Tunable Bandpass Filter
in [Design]
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From: Tim Wescott on 16 Nov 2009 17:17 On Mon, 16 Nov 2009 22:03:03 +0000, 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 Baseband + lowpass will be difficult. Normal I/Q downconversion with the carrier "off to the side" probably won't give you 60dB of rejection. You could get that rejection by using the Weaver method, but only if you could tolerate the DC bias in your baseband signal squatting right in the middle of the signal that you're trying to decipher. OTOH, if you _could_ deal with the DC bias, then it may work quite well, and fit well into your preferred downconversion scheme. -- www.wescottdesign.com
From: ChrisQ on 16 Nov 2009 17:53 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. A crystal filter is still hard to beat on cost and performance, even now :-)... Regards, Chris
From: Joerg on 16 Nov 2009 19:38 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. > 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. > The challenge with that would be to find matched tuning elements. It'll be almost down to PIN diodes which can be had as duals and dual FETs but those aren't so hot when you need a spiffy shape factor because they are resistive elements. Then there's the old scheme of having several resonant filters spread apart like it was done in the old tube-era TV sets. 2-7MHz is feasible but that's close to the reasonable limit. 600kHz BW at 2MHz will be a real stretch with a resonant scheme. Quite frankly, I'd consider a DSP here. > 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... > Don't get me started on that ... it is the reason I have stopped writing for IEEE. -- Regards, Joerg http://www.analogconsultants.com/ "gmail" domain blocked because of excessive spam. Use another domain or send PM.
From: Jim Thompson on 16 Nov 2009 19:41 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 ;-) ...Jim Thompson -- | James E.Thompson, CTO | mens | | Analog Innovations, Inc. | et | | Analog/Mixed-Signal ASIC's and Discrete Systems | manus | | Phoenix, Arizona 85048 Skype: Contacts Only | | | Voice:(480)460-2350 Fax: Available upon request | Brass Rat | | E-mail Icon at http://www.analog-innovations.com | 1962 | If you wanted a President with balls why didn't you elect Hillary?
From: christofire on 16 Nov 2009 19:44
"Joel Koltner" <zapwireDASHgroups(a)yahoo.com> wrote in message news:lxjMm.225580$ua.102021(a)en-nntp-05.dc1.easynews.com... > "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...) The statement 'The middle frequency is in the range 2..7 MHz, and the required bandwidth is 600 kHz' suggests something similar to the front end of an HF or VHF receiver, about which a great deal has been written. I don't have my own library to hand (I use the IET in London) but I daresay Google knows the names of some the relevant books (http://books.google.co.uk/books?ei=v-sBS6KNOYSNjAeUhOGbCw&ct=result&q=RF+tunable+filter&lr=&sa=N&start=10). Of course, a great deal of filtering (!) of the resulting 978 returns is then needed, but that would be a task for the OP. I think the statement 'How do I get started with this?', in the absence of any evidence of effort spent learning the basics, deserves the the response I gave. Chris |