Superheterodyne Receiver Image & Tuning

The superheterodyne radio receiver has an issue referred to as the image: there are two frequencies that can mix with the local oscillator to enter the IF.

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The basic concept of the superheterodyne receiver appears to be fine, but there is a problem. There are two signals that can enter the Intermediate frequency stages.

The unwanted signal that can enter the intermediate frequency stages is known as the image signal.

Removing the image signal is a key requirement in the performance of the superhet radio.

Concept of the superheterodyne receiver image

With the local oscillator set to 0.75 MHz and with an IF of 0.25 MHz, it has already been seen that a signal at 1.0 MHz mixes with the local oscillator to produce a signal at 0.25 MHz that will pass through the IF filter. However if a signal at 0.5 MHz enters the mixer it produces two mix products, namely one at the sum frequency which is 1.25 MHz, whilst the difference frequency appears at 0.25 MHz. This would prove to be a problem because it is perfectly possible for two signals on completely different frequencies to enter the IF.

The unwanted frequency is known as the image. Fortunately it is possible to place a tuned circuit before the mixer to prevent the signal entering the mixer, or more correctly reduce its level to acceptable value.

Removing the image signal in a superheterodyne receiver
RF tuned circuit removes image in a superheterodyne receiver

This RF tuning circuit does not need to be very sharp. It does not need to reject signals on adjacent channels, but instead it needs to reject signals on the image frequency. These will be separated from the wanted channel by a frequency equal to twice the IF. In other words with an IF at 0.25 MHz, the image will be 0.5 MHz away from the wanted frequency.

Superheterodyne receiver image

There are several facts about the superheterodyne receiver image response that can be summarised quite easily.

  • Image is twice the IF away from the wanted signal:   Frequencies that enter the IF are spaced from the local oscillator by an amount equal to the intermediate frequency. Therefore if one frequency is LO + IF, the other one will be LO – IF. The difference between these two is two times IF, i.e. twice the intermediate frequency. It is for this reason high intermediate frequencies may be chosen as it improves the image performance.
    Superheterodyne receiver image is twice the IF away from the wanted signal
    Superheterodyne receiver image is twice the IF away from the wanted signal
  • RF tuning:   In order to ensure that the RF tuning does not introduce any undue signal reduction, and it ensures the image response is reduced by the maximum amount, it tunes in line with the local oscillator. Thus if the LO increases by 1 MHz then the RF tuning must increase by the same amount to ensure it tracks the received signal frequency correctly. Early radio receivers used ganged tuning capacitors consisting of two sections. Attached to the same spindle the capacitance of each section changed by the same amount enabling the RF tuning to track at the same rate as the local oscillator.

The superheterodyne receiver image response is a key performance parameter that is measured in receivers. It is possible to reduce it by significant amounts on high performance so that it does not cause any major problems. Low cost receivers normally have some problems although in recent years receiver image performance levels have improved significantly.

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