Image Frequency Interference in Superherodyne Receivers
It is possible for receivers without an RF amplifier stage to receive two
different stations at the same point of the dial. In a receiver that has no
RF stage, the signal on the antenna is fed through a tuned circuit directly
to the signal grid of the converter tube. Assuming an intermediate
frequency of 455 kHz, the local oscillator will track at a frequency of
455 kHz higher than the incoming signal. For example, suppose the receiver is
tuned to pick up a signal on a frequency of 600 kHz. The local oscillator
will be operating at a frequency of 1,055 kHz. The received and local
oscillator signals are mixed, or heterodyned, in the converter stage and one of
the frequencies resulting from this mixing action is the difference between the
two signals, or 455 kHz, the I-F frequency. This I-F frequency is then
amplified in the I-F stages and sent on to the detector and audio stages.
Any signal at a frequency of 455 kHz that appears on the plate of the
converter circuit will be accepted by the I-F amplifier and passed on.
So on a receiver with no RF amplifier, the input to the converter is rather
broadly tuned and some signals other than the desired signal will get through
to the grid of the converter tube. Normally these other signals will mix with
the local oscillator signal and produce frequencies that are outside the
bandpass of the 455 kHz I-F amplifier and will be rejected.
However, if there is a station operating on a frequency of 1,510 kHz, and
this signal passes through the rather broad tuned input circuit and appears
on the grid of the converter tube, it too will mix with the local oscillator
and produce a frequency of 455 kHz (1,510 - 1,055 = 455). This signal will
also be accepted by the I-F amplifier stage and passed on, thus both signals
will be heard in the output of the receive.
So any station is likely to experience interference from another station that
happens to be on a frequency which is higher than that of the desired station
by twice the I-F frequency. This is known as image-frequency
An RF amplifier stage ahead of the converter stage provides enough selectivity
to reduce the image-frequency response by rejecting these unwanted signals and
adds to the sensitivity of the receiver.
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