[time-nuts] I've designed a GPSDO, but how "good" is it?

Mon Aug 17 09:59:25 EDT 2015

> On Aug 17, 2015, at 4:06 AM, Bob Camp <kb8tq at n1k.org> wrote:
> 
> 
>> On Aug 16, 2015, at 9:40 PM, Nick Sayer <nsayer at kfu.com> wrote:
>> 
>> 
>>> On Aug 16, 2015, at 6:04 PM, Bob Camp <kb8tq at n1k.org> wrote:
>>> 
>>> Hi
>>> 
>>> Here’s some of the “that depends” questions:
>>> 
>>> What is your stability goal? 
>>> 
>>> You talk about the NIST numbers on GPSDO’s. What level of stability are you after?
>> 
>> I believe I’m at or better than the stability I originally sought. Part 1 of my question is whether that’s actually true or whether my naivety is presenting me with a delusion. Part 2 is whether the price point at which I’ve arrived will support the level of stability I’ve achieved, or am I delusional in thinking people would or should pay what I’m asking for what I’m offering.
> 
> The stability can only be verified by careful testing, The design goal (what’s the spec) will in many ways determine
> what sort of testing you will need to set up. A 1x10^-9 design would be tested differently than a 1x10^-13 design.

10^-9 was the target all along. Question number two that I came with was whether or not it was reasonable to ask $175 for 10^-9.

> 
>> 
>>> 
>>> What is your end application? 
>>> 
>>> Is this intended as a lab standard, the reference for a radio, something else entirely? 
>> 
>> A low cost lab standard is what I have in mind. A box you can sit on your workbench with 3 BNC jacks that can feed 10 MHz into your frequency counter or what not.
> 
> Do you anticipate any phase noise sensitive instruments being attached? Most frequency counters are fairly simple.  

I stared with an immediate need, and that was for low frequency testing with sample times in the 10^1 range, so phase coherency has not been anything I’ve been concerned with. The Connor Winfield phase plots at the low end look “good enough” to me, and with a 370 ppt DAC granularity, I can’t ask for a lot of phase coherency at the high end (you can see that in the time lapse videos).

> 
>> 
>>> 
>>> What is the destination? 
>>> 
>>> Is this heading towards a commercial venture or is it a basement project? 
>> 
>> I’ve entered it in the “Best Product” Hackaday 2015 prize contest. That said, I have no intention of attempting to compete with the established commercial firms in this space. I want to stay at around a Q:100 unit cost of around $75, which is where it is right now (the retail price is higher at the moment because I’m not manufacturing them in Q:100 lots yet).
> 
> Ok, so have you taken a good look at your costs and return? Do you have an method set up for
> support of the product and for handling returns / repairs? Overhead costs that do not get factored
> in early can be very painful later on.

Yup. The rule of thumb I have always used is that retail price is double the cost. That’s why I talk about a Q:100 cost of $75 and am charging $175. $175 is not $150 because I am not yet at Q:100 build costs, but that is the eventual goal.

> 
>> 
>>> 
>>> What is the budget?
>>> 
>>> Do you have $200K to spend on this? Did the piggybank run dry at $100?
> 
> 
> For a commercial venture this is a very important factor. You don’t have to share
> the info, You do have to work out a number.
> 
>>> 
>>> What is the timeline?
>>> 
>>> Does the project complete at the end of the summer, no matter what? Is it 
>>> something that is worth another year or two of effort?
>> 
>> I’ve got something now, but I don’t mind revving it to improve it, as long as the budget doesn’t change a lot. A lot of suggestions so far have centered around improvements that could be made regardless of budget. Like I’ve said, I don’t want to try to compete with Trimble.
> 
> I would suggest that you figure on quite a bit of your spare time going into this.

