# Bandpass FIlters

#### C.F.L.

Joined Dec 15, 2011
2
Hi guys, first time post on here. Had a look at the suggested threads and couldn't find what I'm looking for...

Got an assignment from College where we are asked to design two bandpass filters for a given bandwidth, one series, one parallel.

Now, I know I could cascade a low and a high block to chop the frequencies out, and I understand how a pure resonant band pass or stop filter works to pass or block at resonance, but the question has thrown me a bit because I didn't know you could get pure series or pure parallel (i.e. classic RLC circuit) filters with a given bandwidth?

Values given where resonant frequency at 7.5kHz and a bandwidth of 400Hz.

Anyone point me in the right direction?

Edit: Just read the homework help sticky! I'm not after the answers, I'm well aware the only way to understand it is to plough through it yourself. Just don't want to waste my time trying to research something that doesn't exist!

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#### t_n_k

Joined Mar 6, 2009
5,455

Are there any requirements for filter attenuation at the center frequency?
Are there source or load resistances to consider?

Attached is a simple & somewhat contrived series LCR notch filter with spec shown. Not a bandpass - rather a band-stop filter. It does however show you can select values to achieve a goal.

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#### C.F.L.

Joined Dec 15, 2011
2
Hi t n k, thanks for the reply.

No, didn't get any further parameters.

I did design one filter, it had an 88k resistor in series after the voltage source, then a 4.5nF cap and a 100mH coil (with 1Ω coil resistance) connected to ground. I ran a bode plot and it was resonant at 7.5kHz and had an effective BW of about 400Hz, but my query is that is this a series or parallel circuit? To me it's both, so how can they ask me to design a series and parallel one?

I know the Rseries value is a bit high, but I had to work it back using the series and parallel Q factors.

I feel I may just be reading too much into the question.

Edit: Thinking about it further, the only way it makes sense to me is if it only means the filter section itself being series or parallel, not the entire circuit...