[time-nuts] Chinese GPSDO 10 MHz error

Bob Camp kb8tq at n1k.org
Thu Aug 27 19:19:08 EDT 2015


Hi

> On Aug 26, 2015, at 11:54 PM, Bob Benward <rbenward at verizon.net> wrote:
> 
> So how does a frequency lock work?  How is it implemented? Can someone sketch a schematic?
> 
> And what equipment or technique is used to measure a 2hz error at 100GHz?

As with all things, this is a “that depends" sort of thing. You can measure it with a (accurate)
wall clock if you wait for 10^11 seconds (that’s quite a while). If you have a counter than 
can measure 1x10^-9 s (1 ns) then you can get to 1x10^-11 by watching a pps for about a 
hundred seconds. That’s not to bad to do. If you want to do it quickly, there are commercial
test boxes (the TimePod is one of many) that will measure much better than this at 1 second. 
If you want do it in the basement, a DMTD is a cheap way to do it. 

In all of these cases, you need something “better than” the device you are trying to measure to 
use as a comparison standard. Put another way - your counter needs to be calibrated to better than 1 ppm 
to measure 1 ppm. In the case of the GPSDO measurement, checking against a Cs standard or a 
Hydrogen Maser would work. You might also use a “known good” GPSDO. 

The comparison may be between 1 pps outputs or between 10 MHz outputs. In both cases the data 
should be the same. Your choice of 1 pps or 10 MHz needs to match up between your measuring 
gear, your DUT, and your reference standard. 

Lots of choices, lots of ways to do it. Many pieces of gear you could use to get the job done. 

Bob


> 
> Bob
> 
>>>> -----Original Message-----
>>>> From: time-nuts [mailto:time-nuts-bounces at febo.com] On Behalf Of Tim
>>>> Shoppa
>>>> Sent: Wednesday, August 26, 2015 5:18 PM
>>>> To: Discussion of precise time and frequency measurement
>>>> Subject: Re: [time-nuts] Chinese GPSDO 10 MHz error
>>>> 
>>>> Full KE5FX evaluation of BG7TBL GPSDO here:
>>>> http://www.ke5fx.com/gpscomp.htm
>>>> 
>>>> I'm wondering out loud if it might, like many hobbyist GPSDO's, be
>>>> frequency-locked rather than phase-locked and thus susceptible to last-digit-
>>>> counter bobble in some long-averaging counter.
>>>> 
>>>> Tim N3QE
>>>> 
>>>> On Wed, Aug 26, 2015 at 3:18 PM, Perry Sandeen via time-nuts < time-
>>>> nuts at febo.com> wrote:
>>>> 
>>>>> Hi,
>>>>> 
>>>>> 
>>>>> 
>>>>> On the EEVBLOG  (http://www.eevblog.com/forum/index.php)
>>>>> 
>>>>> 
>>>>> 
>>>>> They mention that the "2014-11-06"version GPSDO that was “most
>>>>> extensively tested, so far (by ke5x and others).
>>>>> 
>>>>> 
>>>>> 
>>>>> (Has a) known bug, outputfrequency is not exactly 10mhz
>>>>> (9,999,999.999,800 Hz). This translates to ~2hzerror at 100ghz.”
>>>>> 
>>>>> 
>>>>> 
>>>>> A question is if this bug isjust for this particular model or all
>>>>> other versions suspect?
>>>>> 
>>>>> 
>>>>> 
>>>>> I realize that in and of itsself it is very small error, but errors
>>>>> tend to multiply or cause incorrectconclusions to testing.
>>>>> 
>>>>> 
>>>>> 
>>>>> Another question is will the LHdisplay unit they offer work with other
>>>>> Trimble units such as are offered byRDR?
>>>>> 
>>>>> 
>>>>> 
>>>>> That said, these models seem tobe a very nice turn-key systems.
>>>>> 
>>>>> 
>>>>> 
>>>>> Regards,
>>>>> 
>>>>> 
>>>>> 
>>>>> Perrier
>>>>> 
>>>>> _______________________________________________
>>>>> time-nuts mailing list -- time-nuts at febo.com To unsubscribe, go to
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>>>>> and follow the instructions there.
>>>>> 
>>>> _______________________________________________
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