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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. |
On Sat, 26 Jul 2003 21:55:40 +0100, Paul Burridge
wrote: On Sat, 26 Jul 2003 18:50:45 +0100, Don Pearce wrote: This is a strange way of achieving your aim. You should be going for adjacent channel selectivity in the IF, not the front end. That way you can use a fixed frequency filter, and it can be as lossy as you like - and it will be lossy to achieve these kinds of selectivity with achievable unloaded Q. If you put this filter at the RF stage where it will help with adjacent channel selectivity,m it must be before the first amplifier, and that will have killed your hoped-for sensitivity increase. If you put it after that amplifier, then you may as well leave it where it belongs - in the IF. Thanks, Don. I'm aware this is a daft way of doing it, but I don't have much choice. The IF stages of this rx are not accessable and no schematic is available, either. The makers have declined to make them available and threatened any service personel who do so with termination of their contracts. Faced with these obstacles, I don't see any other option, apart from boosting the tx power instead; a solution which just brings a different load of problems. -- "I believe history will be kind to me, since I intend to write it." - Winston Churchill Hello Paul, it is time to dump that receiver and start again. It is too small and tiddly and I know you do not like surface mount size components. I would imagine that you have plenty of room inside that robot of yours to take a fairly large die cast box. My suggestion is find a radio control receiver that you can get circuit diagrams for or build one from scratch on large pieces of printed circuit board using "ugly construction" or "dead bug construction". Google those terms and you will see what I am talking about. You can then build the various stages of the receiver on separate boards and try all the different things you have been wishing to do but couldn't due to miniature parts, lack of access and lack of information. After you have a prototype ugly construction receiver working, you can miniaturize if you wish on the the next unit. You will need spares anyway. Heh heh heh......it is fighting machine. Have a look at National semiconductor application notes for radio control receiver, they might even give you half a dozen chips as free samples. Google search for radio control receiver schematics. "Buy" is a dirty word for me, but you might be able to buy a receiver in kit form with all the information on construction and alignment as well as a "how it works" description. Have Fun, John Crighton Sydney |
On Sat, 26 Jul 2003 21:55:40 +0100, Paul Burridge
wrote: On Sat, 26 Jul 2003 18:50:45 +0100, Don Pearce wrote: This is a strange way of achieving your aim. You should be going for adjacent channel selectivity in the IF, not the front end. That way you can use a fixed frequency filter, and it can be as lossy as you like - and it will be lossy to achieve these kinds of selectivity with achievable unloaded Q. If you put this filter at the RF stage where it will help with adjacent channel selectivity,m it must be before the first amplifier, and that will have killed your hoped-for sensitivity increase. If you put it after that amplifier, then you may as well leave it where it belongs - in the IF. Thanks, Don. I'm aware this is a daft way of doing it, but I don't have much choice. The IF stages of this rx are not accessable and no schematic is available, either. The makers have declined to make them available and threatened any service personel who do so with termination of their contracts. Faced with these obstacles, I don't see any other option, apart from boosting the tx power instead; a solution which just brings a different load of problems. -- "I believe history will be kind to me, since I intend to write it." - Winston Churchill Hello Paul, it is time to dump that receiver and start again. It is too small and tiddly and I know you do not like surface mount size components. I would imagine that you have plenty of room inside that robot of yours to take a fairly large die cast box. My suggestion is find a radio control receiver that you can get circuit diagrams for or build one from scratch on large pieces of printed circuit board using "ugly construction" or "dead bug construction". Google those terms and you will see what I am talking about. You can then build the various stages of the receiver on separate boards and try all the different things you have been wishing to do but couldn't due to miniature parts, lack of access and lack of information. After you have a prototype ugly construction receiver working, you can miniaturize if you wish on the the next unit. You will need spares anyway. Heh heh heh......it is fighting machine. Have a look at National semiconductor application notes for radio control receiver, they might even give you half a dozen chips as free samples. Google search for radio control receiver schematics. "Buy" is a dirty word for me, but you might be able to buy a receiver in kit form with all the information on construction and alignment as well as a "how it works" description. Have Fun, John Crighton Sydney |
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@@ |
Paul Burridge wrote in message . ..
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. Filter at 40 MHz with 40 KHz BW means a very high Q, up to 1000. Cavity filter at 40 MHz is big! You can try active RF tuned stage (at 40 MHz) with a positive feedback, also known as Q-multiplier. In addition to high selectivity this will also result in a high gain, which may overload your actual receiver. In this case you can attenuate the signal at the output of the RF stage. You have to make sure Q-multiplier would start to oscillate. It is an issue by itself. 73, Vlad kb9olm |
Paul Burridge wrote in message . ..
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. Filter at 40 MHz with 40 KHz BW means a very high Q, up to 1000. Cavity filter at 40 MHz is big! You can try active RF tuned stage (at 40 MHz) with a positive feedback, also known as Q-multiplier. In addition to high selectivity this will also result in a high gain, which may overload your actual receiver. In this case you can attenuate the signal at the output of the RF stage. You have to make sure Q-multiplier would start to oscillate. It is an issue by itself. 73, Vlad kb9olm |
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. |
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. |
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