[hpsdr] High Accuracy Timing for HPSDR

[neil whiting]

A few thoughts on SDR frequency references. I'm not sure the Gibralter PCB
needs to be as complex as suggested.

- Most people who are worried about having GPS-accurate frequency are likely

  to have an existing reference source.

- Many of those who don't have an existing reference are likely to be happy
  with a simple OCXO tweaked to the right frequency. Locking to GPS is not
  necessary for SDR operation.

- If a locked frequency is required (eg for modern digital modes,
  microwave EME, etc)
  - locking to 1pps is tricky/complex and existing solutions may not be
    open source
  - locking to 10kHz as output by some GPS modules (eg. the Jupiter-T,
    which is available surplus) is rather easier, just using a simple PLL

It would seem that a good balance would be to provide 3 options:
- an external reference input,
- an option to use the Gibraltar PCB populated with just an OXCO, no locking

- use Gibraltar with simple PLL locking to 10kHz from a suitable onboard GPS

  module

Does this make sense? Have I missed something?
Are suitable GPS modules available at a reasonable price (new, not surplus)?


73, Neil G4BRK
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