[time-nuts] Some observations of the BG7T BL GPSDO

John Miles john at miles.io
Sun Mar 22 17:54:45 EDT 2015

> Fashinating error. Any details anywhere about how these are built?
> A quick search came up with nothing.

Some of them have been sold on eBay (e.g., item 251802969291) and at http://bg7tbl.taobao.com .
> Is this a FLL design?
> There seems to be a somewhat caotic beat pattern in there, look how the
> phase has relatively flat parts where it seems to be on-track, which
> naturally is when it has it's high frequency periods near the 0 mark.
> Links to the build?

Unfortunately this is all I know about the unit, as it was loaned to me.  There seems to be a subtle 4-6 hour periodicity present, if you load the file and look at the frequency difference plot with a 10K-second averaging time.  If it's real, perhaps it corresponds to interaction between the disciplining time constant and HVAC behavior.... but the effect is so broad that it barely appears as a ripple in the ADEV trace.

Considering the price, the unit seems to offer a good GPS receiver and a very stable DOCXO, assuming the seller is hand-picking their MV89As to reject the bad ones that have been seen on the surplus market.  (That may not be the case, as Bob told me he had to exchange his first unit because it arrived with a bad oscillator.)  It could probably achieve extremely high performance with some optimization and/or bug fixes.  One of those cases where you'd really like to be able to tweak the source code.

-- john, KE5FX
Miles Design LLC

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