Intermodulation Distortion
You are working a DX station on 20m when you suddenly hear what sounds like a second station — an S5 signal right on frequency, speaking in a language you do not recognize. You tune up a few kilohertz and find two powerful domestic stations having a conversation. You tune back to the DX frequency and the mystery signal is still there. You key up and call the DX anyway; the signal disappears during your transmission. When one of the domestic stations goes QRT, the mystery signal vanishes completely. You just experienced intermodulation distortion — a phantom signal created inside your own receiver by two completely real but unwanted strong signals mixing together in a nonlinear device.
Intermodulation distortion (IMD) is one of the most important and most misunderstood phenomena in receiver design and ham radio operating. It explains why some receivers that look impressive on paper fall apart in real-world HF use, why contested bands feel noisier than they actually are, and why the transmitter in your rig must be kept clean if you want to be a good neighbor on the air. This lesson explains exactly how IMD occurs, which products matter and why, and how to recognize and measure it.
Spectrum showing two 40m signals at 7.020 and 7.025 MHz and their IMD products. Second-order products fall far from the originals (at 5 kHz and 14.045 MHz), but third-order products at 7.015 and 7.030 MHz land right in the same band and cannot be filtered.
View LargerWhat is Intermodulation Distortion?
To understand IMD, you first need to understand the difference between an ideal linear device and a real-world nonlinear one. An ideal amplifier or mixer has a perfectly linear transfer function: output = gain × input. Double the input, double the output. This ideal device produces no new frequencies — its output contains only what was present at the input.
Every real electronic device — transistors, diodes, FETs, mixers — departs from this ideal. The relationship between input and output can be modeled mathematically as a power series:
Where:
• a1 is the linear gain term (wanted)
• a2 is the second-order nonlinearity coefficient
• a3 is the third-order nonlinearity coefficient
• Higher-order terms are usually small and often ignored for moderate signal levels
For a single input signal, the nonlinear terms generate harmonics: the second-order term creates a signal at twice the input frequency (2f), the third-order term creates a signal at three times the input frequency (3f), and so on. These are the harmonics you are already familiar with from earlier lessons.
But what happens when two signals are present simultaneously? This is where intermodulation occurs. When two signals at frequencies f1 and f2 are present, the nonlinear terms in the transfer function generate not just harmonics of each individual signal, but also sum and difference frequencies at combinations of f1 and f2. These combination frequencies are the intermodulation products, and the crucial problem is that some of them fall very close to — or directly on — the original signal frequencies, making them impossible to filter without also removing the desired signal.
The magnitude of these IMD products depends on how large the second- and third-order nonlinearity coefficients are (a device property you cannot control once the radio is designed) and on the input signal levels (something you can influence by controlling how much RF reaches the front end).
Second-Order IMD Products
When two signals at f1 and f2 pass through a device with second-order nonlinearity (the a2 × input² term), the following products are generated:
- f2 − f1 (difference frequency)
- f1 + f2 (sum frequency)
- 2f1 (second harmonic of f1)
- 2f2 (second harmonic of f2)
- DC component (zero frequency)
Two signals: f1 = 7.020 MHz, f2 = 7.025 MHz
• f2 − f1 = 7.025 − 7.020 = 0.005 MHz = 5 kHz (audio frequency!)
• f1 + f2 = 7.020 + 7.025 = 14.045 MHz (20m band)
• 2f1 = 14.040 MHz (harmonic)
• 2f2 = 14.050 MHz (harmonic)
Notice that the second-order products land in very different frequency regions: 5 kHz (audio), 14 MHz (a completely different band), and harmonics at 14 MHz. None of these fall within the 40m passband of the receiver's front-end filter. This is why second-order IMD is generally not a problem in narrow-band HF receivers that use a band-pass filter at the front end. The filter easily rejects products outside the receive band.
Second-order IMD becomes a significant problem in two specific situations:
- Direct-conversion (zero-IF) receivers: In a direct-conversion architecture, the mixer outputs baseband (audio frequency) signals. The difference product f2 − f1 = 5 kHz lands directly in the audio passband and cannot be filtered without also filtering the desired audio. This is why second-order intercept point (IIP2) is critical for direct-conversion receivers like those used in many modern SDRs.
