IF amplifier filters are LC resonator stages separated by individual stages of amplification. The principle is called synchronously tuned filters stages. In if amplifier filters there can be any number of resonators per individual stage.

The “m” in Fig 2 is for the number of resonators per stage. That is the number of capacitor and inductor combinations per stage in your if amplifier filter. In Fig 1 there is only one per stage, i.e. one coil / capacitor combination. Therefore m + 1 = 1 + 1 = 2. Substituting the number two into the m + 1 position indicates “the square root”. If we were using double tuned resonators then m = 2 so M + 1 = 2 + 1 = 3. Substituting 3 in place of m + 1 would indicate the cube root. If we had triple resonators then m + 1 = 4 which is the fourth root or the “square root of the square root”.

The “n” indicates the number of stages we have. In fig 1 we have depicted three stages so 1 / n = 1/3 = 0.3333. Now 2 raised to a power of 0.333 = 1.26 (on my calculator the steps are 2 – Yx – 0.333). From the formula we’re told to subtract 1 which leaves us with 0.26. Because it is a single resonator filter m + 1 = 1 + 1 = 2, so we take the square root of 0.26 which of course = 0.51

That’s the hard part over. Earlier I said a 10 Khz wide IF filter at 455 Khz required a Qbp of 455 / 10 = 45.5. This number is NOW multiplied by the 0.51 calculated from Fig 2 which means Qbp = 45.5 X 0.51 = 23.2

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Article by Ian C. Purdie, VK2TIP of http://www.electronics-tutorials.com

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