10MHz GPSDO by "BH3SAP"

[Jdw1953]

I’m continuing to review comments about this specific GPSDO on eBay reviews as well as here. Some are saying that the OFFSET AND PWMSET values that you save via menu are only used as initial values whenever doing your next GPS calibration. A completed calibration is active only as long as GPSDO remains powered up (and locked by saving it in menu at that time) the next power-up however you first need to do another GPS calibration before use (but happens faster this time because you have the previous values for the calibration procedure to start with).

Some people (like myself) have tested this GPSDO with Oscilloscope/Frequency counter and not seen any change in output by just saving the offset (say like 80 Hz for example) but no way it influences measured output but the GPS calibration procedure takes longer to complete when starting with a large saved offset.
These crystal oscillator circuits change some with age, temperature, etc. so makes sense that whatever is learned during calibration is trusted by whatever application that is needing a near perfect 10 MHz at that point in time. Just do not power it down until you are finished using it.

These are just my observations and thinking so far.

[flash2b]

I have tested the accuracy (see some posts above) against a Miller GPSDO and it was within 0.001Hz.

I have also tested the OFFSET function and that changed the output accordingly (see some posts above). I used -0.3Hz. There is however a bug for negative offsets stored, and I do not really use it since I want 10MHz output exactly. I believe the OFFSET should be saved -after- PWMSET is saved. There is no mention of OFFSET in the manual whatsoever.

The PWMSET value after correctly saving, so dial to PWMSET, push the knob, dial to EXIT, push the knob, works and the value is stored. This is however for use after power up without the antenna, so as a "normal" XO. Then this value is used a reference. Once you reconnect the antenna, the GPSDO starts to align the PWMSET again. This is described in the manual (which I posted above).

Above all applicable to the stock firmware.

[Gertjan]

I've now also added access to the GPS module via two pins on the bluepill. The module (ATGM336H version) can be configured via GnssToolkit and it seems like you can configure it for stationary usage.....

Do you think it is possible to get NMEA data out of this GPSDO?
It would be a big improvement to be able to monitor reception (number of satellites, signal strength) etcetera. That would make one more confident in the quality and reliability of it's output signals...

That has been done, but you need to solder wires onto two of the pins of the MCU board (PA2 (TX) and PA3 (RX), 9600 baud). Unfortunately there is no easy way to use the USB port on the board for that. My firmware also shows the number of satellites on the display.

Excellent news that serial communication is available on pins of the MCU board!
Indeed a pity that the standard USB port can not be used. But this is easily overcome with a small Serial to USB board wired to those RX/TX pins.

Off coarse NMEA is not the only way to monitor a GPSDO. Setup and monitoring software of the GPS module manufacturer would probably work too. But still you need access tot the data stream out of the GPS module...

Do you think the signal on the RX/TX pins on the MCU board you mentioned are a direct copy of the serial data out of the GPS module? (Or are these pins simply the MCU pins where the serial port of the GPS module is connected to?  )

regards, Gertjan.

[dankar]

Excellent news that serial communication is available on pins of the MCU board!
Indeed a pity that the standard USB port can not be used. But this is easily overcome with a small Serial to USB board wired to those RX/TX pins.

Off coarse NMEA is not the only way to monitor a GPSDO. Setup and monitoring software of the GPS module manufacturer would probably work too. But still you need access tot the data stream out of the GPS module...

Do you think the signal on the RX/TX pins on the MCU board you mentioned are a direct copy of the serial data out of the GPS module? (Or are these pins simply the MCU pins where the serial port of the GPS module is connected to?  )

regards, Gertjan.

The RX/TX pins is specific to my alternative firmware, and it's a direct copy of the communication with the GPS module. So anything sent will be sent directly to the module and anything out of the module will be sent directly back. You can probably attach directly to the GPS module as well, or at the pins where it's connected to the MCU (should work with the original firmware), but I felt it was safer to "relay" the communication via the MCU.

Using this it's possible to connect with GnssToolkit3 (for the atgm336h module) or u-center (for any u-blox) and test/configure the module.