- Wideband receivers: If the receiver lacks a narrow front-end band-pass filter — as is the case with very wideband spectrum monitoring receivers — then the sum frequency f1 + f2 can fall on another receive frequency of interest.
The second-order intercept point (IP2) is the theoretical signal level at which second-order products would equal the fundamental signals. A high IP2 is most important for direct-conversion architectures. For most conventional superhet HF receivers, IP2 is not the critical specification — third-order products are.
Third-Order IMD Products — The Most Important
Third-order nonlinearity (the a3 × input³ term) is the most troublesome in conventional HF receivers. When two signals at f1 and f2 pass through a device with third-order nonlinearity, numerous products are generated, but the ones that matter most are:
• 2f1 − f2
• 2f2 − f1
These are the near-in products that fall close to the original signals.
Two signals: f1 = 7.020 MHz, f2 = 7.025 MHz (5 kHz apart)
• 2f1 − f2 = 2(7.020) − 7.025 = 14.040 − 7.025 = 7.015 MHz
• 2f2 − f1 = 2(7.025) − 7.020 = 14.050 − 7.020 = 7.030 MHz
Both products land 5 kHz outside the original pair — but still within the 40m band. They cannot be filtered by a 40m band-pass filter without also removing the original signals. If the original signals are strong enough, these phantom products appear as if they are real stations transmitting on 7.015 and 7.030 MHz.
The pattern is clear from this example: the third-order products appear at equal spacing on the other side of each original signal. If two signals are 5 kHz apart, the third-order products appear 5 kHz beyond each signal — one on each side of the pair. The spacing between the products and the nearest original signal is always equal to the spacing between the two original signals. This means:
- Two stations 2 kHz apart generate third-order products 2 kHz away
- Two stations 10 kHz apart generate third-order products 10 kHz away
- Two stations 100 kHz apart generate third-order products 100 kHz away — but these are easier to filter or may be outside the band
The closer together the two strong interfering signals, the closer the IMD products land to those signals, and the more likely a third-order product is to fall on the exact frequency of a weak station you are trying to receive. This is why crowded contest conditions are so much worse for IMD than quiet band conditions — stations are packed closely together and the probability of two strong signals being just 5–20 kHz from your receive frequency is very high.
Why third-order products cannot be filtered
Second-order products land far from the original signals and can be rejected by a simple band-pass filter. Third-order products land at 2f1−f2 and 2f2−f1, which are within the band formed by the two original signals extended by one spacing on each side. Any filter that passes f1 and f2 will also pass the third-order products, because the products are too close to the originals for any practical filter to discriminate between them. The only solutions are:
- Reduce the level of the interfering signals before they reach the nonlinear device (attenuation, better front-end filtering, notch filters)
- Use a more linear device (higher IP3 mixer or amplifier)
- Accept the interference and work around it operationally
IMD Product Level Calculator
Given two equal-level tones and the system IIP3, calculate the third-order IMD product level and how far it is below the original signals (in dBc). Formula: IMD3 (input-referred) = 3×Pin − 2×IIP3
Higher-Order IMD Products
Third-order products are the most troublesome in typical HF receivers, but they are not the only higher-order products. Fifth-order, seventh-order, and higher odd-order products also create near-in interference. The general formula for the frequency of nth-order intermodulation products from two signals at f1 and f2 is:
5th order: 3f1 − 2f2 and 3f2 − 2f1
7th order: 4f1 − 3f2 and 4f2 − 3f1
Each nth-order product falls (n−1)/2 × (f2 − f1) from the outer signal
• 3f1 − 2f2 = 3(7.020) − 2(7.025) = 21.060 − 14.050 = 7.010 MHz
• 3f2 − 2f1 = 3(7.025) − 2(7.020) = 21.075 − 14.040 = 7.035 MHz
The fifth-order products fall 10 kHz from the pair center — twice as far as the third-order products (5 kHz). They are generally lower in amplitude than third-order products and are only significant in heavily overloaded receivers or very high-performance systems where third-order products have been suppressed.
