Tunable Bandpass Filter
in [Design]
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From: Tim Wescott on 16 Nov 2009 16:41 On Mon, 16 Nov 2009 13:19:33 -0800, Guy Eschemann wrote: (top posting fixed) > 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 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. You're kind of painting yourself into a corner, and the corner is called "eight non-overlapping band-pass filters". Or consider that you're already on the wrong side of some active electronics, so you've already levied most of the disadvantages of a superheterodyne receiver against yourself. Why not just go the rest of the way and make it a superhet? Upconvert to something convenient like that 21.4MHz, filter, then use a fixed downsampling scheme. The whole reason that Armstrong invented the superhet was to dodge the difficulty of trying to make a good agile filter at RF -- here you are 75 years later struggling with the same problem, yet the answer may still be the same one. -- www.wescottdesign.com
From: Tim Wescott on 16 Nov 2009 16:42 On Mon, 16 Nov 2009 13:32:59 -0800, Guy Eschemann wrote: >> It would help to know how sharp you need the filtering. > > Ideally: -60dB within the 80kHz guardband. You need a superhet. -- www.wescottdesign.com
From: ChrisQ on 16 Nov 2009 17:03 Tim Wescott wrote: > On Mon, 16 Nov 2009 13:32:59 -0800, Guy Eschemann wrote: > > >>> It would help to know how sharp you need the filtering. >> Ideally: -60dB within the 80kHz guardband. > > You need a superhet. > and a crystal filter, unless you mix down to baseband + a low pass filter. There are probably mobile radio ic's that would do the job, but not really my field. Silicon Labs, Broadcom and others... Regards, Chris
From: ChrisQ on 16 Nov 2009 17:07 ChrisQ wrote: > Tim Wescott wrote: >> On Mon, 16 Nov 2009 13:32:59 -0800, Guy Eschemann wrote: >> >> >>>> It would help to know how sharp you need the filtering. >>> Ideally: -60dB within the 80kHz guardband. >> >> You need a superhet. >> > > and a crystal filter, unless you mix down to baseband + a low pass filter. > > There are probably mobile radio ic's that would do the job, but not > really my field. Silicon Labs, Broadcom and others... > > Regards, > > Chris Thinking again, plug 'software defined radio' into google... Regards, Chris
From: Joel Koltner on 16 Nov 2009 17:13
"ChrisQ" <meru(a)devnull.com> wrote in message news:C2kMm.7618$Gn.1084(a)newsfe26.ams2... > Thinking again, plug 'software defined radio' into google... Yea, but a very real problem that SDRs have is that while, sure, you can get beautiful, near-vertical skirts around a filter, if there's a strong interferer nearby, you have to filter it prior to digitization or at best you lose SNR for the intended signal (desensing)... and at worst that SNR goes negative! Although you probably know this. :-) |