[time-nuts] Which First GPSDO to buy?
kc0wjn at gmail.com
Sun Dec 14 16:20:17 EST 2014
Thanks. See below:
On 12/14/2014 1:48 PM, Bob Camp wrote:
> 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
Good so far.
> 6) Needs a box
Boxes I have.
> 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
> 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)
OK, doesn't sound too expensive.
> 6) Needs a power supply ($20 maybe less)
I have a bunch of DC power supplies sitting in the storage closet. Or I
can build one. It's pretty simple.
> 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.
Yes, I have been thinking about that. Maybe get one of these just to do
it before they are gone.
> 1) You still need to mount the antenna somewhere
The lab is one the second floor; It shouldn't be a problem to add it to
the breakout box going outside the room to the exterior.
> 2) You need to distribute the 10 MHz or 1 pps to your gear
Yes, I haven't figured that out yet. But it will be fun.
> 3) With only one you will always be wondering “what if it’s wrong?”. Having two only confuses this situation ….
One really needs three at a minimum. But acquiring three starts with
> 4) Neither one lets you play much with the loop (filtering), both are pretty much optimum for the hardware as received
I think this where building one oneself comes in. Or buying the Thunderbolt.
> 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.
Yes, and that is affordable while I am still working.
> What to do - get some of each :)
Precisely my conclusion. I need to buy one of each. This is precisely
how I ended up with thirteen Tektronix oscilloscopes. It's the same process.
And right in the middle of Christmas gift-buying season.
A very useful summary. Thanks!
>> 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!
>> 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:
>>>> 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.
>>>> 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.
>>>> 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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