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Unknown GPSDO but BG7TBL logo on the circuit board and OCXO !
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Noy:
There comes no signal except the uart from the GPS Module.

Could it be that the pin they use:
"XCK/T0 – Port D, Bit 4
XCK, USART external clock.
T0, Timer/Counter0 counter source."

Counter/Timer or are they using the NMEA Stream on the UART to measure the phase drift? I'm not sure how often / time synchronized the NMEA message is sent from the module. But the Pin which they are using with the 1kHz input can be used as USART Clk? Any dirty hacks possible with this relation?

UART NMEA Stream <-> OCXO 1kHz?

NMEA Updaterate / 1kHz Clk in? or something?

Maybe the Atmel isn't checking NMEA messages and only the time / sync from package arrival and 1kHz Clk?
rfclown:
I got a second one of these from the same eBay seller (haynswor12). It is the same as the first, with an Altera inside. One of the fails on this design is the serial output connector. It is a stereo jack but puts the RX signal on both the tip and center connector. If you plug in a mono plug, the RX shorts to ground. You have to put the mono plug in only part way for it to work. Also the output is inverted. I decided to fix these two deficiencies.

First try at fixing the jack was cuting the PCB trace between the tip and center on the top side of the jack, and lifting the bottom side center tab off the pad (d_1.jpg). Turns out that didn't work. There must also be a trace between the two center pads underneath the jack. So I also lifted the top center pad (d_2.jpg).

Then I put a second inverter on the RX output line. Q1 is the first interter that was on the board. The GPS RX signal comes to the base through R3. The collector is pulled high to 5V, and the output goes to a via to the top side of the board. I cut the trace between the collector and the via. I scraped a ground pad, and put a NPN  (3904) with emitter on the ground pad, collector on the via. I put a 5.1k from collector to 5V. Connected the base to the output of the first inverter (collector of Q1) though a 27k.

Antenna that came with this one looks identical to the first, but it isn't magnetic (first one was). This one had double sided tape pads on it. Says 3-5V. The unit puts 5V on the antenna.

Does anyone know what battery goes on these? Both mine have that missing on the board. Labeled BAT1 on top side.
Noy:
Its inverted to be "RS232" instead of UART
TurboTom:

--- Quote from: Noy on January 28, 2021, 07:31:02 am ---Its inverted to be "RS232" instead of UART

--- End quote ---

+1
Works straight with an off-the-shelf (DB9F) RS232->USB converter (at least with the "original" units).
I'm surprised the manufacturer meanwhile already uses badly dinged/dented oscillators. The early ones had been sold with oscillator modules in decent shape, though the labels had been removed. But installing a module with a can like this, even though you may obseve some "jerks" from the output  ;), tells stories about the standards of this particular manufacturer / reseller.

Take care where you buy this GPSDO -- I think original BG7TBL units are the better choice.
Noy:
Especially the latest 2020 Version where "no Software" is requiered to get the gpsdo working. Huge benefit if something is broken / you want to modify..
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