Sorry in advance for the "ugly" formatting. Not used to posting via this format...
I had a single BG7TBL Symmetricom Inside GPSDO as a instrument time base. I wanted the ability to test it for accuracy and recently purchased a BG7TBL 2019-03-25 GPSDO for comparison. Initial tests showed the 2019-03-25 unit was about 900uHz low. To confirm accuracy (looks like the song “Does Anyone Really Know What Time It Is” is true), I borrowed a Rhode & Schwarz ED170MP GPSDO and a friend’s (Doug) BG7TBL Symmetricom Inside GPSDO. This post summarizes the results and I have come to the conclusion that the 2019-03-25 unit is not as accurate as the Symmetricom Inside units with a difference of 1E-2 to 1E-3. I don’t profess to be an expert in this area so please let me know your thoughts.
Test Setup
All GPSDOs were powered for at least one week before conducting any tests. All tests run between 24 and 48 hours. The Rohde & Schwarz has its own down converting antenna. The three BG7TBL GPSDOs share a common antenna (Symmetricom 58532A) via a 4 port GPS distribution amplifier (Symmetricom 58536A). Since all 3 share the same antenna, the survey data should be extremely close with respect to position.
Fluke PM6690 Setup
- Time base reference: Rohde & Schwarz ED170MP w/20dB attenuator
- Measurement Time: 10 seconds
- Device under test: 1 of 3 BG7TBLs GPSDO
-BG7TBL GPSDO Date code 2019-03-25 (internal board date code of 2019-01-07 just like the pictures Involute posted)
Photos of BG7TBL 2019-03-25 (1/2)
-BG7TBL GPSDO “Symmetricom Inside” with integral display (Ron’s)
-BG7TBL GPSDO “Symmetricom Inside” (Ron’s)
Results Summary:
Before drilling down into the results of the it appears as though the newest BG7TBL GPSDO (2019-03-25) is only good to 1E-11. In contrast, the earlier “Symmetricom Inside” units which are good to 1E-13 to 1E-14. The presumption here is the Rhode & Schwarz is an accurate reference.
- BG7TBL GPSDO Date code 2019-03-25:
10MHz output: Consistently about 900uHz low, 2 samples provided
1PPS output: Terrible short term (1 minute) stability per the attached scope capture (persistence set to infinite). 2.5 divisions at 20ns per division for a 50ns jitter over a 1-minute interval.
Survey data: Since unit outputs NEMA sentences via the DB9 connector, an analysis of the “$GPGGA” records over about 1000 seconds results in a wide range of positions. This summary and all GPGGA records are in the attached spreadsheet.
Latitude Longitude Altitude
Average 4308.83108 7610.25686 125.8
Min 4308.83043 7610.25545 122.6
Max 4308.83189 7610.25804 131.0
StdDev 0.000430624 0.000811474 1.885299917
- BG7TBL GPSDO “Symmetricom Inside” with integral display (Ron’s)
10MHz output: 14uHz high in the sample provided and typical of other tests
1PPS output: Minimal short term (1 minute) stability with about 1/4 division or 5ns jitter over a 1 minute interval.
Survey data: LAT(N 43:08:49.884) LON(W 76:10:15.426) H(124.50 m MSL)
- BG7TBL GPSDO “Symmetricom Inside” (Doug’s’s)
10MHz output: 1uHz high in the sample provided.
1PPS output: Minimal short term (1 minute) stability with about 1/4 division or 5ns jitter over a 1 minute interval.
Survey data: LAT(N 43:08:49.848) LON(W 76:10:15.384) H(126.00 m MSL)
Ron
WB2WGH