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#1
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![]() Hi chaps, I've decided to bite the bullet and try to build an RF filter for 40Mhz. This filter will ideally have a very, very sharp characteristic at one single spot frequency +-20Khz and attenuate the crap out of anything either side of this. It'll need to be tunable over a range of say 200Khz. Can anyone give me a steer on what type of arrangement would be best suited to fit this purpose? Thanks, p. -- "I believe history will be kind to me, since I intend to write it." - Winston Churchill |
#2
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In sci.electronics.design, Paul Burridge
wrote: Hi chaps, I've decided to bite the bullet and try to build an RF filter for 40Mhz. This filter will ideally have a very, very sharp characteristic at one single spot frequency +-20Khz and attenuate the crap out of anything either side of this. It'll need to be tunable over a range of say 200Khz. Can anyone give me a steer on what type of arrangement would be best suited to fit this purpose? I've got that deja-google feeling all over again: http://groups.google.com/groups?hl=e...com%26rnum%3D6 I like the idea of downconverting to an IF, filtering using standard IF technology, and (if you want the output to be the same frequency band as the input) upconverting using the same local oscillator as the downconverter. Just change the LO frequency (maybe use a frequency synthesizer for stability) to do tuning. Thanks, p. -- "I believe history will be kind to me, since I intend to write it." - Winston Churchill |
#3
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![]() Take a look at lowband filters and duplexers from CellWave, Telewave, etc. Your filter is going to be a coaxial cavity about 2mx20cm with piston for tuning. I am sure there are some better approaches to your task ![]() Vladimir Vassilevsky, Ph.D. DSP and Mixed Signal Design Consultant http://www.abvolt.com Paul Burridge wrote: Hi chaps, I've decided to bite the bullet and try to build an RF filter for 40Mhz. This filter will ideally have a very, very sharp characteristic at one single spot frequency +-20Khz and attenuate the crap out of anything either side of this. It'll need to be tunable over a range of say 200Khz. Can anyone give me a steer on what type of arrangement would be best suited to fit this purpose? Thanks, p. -- "I believe history will be kind to me, since I intend to write it." - Winston Churchill |
#5
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On Sat, 26 Jul 2003 21:20:18 -0700, Watson A.Name - 'Watt Sun'
wrote: In article , mentioned... Take a look at lowband filters and duplexers from CellWave, Telewave, etc. Your filter is going to be a coaxial cavity about 2mx20cm with piston for tuning. I am sure there are some better approaches to your task ![]() How about a helical resonator. They're smaller than a cavity, maybe not as high Q, but still higher than lumped constant tuned circuit. They're tunable, but I'm not sure how much. I'd like to live long enough to see a helical at 40MHz that can 'attenuate the hell" out of signals outside the Fr+/-20kHz channel. |
#6
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On Sat, 26 Jul 2003 21:20:18 -0700, Watson A.Name - 'Watt Sun'
wrote: In article , mentioned... Take a look at lowband filters and duplexers from CellWave, Telewave, etc. Your filter is going to be a coaxial cavity about 2mx20cm with piston for tuning. I am sure there are some better approaches to your task ![]() How about a helical resonator. They're smaller than a cavity, maybe not as high Q, but still higher than lumped constant tuned circuit. They're tunable, but I'm not sure how much. I'd like to live long enough to see a helical at 40MHz that can 'attenuate the hell" out of signals outside the Fr+/-20kHz channel. |
#7
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Watson A.Name - 'Watt Sun' wrote in message ...
.... How about a helical resonator. They're smaller than a cavity, maybe not as high Q, but still higher than lumped constant tuned circuit. I think that's a popular misconception. The resonator Q is essentially the same as the Q of the same part used as a shielded inductor, and the shield actually lowers the Q from what it is with an inductor in free air (so long as it's not large enough to radiate significantly). They're tunable, but I'm not sure how much. They're certainly easily tunable over a few percent, if you need that... But the problem as originally stated implies a filter of fairly high order and low in-band attenuation, which in turn implies resonators of very high unloaded Q. 20kHz bandwidth at 40MHz in a single tank is a loaded Q of 2000, and to keep attenuation low, the unloaded resonator Q should be perhaps 5 times that much. It would be worse for a multi-pole filter. All this tells me it's silly to even think of an LC filter. Add to that the extreme difficulty of getting a set of resonators to tune together. (To get Qu=10000 in a coaxial resonator at 40MHz would take an air-dielectric line nearly half a meter in diameter! Just plain silly.) I'd opt for a front end with very high dynamic range (esp. low third-order intermod products), into a good IF filter, etc., and a communications protocol that optimized whatever performance measure I needed. Talk to the people who build RF communications sytems that go on aircraft carriers. Or talk with hams who design receivers with third order intercepts up in the +50dBm region and higher. By the way, you may do well by putting an ATTENUATOR on the front end, if interference (distortion products), and not desired-signal-strength, is the problem. Distortion products will go down faster than the inserted attenuation. Cheers, Tom |
#8
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Watson A.Name - 'Watt Sun' wrote in message ...
