[time-nuts] TCVCXO Adjustment

Bob kb8tq kb8tq at n1k.org
Sat Apr 14 09:37:29 EDT 2018


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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