Michael Black wrote:

starman ) writes:

I was thinking of a superhet with a single gang cap' to tune the
oscillator and no front-end preselection tuning, which would require
another gang on the cap'.


If they make them, they are going to be rare.

If you don't have front end tuning, the receiver is going to overload
on strong signals. If the IF is too low compared to the signal frequency,
you also will never know which signal you are receiving is the one you
want, and the image frequency that you don't want.

If you've got a receiver with a low IF frequency, but front end tuning
not ganged to the local oscillator tuning, you may be able to null
out the image frequency, but you will have to keep adjusting both knobs.
It would be easy to mistune the front end tuning, and tune in the image
frequency. That's why all receivers have ganged tuning, at least after
it was invented decades ago.

If the IF is higher in frequency, of course one can use other techniques.
Put the IF in the HF range, and the image response will be MHz away, and
the front end will not need constant tuning. That lead to the separate
front end tuning in the sixties, where it only needed peaking every so often.
For limited range receivers, such as for only the ham bands, a suitably
high IF could mean that one could use bandpass filters at the front end,
ie they tuned a fixed 500KHz or so segment, and did not need tuning
as you crossed the band.

Or put the IF above the shortwave frequencies, and you have more leeway.
There, the image frequency is the other side of the IF, so one could use
a low pass filter, with a cut-off of 30MHz, though that still means
the active stages before the first IF filter see a 30MHz range of frequencies,
which may lead to overloading. At least some receivers, once first IFs
went that high, allowed for a low pass filter and some sort of preselection,
so you could choose.

Michael

The most common tube type AM radios (5-tubes) used a two gang tuning
cap' for the front-end tuning and oscillator but I think there were some
cheap models with no front-end RF amp' or preselection tuning. The
antenna was connected to the mixer which had some gain to off set the
lack of an RF amp' stage.


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