Wednesday, 8 May 2013

Fun with coupled-pair tuned circuits.

I'm not being rude! It's a couple of these, uh, coupled together...
LC Tuned Circuit.
A coupled-pair tuned circuit has a frequency response something like...
Passes signals at fo. Attenuates signals at other frequencies.
One of my missions was to design a frequency-agile (capable of changing frequency in less than 1ms) coupled-pair tuned circuit that maintained a constant width (Δf) as fo varied.

As Q = fo/Δf, for Δf to remain constant requires fo/Q to remain constant i.e. Q must vary in proportion with fo.

In the top diagram, inserting a resistance of value "R" in series with the L and the C sets the Q.

Q = ωL/R, where ω = 2*π*fo. For Q to vary in proportion with fo requires R to be constant.

This was achieved using wideband RF transformers to transform the source & load impedances from 50Ω down to a suitable "R" value. Here endeth today's trip around my brain.

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