Quote:
Originally Posted by Radio Wrangler
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.
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That's true, however, single tuned circuits with a buffer-isolator between them, don't offer the versatility in the response of two circuits partially coupled together.
If you couple them tightly, it certainly does degrade to a single tuned circuit response. Loosely, and you get the product of the two responses. Very loosely, and you get the same, with more loss. But there's a range of coupling where the response broadens without significantly increasing in amplitude.
It's impossible to get a Butterworth response (maximally flat) with two tuned circuits separated by an amplifier. But properly coupled with no amplifier between, you can.