Image Frequency in a Superheterodyne Receiver: Why It Appears
Sometimes a receiver plays a trick: a strong station that should be on one frequency also appears on another, far from it, often where it makes no sense. A broadcast station shows up in the middle of a ham band, or an aircraft voice is heard on the 2 metre band. In many cases this is the image frequency, a fundamental weakness of the superheterodyne receiver. It is not a fault of a particular radio, but a result of the way the receiver works, and the designer’s job is to push it down to a harmless level. In this guide we explain where the image comes from, how to calculate it, how designers deal with it and what you can do about it in your own station.
How a Superheterodyne Receiver Works
Nearly every radio built in the last hundred years uses the superheterodyne principle. The incoming signal is mixed with a local oscillator, and the mixer produces the difference frequency, which is the intermediate frequency, or IF. The IF stage has fixed filters and amplifiers, which can be made to work very well because they never change frequency. Tuning the radio means changing the local oscillator so that the wanted signal always ends up at the IF. The basic idea is described in the article on the superheterodyne receiver, and our comparison of direct sampling SDR and superheterodyne receivers shows where the design stands today.
The weak point is the mixer itself. It does not care whether the signal is above or below the oscillator: it produces the difference in both cases. A wanted signal at the LO frequency plus the IF gives the IF, but so does a signal at the LO frequency minus the IF. These two frequencies are mirror images of each other around the local oscillator, hence the name. One of them is the wanted signal, and the other one is the image.
The Formula
If the local oscillator is above the signal, which is the common case for up-conversion HF receivers, then
fLO = fsignal + fIF and fimage = fsignal + 2 × fIF.
If the local oscillator is below the signal, then
fLO = fsignal − fIF and fimage = fsignal − 2 × fIF.
In both cases the image is two IFs away from the wanted frequency. This simple fact explains everything that follows: the further the image is from the signal, the easier it is to remove it with a filter in front of the mixer, and the higher the IF, the further away it lies.
Example on a broadcast receiver
A simple medium-wave or shortwave receiver with an IF of 455 kHz, tuned to 7.100 MHz with the oscillator above, uses an LO of 7.555 MHz. The image is at 7.100 + 2 × 0.455 = 8.010 MHz. The antenna circuit has to attenuate 8.010 MHz while passing 7.100 MHz, with a separation of only about 13 percent. That is a demanding task for a simple tuned circuit, and many cheap receivers show images of strong stations.
Example on 2 metres
Take a VHF receiver with the 10.7 MHz IF common in FM equipment. For a signal on 145.000 MHz and an LO below it, at 134.300 MHz, the image is at 145.000 − 21.400 = 123.600 MHz, in the middle of the aeronautical voice band. A receiver with poor selectivity at its input could therefore let an aircraft conversation break into the 2 metre band. With an LO above the signal, the image would be at 166.4 MHz instead, where it may meet other services. The designer chooses the side that places the image in the least troublesome spot, and relies on the filtering to do the rest.
Choosing the IF to Move the Image Away
The table shows how the choice of IF changes the distance between the signal and its image for a signal of 14.200 MHz. The larger the IF, the further away the image, and the easier it is to reject. The price is that the IF filters must work at a higher frequency, where it is harder to make narrow ones, and this is why most high-performance HF receivers use two or three conversions.
| Receiver type | IF | Image for 14.2 MHz | Separation from signal |
|---|---|---|---|
| Old shortwave set | 455 kHz | 15.11 MHz | 0.91 MHz |
| Classic SSB transceiver | 9 MHz | 32.2 MHz | 18 MHz |
| Up-conversion receiver | 45 MHz | 104.2 MHz | 90 MHz |
| High-end up-conversion | 70 MHz | 154.2 MHz | 140 MHz |
The extreme case is the up-conversion receiver, in which the first IF lies above the whole HF range, at 45 or 70 MHz, for instance. The image of every HF frequency then falls in the VHF region, where a simple low-pass filter at the input removes it completely. This is the reason why most modern HF transceivers use this design. It makes the image problem almost disappear and lets the designer put a roofing filter on the first IF, as described in our article on receive filter bandwidth and the roofing filter.
Image Rejection and What the Number Means
Image rejection is the ratio between the sensitivity of the receiver at the wanted frequency and its sensitivity at the image frequency, expressed in decibels. A rejection of 60 dB means that a signal on the image frequency must be a million times stronger in power than a wanted signal to produce the same output. Good HF transceivers reach 70 dB or more, and simple VHF handhelds may reach only 40 to 50 dB, which is why handhelds more often hear images.
The number is only half of the story. A rejection of 60 dB sounds excellent until you meet a local broadcast transmitter that is 70 dB or more above the level of a weak amateur signal. Then even a good rejection leaves a readable image. The relevant question is therefore how strong the interfering station can be at your site, and how much rejection you need to push it below the noise floor. The stronger the signals near your antenna, the more attention the image behaviour deserves. In related terms, the overall performance against strong signals is covered in our guide to receiver dynamic range, blocking and IMD.
