Let’s look at SDR intermodulation products, and also how these are different from analog receivers. These phantom signals are a problem for low HF and higher MF listening.
If you tune across 1.9 ±0.2 MHz on your SDR receiver at any time, you will see a wonderful selection of many strong signals. Unfortunately, few of them are real. These are phantoms arising from second and third order intermodulation between strong AM broadcast signals.
If you process any two signals (F1, F2) through a nonlinear circuit, mixing products occur according to the mF1± nF2 rule, where m and n are zero or integers. These are the SDR intermodulation products seen and heard as the mixing of two strong AM signals.
You can make a list of lower-order products as follows:
- Second Order Products: F1±F2, 2F1, 2F2
- Third Order Products: 2F1±1F2, 2F2±2F1, 3F1, 3F2
As these are very simple formulas, you can use a spreadsheet to calculate where you will hear these phantoms. Select the five strongest local AM signals, typically with strength above -20dBm. Calculate the IMD products created by each pair of these stations. Then, look at your spectrum display.
You will find all of these interference products showing up exactly as the formulas predict. You can even hear the audio from both local stations in each pair mixed together.
SDR Intermodulation Products – Analog Versus Digital
With analog receivers, the strength of IMD products rises and falls exponentially with the source signal strengths on F1 and F2. Second order products generate strength as a square p² and third order as a cube p³. So, if you add attenuation, you can dramatically reduce the strength of the IMD faster than the source signals.
Unfortunately, SDR ADC do not work this way. Basically, once you reach a certain overload threshold, the strength of the SDR Intermodulation products remains constant and is nearly independent of input power. You can check this out for yourself. If you add 20 dB attenuation ahead of your wideband SDR, both real signals IMD products will reduce by 20 dB only.
Sources of SDR intermodulation products include preamplifiers and analog components associated with or even inside the analog to digital converter. Indeed, even the sampling process ADC bit size results in integral intermodulation, something that can be improved a few dB with dithering.