[time-nuts] Rb as source for ADEV?

[Bob Camp]

Hi

I pretty much wire them up on a piece of perf board when I need them. I’ve never bothered to do a real pc board. The OP-27 and OP-37 op amps are getting a bit old / expensive. If I was going to do one, I’d probably do some digging on the op amps first. There may be new(er) parts that are cheaper. 

Like anything else, done one up there’s a lot more pain involved doing one than doing a batch. PC boards make things easier.  

———

The real benefit comes from a couple things:

1) Mixing down to a lower frequency. The counter gives you 9 digits a second no matter what the (> 1Hz) input. Lower frequencies give you more digits. Any reasonably quiet mixer will do this for you. That includes the DBM’s with connectors on them.

2) Amplifying the beat note as much as you can ahead of the limiter. Slew rate matters. Any *quiet* audio amp will do this for you, provided it’s got the frequency response and is very quiet at the low end.

3) Limiting with a circuit that has good noise performance at low frequencies. The poor guy who did the counter could not just focus on the low end. We can. 

There’s lots of ways to do each of those things. You can easily improve any of the three “chunks” over the very simple circuit I outlined. You can easily get caught up in the optimization process and turn this into a very complex project. For one second ADEV at the 1.0 x 10^-12 level (1x10^-13 at 10, 1x10^-14 at 100, 1x10^-15 at 1000) a very simple circuit will do the job. 

—————

The other alternative is to get something brand new with warranty and support like a TimePod. Symmetricom will happily sell you one. They work amazingly well and there’s no muss no fuss comparing a pair of devices. They are just a bit over the $10 to $15 budget (like by three zeros). Accuracy wise it will crush my little circuit, and mine doesn’t come with a cool GUI that reports power levels and the like. 

Am I trying to sell you one of these - no of course not. The point is that there’s a massive step cost wise going to new gear. Even at ten cents on the dollar there’s still a massive step. (and yes in this case you *can* take that as a “anybody want to sell a TimePod for 10 cents on the dollar?” question, I’m in a shopping mood … contact off list of course). I’m not holding my berth for the inbox filling up with offers. Finding this kind of gear used in good condition at a good price is not easy. 

—————

One other approach would be group builds. There are a number of these going on off list and aimed at selling the result. Because of the off list nature of this, it’s often tough to know what people are doing and how well they are doing. They tend to turn into limited production run builds and then vanish. 

Short runs make it tough for newcomers to stock up on stuff. There simply isn’t enough need for these things to keep a steady supply readily available. That’s not at all unique to this area. A *lot* of hobby electronics (and mechanics and ..) has exactly the same issue. I could fill several pages with examples. 

—————————

Long rambling reply to a simple question - sorry about that ….

Bob

 




On Feb 8, 2014, at 6:02 AM, Jimmy Burrell <jimmydburr at gmail.com> wrote:

> Somewhat like Bob (Stewart) I've spent my life in IT with a little background in Ham Radio. Now, infected with the Time Bug, I'm slogging through tf.nist.gov, wenzel, and leapsecond to try and level up.
> 
> I'd like to ask Bob (Camp) if he has a readily available link for a suitably quite homebrew mixer, similar to the one whose parts list he was discussing?
> 
> Thanks to Magnus for his links (later in this thread), to which I'd like to add the following for those on the list, like me, who are neophytes.
> 
> Fundamentals of Time and Frequency: http://tf.nist.gov/general/pdf/1498.pdf
> 
> Using a Time Interval Counter to Measure Frequency Stability: http://ipnpr.jpl.nasa.gov/progress_report/42-90/90S.PDF 
> 
> The later link, while a little dated, still does a good job of covering concepts.
> 
> Thanks again,
> 
> Jimmy...
> N5SPE
> 
>> On Feb 6, 2014, at 8:57 PM, Bob Stewart <bob at evoria.net> wrote:
>> 
>> I think I see the problem.  I was wondering about using the 1PPS output from my Rb in a test.  Cobbling all that together would be a quick bit of work for you, but I spent my life in IT.  I'm good with a soldering iron, but I readily admit my shortcomings at hardware tinkering.
>> 
>> 
>> Bob
>> 
>> 
>> 
>> 
>>> ________________________________
>>> From: Bob Camp <lists at rtty.us>
>>> To: Tom Van Baak <tvb at leapsecond.com>; Discussion of precise time and frequency measurement <time-nuts at febo.com> 
>>> Sent: Thursday, February 6, 2014 7:43 PM
>>> Subject: Re: [time-nuts] Rb as source for ADEV?
>>> 
>>> 
>>> Hi
>>> 
>>> My intent was certainly not to stop anybody from doing anything with what they have. My concern is that this is a lot of work for modest return. A simple single mixer setup for  $20 would dramatically change things….
>>> 
>>> $3 mini circuits double balanced mixer
>>> $3 op amps x 2
>>> +/- 15 V supply you already have (hopefully).
>>> $5 for a piece of perf board
>>> $3 for 3 BNC connectors. 
>>> $3 left over for resistors and capacitors. 
>>> <snip>
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