How Designers Reduce the Image
There are four main methods. The first is a selective filter in front of the mixer, called a preselector. For a fixed band it can be a band-pass filter with a few sections. For a receiver that tunes over a wide range, the preselector is tracked, with switched filters for each band, as in most amateur transceivers. The second method is the choice of a high first IF, as in the up-conversion design above. The third is double or triple conversion, in which the first IF is high for the image rejection and a later IF is low for the narrow filters. The fourth method is the image-reject mixer, built with two mixers and a phase-shifting network, which cancels the image by the arrangement of phases. The Hartley and Weaver architectures are well-known examples, and they are common in modern integrated circuits.
A direct-conversion receiver, with an IF of zero, has its own version of the same problem. The image is then the opposite sideband, folded onto the wanted signal. Software defined radios with I and Q channels remove it by calculation, and the quality of the removal depends on how well the two channels are matched. A small imbalance leaves a faint mirrored copy of strong signals on the other side of the centre frequency, usually 40 to 60 dB down, which you may see on a panadapter as a ghost. Many SDR programs include a calibration for this, and the guide to the panadapter and waterfall display shows how such ghosts look on screen.
Recognising and Testing for Images
A true signal behaves in a predictable way. If you tune the radio by 1 kHz, a real signal moves by 1 kHz. An image moves in the opposite direction when the LO is above the signal, because it lies on the other side of the oscillator. A station that appears at two places on the dial, and whose tone changes in opposite directions, is almost always an image. A second clue is the dependence on the antenna and the preselector: a real signal is attenuated when you detune the front end, but an image can behave differently, since the image is rejected by the same circuit.
To measure the rejection, use a signal generator. Set it to a level that gives a clear reading at the wanted frequency and note the level. Then change the generator to the image frequency and raise the level until you get the same reading. The difference in decibels is the image rejection. If you have no generator, a stable local signal with a known frequency and your calculation of the image can give an approximate answer. With a spectrum analyser, such as the one in our guide to the tinySA, you can also check the filter response at the input of the receiver.
What You Can Do at Your Station
If a particular image troubles you, first calculate where it comes from, using the formula and the IF of the receiver. Knowing the IF is easy for older radios, since it is in the manual, and for new radios you can often find it by searching the specification. Then find which signal sits on the image: a broadcast station, a paging transmitter, a radar. Use a band-pass or notch filter in the antenna line, designed for the image frequency. A high-pass filter works if the image is below your band and a low-pass filter if it is above. A simple attenuator can also help when the interfering signal is very strong, because the mixer may be overloaded by it.
When you buy a receiver, check the specifications for image rejection and also for the dynamic range. A number of 70 dB or more on HF is good. For VHF and UHF handhelds, a lower figure is typical, and a filter on the antenna port for a particular strong local transmitter is often the cheapest improvement.
Images and the Wider Band Plan
The image problem is also the reason why some frequency plans work better than others. A receiver that covers 118 to 137 MHz and 144 to 148 MHz with the same front end needs to keep these two ranges apart, and a scanner that covers hundreds of megahertz has many possible images at once. Experienced users learn the typical images of their own radios: a strong local FM broadcast station can show up as a carrier on a certain VHF channel, a paging transmitter on a certain UHF frequency. Writing down the pairs when you meet them saves time later, because the next time you recognise the ghost at once. The image response article gives more background on the effect in general.
The Bottom Line
The image frequency is the mirror of the wanted signal around the local oscillator, two intermediate frequencies away from it. A superheterodyne receiver cannot tell the two apart in the mixer, so it must be separated by filtering in front of it. Designers push the image far away by choosing a high first IF, add preselector filters, or use image-reject mixers. As a user, remember the formula, check where a ghost signal would fall, and put a filter in the antenna line for a strong interferer. A figure of 60 to 70 dB of rejection is good, but its value depends on how strong the local signals are.
Frequently Asked Questions
What is the image frequency?
It is the second frequency, two IFs away from the wanted one, that a superheterodyne receiver converts to the same IF. It is the mirror of the signal around the local oscillator.
How do I calculate the image frequency?
Add or subtract twice the IF from the signal frequency: add if the local oscillator is above the signal, subtract if it is below.
Why do modern HF radios use a high first IF?
A high first IF, such as 45 or 70 MHz, puts the image far above the HF range, where a simple input filter removes it. It also allows a roofing filter on the first IF.
What is a good image rejection figure?
Good HF transceivers reach 70 dB or more. Simple VHF handhelds may have only 40 to 50 dB, so they are more often troubled by images.
How can I tell an image from a real signal?
Tune the radio a little. A real signal moves in the direction you tune, while an image moves the opposite way when the oscillator is above the signal. Images also depend on the front-end filter.
Do software defined radios have an image?
Yes, in the form of a mirrored copy caused by imbalance between the I and Q channels. It is usually small, and many programs offer a calibration to reduce it.