Electronic – Changing LC filter passband width

filter

I am building a circuit that uses an LC bandpass filter (with the inductor and the capacitor in parallel between the signal and ground lines). However I am having a problem in that there is no way that I can tell to change the passband width of the filter.

I have given details of my specific situation below, I am really wanting a general answer: how does one change the passband width of a parallel LC filter? This seems like such a simple question, but Google has failed to give me the answer.

Specific situation: I am trying to build a shortwave radio receiver. As such, I am needing a passband width of somewhere around 5kHz to 10kHz. However with my current component values, which is a 2.844uH inductor and a variable capacitor which at 50pF is supposed to give a tuned frequency of 13.333MHz for the receiver, the passband width is somewhere around 5MHz which is obviously way too high. According to a simulation which I ran, I need to put a 1 megaohm resistor between the antenna and the LC network, but that blocks most of the signal from the antenna.

Best Answer

Adding resistance to the LC "tank" circuit will widen the bandwidth, but will ruin the Q of the filter at the same time. This is a basic tradeoff. The "Q" (or quality factor) is a measure of how much on-frequency signal gets through, versus how much it can reject the off-frequency signals.

Real schemes to produce wider bandwidth filters use multiple LC's in various combinations, producing a "filter network". It's still difficult to vary the bandwidth without switching components in or out.

Modern digital signal processing techniques can accomplish these things, as they are substituting "adding more circuits" with "adding more logic elements" and program code.