Image rejection is an important aspect of the design of any radio using the superheterodyne principle.
As a result of the mixing or RF multiplication process, two signals can enter the receiver signal chain - the wanted signal and the image.
To ensure that the image rejection is sufficiently high, adequate RF selectivity must be in place.
What is image response?
To look at image rejection and how the need for it arises, it is necessary to look at the way in which a superheterodyne radio receiver works.
Note on the Superheterodyne Radio:
The superheterodyne radio uses a technique where incoming signals are mixed or multiplied with the signal from an internally generated local oscillator. In this way the signals can be converted in frequency to an intermediate frequency where they can be filtered. By using a variable frequency local oscillator, a fixed frequency intermediate frequency filter can be used.
Read more about the Superheterodyne Radio.
The basic concept of the superhet radio means that it is possible for two signals to enter the intermediate frequency, IF, amplifier. For example with the local oscillator set to 5 MHz and with an IF of 1 MHz it can be seen that a signal at 6 MHz mixes with the local oscillator to produce a signal at 1 MHz that will pass through the IF filter. However if a signal at 4 MHz is also able to produce an output at 1 MHz. It is clearly unacceptable to receive signals on two frequencies at the same time and it is possible to remove the unwanted one by the addition of a tuned circuit prior to the mixer
Fortunately this tuned circuit does not need to be excessively 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 1 MHz, the image will be 2 MHz away from the wanted frequency.
Effect of poor image rejection
A receiver with a poor level of image rejection will suffer from much higher levels of interference than one with a high level of image rejection. In view of this, radio receivers to be used in high performance radio communications applications need to have a good image rejection performance.
When a radio receiver has a poor level image rejection signals which should not be received as they are on the image will pass through the IF stages along with the required ones. This means that unwanted signals are received along with the wanted ones and this means that the levels of interference will be higher than those with a high level of image rejection.
In addition to this the image signals will "tune" in the opposite direction to the wanted ones. When they interfere heterodyne notes will be heard and as the receiver is tuned, the pitch of the signals will change. In view of this it is very important to reduce the image response to acceptable levels, particularly for exacting radio communications applications.
It is clearly important to specify the level of image rejection. The specification compares the levels of signals of equal strength on the wanted and image frequencies, quoting the level of rejection of the unwanted signal.
The image rejection of a receiver will be specified as the ratio between the wanted and image signals expressed in decibels (dB)at a certain operating frequency. For example it may be 60 dB at 30 MHz. This means that if signals of the same strength were present on the wanted frequency and the image frequency, then the image signal would be 60 dB lower than the wanted one, i.e. it would be 1/1000 lower in terms of voltage or 1/1000000 lower in terms of power.
The frequency at which the measurement is made also has to be included. This is because the level of rejection will vary according to the frequency in use. Typically it falls with increasing frequency because the percentage frequency difference between the wanted and image signals is smaller.
Another problem which can occur with a superhet occurs when signals from the antenna break through the RF sections of the set and directly enter the IF stages of the radio receiver. Normally intermediate frequencies are chosen so that there are likely to be no very large signals present which might cause problems. However when the radio receiver has a fixed frequency first local oscillator this is not easy to ensure as it will sweep over a band of frequencies.
The specification for breakthrough is quoted in the same fashion as image rejection. Normally it is possible to achieve figures of 60 to 80 dB rejection, and on some receivers figures of 100 dB have been quoted.
The image response or image rejection and IF breakthrough specifications of a superheterodyne radio are of great importance in many radio communications applications. Any receiver with a poor image rejection specification will appear to receive many more signals than one with a high level of image rejection. As many of these signals will result from the poor image rejection, they are not on the actual frequency to which the receiver is tuned.
More Essential Radio Topics:
Radio Signals Modulation types & techniques Amplitude modulation Frequency modulation RF mixing Phase locked loops Frequency synthesizers Passive intermodulation RF attenuators RF filters Radio receiver types Superhet radio Radio receiver selectivity Radio receiver sensitivity Receiver strong signal handling
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