Symmetrical “Ghost” Signals or IMD?
Intermodulation Distortion from an Overloaded Receiver Front End
You tune across the band and suddenly you see it: extra signals popping up on both sides of a real carrier, often with eerily equal spacing. On a waterfall, they can look like “mirror copies” or symmetric side-products around strong stations.
In many cases, those “ghosts” are not real transmitters at all. They are intermodulation distortion (IMD) products created inside your own receiver when the front end is being driven beyond its linear range.
What’s actually happening
A modern receiver or SDR front end is only “clean” while it remains linear. Once strong signals push an RF amp, mixer, and/or ADC into compression or clipping, the radio stops behaving like a faithful pass-through device.
In that nonlinear region, the front end effectively becomes a signal blender... it starts mixing incoming signals together internally and creates new frequencies that were never on the air.
If two strong signals exist at
f1 and f2, common distortion products include:• Second-order:
f1 + f2, |f1 − f2|• Third-order (often the most troublesome):
2f1 − f2 and 2f2 − f1Third-order products often land close to the real signals, so they look like nearby “ghosts” and can appear symmetrically spaced.
What you see on the spectrum
Overload IMD typically shows up as extra peaks that:
- appear on both sides of strong carriers (often with equal spacing),
- grow rapidly as strong signals increase,
- move when the strong signals move (because they’re mathematically related),
- can “come and go” depending on band conditions and gain settings.
Why they’re not harmonics or fixed spurs
| Artifact type | What it is | How it behaves |
|---|---|---|
| Harmonics | Integer multiples of one signal (2×, 3×, 4×...) | Land at predictable multiples (e.g., 2f, 3f) of a single strong carrier |
| Fixed spurs | Radio-generated artifacts (clocks, LO leakage, digital noise) | Often stay at the same frequencies (or follow predictable tuning relationships) |
| IMD products | New frequencies created by signal interaction inside a nonlinear stage | Change with the strong signals present; “ghosts” shift as f1/f2 shift |
How to recognize overload IMD in seconds
- Ghosts worsen when strong signals are present (broadcast stations, contest pileups, nearby transmitters, etc.).
- RF attenuation helps a lot: reduce RF gain, disable preamp/LNA, or enable input attenuation... and the artifacts drop sharply.
- They track the band: tune around and watch the “ghosts” move with the strong signals.
1) Note one obvious “ghost” peak.
2) Enable 10–20 dB attenuation (or reduce RF gain noticeably).
3) If the “ghost” collapses faster than normal signals (or vanishes), you’re very likely seeing overload IMD.
How to reduce or prevent it
The fix is almost always about keeping the front end in its linear region and preventing out-of-band energy from hitting vulnerable stages.
- Turn down RF gain and disable any preamp/LNA unless you truly need it.
- Enable input attenuation (if available). This is often the fastest, most effective cure.
- Use band-pass filtering or a preselector so out-of-band power never reaches the front end.
- Be mindful with “better antennas”: more capture can improve weak-signal reception, but it can also raise total RF at the receiver and trigger overload.
- Choose a receiver with stronger dynamic range (higher overload resistance, higher IP3) if your RF environment is harsh.
A very common trap: adding an external LNA to “fix” reception can actually make overload and IMD worse if the receiver is already the limiting factor.
One extra nuance: true mirroring from I/Q imbalance
Some SDR architectures can show a real “image” across the tuned center frequency due to I/Q imbalance. That looks like a mirrored copy on the opposite side of the center frequency, even when signals aren’t particularly strong.
The practical difference:
- I/Q image: tends to be tied to the center frequency symmetry and can persist even at modest signal levels.
- Overload IMD: appears or explodes when strong signals arrive... and improves immediately with attenuation or gain reduction.
Why this matters in real stations
Once your receiver is overloaded, the spectrum becomes unreliable: you may chase “signals” that are actually internal products, miss weaker stations buried under distortion, or misinterpret band conditions.
Keeping the front end linear is not just about “clean audio”... it’s about preserving the receiver’s ability to distinguish real RF from internally created RF.
Mini-FAQ
-
Why do the ghosts look symmetric? Because common third-order IMD products often appear at predictable spacings relative to strong signals (e.g.,
2f1 − f2and2f2 − f1). - Why does attenuation “fix” it so well? Distortion rises rapidly as stages approach compression. Reducing input level can move the front end back into its linear region, collapsing IMD products.
- Is this a problem with my antenna? Not necessarily. A strong antenna can simply deliver more RF than the receiver can handle in your environment. The real fix is gain structure and filtering.
- Do I need an LNA? Only if you are truly noise-limited and the receiver is not already near overload. In many urban or high-signal environments, an LNA makes things worse.
Interested in more technical content? Subscribe to our updates for deep-dive RF articles and lab notes.
Questions or experiences to share? Feel free to contact RF.Guru.