In most practical ham radio situations, if you have a receiver or transmitter with good third-order performance, fifth- and higher-order products will be negligible. The rule of thumb: fix the third-order problem first, and higher-order products usually take care of themselves.
The order of IMD products and their amplitudes
| IMD order | Product frequencies | Rise rate (dB per dB input) | Typical level vs. 3rd order |
|---|---|---|---|
| 2nd order | f1±f2, 2f1, 2f2 | 2 dB/dB | Falls far from f1, f2 |
| 3rd order | 2f1−f2, 2f2−f1 | 3 dB/dB | Reference (most important) |
| 5th order | 3f1−2f2, 3f2−2f1 | 5 dB/dB | Typically 10–30 dB below 3rd |
| 7th order | 4f1−3f2, 4f2−3f1 | 7 dB/dB | Typically 20–50 dB below 3rd |
The 3:1 Slope — Why IMD is So Input-Level Sensitive
The 3:1 rise rate of third-order IMD products is the most important characteristic to understand about IMD behavior. It is what makes IMD unpredictable and operationally significant: a modest increase in the level of interfering signals can cause a dramatic increase in IMD interference.
Consider what happens as you increase the level of two input signals simultaneously, one decibel at a time:
| Input signal level (dBm) | Signal level change | Third-order IMD level (dBm) | IMD change |
|---|---|---|---|
| −80 | — | −220 | — |
| −70 | +10 dB | −190 | +30 dB |
| −60 | +10 dB | −160 | +30 dB |
| −50 | +10 dB | −130 | +30 dB |
| −40 | +10 dB | −100 | +30 dB |
| −30 | +10 dB | −70 | +30 dB |
(Assuming IIP3 = +10 dBm; IMD3 = 3×Pin − 2×IIP3)
The dramatic implication: when the two input signals are at −80 dBm, the third-order IMD level is −220 dBm — hopelessly below any noise floor, totally invisible. As the signals strengthen to −30 dBm (a 50 dB increase, corresponding to going from S1 to S9+36 dB), the IMD products rise by 150 dB to −70 dBm. At this level, the phantom IMD signals are stronger than most stations on the band.
This rapid rise explains several practical observations:
- Problems seem to appear suddenly as band conditions improve and distant stations' signals strengthen
- Moving the antenna a few feet (changing the direct path to a strong nearby station) can eliminate interference that seemed intractable
- Adding 10 dB of attenuation reduces IMD products by 30 dB, often eliminating audible interference
- The interference appears and disappears with the propagation of the two strong interfering signals — it is not a constant problem
The theoretical IP3 intercept point
The third-order intercept point (IP3) is defined graphically: if you plot signal output power and third-order IMD output power versus input power on a log-log scale, you get two straight lines. The signal line has a 1:1 slope; the IMD line has a 3:1 slope. These lines, extrapolated, cross at a point called the intercept point. At the input level corresponding to IP3, the IMD products theoretically would equal the signal. In practice, the receiver saturates long before this point, so IP3 is always an extrapolated value, not a directly measurable one under normal conditions.
The higher the IP3, the better the large-signal performance. IP3 is the subject of the next lesson. The key relationship to remember here is:
• Pin = level of each input tone (dBm)
• IIP3 = input third-order intercept point (dBm)
• IMD3 level relative to input signals = 2 × (Pin − IIP3) dBc
Recognizing IMD in Practice
When IMD is causing phantom signals in your receiver, there are several reliable diagnostic signs that distinguish it from real interference. Knowing these signs lets you quickly identify IMD rather than spending time chasing a ghost transmission.
The characteristic symptoms
1. The phantom signal follows the strong stations: When either of the two strong stations causing the IMD changes frequency slightly, goes QRT, or fades in propagation, the phantom signal immediately changes or disappears. A real station would not disappear just because your neighbor stopped transmitting on a different frequency.
2. Disappears when you transmit: If you are on a half-duplex radio and you key up, you are no longer receiving. The phantom signal disappears during your transmission. When you unkey, it reappears. Real stations continue to exist whether or not you are transmitting.