That never happens. :)

> 
>> 
>>> 
>>> What is your background? 
>> 
>> Mostly software, but in the last few years I’ve become reconnected to my nascent hardware side. I’ve been selling stuff in my Tindie store for a while now - a fairly eclectic mix of different projects that interest me. What led me to this project was another one - my Crazy Clock. I discovered a rather embarrassing design error that was causing errors on the order of dozens of ppm (I expected under ten). The first step in coming to grips with that issue was determining its scope, and that meant an extremely accurate low frequency counter, and that led me to needing a frequency standard. But I don’t have any way to test something I buy off eBay, so I wanted a GPSDO. But I couldn’t find any that weren’t way out of budget, so I set out to design one. And because I figured I wasn’t the only maker that needed something like this, but didn’t have the need or budget for something 2 orders of magnitude better, I thought I’d try this.
>> 
>> Now where I am is trying to determine if I am correct in my assertions, and if my cost-benefit analysis of this as a product makes sense or not.
> 
> 
> That’s what the market will tell you ….
> 
>> 
>>> 
>>> Does all of the stuff we’ve been tossing around make perfect sense? (= you do
>>> something like this for a living). Are we talking about a bunch of stuff that makes 
>>> very little sense? (= you are just getting started at this sort of thing). 
>> 
>> No, it’s all perfectly sensible.
> 
> Ok, so here’s the next layer:
> 
> If you are selling a frequency standard, most of your customers will probably be
> interested in it’s frequency stability. Unfortunately this opens a major can of worms
> spec wise. There are a number of measures that characterize various types of
> noise. None of them directly address “if I measure 1.23 x10^-10 is it correct”. You
> see numbers like “< 1x10^-10 99% of the time” in spec sheets. 
> 
> If you dig into the archives, you can find numbers posts from people who are 
> disappointed that their "1x10^-13 GPSDO” can not deliver a 1x10^-10 frequency 
> measurement 99% of the time.

Ok, well, with the advice I’ve gotten so far here I’ve decided I am going to see if I can afford a TimePod and I’m going to try and get one of those FE-5680A rubidium standards off eBay. If that works out, then my perception (correct me if I’m wrong), is that that will be enough to characterize 10^-9 levels of expected performance.

I’ve also mailed a unit to a kind volunteer here who said he’d take a look at it. If his opinion differs wildly from what I think is the current state of affairs, then I can stop and take a serious look at whether a new design would be able to better reach the same goals with the same pricing.

> 
> 
>> 
>>> 
>>> Each of these twists and turns heads you off into a different set of further issues and 
>>> likely some more questions. For a commercial venture, buying custom oscillators in 
>>> bulk is a very normal thing to do. For a battery powered balloon carried reference, you
>>> do things different than for a rack mount standard. Each of these projects people come up
>>> with have its own unique drivers. 
>>> 
>>> Each of us in our replies, tries to guess what your constraints are or are not. In doing 
>>> so we likely substitute our constraints for yours. The further our constraints  diverge from 
>>> your constraints, the further off base our advice and answers will be.
>> 
>> I appreciate that. I came here with a narrow question in mind, but perhaps it wasn’t the correct one.
> 
> There is only so much you can pack in a single message …
> 
> Bob
> 
>> 
>>> 
>>> Bob
>>> 
>>> 
>>> 
>>>> On Aug 16, 2015, at 3:39 PM, Nick Sayer via time-nuts <time-nuts at febo.com> wrote:
>>>> 
>>>> 
>>>>> On Aug 16, 2015, at 12:31 PM, Tom Van Baak <tvb at LeapSecond.com> wrote:
>>>>> 
>>>>> Anyway, to answer your question -- to measure its true performance you only need two things. 1) a phase meter (or time interval counter) that's good to 1 ns or better, and 2) a local reference standard that's maybe 10x better than the TCXO and the Adafruit GPS. Usually that means a cesium standard, or supremely qualified GPSDO, or equivalent.
>>>> 
>>>> I have a frequency counter, but it’s not a phase meter. I have a scope, but I assume that trying to use a ruler with scope traces isn’t the textbook way of doing that. :D
>>>> 
>>>> I have considered in the past buying a used rubidium standard off eBay, but have hesitated because I don’t know how much life there is left in the tube, and I just have to take it on faith that it’s stable and accurate. I have somewhat more faith in the GPS PPS, but clearly that has limits.
>>>> 
>>>>> 
>>>>> A number of us here on the time-nuts list have such equipment at home. And unlike professional labs, we will do it for free/fun if you loan the GPSDO for a week.
>>>> 
>>>> I will happily *give* one to someone if they would be willing to help a relative newbie with this stuff.
>>>> 
>>>> Just one though. They’re kind of expensive to build. :D
>>>> 
>>>> _______________________________________________
>>>> time-nuts mailing list -- time-nuts at febo.com
>>>> To unsubscribe, go to https://www.febo.com/cgi-bin/mailman/listinfo/time-nuts
>>>> and follow the instructions there.
>>> 
>> 
> 