Enter the GPSDO

So far in this series, we’ve seen the strengths and weaknesses of both crystal oscillators and GPS receivers^[1]I mentioned in part 1, but it’s time to reiterate — I’m using “GPS” generically. It’s perfectly possible to use the GLONASS, Galileo, or Beidou satnav systems to
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Comparing Crystal Oscillators with GPS

In part 1 and part 2 of this series, we saw that crystal oscillators and GPS receivers both have strengths and weaknesses as frequency standards.

Now, let’s compare them to understand how they differ, and how they complement each other. But first…

A slight detour:
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GPS as a Time and Frequency Standard

In part 1 of this series, I described the strengths and weaknesses of crystal oscillators. The moral of that story was that a number of processes affect a crystal’s frequency, and that you need something to calibrate and measure the XO against.
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Introduction

A lot of my recent time and frequency measurement work has been related to GPSDOs — GPS Disciplined Oscillators.^[1]There are now multiple GNSS (Global Navigation Satellite System) systems so saying “GPS” is a bit US-centric. But at least for now, virtually all GNSSDOs use GPS.
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