[time-nuts] GPSDO question

[Karl Lautman]

>I just got a BG7TBL.  I am in the process of evaluating it now.  Preliminary result: +/- 10 mHz after calibration 
>(and powered up for one week).  If your RTC is good to 1-1/2 ppm over a 50 degree range it should be better 
>over a 5 degree range indoors.
>
>Andy Backus

Hi, Andy.  What's the date on your front panel?  Mine is 2019-03-25 (see pic here:  http://tinyurl.com/y4cf69k7).  How did you calibrate yours?  I thought the GPS did that.

-----Original Message-----
From: time-nuts <time-nuts-bounces at lists.febo.com> On Behalf Of Andy Backus
Sent: Sunday, June 2, 2019 2:11 PM
To: Discussion of precise time and frequency measurement <time-nuts at lists.febo.com>
Subject: Re: [time-nuts] GPSDO question

I just got a BG7TBL.  I am in the process of evaluating it now.  Preliminary result: +/- 10 mHz after calibration (and powered up for one week).  If your RTC is good to 1-1/2 ppm over a 50 degree range it should be better over a 5 degree range indoors.

Andy Backus

________________________________
From: time-nuts <time-nuts-bounces at lists.febo.com> on behalf of Karl Lautman <karl at lautman.com>
Sent: Sunday, June 2, 2019 10:18 AM
To: time-nuts at lists.febo.com
Subject: [time-nuts] GPSDO question

Time newb here.  I'm designing a digital clock based on a microprocessor and external time source.  I only need about 1 minute/year accuracy, so I settled on the Micro Crystal RV-8803-C7<https://www.mouser.com/ProductDetail/Micro-Crystal/RV-8803-C7-32768kHz-3PPM-TA-QC?qs=7bTaA%2FLYtSY57sitRg9%2Fow%3D%3D>  RTC which contains a TCXO and is spec'd at 1.5 ppm at 0 - 50 C.  I need to be able to verify the timebase accuracy, so I bought a BK Precision 1823A frequency counter.  It's only 9 digits, but I only need 7 for this test.  I have a 200 MHz scope, which is fine for comparing signals, but it's not nearly accurate nor precise enough for this sort of measurement.

The BK's frequency reference is a 1 ppm TCXO, but the counter only provides 8 digits of precision when running on the internal reference; to get the 9th digit you need to use an external reference.  Even though I don't strictly "need" this 9th digit, I became curious about how to access it (I sense this sort of thing is how people become time nuts).  Long story short, I ended up with this<https://www.ebay.com/itm/172933685909> BG7TBL 10 MHz GPSDO.  Measuring its output with the BK, on the BK's internal reference, it reads as 10 MHz, so it's not totally bogus, like, 8 MHz.  It has a couple of LEDs which allegedly indicate it's GPS locked and its accuracy is better than .05 Hz.

1.  I assume that .05 Hz quality spec is embedded in the GPS signal.  Is that correct?  If not, how is it calculated?

2.  Any relatively inexpensive ways to validate the GPSDO's accuracy?  I've ordered a cheap, basic ublox GPS receiver<https://www.banggood.com/Satellite-Positioning-GPS-Module-For-Arduino-51MCU-STM32-p-1203610.html?utm_design=41&utm_source=emarsys&utm_medium=Shipoutinform171129&utm_campaign=trigger-emarsys&utm_content=Winna&sc_src=email_2671705&sc_eh=cc23797f8242d0691&sc_llid=13041562&sc_lid=104858042&sc_uid=vu2Rtn59cL&cur_warehouse=CN> from China with the intension of comparing its output to the GPSDO's on the scope.  I realize the ublox signal might be a little jittery, especially if I use 10 MHz, but it should still be useable for my purposes.  Am I mistaken?

3.  Anything else I should be aware of with this GPSDO?

Thanks.
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