[time-nuts] mixers for frequency measurement
Bob Camp
lists at rtty.us
Fri Jan 20 17:12:30 UTC 2012
Hi
Your SR620 is in better shape than mine...
Bob
-----Original Message-----
From: time-nuts-bounces at febo.com [mailto:time-nuts-bounces at febo.com] On
Behalf Of Ulrich Bangert
Sent: Friday, January 20, 2012 8:54 AM
To: 'Discussion of precise time and frequency measurement'
Subject: Re: [time-nuts] mixers for frequency measurement
Bob,
> ~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.
In general I agree to all these numbers. I just want to point to the fact
that a HP5370 or a SR620 allow for a kind of "high resolution mode". This is
a mode in which the counter is externally armed to make 1000 TI measurements
per seconds and display the mean of them. Which gives a SQRT(1000)
improvement of all counter related non systematic errors. My experiments
with a SR620 indicate a 6E-13 noise floor for Tau = 1s without any need for
mixing for two 10 MHz sources. Naturally this works ok only for source
frequencies >= 1000 Hz ( The higher the frequency the less trigger noise ).
For those of us who have no access to H2-masers or BVA-oscillators as a
reference this may be not exactly an overkill but quite good to characterize
HP10811/FTS1000(1200) or the like not to mention anything worse than that.
Best regards
Ulrich
> -----Ursprungliche Nachricht-----
> Von: time-nuts-bounces at febo.com
> [mailto:time-nuts-bounces at febo.com] Im Auftrag von Bob Camp
> Gesendet: Donnerstag, 19. Januar 2012 18:37
> An: 'Discussion of precise time and frequency measurement'
> Betreff: Re: [time-nuts] mixers for frequency measurement
>
>
> 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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