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  1. CommanderLake

    AOCJY2 10 MHz OCXO time source

    So after all this time and a few calibrations later it's currently at about 0.8ms slow 36 days after its last calibration which (correct me if I'm wrong) comes to 0.000000000257 seconds per second or 0.257 PPB from an OCXO rated at 5PPB!? Maybe they never tried calibrating it using a DAC eh? I...
  2. CommanderLake

    AOCJY2 10 MHz OCXO time source

    I designed a PCB for the oscillator, PICAXE, DAC, op-amp and regulators, for this I used an OPA388 "zero-drift" op-amp which has extremely low offset and drift, perfect for this application. I also added a few SMA connectors so I can get a secure connection to my oscilloscope to verify the...
  3. CommanderLake

    AOCJY2 10 MHz OCXO time source

    In my quest to find a more precise control voltage source than a trimpot I found the TI DAC8830 16bit single ended DAC which has a reference input so I can use the 2.8V reference from the VCOCXO as full scale which makes it super stable. It has a high output impedance so I'm still using the...
  4. CommanderLake

    AOCJY2 10 MHz OCXO time source

    I'm trying to measure with my scope exactly how much adjustment the 10MHz VCOCXO has using a 100ms trigger delay and a 200ns collection time... The adjustment range is about 860ns/s in the range of 1-2 volts on the control voltage pin. The datasheet states the frequency pull range is +-0.7ppm...
  5. CommanderLake

    AOCJY2 10 MHz OCXO time source

    Got the 12MHz oscillator, works great, returns the data in just 6.65μs!
  6. CommanderLake

    AOCJY2 10 MHz OCXO time source

    It's working perfectly with the pull-down resistors, it responds with all 64 bits of data in just 10μs at 8MHz! The only thing holding it back is the latency of the USB serial interface, most of the time it responds with the data in about 1.1ms but it often returns in 0.12-0.15 ms, low response...
  7. CommanderLake

    AOCJY2 10 MHz OCXO time source

    So I decided to dump the 10:1 decade counter frequency divider for the serial clock and try it at 10MHz at which I thought it would be a jumbled mess but actually it works just fine at 10MHz according to my scope but as my serial adapter is 12Mbps its clock doesn't divide well into 10 so... I...
  8. CommanderLake

    AOCJY2 10 MHz OCXO time source

    Actually you just got me thinking, I could replace the register selection inverters with a flip-flop which would keep at least one active at all times and switch between them but I think it's one of the shift register inputs remaining high that causes its output to end up in the high state for a...
  9. CommanderLake

    AOCJY2 10 MHz OCXO time source

    Unfortunately it couldn't stay so neat and tidy, it pretty much works but the counter outputs need some pull-down resistors because they're 3-state and the CMOS inputs of the shift registers don't like the gradual discharge on the inputs.
  10. CommanderLake

    AOCJY2 10 MHz OCXO time source

    I had a sudden realization as I woke up today, the shift registers wont load the second byte of each 16bit counter because the load pin doesn't get cycled, luckily I always get a spare of each component and all it takes is an AND gate on each flip-flop to fix it which means 1 more IC which I...
  11. CommanderLake

    AOCJY2 10 MHz OCXO time source

    Work has started on time machine v2, I just cut, stripped, bent and inserted 32 custom length jumper wires for the register buses from the counters to the shift registers
  12. CommanderLake

    AOCJY2 10 MHz OCXO time source

    I just found out the voltage reference pin on the OCXO is for the trimpot supply so it has a stable voltage reference for the control voltage, now its so stable I can adjust it to within 1mV, much more stable than the 3.3V LM1084 which is already pretty good! 1mV on the control pin is about 0.42...
  13. CommanderLake

    AOCJY2 10 MHz OCXO time source

    A revised logic circuit to fix the RCLK which remains high for the duration of the serialization process and to accommodate the fact there are only 10 bit ring counters (decade counter) which was a pain: https://tinyurl.com/y3qyhe45
  14. CommanderLake

    AOCJY2 10 MHz OCXO time source

    If you don't mind me sharing something else I've been working on with the Falstad circuit simulator (yes I know its a childish graphical thing) I came up with a circuit that synthesizes a serial interface which outputs the counter values, its not quite complete it doesn't roll over the counters...
  15. CommanderLake

    AOCJY2 10 MHz OCXO time source

    The voltage fluctuation of the pot is determined by the load which is 10kΩ on the control voltage input, its basically a voltage divider.
  16. CommanderLake

    AOCJY2 10 MHz OCXO time source

    Putting the trimpot through the OPA197 made a huge difference now its extremely stable over a wide temperature range.
  17. CommanderLake

    AOCJY2 10 MHz OCXO time source

    I assumed it wouldn't need a control voltage but its far less accurate without one, the trimpot I'm using has a 100ppm/C stability, a 10ppm/C one looks to be about £60. Maybe there's some sort of digitally controlled temperature compensating regulator a bit like a TI PGA2310/11 volume control IC...
  18. CommanderLake

    AOCJY2 10 MHz OCXO time source

    The problem I'm seeing now is the slightest temperature variation of the trimpot affects the control voltage and therefore the accuracy of the clock so I need some form of voltage source that's not affected by temperature fluctuation.
  19. CommanderLake

    AOCJY2 10 MHz OCXO time source

    So far the drift is less than 1ms per day and I've optimized the PICAXE program to the point it takes less than 1ms to read and return the 8 bytes of data, the tricky part was the high speed serial output was occupied by one of the pins connected to the second counter bus so I used a 1k resistor...
  20. CommanderLake

    AOCJY2 10 MHz OCXO time source

    I suppose I should post a picture... Its hard to tell due to the error from inconsistent network latency to the time servers but it now appears to have drifted forward by 2-3ms in about 13.5 hours
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