Crystal drift

[Ken Smith]

As I type this, my Stanford Research generator vs. a Digikey 100MHz
oscillator comparison is drifting by about 5 parts in 10^10 per second.

While watching this I have a thought. If I could get a hold of the
temperature set point of an OCXO, and could connect it to a DAC, can I
make a very slow very low noise sweep generator with a very narrow range
or did I miss something?

I need to be able to move a frequency about one part in 10^7 or 10^8 very
smoothly. Any thoughts?

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[John Larkin]

On Thu, 23 Sep 2004 17:44:24 +0000 (UTC), kensmith@green.rahul.net
(Ken Smith) wrote:

As I type this, my Stanford Research generator vs. a Digikey 100MHz
oscillator comparison is drifting by about 5 parts in 10^10 per second.

While watching this I have a thought. If I could get a hold of the
temperature set point of an OCXO, and could connect it to a DAC, can I
make a very slow very low noise sweep generator with a very narrow range
or did I miss something?

I need to be able to move a frequency about one part in 10^7 or 10^8 very
smoothly. Any thoughts?

Most any serious OCXO also has a VCO input that you could tap into. Or
a rubidium, if you're really serious; they generally have an
electrical tweak input with extreme resolution.

SRS makes a brick-type SC-cut OCXO with an electrical VCO input; it's
very stable. $400 roughly, as I recall.

Or build a quadrature mixer and offset an oscillator output with the
output of a function generator.

John

 

[John Larkin]

On Thu, 23 Sep 2004 19:08:31 +0000 (UTC), kensmith@green.rahul.net
(Ken Smith) wrote:

Two rubidium clocks with one being tweeked may be what I need. Getting
the money to buy them may be a bit of an issue. I'll try the $100-$1000
range solutions first.

I got a nice Efratom unit on ebay a while back, $350 or some such. It
drifts nanoseconds per hour against a cesium, and has an electrical
tweak input.

What are you doing?

John

 

[Ken Smith]

In article <lg36l0tvvh5cve0q8cn26g3bflg1fjja09@4ax.com>,
John Larkin <jjlarkin@highSNIPlandTHIStechPLEASEnology.com> wrote:
[...]

Most any serious OCXO also has a VCO input that you could tap into.

Oh!. I'll have to check into that. I know that the extreme low noise one
I had custom made for this project doesn't because the variactor diode
makes a path for noise to get in. If I can find a fairly low noise one
with a VCO input that may work for me. I was just now considering a
non-OCXO and putting in in my own oven for testing.

Or
a rubidium, if you're really serious; they generally have an
electrical tweak input with extreme resolution.

Two rubidium clocks with one being tweeked may be what I need. Getting
the money to buy them may be a bit of an issue. I'll try the $100-$1000
range solutions first.

Or build a quadrature mixer and offset an oscillator output with the
output of a function generator.

I had rejected that on a noise worry. I'll have to take another look.

--
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[Ken Smith]

In article <c8c6l05ejbv7mroikr8m272smhrto81dat@4ax.com>,
John Larkin <jjlarkin@highSNIPlandTHIStechPLEASEnology.com> wrote:

On Thu, 23 Sep 2004 19:08:31 +0000 (UTC), kensmith@green.rahul.net
(Ken Smith) wrote:



Two rubidium clocks with one being tweeked may be what I need. Getting
the money to buy them may be a bit of an issue. I'll try the $100-$1000
range solutions first.


I got a nice Efratom unit on ebay a while back, $350 or some such. It
drifts nanoseconds per hour against a cesium, and has an electrical
tweak input.

Nice!

I need a good AVAR number for times in the 0.01 to 10 seconds range.

What are you doing?

Can't say yet.


--
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kensmith@rahul.net forging knowledge

 

[Mike Monett]

Ken Smith wrote:

In article <lg36l0tvvh5cve0q8cn26g3bflg1fjja09@4ax.com>,
John Larkin <jjlarkin@highSNIPlandTHIStechPLEASEnology.com> wrote:

[...]

Or build a quadrature mixer and offset an oscillator output with the
output of a function generator.

I had rejected that on a noise worry. I'll have to take another look.

kensmith@rahul.net forging knowledge

You probably have a huge library already, but Philips has a brief summary
on quadrature mixers including the Weaver method at

http://www.semiconductors.philips.com/acrobat/applicationnotes/AN1981.pdf

One advantage of the Weaver method is a lowpass filter is used instead of a
phase shift network. This might help when generating small frequency offsets.
But with any scheme using mixers, the additional spurs and noise may degrade
the output too much. Please let me know your conclusions - I have a similar
problem with generating high quality signals with a small frequency offset.

Rick Karlquist shows a repeated mix-and-divide method using inexpensive
ceramic filters at

http://www.karlquist.com/FCS95.pdf

This may give a low noise output signal, but the method requires one stage for
each digit in the output, so 10 stages plus shielding might take up some room.

