[time-nuts] Which First GPSDO to buy?
Bob Camp
kb8tq at n1k.org
Sun Dec 14 20:48:11 UTC 2014
Hi
Ok so down to two choices:
LTE Lite:
1) Comes up nice and fast (it’s a TCXO)
2) Modern GPS receiver
3) Good documentation
4) Very low power
5) Nice small size
6) Needs a box
7) You know where to find Jackson Labs if there is a problem
8) Getting a couple more in a couple years may be possible
Lucent:
1) More accurate if you keep it on (it’s an OCXO)
2) Spare parts set (= the second box) when you buy the pair
3) Comes with a (clunky) enclosure
4) Outputs are already isolated / buffered
5) Needs an antenna ($30 or so for a good one, $3 for a simple one)
6) Needs a power supply ($20 maybe less)
7) Hook it up and check it out fast, there’s a 30 day warranty (which the guy does honor)
8) Once this guy sells out, there may not be many more.
Both:
1) You still need to mount the antenna somewhere
2) You need to distribute the 10 MHz or 1 pps to your gear
3) With only one you will always be wondering “what if it’s wrong?”. Having two only confuses this situation ….
4) Neither one lets you play much with the loop (filtering), both are pretty much optimum for the hardware as received
5) Price wise not a lot of difference. Both will be $200-ish once you get them delivered and set up without the distribution stuff.
What to do - get some of each :)
Bob
> On Dec 14, 2014, at 12:49 PM, Dave Daniel <kc0wjn at gmail.com> wrote:
>
> Well, thanks, everyone, for the information. I appreciate the help.
>
> First, I am presently not up to adding another project to my long list of projects. I get whiplash every time I walk into the lab. Building a GPSDO sound like fun. Perhaps down the line.
>
> I figured I should add some information about myself: I am an electrical engineer (currently employed) with a lot of digital/Verilog experience and a fair bit of analog experience (but less than my digital experience) and quite a bit of software experience, all of this from working for about thirty-eight years on various embedded systems. Currently, I shy away from writing code just because I don't enjoy it much and have done too much professionally. But I know that eventually I will need to write code in my lab. Presently, I am in the process of restoring some older ham radio gear, but I became sidetracked from that by the necessity to repair a bunch of vintage test equipment which effort has somehow taken on a life of it's own.
>
> What I need right now is a frequency standard that is accurate enough to use as a reference for things like calibrating test gear. I also want to "play" with one before I build one. Just going through all of the educational material is a daunting task. I figured I'd combine an interest with GPSDOs in general with a need for an accurate enough "standard" (I use the term loosely here) to get some instruments calibrated.
>
> Thanks again for all the information!
>
> Cheers,
> DaveD
>
> I had forgotten about the LTE-lite; I should add that to the list of choices. I'm tending towards either a 10 MHz version of that or the Lucent boxes.
> On 12/14/2014 8:00 AM, Bob Camp wrote:
>> Hi
>>
>>> On Dec 14, 2014, at 12:47 AM, Chris Albertson <albertson.chris at gmail.com> wrote:
>>>
>>> I tried to see just how simple, low cost and self contained I could make a
>>> GPSDO. I started with the Lars Walenius design then removed everything I
>>> could from it. I replace all the software with just a small loop with
>>> about a dozen lines of code so it would be easy to understand.
>>>
>>> My goal was to make something that could be built and tested using just
>>> basic equipment. The question is of course "How do you know the unit is
>>> making a 10 MHz signal if you don't already have a 10MHz reference to
>>> compare it to?" Well you can assume that your 1PPS reference is accurate.
>> Except that the GPS PPS is *not* perfect, far from it. It’s only reasonably accurate over very long time spans. Over short spans the pps moves around a lot.
>>
>>> Then you count and make sure you see EXACTLY 10,000,000 oscillator cycles
>>> per each PPS.
>> If you do a tight lock (“EXACTLY”) against a GPS PPS that is moving +/- 10 ns, your frequency will swing +/- 1x10^-8 every second
>>
>>> Count both for a few days and verify the ratio remains at
>>> ten million to one, exactly.
>> Ok, that’s looking at the long term where GPS is indeed accurate. That’s the easy part on any GPSDO design.
>>
>>> I ran mine for about 8 weeks and it stays at
>>> the desired ratio. I know this is not a perfect test because it could
>>> have been running at zero hertz for 30 seconds and then 20MHz for 30
>>> seconds but I assume the OCXO is better than that. The point is that once
>>> you have the GPS working you DO have a pretty good 1Hz reference.
>> Well, not quite so fast. You just jumped over a massive amount of work that normally gets done on a GPS. A unit that *was* swinging +/- 1x10^-8 every second would pass your test. (which is not in any way to say that your design actually does that). It would make a lousy GPSDO for most uses. You very much *do* need to check the ADEV (or what ever) close in and tune your filter up to match your parts.
>>
>>> Cost:
>>> Motorola Oncore GPS $18
>>> magnnetic patch antenna 6
>>> OCXO (eBay) 19
>>> Arduino, mini 3
>>> PLL chip 2
>>> TTL diver chip 1
>>> Plug-in power cube 0
>>> perf-board 1
>>>
>>> Total cost of GPSDO $50
>> Just a side note - A *lot* of the $19 OCXO’s I have from eBay are in very poor shape spec wise. Testing them before using them would be a very good idea.
>>
>> Bob
>>
>>
>>> Actually I do have A Thunderbolt. I place the 10MHz output of the above
>>> unit and the TB on my dual channel scope and was able to see the phase of
>>> the two 10MHz references was locked. I saw the phase drift over about an
>>> hour but then it would pull back. But I made this very simple and it
>>> could be better.
>>>
>>> Actually I've added some features to it like a 2 line by 16 character LCD
>>> display and some status LEDs. And I can log data to a computer via a USB
>>> cable so it is easy to plot data and it is using my more expansive mast
>>> mounted timing antenna.
>>>
>>> The Arduino based design is OK for controlling an OCXO but I think it is
>>> best used for controlling my Rubidium oscillator. The RB is so stable I
>>> should only update the frequency control every few hours at most.
>>>
>>> On Sat, Dec 13, 2014 at 7:21 PM, Jim Harman <j99harman at gmail.com> wrote:
>>>> On Sat, Dec 13, 2014 at 9:36 PM, Bob Camp <kb8tq at n1k.org> wrote:
>>>>
>>>>> The problem with “build it yourself” is that there is no way do know if
>>>>> you got it right unless you have something to compare your design to. You
>>>>> *will* make mistakes as you build one of these….
>>>>
>>>> I think you will have the same problem with an off-the-shelf unit if you
>>>> don't have at least one reference for comparison. However speaking from
>>>> experience with Lars Walenius' Arduino-based design, I can say that it is
>>>> not hard to make a working system, even without another reference. Along
>>>> the way you will learn a tremendous amount about how these systems work,
>>>> plus a lot about Arduino programming.
>>>>
>>>> Lars' design will run stand-alone, but if you want it can send very useful
>>>> logging data to a PC, much more informative than a "locked" led on a
>>>> commercial unit.
>>>>
>>>> Total cost including processor, Adafruit GPS shield, and $25.00 ebay OCXO
>>>> is about $100.00
>>>>
>>>>
>>>> --
>>>>
>>>> --Jim Harman
>>>> _______________________________________________
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>>>>
>>>
>>> --
>>>
>>> Chris Albertson
>>> Redondo Beach, California
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