Fwd: [time-nuts] Frequency counter recommendation

[Bob Bownes]

---------- Forwarded message ----------
From: Bob Bownes <bow...@gmail.com>
Date: Tue, Dec 21, 2010 at 12:04 AM
Subject: Re: [time-nuts] Frequency counter recommendation
To: Discussion of precise time and frequency measurement <time...@febo.com>


Comments inline.

On Mon, Dec 20, 2010 at 11:17 PM, Chris Albertson <alberts...@gmail.com> wrote:

I looked.   I think we should keep the design goals modest for a first
revision.  Shoot for a spec that can be hand built on perf board.  So
I'd relax those numbers by a factor of 1000.  The top frequency is in
Mhz, not Ghz and the time resolution closer to ns than ps.  It's good
to have a cheap option.  Many people are happy with an FCC1  Try for
the next step after that with a goal of actually matching the state of
the art in steps.

 

My initial thinking was to be better than a pictic ii, preferably on par with, or better than  a 5370. I'm not sure you can do that on perfboard. I suppose if the speed is kept low it can be done that way.

Others have pointed out (offline) 20Ghz isn't reasonabe with a decent noise figure or prescalers. What do people think a reasonable number is? What about resolution? I'd like to get better than a ns, preferably a lot better.

Why no through holes?  I don't see the point of banishing them.  I to
agree with the rest.  SMT that is hand solderable by a skilled tech
but now reflow ovens or solder past masks should be required .  You
might place a limit on component size too like 0.5mm lead pitch or
whatever is reasonable.

I've been prototyping a lot of late and restricting the number of through holes makes the job much much easier and quicker. No other real reason.

I didn't place a limit on the lead pitch because a) I felt that limited the component selection and b) pretty much even the finest pitch can be hand assembled with care, solder wick, and 20x magnification. But if folks are very against it, it can go in the 'desired qualities' list. My only fear is the limit it might put on critical parts like a FPGA.
 

Mechanical assy is going to be a killer. Let's start with overall form factor.

      Rack mount or bench format?

      If rack mount, 1U or more?

      Commercial project enclosure (ala the VNA Hammond box) or do we take an existing form factor like a disk drive as you suggest.

Heck, if we go with a disk drive size, it could be built/slid into anyone's lab PC case and use ribbon cable as a back plane... 1/2 :)

I like 1U because it matches up with the rest of the test equipment on the bench and it gives it a professional feel. And there are many many surplus 1U cases out there with decent +/-12vdc,+5vdc (even some with 3.3vdc) power supplies.
 

Bob

On Mon, Dec 20, 2010 at 6:20 PM, Bob Bownes <bow...@gmail.com> wrote:
> Well said Chris. Take a look at the initial spec in the OpenCounter Gogle
> group and tell me what you think with respect to your item #1. I think the
> core counter is going to be the really difficult part of the module list.
>
> Item #2 is going to be a tough one methinks. I love Eurocard, but, as you
> say, it is very expensive, if only for the connectors. In cases like this
> I'm a fan of either repurposing commercially available connectors (PCI and
> memory DIMMS are two I have used in the past) because they can be a)
> purchased off the shelf, b) are manufactured in enough volume to make cheap,
> and c) are common enough that the really cheap amongst us can get them off
> of scrap boards someplace for little or nothing. The N2PK VNA is built to
> fit into a particular HAmmond enclosure that I like but again, there are
> many options. My feeling is that the enclosure should not dictate any
> functional design decisions.
>
> #3 - I've created a group and appointed myself benevolent dictator. We can
> discuss things, propose designs or design criteria, call for a concensus,
> accept, and draft volunteers to design that section to the defined spec. If
> there are multiple competing designs, so much the better, as long as we all
> agree on the interfaces. Sound like a process? Can you tell I've done this
> once or twice? :)
>
> Step one will be to agree on the overall functional spec. If we get enough
> participants, I'd like to nail that down by mid January. The next step is to
> agree on the interfaces between the modules. Same process, discuss, propose,
> draft, get concensus, close and move on.
> Then we get folks working on the individual modules.
>
>
> On Mon, Dec 20, 2010 at 8:43 PM, Chris Albertson
> <alberts...@gmail.com>wrote:
>
>> On Mon, Dec 20, 2010 at 2:51 PM, Dave M <dgmi...@mediacombb.net> wrote:
>> >
>> .. never got off the launchpad because of their inability to come to
>> > consensus on a set of features. I had to conclude that too many cooks
>> > spoiled the broth. Everyone that had input to the project was unwilling
>> to
>> > yield on anyone else's ideas.   Hopefully, our project won't degrade into
>> > another such fiasco.
>>
>>
>> THAT is the number one problem to solve.  Technical issues are easy
>>
>> I think the solution is to
>> (1) chop the project up into small enough parts, each on it's own PCB
>> so that each part is "easy" and has some wider user outside the
>> project.
>> (2) Find a mechanical standard so all the PCBs can be mounted in some
>> kind of chassis.  I'm thinking now that maybe a 160-3U Eurocard would
>> be about right size.  But the parts are expensive.
>> (3) Need some sort of design process that allows everyone to
>> contribute.   And everyone can.  Projects always are lacking technical
>> writers and quality control people
>>
>> Of those a "process" and "mechanical standard", I think are the
>> hardest.   We always give managers a hard time but that is what is
>> needed.  The person who will make this happen will be a manager and
>> organizer maybe not a designer.
>>
>>
>> --
>> =====
>> Chris Albertson
>> Redondo Beach, California
>>
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--

=====
Chris Albertson
Redondo Beach, California

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