Products > Test Equipment
Unknown GPSDO but BG7TBL logo on the circuit board and OCXO !
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Leo Bodnar:
Somebody was badgering me about comparing our mini GPS clock phase noise to "BG7TBL NEW PLL-GPSDO GPS Tame Disciplined Watch Sine GPS Receiver 10M" so I got one to test off eBay https://www.ebay.co.uk/itm/123519078474

Disclaimer: I have not done any tweaking or setting it up.  I just took it out of the box, used its supplied PSU, connected the antenna and let it sit for a few hours after ALM LED went off.

Cheers
Leo
jpb:
I like the bottles of wine in the lab - essential equipment! :)
Leo Bodnar:
Here is BG7TBL phase noise  plot taken on a different system (Agilent E5500) I use at home.
It's pretty much identical apart from 900Hz spur is absent this time - it might be an MCU correction loop artefact that comes and goes.
This measurement system has much flatter response between 60kHz and 100kHz - HP3048A has a few dB fall-off there.
Leo
jeffsf:

--- Quote from: Electroalice on October 15, 2018, 02:01:17 am ---The 10 MHz is good but the 1 PPS has a constant jitter approx. 20ns. Every 1 second it shifts 3  ns then on the 6th second jumps back and repeats continuously.

--- End quote ---

Good to know about the behavior of the PPS output. I'm not sure I'd call it "jitter" as it sounds like it's deterministic in behavior.

Might be similar to, for example, the Trimble Resolution T timing GPS that uses a 12.504 MHz internal clock and the PPS output is always coincident with an edge of that clock. See page 34-35 (36-37 of the PDF) of Resolution T GPS Embedded Board.

Edit: Checking https://www.u-blox.com/sites/default/files/products/documents/NEO-6_DataSheet_(GPS.G6-HW-09005).pdf shows a 21 ns granularity in the "timepulse" signal of that unit, used in several of the BG7TBL units, from what I've read in the "master thread" (linked above).
JBooth:

--- Quote from: Electroalice on October 15, 2018, 02:01:17 am ---Hi Purchase one of these units dated 2018-06-05. Compared the output to an XLI GPS. The 10 MHz is good but the 1 PPS has a constant jitter approx. 20ns. Every 1 second it shifts 3  ns then on the 6th second jumps back and repeats continuously.
Is this common to all units or is mine faulty?

--- End quote ---
The 10 MHz output on mine is really ugly.  Not clean at all.  There is jitter on the signal when the GPS is locked and the alarm light is off.  When the GPS lock is lost and the alarm light comes on, the frequency jumps down much as 10 KHz within seconds and drifts erratically around!  I'm wondering if the internal oscillator is bad since the 10 MHz output is so noisy when locked to GPS, and so far off frequency as soon as the GPS signal is lost.

Here are some screen captures from my oscilloscope to show you guys what I'm talking about.  Unfortunately I can't show you the jittering motion in a still picture, but you can sort of tell that it is definitely not a stable sine wave.  The last 2 pictures help show the amplitude of the low frequency that is modulating the 10 MHz output signal:

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