SDR++ PPM Frequency Correction Guide

[Pavel]

Short answer: PPM correction compensates a receiver's oscillator error; it should be measured against a stable known-frequency signal after warm-up. Do not use PPM to move an incorrectly selected channel or to compensate a transmitter whose frequency is unknown.

Before you start​

• First prove that the operating system and the hardware vendor's basic utility can see the SDR. An application cannot fix a missing USB, network or driver layer.
• Record the exact application build and SDR driver or support-library version. Plug-ins and source modules are often coupled to a particular build or architecture.

Step-by-step method​

1. Step 1: Warm the SDR for 15 to 30 minutes in its normal enclosure and temperature. Select a stable reference near the band of interest with accurately known frequency.
2. Step 2: Use a narrow enough mode and zoom to estimate the frequency error. Calculate PPM as measured error in hertz divided by reference frequency in hertz, multiplied by one million.
3. Step 3: Enter the correction with the sign that moves the displayed signal onto the known frequency, then retune and measure again. If the error doubles, reverse the sign.
4. Step 4: Repeat at another stable frequency and after a cold start. Save a conservative value for that individual receiver and note temperature drift instead of forcing one perfect instant.

Concrete example​

If a 100.000 MHz reference appears 1.5 kHz high, the magnitude is 1,500 / 100,000,000 × 1,000,000 = 15 PPM. Apply the sign required by SDR++ to move the peak back by 1.5 kHz, then verify rather than assuming the sign convention.

How to judge the result​

A valid correction places multiple stable references close to their assigned frequencies after warm-up. If different bands require wildly different values, suspect the reference, tuning method, Doppler or an SDR spur rather than ordinary constant oscillator error.

What to record​

• A clean software baseline can start and stop the device repeatedly for ten minutes without dropped samples or a device-busy error.
• Confirm the displayed center frequency, sample rate and audio device rather than assuming saved settings still match the current hardware.
• When testing a change, close other SDR programs so only one process owns the receiver.

Common mistakes​

• Changing several hardware, software or RF variables at once, which removes the controlled comparison needed to identify the cause.
• Treating one autoscaled screenshot or one unusually good result as proof without recording the settings and repeating the test.
• Calibrating against an ordinary voice carrier, moving satellite or loosely specified consumer transmitter can produce a precise-looking but incorrect PPM value.

Final check​

Restart the application and computer once, then repeat the exact software workflow. Keep the configuration only when the correct source, device and output return without hidden manual repairs.