3. The 3-to-1 attenuator test: This is the definitive diagnostic. Insert a precision attenuator (or use your radio's built-in RF attenuator if it is calibrated) and observe what happens to both the strong real stations and the phantom signal. If inserting 10 dB of attenuation reduces the phantom signal by approximately 30 dB while reducing the strong real stations by only 10 dB, the phantom signal is third-order IMD. If the phantom signal drops by the same amount as the strong stations (10 dB for 10 dB of attenuation), it is a real transmission. The 3:1 ratio is a fingerprint of third-order IMD.
4. Frequency relationship test: Calculate where third-order products would fall for any two strong nearby stations. If the phantom signal is at exactly 2f1−f2 or 2f2−f1 where f1 and f2 are the two nearby strong stations, it is almost certainly IMD. With a calibrated receiver or SDR waterfall, you can identify these relationships precisely.
You are tuned to 7.018 MHz and hear an S5 signal that sounds like a QSO. You check and find strong station A on 7.020 MHz (S9+10 dB = approximately −63 dBm) and strong station B on 7.022 MHz (S9+10 dB = approximately −63 dBm).
Third-order product from A and B:
2fA − fB = 2(7.020) − 7.022 = 14.040 − 7.022 = 7.018 MHz
This is exactly where your mystery signal is. It is IMD. You insert 10 dB of attenuation. Stations A and B drop from S9+10 to S9 (10 dB less). The mystery signal at 7.018 drops by about 30 dB — well below your noise floor. Confirmed: third-order IMD.
Practical remedies
Once you have confirmed IMD, your options are:
- Use the RF attenuator: As shown above, 10 dB of attenuation reduces third-order IMD by 30 dB. This is effective when external noise sets the noise floor (as it typically does on HF below 30 MHz during the day).
- Use a bandpass or bandstop filter: If one or both of the strong stations is out-of-band (e.g., a BC station on medium wave creating IMD on 160m), a high-pass or band-pass filter removing the out-of-band signal eliminates the problem.
- Use a notch filter: If one strong station is the dominant cause, a narrow notch filter tuned to that frequency reduces its contribution to IMD significantly.
- Upgrade the receiver: A receiver with a higher IIP3 mixer creates fewer IMD products for the same input levels. For a station in an RF-rich environment, a high-IP3 receiver is a genuine solution.
IMD in Transmitters
IMD is not only a receive-side problem. Transmitters also produce IMD products, and when your transmitter sends them, they become interference for every other station within range. Understanding transmitter IMD makes you a better-mannered operator and helps you understand the complaints you might receive on the air.
The two-tone test for SSB transmitters
The standard method for measuring transmitter IMD performance is the two-tone test. A two-tone generator sends two audio tones (typically at 700 Hz and 1900 Hz, or 1 kHz and 2 kHz) simultaneously into the microphone input of the transmitter while it is in SSB mode. Ideally, the transmitter would produce only two RF signals, separated by the audio tone spacing. A real transmitter with nonlinear stages also produces third-order products at:
- 2ftone1 − ftone2 (e.g., if tones are at 700 Hz and 1900 Hz: 2×700 − 1900 = −500 Hz — below audio, folds back to negative frequency)
- 2ftone2 − ftone1 (e.g., 2×1900 − 700 = 3100 Hz — inside SSB passband)
- Higher-order products at various audio frequencies
On the transmitted RF spectrum, these audio IMD products become RF splatter — additional signals at frequencies outside the desired SSB bandwidth. A clean SSB transmitter at rated power should have third-order IMD products at least 31 dBc below the two-tone test signal level. The FCC requires this as a minimum standard; well-designed transceivers achieve −35 to −45 dBc or better.
What drives transmitter IMD
Transmitter IMD is most commonly caused by nonlinearity in the power amplifier (PA) stage, particularly when operating near maximum power. The final PA transistors are the highest-power stage and the most likely to depart from linear operation. Key causes include:
- Overdriving the microphone gain: Clipping the audio in the microphone amplifier stage before the balanced modulator creates a complex waveform that overdrives the final PA and generates heavy splatter. Reduce your microphone gain until the ALC meters are just barely moving, not pegged.
