March 20, 2015
The plots below depict a 3-day simultaneous comparison between a BG7TBL GPSDO, a modified Trimble Thunderbolt GPSDO, and an HP 5071A cesium-beam primary standard. You can click on any of the images to enlarge them.
The corresponding .TIM files may be downloaded here (60 MB). This test was performed with a relatively poorly-situated GPS antenna; consequently a few hours of low-quality data acquired during a holdover period have been removed for clarity.
The BG7TBL GPSDO is equipped with a surplus Morion MV-89A DOCXO, capable of superb performance on its own. As a result, the designer has chosen to use a very long loop time constant -- several thousand seconds at least. It's quite a bit more stable at intervals between t=1s and t=10s than the HP 5071A that served as a master reference. (Like the BG7TBL unit, the Thunderbolt GPSDO also uses a high-quality OCXO with a long time constant.)
The noise level in the two frequency-difference traces representing the BG7TBL unit is significantly higher
than the trace corresponding to the 5071A and Thunderbolt. This is largely as expected, given the three units' relative ADEV performance between t=1000s and t=10000s.
March 8, 2016
BG7TBL has fixed the "math issue" observed above, according to an email I've received from Kevin, N9IAA. Customers with older units may wish to contact the factory for information on possible firmware updates.
May 28, 2016
BG7TBL has not yet fixed the "math issue" observed above, according to a post on EEVBlog. :-P
Dec 9, 2018
Some additional GPSDO comparison tests run by Mark Sims can be found here.
Feb 15, 2020
A detailed, up-to-date comparison of several u-blox modules by TAPR's John Ackermann: Timing and Location Performance of Recent u-blox GNSS Receiver Modules (3 MB .PDF).