The PCBs and multiple parts arrived. A GPSDO was assembled and connected up to GPS. It worked!! For a while ...
It took nearly 3 hours to assemble. I am not an experienced kit builder, so maybe I should have expected it. Soldering looked good, nothing showing up on the multimeter to worry about, power on. Attached the PICkit 3 and programmed successfully. Sailed through the 3 hour calibration, started to settle into run mode, then serial output stopped. I had noted that serial input from the user wasn't acknowledged, what was wrong?
What was wrong was an oversight on my part. There are no markings on the PCB to show which connector pin does what (should this prove popular, will be fixed in the next run of boards). I had wired the serial input/output incorrectly and it looks like it fried the PIC UART. The system continued to run minus serial output and after a while the LED indicated it had settled to better than 10MHz+-0.01Hz, (the target error of less that 1 part in 10E9). But with no serial output, that was all that could be said. Success of a sort.
Fortunately or unfortunately whichever way you looks at it, it looks like the PIC serial port failed rather than the serial/USB converter. After the disaster, the converter was hooked up to an older GPSDO (carefully, correctly) and is working. Fortunately because I only have one serial/USB converter, but several spare PIC16F1455 processors. Unfortunately as now I have to figure out how to remove and replace a 14 pin part without damaging the PCB.
It looks like a kit can be created for about $16AU+postage (remembering a kit does not include a power supply, a GPS receiver, and the (optional) serial/USB adapter and case). A kit is the PCB, OCXO (OSC5A2B02), PIC16F1455, 74HC04, Panel mount BNC connector, resistors, capacitors, and pin connector strip. I was hoping to include a micro USB connector for power but these are in transit. For testing I supplied power through a barrel connector (the PCB also has provision for it) but I don't have spares of them. A builder could attach wires direct to the board, maybe with a USB A plug. I may include the LED I ordered, it works but not very bright. The LEDs I used before were cannibalised from solar powered Christmas lights and they were much brighter. Maybe one of those can go in, they come in strings of hundreds, just need to remove the wires.
So it's close, folks. I'd like to replace the PIC on the board, test again with serial output working. Report back in a day or two. I've ordered what is claimed to be a NEO-7 module marked BOAR, the O looking like a paw print. I believe these have a preamp for the patch antenna, may get reasonable reception indoors. All in the interests of experimentation.
The system being tested in the last post was turned off to allow this test. That test revealed the system is sensitive to temperature changes. This explains a problem I was having a few weeks back, the system is set up not far from a thermostatically controlled heater. As the heater cycled on and off, it was reflected in the processor raising and lowering the control voltage. I thought at first it was electrical interference but not so. Is it worth doing something about it? probably not. This is a budget build and it meets the specification under normal circumstances (even heaters turning on and off).