Rick has updated his web page to include some of his patents pending:

http://www.karlquist.com/

Regards,

Mike

 

[Phil Hobbs]

John Larkin wrote:

On Thu, 23 Sep 2004 17:44:24 +0000 (UTC), kensmith@green.rahul.net
(Ken Smith) wrote:


As I type this, my Stanford Research generator vs. a Digikey 100MHz
oscillator comparison is drifting by about 5 parts in 10^10 per second.

While watching this I have a thought. If I could get a hold of the
temperature set point of an OCXO, and could connect it to a DAC, can I
make a very slow very low noise sweep generator with a very narrow range
or did I miss something?

I need to be able to move a frequency about one part in 10^7 or 10^8 very
smoothly. Any thoughts?



Most any serious OCXO also has a VCO input that you could tap into. Or
a rubidium, if you're really serious; they generally have an
electrical tweak input with extreme resolution.

SRS makes a brick-type SC-cut OCXO with an electrical VCO input; it's
very stable. $400 roughly, as I recall.

Or build a quadrature mixer and offset an oscillator output with the
output of a function generator.

John


Sideband locking is a possible strategy here. If you mix the two oscillators

together, you can phaselock the beat note to a function generator with a
frequency-phase detector, e.g. a 4046--it won't lock up to the image
frequency, because the sign of the loop gain is opposite for the two
sidebands. By making the loop narrow enough, you can avoid having the beat
note modulate the VCXO significantly. You do need a large enough frequency
offset to make this work, and although it won't lock up to the image
frequency, it can lock to higher IM products if you don't take some care to
prevent this (e.g. by acquiring lock at a high enough beat frequency that
only one product is within the tuning range of the VCXO, then sweeping as
desired).

If you need to have the beat frequency go through 0, then John's quadrature
idea is a good one--otherwise you could mix with a constant offset frequency,
filter, then do the same sideband locking trick.

Cheers,

Phil Hobbs

 

[Jason Rosinski]

Are you stuck to sweeping the OCXO, or could you just sweep a digital frequency synthesizer running
off the OCXO?

Ken Smith wrote:

As I type this, my Stanford Research generator vs. a Digikey 100MHz
oscillator comparison is drifting by about 5 parts in 10^10 per second.

While watching this I have a thought. If I could get a hold of the
temperature set point of an OCXO, and could connect it to a DAC, can I
make a very slow very low noise sweep generator with a very narrow range
or did I miss something?

I need to be able to move a frequency about one part in 10^7 or 10^8 very
smoothly. Any thoughts?

 

[Ken Smith]

In article <41541389.30007@zarlink.com>,
Jason Rosinski <Jason.DOT.Rosinski@zarlink.com> wrote:

Are you stuck to sweeping the OCXO, or could you just sweep a digital
frequency synthesizer running
off the OCXO?

I am using a synthesizer but I need about 12 digits of adjustment. By
picking the right numbers I can make the frequency something like:

123456.7000008 KHz

so nudging the OCXO can give me the bottom digit and the synthesizer can
give me the upper ones.

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[John Larkin]

On Fri, 24 Sep 2004 11:28:06 -0400, Phil Hobbs
<pcdhSpamMeSenseless@us.ibm.com> wrote:

Also, depending on the frequency, a simple D-flop makes an excellent digital
mixer. Put one input on the clock, the other on the D. The Q output will switch
at the difference frequency.

Not with a PFD--metastability will blow you right out of the water. Every
lost cycle equals lost lock. The D flipflop trick can work with a narrow loop
and a multiplying phase detector though--I used it when I built a pilot tone
generator for what I think was the first commercial direct broadcast satcom
system, in about 1982. (I had just got my bachelor's degree in astronomy and
physics at the time, and they hired me to look after all the ultrastable PLL
stuff--talk about being thrown in the deep end. For the frequency reference
board (different from the PTG) I had to invent a fractional-N synthesizer
based on resynchronized rate multipliers. It worked great, eventually.)

This one

http://www.highlandtechnology.com/DSS/V880DS.html

uses an EclipsLite ff as the phase detector, locking the local 155 MHz
rock to the incoming OC-3 data stream. BW is about 10 KHz for acquire,
2 KHz when tracking, with the uP switching loop filters when it feels
it ought to. About 200 of these are scattered all over the NIF laser
to fire everything at the right time. You can barely make out the
crystal oscillator sitting up on tiny springs.

Biggest hassle was the (expletive deleted) 850 nm VCSELs for the
optical-out option. What nasty parts!

John

 

[Ken Smith]

In article <41544E88.7020005@zarlink.com>,
Jason Rosinski <Jason.DOT.Rosinski@zarlink.com> wrote:
[...]

to trim the 100MHz clock as well, you'd need some other way. Is it just
a differential frequency
difference that you need?

I can nudge either input for the fine adjustment and the circuit won't
know which I've moved. I can't move the 100MHz by 0.1% or anything like
thant though.

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