.... How about a helical resonator. They're smaller than a cavity, maybe not as high Q, but still higher than lumped constant tuned circuit. I think that's a popular misconception. The resonator Q is essentially the same as the Q of the same part used as a shielded inductor, and the shield actually lowers the Q from what it is with an inductor in free air (so long as it's not large enough to radiate significantly). They're tunable, but I'm not sure how much. They're certainly easily tunable over a few percent, if you need that... But the problem as originally stated implies a filter of fairly high order and low in-band attenuation, which in turn implies resonators of very high unloaded Q. 20kHz bandwidth at 40MHz in a single tank is a loaded Q of 2000, and to keep attenuation low, the unloaded resonator Q should be perhaps 5 times that much. It would be worse for a multi-pole filter. All this tells me it's silly to even think of an LC filter. Add to that the extreme difficulty of getting a set of resonators to tune together. (To get Qu=10000 in a coaxial resonator at 40MHz would take an air-dielectric line nearly half a meter in diameter! Just plain silly.) I'd opt for a front end with very high dynamic range (esp. low third-order intermod products), into a good IF filter, etc., and a communications protocol that optimized whatever performance measure I needed. Talk to the people who build RF communications sytems that go on aircraft carriers. Or talk with hams who design receivers with third order intercepts up in the +50dBm region and higher. By the way, you may do well by putting an ATTENUATOR on the front end, if interference (distortion products), and not desired-signal-strength, is the problem. Distortion products will go down faster than the inserted attenuation. Cheers, Tom |
#9
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In article , mentioned...
Take a look at lowband filters and duplexers from CellWave, Telewave, etc. Your filter is going to be a coaxial cavity about 2mx20cm with piston for tuning. I am sure there are some better approaches to your task ![]() How about a helical resonator. They're smaller than a cavity, maybe not as high Q, but still higher than lumped constant tuned circuit. They're tunable, but I'm not sure how much. Vladimir Vassilevsky, Ph.D. Paul Burridge wrote: Hi chaps, I've decided to bite the bullet and try to build an RF filter for 40Mhz. This filter will ideally have a very, very sharp characteristic at one single spot frequency +-20Khz and attenuate the crap out of anything either side of this. It'll need to be tunable over a range of say 200Khz. Can anyone give me a steer on what type of arrangement would be best suited to fit this purpose? Thanks, p. -- -- @@F@r@o@m@@O@r@a@n@g@e@@C@o@u@n@t@y@,@@C@a@l@,@@w@ h@e@r@e@@ ###Got a Question about ELECTRONICS? Check HERE First:### http://users.pandora.be/educypedia/e...s/databank.htm My email address is whitelisted. *All* email sent to it goes directly to the trash unless you add NOSPAM in the Subject: line with other stuff. alondra101 at hotmail.com Don't be ripped off by the big book dealers. Go to the URL that will give you a choice and save you money(up to half). http://www.everybookstore.com You'll be glad you did! Just when you thought you had all this figured out, the gov't changed it: http://physics.nist.gov/cuu/Units/binary.html @@t@h@e@@a@f@f@l@u@e@n@t@@m@e@e@t@@t@h@e@@E@f@f@l@ u@e@n@t@@ |
#10
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Paul Burridge wrote in message . ..
I've decided to bite the bullet and try to build an RF filter for 40Mhz. This filter will ideally have a very, very sharp characteristic at one single spot frequency +-20Khz and attenuate the crap out of anything either side of this. It'll need to be tunable over a range of say 200Khz. Can anyone give me a steer on what type of arrangement would be best suited to fit this purpose? It'll need to be a crystal filter, and your requirement that it must be tunable means that you will convert down/up to an IF frequency and back up/down again. (Well, you may not have to convert back up again but you don't tell us your application). Problem with the IF and conversion is the production of images. Images won't be a killer problem because your tuning range is really quite narrow. If you wanted to really cheapskate out some ceramic IF filters also seem to meet your stated needs. Tim. |
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