Re: Double Super Hetrodyne - What does this mean in a CB Radio

"Jan Panteltje" <pNaonStpealmtje@xxxxxxxxx> wrote...

In a 'double' superhet the signal is again mixed to a lower frequency.
So 2 mixers and 2 intermediate frequencies.
This could be because the first IF (intermediate frequency) is for example the
width of a whole band, while the second is the width of one station (provides
the selectivity).

A low IF gives good selectivity but, unfortunately, puts the image frequency only a short
distance away. A higher IF puts the image out of range, but is not so selective. CB, with
just 10KHz spacing, needs selectivity but, with the bandwidth needed in the front-end,
could have trouble rejecting an image just 910KHz away.
A dual conversion superhet CB uses the first If (10.695MHz) to put the image well
out of the rx bandwidth and the second IF (455KHz) to get the desired selectivity. The image
of the first IF is rejected by the front end, the image of the 2nd IF is rejected by the first
IF filter, and adjacent channels are rejected by the 2nd IF filter.
At least in theory, but too many sets have crap filters and some have crystal 1st IF filters to
get selectivity at that point. British sets are now made with crystal filters in order to
meet the legal requirements for adjacent channel rejection.

So you may find stations 2x if it is not done correctly.

Somewhere in my radio collection, I have an old dual Superhet that used no 1st IF filter for
receiving FM... it was straight through from the 1st mix to the second. The set picks up
signals 910KHz away just as if they were on the channel it is supposed to be receiving.
I could get a filter for it, Lou does one that will work in it, but I cannot be arsed as it's
an old radio.


Regards,

Peter.
http://www.citizensband.radiouk.com/



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