[time-nuts] LTE-Lite module

Bob Camp kb8tq at n1k.org
Sun Oct 19 20:40:27 EDT 2014


We seem to have swung from “it’s impossible, don’t even try” to “it’s trivial, you should have it done in a few minutes”  :)  (Yes I know that’s *not* at all what was said in either case. We have swung a ways though)

Yes, I can do it for less than $1 in parts. That’s not to say it’s the *right* way to do it. Yes, I can have it “done" (locked up) in a few hours (from scratch, including the parts). That’s not to say you *should* do it that way. My way most certainly should not be your way. The stuff I have sitting around is not the stuff you have lying around. What I paid may not be what you pay. 

We spend a lot of time playing “I can do it cheaper”. Unless a few months of your time *really* is worth $10, the “cheaper” part simply does not count past some point. The cost of even one meal out over several months will wipe out that advantage. Doing it with a part that is running a 10 bit ADC that really gives you 8 bit performance will indeed impact the result. It’s cheaper, but how much struggle will there be to make it work well? Will it add a month or three to the project? Will you start over from scratch? Who knows. Are we comparing a board anybody can get for < $15 to just the cpu on another board .. maybe we are. If what counts is a price that somebody got once, I have  boards that I got for free. Do they count as $0 in a project? There’s really no value even going down that road. 

Each time this comes up on the list, we typically spend a month with everybody tossing up their favorite board. We each post several messages talking about the great deal we got. We never seem to get around to actually doing much with those cheap boards compared to the time everybody spends extolling their virtues (and ignoring their drawbacks). A $50 board is no different than a $1  board in this case. They both have near zero impact on the total investment in the project. If they did / do - buy a $135 OCXO based GPSDO rather than the $185 LTE board. That puts you $50 and months of your time ahead. 

If you want to start from scratch and get a result that is “OCXO” caliber, it will take a while. 1x10^-10 is not your target. The LTE part pretty much does that. Your target is at least 1x10^-11 short term and much better a you go to a few hundred seconds. In order to say you have hit it, you need to test it and verify that you have hit it. No I can’t do a run that takes a month to verify a part I build in a short time.Nobody can do that, it takes time. No I don’t have a gizmo that’ stable to 1x10^-13 over a month sitting in the basement. If I already had that, why would I need to put an OCXO on a LTE board? I have to do some work simply to do the test (like build several and cross check them). 

How much time does the testing take? You want something around 100 samples for a good ADEV number. You need data out to 1,000 seconds (and more likely 10,000 seconds) to check the loop out. Each run will be in the 1 to 10 days range. Once you have it “right” you really need to check it over a month or two to watch for GPS “once a day” issues. If you have a really good setup, you will get good data 4 runs out of 5. With a basement setup, that may drop to 2 in 5. 

The job is not done once the first one is locked. That’s the quick and easy part. The full job is only done once you have it optimized and know you have done so from measured data. That’s true if you are making one, or making a few hundred thousand of them. 


> On Oct 19, 2014, at 6:14 PM, Chris Albertson <albertson.chris at gmail.com> wrote:
> At the low end of the spectrum, I tried to make the simplest possible
> GPSDO what would still work.  Assuming you have a GPS with  1PPS
> output, an OCXO and a small DC power supply I was able to get the
> entire parts for the controller, board, hookup wire and all for under
> $5.  I purposely took the lowest cost solution at each decision point
> just to see what you'd end up with.   Part were from eBay.
> The result is not bad. but I don't have a really good way to test it.
> I'm using a Thunderbolt for the 1PPS and a pretty decent OXCO part.
> Why build a low-end GPSDO when yo have a Thunderbolt?  It's and
> experiment.   The way I test is to place the sine output from the TB
> and from my GPSDO both on a dual channel scope and adjust it so the
> two sine waves are superimposed.   Then I wait for them not to be
> superimposed.  What I see is that over 1/2 hour or so they get
> slightly out of phase but then drift back in phase,  This happens
> cyclically.   It is because of the VERY simply controller.  I tried to
> minimize lines of C++ code.  It's running about 16 lines of code, more
> or less.      Using my counter I think the GPSDO is good to  1E-10.
> Rather than using a $15 ARM MCU board I used a $3 AVR board and used
> 100% 16-bit integer math in a very simple control loop.  There is one
> external chip because the little AVR could not deal with the 10MHz
> signal from the OCXO so I used a divider chip.  I use two 8-bit DACs
> to control the EFC on the OCXO.  One is curse adjustment, one fine.
> Added with a resister network and an RC filter with almost a 1 second
> time constant.
> If you can spend $35 you can build a very sophisticated controller
> that logs internal diagnostic data to a computer over USB and displays
> it's internal status on a graphic LCD panel.   Well, actually my
> controller has an LCD status display and logs data to a PC.  But with
> those parts plugged in the cost is closer to $10.
> On Sun, Oct 19, 2014 at 2:13 PM, Bob Camp <kb8tq at n1k.org> wrote:
>> Hi
>>> On Oct 19, 2014, at 5:00 PM, Jim Lux <jimlux at earthlink.net> wrote:
>>> On 10/19/14, 1:08 PM, Bob Camp wrote:
>>>> Hi
>>>>> On Oct 19, 2014, at 3:35 PM, Charles Steinmetz
>>>>> <csteinmetz at yandex.com> wrote:
>>>>> Bob wrote (alluding also to something Poul-Henning wrote):
>>>>>> The phase comparison part of the PLL is pretty straightforward if
>>>>>> you are looking at two RF frequencies. An XOR gate is one
>>>>>> solution, there are many others. Getting something like 100 to
>>>>>> 200 ns full scale on the phase comparator makes the rest of the
>>>>>> gizmo much easier.
>>>>> All true.  However...
>>>>>> A 12 bit ADC on a MCU will get you to 100's of ps per bit.  That
>>>>>> is more resolution (it's < 1 ns) than you need for this.
>>>>> Getting an ADC to sample fast and accurately enough to provide that
>>>>> honest resolution is not trivial.  And if you have that, you'll
>>>>> almost certainly have the resources to do the phase comparator
>>>>> digitally, too, which brings many advantages -- so I see no reason
>>>>> to use an analog PC.
>>>> If you take a look at some of the newer ARM MCU’s they are getting
>>>> 13+ solid bits out of their ADC’s at a > 10 KHz rate. That’s more
>>>> than good enough for anything you are trying to do with this design.
>>>> There’s no need to make it any more complex.
>>> I'm using the Freescale Kinetix K20 parts, which have 16 bit differential input ADCs, and built in averaging.  The raw ADC can sample at about 400kHz.
>>> You can easily get 14 bit performance from these at tens of kHz rates.
>>> I need I/Q, so I sample two inputs at 50 kHz (read one, then the other) without averaging (so they're about 2.5 microseconds apart), and then decimate them through a 2 stage CIC and a 13 tap FIR filter down to 200 Hz.  This takes about 60% of the processor running at 48MHz.
>> I’m using parts from the same family, but not doing the whole DDS thing. Single input and control loop - the part sleeps about 98% of the time. The demo boards (Freedom boards) are all below $15 and free if you go to one of their (often free) classes.
>> Bob
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> -- 
> Chris Albertson
> Redondo Beach, California
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