[time-nuts] mixers for frequency measurement

Bob Camp lists at rtty.us
Fri Jan 20 12:12:30 EST 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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