Choice of 455 kc and 10,7 mc as IF frequencies

[Joe Buch]

At 20:50 3/19/99 -0700, Will White wrote:
>Just an idle thought I had, as I was browsing my radio books: why are so
>many, and at one time nearly all, radio receivers designed to use IF
>frequencies of 455 kc and/or 10.7 mc? There must be some fundamental
>electronic principle that applies to this choice, I can't imagine these
>IFs are only a matter of convention and interchangeability of parts. So,
>how were these frequencies chosen, and why have they remained the
>overwhelming favorites?
>--

The frequencies were standardized for the usual economic efficiencies, but
they were chosen to accomodate sound engineering design requirements.

The 10.7 MHz IF frequency was chosen as the lowest IF frequency which would
still prevent in band images in the FM broadcast band.  The FM band for
most of the world goes from 88 to 108 MHz.  A receiver tuned to 88 MHz
would have an image response 21.4 MHz higher at 109.4 MHz which is out of
band.  This band is used internationally for aeronautical navigation aids.
Yeah I know they could also have chose 10.5, 10.4 or some other arbitrary
frequency in this range.  I do not have any info on why they standardized
on precisely 10.7.

The choice of 455 KHz for AM receivers is a compromise between two design
factors.  The medium wave broadcast band now runs from 540 to 1700 KHz.
Back in 1933 US stations were licensed between 550 and 1500 KHz.  This was
about the time the superheterodyne circuit was becoming the standard design
for broadcast receivers.  A receiver tuned to 550 KHz with a 455 KHz IF
would have an image response 910 KHz higher at 1460 KHz.  To eliminate
in-band images, the IF frequency would need to be higher.  On the other
hand stations were spaced on 10 KHz channel assignments.  The higher the IF
frequency, the wider the selectivity for a given number of stages and IF
transformer Q.  Some automobile receivers in the old days used 256 KHz IF's
to improve selectivity.  The Hammarlund HQ-180 uses a final IF of 60KHz to
provide optimal CW and SSB selectivity.  So the choice of 455 KHz was a
compromise between in-band image rejection and selectivity.

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