Frequency error and SDR calibration

A small factor, an audible error

One ppm is one millionth of the reference value. A 1 ppm oscillator error corresponds to 145 Hz at 145 MHz and 435 Hz at 435 MHz. Relative error stays constant while absolute error grows with frequency. This can be noticeable with narrow signals.

A constant offset in hertz is not automatically a ppm error. Mixing stages, an incorrect display setting or the transmitted frequency itself may also contribute. Where possible, compare several known references and record both the expected and observed values.

Reference and warm-up

Allow the receiver to reach thermal stability first. Use a signal with a reliably known frequency and stability. An arbitrary voice station is not an accurate reference. Read the carrier with sufficient spectrum resolution and repeat the observation before saving a permanent correction.

Calculate relative error as (observed − reference) / reference × 10⁶. Then check your software’s sign convention: a field may expect the measured error or the opposite correction. A controlled small adjustment followed by another measurement establishes the direction.

What correction does not fix

A single correction does not eliminate temperature drift under every operating condition. Nor does it automatically calibrate the level display. Doppler shift from a moving transmitter and changing propagation should not be saved as a stationary receiver error. Record the reference, temperature conditions and date alongside the correction.