Intermediate Frequency (I.F.) filter normally passes either the sum or
difference frequency, and this is then processed into the sound we hear.
Because nothing is perfect, certain "harmonics" will also get through if
they are strong enough. For example, if a radio's I.F. is 10.7 MHz, we
might be able to tune to a frequency 21.4 MHz (2 x I.F.) above (or below,
depending on the radio's design) a strong signal and hear it! This is more
evident in a double-conversion radio than a triple-conversion radio,
because the triple-conversion radio's 1st intermediate frequency is quite
high. This causes the image to be so far off frequency that it is easy to
effectively filter it out.
So remember that just because a radio doesn't receive something which
another does is not necessarily an indication of a problem. The one radio
may simply not be "tricked" into picking up an image! This rejection of
undesired signals is one reason that a triple-conversion receiver costs
more than a similar dual-conversion model.
You might be interested in finding more out about radio. One good
location to start looking is your local public library. You might also
wish to contact the A.R.R.L., as they are an excellent source of
informative texts on the subject.
Contact:
Amateur Radio Relay League
225 Main St.,
Newington, CT 06111-1494
USA
(CD EB 2/22/00)
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