The Second Receiver Test: “Sniffing the Local Oscillator” of LMR SDR 1.1

| April 29, 2026 | Amateur Radio (Ham Radio) | No Comments

If you have another HF radio or a cheap RTL-SDR dongle, you can verify the Si5351 is actually oscillating.

  1. Place the antenna of a second receiver very close to the Si5351 on your LMR SDR board.
  2. Tune the second receiver to 4 times your LMR SDR’s displayed frequency (e.g., if the SDR is set to 7.000 MHz, tune the second radio to 28.000 MHz).
  3. Result: You should hear a strong, unmodulated carrier (S9 signal). If you don’t hear anything, the Si5351 is not initialized, likely due to an I2C communication error with the Arduino.