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Budget GPSDO - a work in progress

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MIS42N:

--- Quote from: conrad550 on September 20, 2021, 10:37:02 pm ---Thanks for the info. I don't need a 10MHZ ref. Any frequency will work for my calibration needs. Do the clone
GT-U7 and clone NEO 7/8 units have this capability?

--- End quote ---
I don't know. I know it is talked about in a few other topics on this site, but it wasn't relevant to what I'm doing so I didn't bookmark them. IIRC some clone modules work, most don't retain changes, some don't work at all. You'll have to do your homework.

MIS42N:
A new challenge. The software is basically finished, I have flashed a few units with a PICkit3 and no problems. However it seems PICkit3 is not that common and people have trouble with that final step of getting the software into the PIC. At first I thought, well that's OK I'll just mail out a loaded PIC, no problem. But what if the software changes?

Looking around, a few quite reputable companies (Bosch, Cisco, etc.) used Tera Term and Xmodem to update their software. Could this work for the PIC? Technically it seems so but I couldn't find an assembler implementation so I am writing one. So far, so good - the Xmodem logic has been implemented - the Tera Term Xmodem sends the PIC a file (which in the final version will be the hex file output by MPLAB X) and the PIC receives it. Tera Term thinks it has completed and so does the PIC. Now all that has to be done is decode the hex file (it is well documented), flash the program. Should keep me going for a week or two.

I sent a unit to Silicon Chip magazine to see what they make of it. Also getting some good feedback on a few PCB improvements. The current version is V0.0, maybe there will be a V0.1 if there's enough interest.

de_light:
Quick update. I have received one of the trial kits from @MIS4N which came very quickly and well-packaged. After some struggles borrowing a Pickit programmer, I used an Arduino nano to flash the PIC which worked fine. I have installed it in a plastic Jiffy box available locally, but one of the suggestions I made was whether the board layout could be optimised a little and whether the board could fit in some readily available housing (Australia Jaycar jiffy boxes or an extruded aluminum housing or something).

Total current draw is about 450mA when the OCXO is heating and this drops to about 250mA after a few minutes. I created a serial port outlet with a JST connector and this works well with PuTTY. The LED works well and clearly displays the operation. MIS4N has written a very comprehensive manual that is highly useful. I have an active antenna that has arrived today so hopefully can properly fire the device up.

de_light:
I'm pleased to say that this device works exactly as intended. I had a few hiccups along the way, mainly with supply of recycled NEO-6M devices from Aliexpress (poor soldering/shorted chips, shorted voltage regulators...you name it).

Apart from that, the device works very well. It is sitting, stable, on my desktop. It does lose calibration a few times a day (been running 48 hours) but quickly regains it with the inbuilt algorithm. It also seems sensitive to the activation of other test devices (esp clock-driven) on the bench, so could do with some sort of shielding.

Overall: excellent project. The fact that this is working for the price is mind-boggling. I feel this thread needs some traction.

HB9EVI:

--- Quote from: conrad550 on September 20, 2021, 10:37:02 pm ---Thanks for the info. I don't need a 10MHZ ref. Any frequency will work for my calibration needs. Do the clone
GT-U7 and clone NEO 7/8 units have this capability?

--- End quote ---

neither the clones nor the NEO7 or NEO8 give a jitter-free signal @ 10MHz; and like the others said - a GPSDO is like the 'best of both worlds' - short term stability via OCXO and long term stability by disciplining it to GPS.

the signals from those GPS modules may be accurate in frequency, but also jitter-free frequencies like 12MHz or 2MHz are not clean enough to be used as a reference clock for other devices, while most OCXO have a rather high spectral purity. you just have to watch out to not 'mess up' things when disciplining a OCXO ;)

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