Tunable Bandpass Filter
in [Design]
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From: Joerg on 16 Nov 2009 19:44 Tim Wescott wrote: > 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. > Yup. Superhet and then one of these puppies, under a buck: http://media.digikey.com/pdf/Data%20Sheets/Murata%20PDFs/CDSCB10M7GA085-R0.pdf http://www.abracon.com/Filters/SAW%20FILTERS/AFS315E.pdf -- Regards, Joerg http://www.analogconsultants.com/ "gmail" domain blocked because of excessive spam. Use another domain or send PM.
From: Joerg on 16 Nov 2009 19:47 Jim Thompson wrote: > On Mon, 16 Nov 2009 16:38:34 -0800, Joerg <invalid(a)invalid.invalid> > wrote: > >> Joel Koltner wrote: >>> "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...) >>> >> For ultrasound engineers and Radar guys it's routine, except that we >> call them tracking filters. They consist of a fixed lowpass and a >> highpass that's tuned downwards while echoes are received. The challenge >> is to make them reproducible in production without any alignments. Many >> tricks there, such as servo or pilot tones, but that's as far as I am >> allowed to speak in public. >> >> > [snicker] > > Probably the same way I do sonar ;-) > Sonar? In Arizona? You guys don't even have an ocean :-) -- Regards, Joerg http://www.analogconsultants.com/ "gmail" domain blocked because of excessive spam. Use another domain or send PM.
From: Joel Koltner on 16 Nov 2009 19:55 "Joerg" <invalid(a)invalid.invalid> wrote in message news:7me9k9F3i7n2mU1(a)mid.individual.net... > For ultrasound engineers and Radar guys it's routine, except that we call > them tracking filters. They consist of a fixed lowpass and a highpass that's > tuned downwards while echoes are received. What frequency ranges and how steep are the skirts though? And what is the tuning mechanism? > The challenge with that would be to find matched tuning elements. Yep. > Don't get me started on that ... it is the reason I have stopped writing for > IEEE. Did you have much luck with their insurance offerings? ---Joel
From: Tim Wescott on 16 Nov 2009 19:59 On Mon, 16 Nov 2009 22:53:17 +0000, ChrisQ wrote: > Joel Koltner wrote: >> "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. :-) >> >> >> > From analog radio days, yes, but have no experience of sdr at all. Just > something i've been reading about in the last few months. From what I > can see, it doesn't get round the need for a low phase noise lo to > prevent filter washout, irrespective of how the filter and processing > are implemented. Yup. Not to mention that the dynamic range of most ADC's is a huge restriction. The more you can control the bandwidth before conversion the better off you'll be. > A crystal filter is still hard to beat on cost and performance, even now > :-)... Or a ceramic one, depending on your app. -- www.wescottdesign.com
From: Joerg on 16 Nov 2009 20:13
Joel Koltner wrote: > "Joerg" <invalid(a)invalid.invalid> wrote in message > news:7me9k9F3i7n2mU1(a)mid.individual.net... >> For ultrasound engineers and Radar guys it's routine, except that we call >> them tracking filters. They consist of a fixed lowpass and a highpass that's >> tuned downwards while echoes are received. > > What frequency ranges and how steep are the skirts though? > Typical ranges are 4-6MHz, 6-9MHz, 11-15MHz, 17-22MHz and similar. The steepness isn't that great, something like 8MHz at -6dB and -60dB at 9MHz, for example. If the OP needs stellar channel rejection he has IMHO only two options, superhet or DSP. > And what is the tuning mechanism? > Varicaps, usually. However, purchasing duals in order to be able to servo has become a bear. And one has to remain friends with the purchasing department since those are the guys who get all the Christmas bonbons :-) >> The challenge with that would be to find matched tuning elements. > > Yep. > >> Don't get me started on that ... it is the reason I have stopped writing for >> IEEE. > > Did you have much luck with their insurance offerings? > Nope, they couldn't do it :-( -- Regards, Joerg http://www.analogconsultants.com/ "gmail" domain blocked because of excessive spam. Use another domain or send PM. |