Field Guide · hardware

Also known as: GPSDO, GPS-disciplined oscillator, GPS disciplined oscillator

A GPSDO (GPS-disciplined oscillator) is a local oscillator — usually an OCXO — whose long-term frequency is continuously steered to the atomic-clock timing carried by GPS satellites.1 It fuses two strengths: the clean short-term stability of a good crystal oven and the near-perfect long-term accuracy of the GPS time base, reaching fractional accuracies around 1e-11 to 1e-12 over a day. That makes it the practical gold-standard reference for SDR work that must be exactly on frequency and, when several radios share it, phase coherent.

GNSS GPS rx 1 PPS phase compare OCXO steer 10 MHz + PPS
A GPSDO compares its OCXO against the satellite-derived 1 PPS and slowly steers the oven so long-term frequency tracks GPS atomic time.

Overview

GPS satellites carry atomic clocks and broadcast time so precisely that a ground receiver can recover a one-pulse-per-second (1 PPS) tick whose long-term rate is essentially exact. The problem is that GPS timing is noisy second-to-second and can drop out entirely. A crystal OCXO is the opposite: beautifully quiet over seconds but slowly aging over days. A GPSDO marries them, using a slow control loop so each covers the other’s weakness.

How it works

  • A GPS receiver produces a 1 PPS output locked to satellite atomic time.
  • A phase/frequency comparator measures how far the local OCXO has drifted against that PPS, accumulated over a long time constant (seconds to many minutes).
  • The loop applies a small steering voltage to the OCXO, nudging it back on track. Because the loop is slow, it corrects the OCXO’s long-term aging while the OCXO’s own short-term stability filters out the GPS jitter.
  • Holdover: if GPS is lost, the loop freezes its last correction and the OCXO coasts, drifting only slowly until lock returns.

In practice

A typical GPSDO outputs a disciplined 10 MHz sine and a 1 PPS tick, both used to clock and synchronise instruments and radios. It needs a sky-view GNSS antenna and a few minutes (sometimes longer) to acquire satellites and settle its loop. The short-term frequency stability is set by the OCXO, so a better oven still matters; GPS supplies the long-term truth.

Relevance to SDR

A GPSDO is what a serious SDR station uses when frequency error must be negligible and, especially, when multiple receivers must share one reference — coherent direction finding, passive radar, or multi-band monitoring where all radios must agree on frequency and phase. SDRs with a 10 MHz external-reference input lock straight to it. GopherTrunk decodes the samples the front end delivers and does not drive the oscillator, but a GPSDO removes reference drift as a variable entirely: the control channel sits exactly where the system map says it should, and nothing the software’s automatic frequency correction does has to compensate for a wandering clock. Below a GPSDO, an OCXO or TCXO covers less demanding stations.

Sources

  1. GPS disciplined oscillator — Wikipedia, on disciplining an OCXO to GPS 1 PPS, long-term accuracy, holdover, and 10 MHz/PPS outputs. ↩

See also