- Running the PA at the edge of its linear operating region: All Class AB PAs produce increasing IMD as power output approaches the rated maximum. Running at 80% rated power produces significantly lower IMD than running at full rated power.
- Poor bias adjustment: If the PA transistors are not biased correctly (wrong quiescent current), the amplifier may operate in a more nonlinear region of the transistor characteristic curve.
- External linear amplifiers: Adding a linear amplifier multiplies the output power but also multiplies (and can increase) IMD problems if the driver level is set too high.
How to check your own transmitter IMD
The most direct way to check your transmitted signal quality is to listen to yourself. Ask a nearby ham friend to listen to your signal using a good receiver while you transmit SSB speech. Alternatively, use a spectrum analyzer connected to your transmitter via a suitable attenuator (never connect a spectrum analyzer directly to a 100 W transmitter output — use at minimum 40 dB of attenuation). Compare what you see to the typical transmitter specifications in the table below.
| IMD specification (2-tone test) | Meaning | Operational effect |
|---|---|---|
| −25 dBc | Poor — minimum acceptable | Audible splatter on adjacent channels; complaints likely on crowded bands |
| −30 dBc | Acceptable | Some splatter in adjacent channels; within FCC rules for HF |
| −35 dBc | Good | Minimal splatter; clean signal on most receivers |
| −40 dBc | Very good | Clean signal; considered excellent for a tube or transistor PA |
| −45 dBc or better | Excellent | High-linearity Class AB or push-pull design; ideal for contesting |
A quick real-world check: during an SSB QSO, ask the other station "how does my audio sound?" and "is there any splatter on adjacent frequencies?" A complaint of "splattering 5 kHz up" is a strong indicator of transmitter IMD. Reduce your microphone gain or power output and repeat the test.
Frequently Asked Questions
I hear a signal but when I tune around I can find two strong stations that seem to cause it — is this IMD?
Almost certainly yes — especially if the phantom signal's frequency matches the calculation 2f1−f2 or 2f2−f1 from the two strong stations. To confirm, apply the attenuator test: insert 10 dB of receiver attenuation and observe what happens. If the phantom signal drops approximately 30 dB while the two strong stations only drop 10 dB, it is third-order IMD created in your receiver's front end. The 3:1 ratio (30 dB change in IMD for 10 dB change in input) is the definitive fingerprint of third-order intermodulation distortion. If the signal drops by only 10 dB (same as the strong stations), it is a real transmission from somewhere and not IMD.
Why is third-order IMD worse than second-order for HF?
Second-order IMD products land at f1+f2 and f2−f1. For two HF signals, these products fall either far above the HF spectrum (at the sum frequency, e.g. 14 MHz for two 7 MHz signals) or at audio frequencies (the difference, e.g. a few kHz). A standard front-end band-pass filter rejects both, so second-order products are rarely a problem in conventional superhet HF receivers. Third-order products at 2f1−f2 and 2f2−f1 fall right within the same band as the original signals — just a few kHz away. No front-end band-pass filter can reject these without also rejecting the desired signals. They can only be avoided by having a more linear receiver (higher IIP3) or by reducing the level of the interfering signals before they reach the nonlinear device.
Does more transmit power in my radio cause IMD in other people's receivers?
Yes, if two transmitters on nearby frequencies are both strong at someone else's antenna. The receiver does not care whether the two strong signals are from the same station or from two different stations — any two strong RF signals entering the receiver's front-end mixer simultaneously will generate third-order IMD products. If you are transmitting 500 W on 7.020 MHz and your neighbor is transmitting 100 W on 7.025 MHz, your combined signals at a receiver nearby can create a phantom signal at 7.015 MHz. You are not doing anything wrong individually, but the combination of your signals and your neighbor's signals causes IMD in receivers that are not sufficiently linear. This is one reason high-performance receivers with high IIP3 are especially valued by operators in urban or suburban areas where there are many nearby transmitters.
Test Your Knowledge
Answer the questions below to check your understanding. Every answer can be found in the lesson above.