[time-nuts] mixers for frequency measurement

Bob Camp lists at rtty.us
Thu Jan 19 12:36:44 EST 2012


Hi

It's always a "that depends" sort of thing. What do you have to measure? If
you happen to have a pair of Hydrogen Masers in your basement, that's going
to be more of a challenge than a pair of telecom rubidiums. 

In general, the mixer is less of an issue than the circuit you follow it up
with. Squaring up a 1 Hz sine wave isn't as easy as you might think, if you
want to keep it quiet. A Mini Circuits RPD-1 will do a pretty good job until
you get to fairly exotic measurement levels. 

A diode mixer squares up the sine wave you feed it. If you are running in
saturation, there is very little difference between sine and square feed.
Again for ultra fancy stuff, you would want to run sine wave and drop the
levels a bit. Saturation brings in some side effects. Either way, you will
have some sensitivity to input levels.

All of this will really only give you an answer to - how good is this pair
of gizmos right now? Coming up with an answer that's specific to gizmo A vs
gizmo B is a bit more complex. If you have the luxury of a "perfect"
reference for gizmo A, then you can blame what ever you see on gizmo B. Even
then you can't really tell how good gizmo A is. 

So what sort of units does this all have? Sticking at one second tau and
talking about accurate data (not resolution) on a fairly normal part:

~2x10^-10 you can do this with a good frequency counter, no mixers needed.
~2x10^-11 you can do this with a very good /hard to find / expensive
frequency counter.
1.0x10^-11 pretty easy, nothing very fancy required for a single mixer
approach.
1.0x10^-12 works fine with an RPD-1 and some care, but not a lot of crazy
stuff
1.0x10^-13 you need some attention to detail, and may need a better mixer.
1.0x10^-14 can I come live at your house? If you have this sort of stuff,
the cost of a fancier test setup should be a minor issue.
1.0x10^-15 indeed people do measure this stuff. Proving accuracy at this
level involves a lot of work on secondary effects. 

Again, that's all at one second tau. Change the tau and *all* the numbers
move. 

One other notes. If you have two equal / identical devices and they measure
1.4x10^-11, then they both are at 1.0x10^-11. Unless you take a lot of data,
your 1.0x10^-11 reading may only be accurate to > 10%. Getting good data at
longer tau's takes a lot of time.

Bob

 

-----Original Message-----
From: time-nuts-bounces at febo.com [mailto:time-nuts-bounces at febo.com] On
Behalf Of Chris Albertson
Sent: Thursday, January 19, 2012 12:04 AM
To: Discussion of precise time and frequency measurement
Subject: [time-nuts] mixers for frequency measurement

I want to get set up to make accurate frequency stability measurements
at reasonable cost.  I think mixers are the way to do that.   I'll set
up a single mixer first them later learn to use the double mixer
technique.   But I will use the simpler single mixer to try out the
parts.  My goal is a multi-channel double mixer setup.

First question:  Which type of mixers work best for this.   I have
some SA612 chips in the parts bin.  Or should I be using diodes or
something else.  Yes I know I could simply buy one from Mini Circuits
but that defeats the purpose which is learning how to do this

2nd question.  if the device under test is a square wave oscillator is
it best to run the square wave right into a mixer or filter them to
sine waves first. I see pros and cons of each.  What do the experts
do?

-- 

Chris Albertson
Redondo Beach, California

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