Tunable Bandpass Filter
in [Design]
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From: Guy Eschemann on 16 Nov 2009 16:19 Joel, There are 8 non-overlapping analog channels in the range between 2 and 7 MHz. Each channel is approx. 600 kHz wide. I'm not sure about the power levels yet, but the channel selection filter comes after the preamplifier and the receiver main amplifier (AGC), so the amplitude is pretty much controlled at this point. If possible, I'd like to avoid any mixing up and down. I'm actually considering a mixerless approach (bandpass sampling) to translate the channel of interest down to DC, so it would be really annoying to mix the signal up and down just for filtering. Also, I don't want to use a digital filter at this stage. This would require sampling the band of interest at something like 30 MHz, which is bad for power consumption. Thanks for your help! Guy. > Couple of questions: > > -- Is the entire analog channel also 2-7MHz? Or wider? > -- What power levels are you dealing with? > > A few approaches are: > > -- Mix your signal with an LO of, say, 19.4-14.4MHz such that the band center > of interest is at 21.4MHz (use a low pass filter so that you don't pick up the > image frequencies above 7MHz). Use a cheap off-the-shelf 21.4MHz IF filter > (probably ceramic) to get your 600kHz passband (this is a Q of > 21.4MHz/600kHz=36 -- easy peasy). Mix again with the same LO to put your > center band back where it came from. (High power levels -- much above, say, > 0dBm -- start creating intermods and compression problems from the mixers..) > -- Build yourself a bank of switched capacitor and inductors that get switched > in and out as appropriate to "build" a bandpass filter wherever you need it. > (Use PIN diodes or MMIC switches for the switching.) If you need very fine > control you'll end up using a varactor diode (or perhaps a DC bias on an > inductor) to set the exact center frequency. (High power levels here push > your varactor or inductors far enough outside of their linear ranges that get > start getting frequency responses that are functions of power levels as well > as intermods.) > -- Same as above, but use relays for switching inductors and capacitors in and > out and motorized variable capacitors (or slug-tuned inductors) if you need > fine tuning. (Higher power levels are attainable, but you end up consuming a > lot of physical space and tuning is slow.) > > If the filter is simple enough, you *might just* be able to get away these > days with an FPGA-based "all digital" implementation: Feed your signal to an > ADC, have the FPGA run a FIR or IIR filter, and spit it back out to a DAC.. As > with most things "DSP," there are a lot of upsides, although your signals are > at a high enough frequency you'll probably consume a fair amount of power > running all the multipliers in your FPGA, and it isn't going to be the > "bargain basement price" series of FPGAs that'll have enough horsepower to > pull it off. > > ---Joel
From: ChrisQ on 16 Nov 2009 16:19 Guy Eschemann wrote: > I need to design an analog channel selection filter (tunable bandpass > filter) for a communications application. The middle frequency is in > the range 2..7 MHz, and the required bandwidth is 600 kHz. How do I > get started with this? > Many thanks, > Guy. I remember seeing an article years ago about a dual mixing technique that used two minicircuits sbl1 type balanced mixers in cascade with a common variable local oscillator and a crystal filter interspersed between the two mixers. Draw it out on paper and do the sum / differences to see how it works. You don't say how many db/octave at the edges, but a crystal filter can provide a very sharp rolloff. The 600Khz passband may be better handled by an lc or active filter with fast opamps, depending on requirements. Otherwise, how about using fast opamps in a byquad or state variable configuration ?... Regards, Chris
From: Joel Koltner on 16 Nov 2009 16:32 "christofire" <christofire(a)btinternet.com> wrote in message news:nvGdnUeJQYfUXZzWnZ2dnUVZ8k-dnZ2d(a)bt.com... > How about visiting a library and reading some relevant books? I'd love to hear it if you could point to any book that has a large amount of text specifically devoted to *tunable* filters. I have plenty of filter books (including many of the "classics"), and most give little more than passing mention to them. (I suppose because -- other than the "mix it up to a fixed frequency with a good filter" method than Jan and I mentioned -- most implementations I'm aware of are some variety of the "brute force" method anyone would think of, so perhaps there's not a whole lot to say...) One approach I forgot to mention: I have seen people build active filters with multiplying DACs as the tuning elements up to better than a MHz, but I think 7MHz would be quite a stretch (the DAC's parasitics start to eat you alive). I've messed around with gyrators occasionally, and while you can build them to tens of MHz with fast op-amps, tuning is still tricky -- the last time I went down that path I convinced myself a way to make it work might be to bulid a set of two filters with matched tuning elements, have one be the "real" filter, and the other servoed to it via its twin that's constantly seeking to peak a synthesized signal (from a DDS or whatever) that's going through it. Alas, this approach is best for small signals and an IC implementation. Speaking of which... the IEEE has plenty of articles on tunable filters, but most are oriented towards IC implementations. Too bad the standard membership fee of $176/yr (!) doesn't get you *any* on-line access to the them... ---Joel
From: Guy Eschemann on 16 Nov 2009 16:32 > > It would help to know how sharp you need the filtering. Ideally: -60dB within the 80kHz guardband.
From: Joel Koltner on 16 Nov 2009 16:39
"Guy Eschemann" <guy.eschemann(a)gmail.com> wrote in message news:07f36aa1-807a-4b02-a85a-" > There are 8 non-overlapping analog channels in the range between 2 and > 7 MHz. Each channel is approx. 600 kHz wide. For 8 channels I think I would just build 8 lumped-element filters and switch them in and out. -- It's easier to get decent repeatibility using, e.g., 2% inductors and capacitors when each component only influences one frequency band rather than a single "switching" filter where multiple inductors and capacitors may interact and their tolerances then build on one another. Depending on what order filter (how sharp) it needs to be, 8 filters can still be pretty compact. ---Joel |