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Old 23rd Apr 2021, 9:46 am   #10
Radio Wrangler
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Join Date: Mar 2012
Location: Fife, Scotland, UK.
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Default Re: Where is the RF amplifier?

You can have two resonant tanks, each tuned and couple them lightly together. This gives you narrower (= better) bandwidth than you'd get with a single tank. If you couple them too tightly together it degrades into just a single tuned circuit response and you've wasted a whole coil and one gang. If you couple them too weakly, the bandwidth narrows further but the insertion loss shoots up and your receiver becomes deaf.

Having an RF stage valve between the two tuned circuits acts as an isolator so it's easy to run them at high Q and good selectivity without light coupling and associated losses.

So the change in sensitivity of a set with a true RF stage is even better than the gain of the valve would imply, if the selectivity is kept the same.

This is one of those effects of a design compromise which isn't obvious on the circuit diagram.

Some comms receivers like the AR88 have TWO RF stages. Not because they need the gain, but because they need the isolation between RF tuned circuits in order to get more RF selectivity.

David
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