The Appearance of the
Image-Frequency
Heterodyning is the combining of
the incoming signal with the local oscillator signal. When
heterodyning the incoming signal and the local oscillator signal
in the mixer stage, four frequencies are produced. They are the
two basic input frequencies and the sum and the difference of
those two frequencies. It is possible for superheterodyne
receivers to receive two different stations at the same point of
the dial therefore.
| |
- fif = frx - flocal
oscillator
- fif = flocal oscillator - frx
|
There aren't any components which can distinguish a
negative frequency of a positive frequency. We can
measure the magnitude of the frequency only
therefore:
fif = | flocal oscillator - frx | |
The result is a second reception
frequency as a �mirror image� around the intermediate frequency.
Assuming an intermediate frequency of 60 MHz, the local
oscillator will track at a frequency of 60 MHz higher than the
incoming signal. For example, suppose the receiver is tuned to
pick up a signal on a frequency of 1030 MHz. The local
oscillator will be operating at a frequency of 1090 MHz. 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
60 MHz, the IF frequency. This IF frequency is then amplified in
the IF stages and sent on to the detector and audio stages.
Any signal at a frequency of
60 MHz that appears on the plate of the converter circuit will
be accepted by the IF 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 input jack of the converter stage.
Normally these other signals will mix with the local oscillator
signal and produce frequencies that are outside the bandpass of
the 60 MHz IF amplifier and will be rejected. However, if there
is a station operating on a frequency of 1150 MHz, and this
signal passes through the rather broad tuned input circuit and
appears on the input jack of the converter stage, it too will
mix with the local oscillator and produce a frequency of 60 MHz (1150 - 1090 = 60). This signal will also be
accepted by the IF amplifier stage and passed on, thus both
signals will be indicated on the screen.
Calculation of the
Image-Frequency
at the example of the FM radio
frequencies (87,5 - 108 MHz):
If an oscillator frequency is assumed above the reception
frequency, then intermediate frequencies have to be expected
from two reception frequencies at the exit of the mixer stage.
- f1 = foszillator +
fZF
- f2 = foszillator -
fZF
|
|
(Only the algebraic sign would change at an
oscillator frequency below the reception frequency
here.) |
If I want to receive the highest
frequency, then the image frequency must be below the receiving
frequency more than the bandwidth (and reversed). Well, if i
want to receive a frequency of 108 MHz, then a strange
transmitter may send at approximately 87 MHz without to
disturbe.
I insert these two frequencies into the equations and transpose
the oscillator frequency:
- 108,5 MHz = foszillator + fZF
- 87,0 MHz = foszillator - fZF
|
- foszillator = 108,5 MHz - fZF
- foszillator =  87,0 MHz +
fZF
|
I can convict both relations in
one equation now:
108,5 MHz - fZF = 87,0 MHz + fZF
... and can transpose to the IF:
2fZF = (108,5 - 87,0) MHz
fZF = 10,75 MHz
By a fluke: All the radio sets
for this FM-waveband work on the intermediate frequency of
10,7 MHz!
| If frx>fLocal Oscillator then: |
fimage=ftrx - 2� fIF |
| if frx<fLocal Oscillator then: |
fimage=ftrx + 2� fIF |
