  Could you
please elaborate?

I blessed to have HP 5370's, rubidium standards, a HP 3336B and GPS
receivers and don't really need to fabricate any equipment.  My reason for
posting is to come up with a relatively simple test equipment set-up that
could be used by those who have a more modest budget or live where the test
equipment we in North America can afford is unobtainiumly expensive.

Thanks to all who have commented as shared the actual results of their 10811




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