# bandpass filter

I was wondering why there is a low end pass ; low end stop; high end pass; and high end stop.

I have never taken a analog filter class before, But if I want a bandpass filter wouldnt I just want a Lower bound and upper bound for what passes? what would i even set the other 2 points for?

Any help is much appreciated I have been reading for hours.

• You can't have good frequency-domain behaviour and good time-domain behaviour. – immibis Jul 24 '16 at 22:16
• okay so yeah a better filter takes longer? so what would i set those other values to if i wanted a range of a to b passing? – lightro Jul 24 '16 at 22:19
• Don't get too lost in the math,as the software assumes ideal conditions. The equations are only guidelines. Real-world conditions will throw you curve balls, such as ringing, over/undershoot, standing and reflected waves, phase shifting, etc. – Sparky256 Jul 24 '16 at 22:41
• A bandpass is used for suppressing unwanted frequencies. But what means "suppress"? It is necessary to specify for the unwanted frequencies the minimum amout of damping that is required. This is the purpose of specifying the (max. allowed) stop band gain – LvW Aug 24 '16 at 7:53
• It's not clear to me what you're asking for - is it about the fact that the attenuation has no sharp edge but tapers off in both directions? Please clarify. – try-catch-finally Oct 23 '16 at 10:14

When you build an ideal filter, of any type, you usually have two regions: the pass band, and the stop band.

In an ideal filter in the pass band you want a very precise, flat gain, maybe 0dB, while in the stop band you want no gain at all, ideally $-\infty$dB.

Now, this is unfortunately not possible in real life. When you build a filter it will have some gain even in the stop band, gain in the pass band will not be flat, and the transition between pass band and stop band can't be infinitely small.

These problems mainly comes from the fact that an ideal filter is not a "nice" function in the f-domain, i.e. it has discontinuities, sharp turns (is this how you call them?) and so on. This needs an infinite time domain response, and this is of course impossible.

Your filter building tool takes care of all these problems by letting you specify all the parameters to build a realistic filter.

Gain is guaranteed to be at most stop band gain for frequencies below low end stop and high end stop, while it is guaranteed to be pass band gain$\pm$half of pass band ripple for frequencies in the low end pass..high end pass interval.

Please note that this is far from being an ideal filter: gain is not flat (it has ripple!) and you have some transition regions between stop band and pass band, and vice versa.

To better understand what is going on, try to input two very near frequencies for low end stop and pass. What happens to the order of the filter? Try to input 0dB as gain in the pass band, and -200dB as gain in the stop band, again, what does the order look like?

The other points are to set a value for how fast the filter rolloffs are. Typically the low (or high) end pass frequencies are values for which the response is 3 dB down from the center frequency. If that is all you specify the behavior of the filter beyond those frequencies is unknown. By also specifying the low (or high) end stop frequencies, you are setting how fast the filter rolls off beyond the pass frequencies. Typically the stop frequencies are values for which the response is 60 dB down from the center frequency. Thus with all these frequencies specified, you now know the 3 dB bandwidth of the filter and also for which frequencies, on both sides of the bandpass, where the response will be at least 60 dB down.

You ask why passband and stopband have to be specified, when you're only interested in the passband. If this were for a TV station receiver, you would want to pass the channel allocation. You could ignore the adjacent channels' frequency bands, because transmission licenses disallow the transmission on two adjacent channels in any locality. But the alternate channels (target_channel +2 and target_channel -2) might have transmissions, so you want to exclude those.

The stopband spec allows you to specify how much attenuation and where in frequency it matters. It MUST matter, or you wouldn't need a passband filter at all!