[time-nuts] TCVCXO Adjustment
Bob kb8tq
kb8tq at n1k.org
Sat Apr 14 13:26:26 EDT 2018
Hi
> On Apr 14, 2018, at 11:10 AM, Adrian Godwin <artgodwin at gmail.com> wrote:
>
> What if you iterated toward a suitable minimum-error setting, then looked
> for cyclic corrections with a period of weeks to months. Once you start to
> see that, choose the centre of the cycle and track it (or perhaps just
> increase the time constant).
If you only get one data point per month, then it will be a pretty basic plot.
Even with a data point a week, it will be pretty sparse.
We get very used to “data everywhere” sorts of situations. With most of the
proposed approaches *if* tight set is desired the data will be pretty sparse.
It’s still a bit unclear what the target accuracy is (or even if the TCXO needs
setting at all …).
Bob
>
> On Sat, Apr 14, 2018 at 2:37 PM, Bob kb8tq <kb8tq at n1k.org> wrote:
>
>> Hi
>>
>> The gotcha is that you do not have a calibrated adjustment. Put another
>> way,
>> there isn’t a perfect correlation between DAC bits and ppm. Each
>> adjustment
>> you make is subject to a bit of error. When you are trying to get within a
>> ppm,
>> your measurements are quicker, so the larger error ( percentage of step)
>> may
>> not be as big a deal. When you get close, it is likely to become a big
>> deal.
>>
>> You could track all of your changes (month to month). The issue there is
>> that the
>> drift in the TCXO month to month is not likely to be the same. Sorting all
>> of that out
>> could be a bit nasty …..
>>
>> ====
>>
>> TCXO drift is not the only contributor to the accuracy of a time code
>> generator.
>> The other obvious one is setting it to the correct time in the first
>> place. If the
>> objective is to compare data from different locations, getting it set may
>> be as
>> big an issue as the TCXO. If it’s a single location and the time is
>> arbitrary, then
>> maybe not so big a deal. If it’s all arbitrary … why worry about drift? ….
>>
>> GPS on the board looks like a good thing to have to me ….
>>
>> Bob
>>
>>
>>> On Apr 14, 2018, at 6:35 AM, Adrian Godwin <artgodwin at gmail.com> wrote:
>>>
>>> If you compare VCXO time with UTC or GPS once a month to an accuracy of
>> 1s
>>> (with NMEA or even a time signal and manual pushbutton) and make a
>>> correction for the 2.5 million seconds that occurred since the last
>>> correction, you'll be better than 0.5 ppm.
>>>
>>> Is that good enough ?
>>>
>>> On Sat, Apr 14, 2018 at 5:59 AM, Hal Murray <hmurray at megapathdsl.net>
>> wrote:
>>>
>>>>
>>>> kb8tq at n1k.org said:
>>>>> The alternative is to plug a USB GPS into the mac and do a bit of code
>> to
>>>>> compare things. If you want to pass the gizmo around to your friends ….
>>>> that
>>>>> can be done. Pretty good ones are “sub $10” delivered.
>>>>
>>>> USB GPS gizmos generally don't have a PPS and the timing on the NMEA
>>>> sentences is generally crappy. (I may be biased by a few bad examples.)
>>>>
>>>> Does anybody have a list of ones known to work well?
>>>>
>>>> There is at least one GPS-USB with PPS, the Navisys
>>>> GR-601W/GR-701W/GR-801W,
>>>> They were hard to get retail. Looks like idealez sells them Taiwan for
>>>> $47
>>>> for the 701W (Ublox 7), $50 for the 801W (Ublox 8) (plus shipping). I
>>>> haven't tried ordering through them. Gary and/or Mark may have some for
>>>> sale.
>>>>
>>>> ----------
>>>>
>>>> Plan B is a GPS breakout and a USB-serial breakout and a few wires. 4x
>>>> less
>>>> USB jitter if your USB-Serial chip is full-speed.
>>>>
>>>> I got mine from SparkFun:
>>>> Venus GPS with SMA Connector
>>>> https://www.sparkfun.com/products/11058
>>>> USB to Serial Breakout - FT232RL
>>>> https://www.sparkfun.com/products/12731
>>>>
>>>> ----------
>>>>
>>>> Google found this. I don't know anything more:
>>>> https://www.zti-communications.com/z050-gps-dongle/
>>>>
>>>>
>>>> --
>>>> These are my opinions. I hate spam.
>>>>
>>>>
>>>>
>>>>
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