[time-nuts] Frequency Counter Choice

Thu Oct 22 17:29:07 UTC 2020

Hi

One approach to a mixer setup is the DMTD (dual mixer time difference). 
It has a “little cousin” that also does very will running one mixer instead of 
two. The limit there being that you need to be able to tune one or both the
devices you are testing to get a beat note in the 2 to 10 Hz range. 

There are lots and lots of posts on DMTD’s so I’lll leave that to others. Single
mixer sort of works like this:

Grab a MiniCircuits RPD-1 and drive both ports into saturation ( about +7 dbm).

What comes out will be a triangle wave. To get the max voltage out ( which is
a good thing in this case) terminate the output into >= 5K ohms at audio / DC. 
You will need some RF filtering on the output to knock down the 2X input frequency
RF on the output. 

Any of a number of op-amps from the good old OP-37 on will do a fine job of 
acting as a preamp for the audio. My preference is to amplify it up pretty far
(like 10X) and then feed it into a multi stage limiter. 

There is a paper running around behind the IEEE paywall by Collins that details
one approach to designing multi-stage limiters. There are other ways to do it. 
The key is to not go to crazy all in one stage. 

The output of the last limiter stage feeds your counter. With a good limiter
you might get 6 fairly solid digits on your 5335 (off of a pair of 10 MHz OCXO’s).
Without a good limiter, you are doing well at three solid digits on the 5335. 

Fun !!!

Bob

> On Oct 22, 2020, at 11:00 AM, Giorgio Barinetti <giorgio at barinetti.it> wrote:
> 
> Hi Bob,
> 
> 
> 
> Nice explanation. I'll see what I can do to find a 53132.
> 
> 
> 
> Maybe, in the meantime, if I'll be lucky to find a 5370, I'll play on it.
> 
> 
> 
> Regarding the mixer based setup, I'm already in progress for setting up a suitable test jig.
> 
> Please point to some information, if you have it handy.
> 
> 
> 
> Regards,
> 
> Giorgio.
> 
> 
> 
> 
> 
> Hi
> 
> 
> 
>> On Oct 22, 2020, at 7:50 AM, Giorgio Barinetti <giorgio at barinetti.it<http://lists.febo.com/mailman/listinfo/time-nuts_lists.febo.com>> wrote:
> 
>> 
> 
>> Hi,
> 
>> 
> 
>> As a newbie in the field, I've collected, by chance, some frequency standards.
> 
>> 
> 
>> Now is the time to measure them, and see how they perform.
> 
>> 
> 
>> I've inherited a 5371, but something tells me that is not the right instrument - and maybe is even faulty.
> 
>> 
> 
>> So, I'm in the need to buy a counter to use togheter with TimeLab.
> 
>> 
> 
>> Choices are many, but I'll try to avoid the "older" machines lile 5370 or 5335. The 531xx series seems nice ( money apart )
> 
>> But again : which one between the 3 ? 53131, 53132 or 53181 ?
> 
> 
> 
> 53181 = single channel 400 ps counter
> 
> 53131 = dual channel 400 ps counter
> 
> 53132 = dual channel 100 ps counter
> 
> 
> 
> 5370 (when working) = dual channel 20 ps counter
> 
> 
> 
> The gotcha is that none of them are good enough to properly measure 1
> 
> second ADEV on a good ( but still could be cheap ) OCXO. A mixer based
> 
> setup is a cheap way to get things done, even with a 5335.
> 
> 
> 
> The 531xx counters all share a couple of issues:
> 
> 
> 
> 1) The power supply was made by who knows who and their quality is not
> 
> the best. Good news is that the power supply out of a (cheap) 53181 will
> 
> swap over to revive an expensive 53132.
> 
> 
> 
> 2) They have a fan, it plugs with dust. When that happens it gets hot inside
> 
> the cabinet. Things (like the power supply) die as a result.
> 
> 
> 
> 3) They have a VFD display. It's a nice one, but they do wear out. Replacing
> 
> them is problematic.
> 
> 
> 
> 4) They have a fancy setup to add digits to a frequency measurement. It
> 
> can lead you astray. It also tends to go deaf right at 10 MHz. There are app
> 
> notes out there that explain the details.
> 
> 
> 
> 5) With *any* counter, a good external reference is the way to go for "Time Nut"
> 
> grade measurements.
> 
> 
> 
> None of that is to say they are a bad counter, far from it. I have a number
> 
> of them and have been using them here and at work for decades. The 532xx
> 
> counters are the latest and greatest. They are on eBay, but not cheap.
> 
> 
> 
> Bob
> 
> 
> 
> 
> 
> 
> 
>> 
> 
>> Can somebody shed some light, and maybe help even to found a baseline for us beginners ?
> 
>> 
> 
>> Regards,
> 
>> IZ2JGB
> 
>> Giorgio
> 
>> 
> 
>> My NTP servers: (ntpd on FreeBSD - PPS in via Serial)
> 
>> 
> 
>> HP/Symmetricom 55300A  https://www.ntppool.org/scores/93.41.196.243
> 
>> 
> 
>> Efratom Rb/Xc GPSDO https://www.ntppool.org/scores/95.255.136.126
> 
>> 
> 
>> 
> 
>> 
> 
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> 
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