Also known as: PPM correction, PPM

PPM frequency correction compensates for the small error in an SDR’s reference oscillator, measured in parts per million, so signals land on their true frequency.¹ At UHF a 30 PPM error is several kilohertz — more than a channel’s width.

PPM correction compensates the dongle's oscillator error so signals land on their true frequency.

How it works

Setting the right PPM shifts the local oscillator so a known reference sits exactly where it should. The error drifts a little with temperature, so warm-up matters.

Relevance to SDR

A wrong PPM produces the classic rotating constellation that won’t lock — fixed by calibration, not a better antenna.

Sources

1. Clock drift — Wikipedia, on oscillator frequency error and drift measured in parts per million. ↩

See also

• Local oscillator
• Frequency
• Costas loop
• Constellation diagram

Learn it step by step

• Calibration & troubleshooting