Amateur Radio: Intermodulation Distortion Explained
In amateur radio, Intermodulation Distortion (IMD) is a form of signal interference that occurs when two or more signals interact within a non-linear device—such as an overdriven power amplifier or a poorly designed mixer. This interaction creates “sum and difference” frequencies that didn’t exist in the original signals, often landing right on top of other users’ frequencies. There could also be higher order intermodulation products at sums and differences of multiples of those frequencies.
This wonderful Creative Commons Attribution-Share Alike 4.0 International animation from Wikipedia by Ice Ardor shows that non-linear amplifier produces undesirable intermodulation products (D3, D4) at 2f1-f2 (D3) and 2f2-f1 (D4). f1 and f2 are the fundamental frequencies which are interacting. Note that the 3rd order intermods grow by 3 dB for every 1 dB increase in input power, while the carriers increase by 1 dB for every 1 dB increase in input power. The spectrum becomes highly distorted at higher power levels as the odd order intermod products (3rd, 5th, 7th, 9th, …) increase in level with respect to the carriers. An nth order intermod grows by n dB for every 1 dB increase in input power.
How IMD Occurs
When a circuit is linear, the output is a perfect, scaled replica of the input. However, every electronic component has a limit. When a signal is too strong (driving an amplifier into “clipping”), the device becomes non-linear. When two frequencies, f1 and f2, interact in this non-linear environment, they mix to produce:
Second-order products: f1 + f2 and f1 – f2.
Third-order products: 2f1 – f2 and 2f2 – f1.
The most problematic for hams are the Third-Order Products. These products are dangerous because they usually fall very close to the original operating frequencies, making them impossible to filter out with standard low-pass or band-pass filters.
Why It Matters to Hams
IMD isn’t just a technical curiosity; it has real-world consequences on the bands:
- Splatter: On SSB (Single Sideband), high IMD causes your signal to “bleed” into adjacent channels. You might be tuned to 7.150 MHz, but your IMD products are causing interference for someone at 7.144 MHz and 7.156 MHz.
- Reduced Dynamic Range: In a receiver, IMD occurs when two strong nearby signals mix inside your radio, creating a “phantom” signal on the frequency you are trying to monitor. This makes it difficult to hear weak DX stations.
- Signal Quality: High IMD in a transmitter degrades the “cleanliness” of your voice or data, making it harder for the receiving station to decode you, especially in noisy conditions.
Common Causes and Fixes
| Cause | Solution |
| Overdriving the Amp | Reduce the “Drive” or “ALC” levels. Never run an amp at 100% if it sacrifices linearity. |
| Excessive Speech Compression | Turn down the processor. Over-compression squares off the waveform, creating harmonics and IMD. |
| Non-linear Components | Ensure transistors (like the IRFP150 or similar MOSFETs) are properly biased into their linear region. |
| Passive Intermodulation (PIM) | Sometimes caused by “rusty bolt effect”—corroded antenna connections acting as a diode and mixing signals. |
Measuring IMD: The Two-Tone Test
To check a transmitter’s linearity, hams use a Two-Tone Test. Two non-harmonically related audio tones (e.g., 700 Hz and 1900 Hz) are fed into the mic input. The output is then viewed on a spectrum analyzer.
A “clean” modern transceiver should ideally have third-order IMD products at least 30 dB below the level of the two main tones (referenced to PEP).