need help with active filters

Discussion in 'Wireless & RF Design' started by yourownfree, Jul 31, 2009.

  1. yourownfree

    Thread Starter Active Member

    Jul 16, 2008
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    I would like a narrow bandwidth just like a notch filter, but I want that signal to pass not be stopped. The bandpass filter is not narrow enough. The frequencies in question are for the 1750 meter band. 160 kHz to 190 kHz. I would like it to be variable so as to be able to dial in on a specific frequency.
    I would like to use an active filter rather than an LC type. I am just not seeing what i am looking for when I searched the forum or search engines.
    I am very interested in the 1750 meter band operation. Any help thanks.
     
  2. beenthere

    Retired Moderator

    Apr 20, 2004
    15,815
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    Forgive my ignorance, but I always thought that was the tuner's job?
     
  3. steveb

    Senior Member

    Jul 3, 2008
    2,433
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    You should post more details about what you are trying to do. Designing active filters at that frequency is not easy. Passive filters with buffers/amplifiers would be easier, but still far from trivial. You need coils and caps with very low parasitics at that frequency, if you want narrow bandwidth. Are you just trying to receive the signals? If so, receive them and downconvert the frequency before filtering.
     
  4. rjenkins

    AAC Fanatic!

    Nov 6, 2005
    1,015
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    That is the problem with a directly-tuned receiver and why Superhetrodyne receivers are usually preferred.

    If you use a broad front-end filter then mix with a tunable Local Oscillator covering 260 - 290KHz, you will get an output product from the mixer at 455KHz which is a standard intermediate frequency.

    You can get numerous coils & filters for 455KHz, anything from tens of khz wide down to crystal filters giving a few hundred hz bandwidth, usually used for CW (Morse code) reception.

    Doing it like this, the 1750 Meter filtering only has to be good enough to block the 'image' freqencies around 715 - 745 KHz that could also mix with the local oscillator to give a product at 455KHz.
     
  5. yourownfree

    Thread Starter Active Member

    Jul 16, 2008
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    Sometimes I get off on different tangents and head to places unknown.
    I Think the direction I was going was just to see if I could do it I guess, not sure now. I dont what the heck I was thinking anyway. I guess it would be easier to convert then do the active in the audio section. You know just curious if I could use an rf stage then use the active filter then convert and run it through an active filter again also some noise processing. I read where a guy extracted for instance 60 hz hum, fed it to the input of an amlifier and was able to cancel it out that way. So I would probably try that as well for the noise section. Since two signals the same cancel. I thought he had a great idea. I have an old National radio receiver that is really heavy, it will go down to about 10 khz maybe slightly lower. As I recal there is no conversion in the thing, but man does it work good. Anyhow 160-190 khz is a band that needs no license and is for experimental purpose, 1 watt input power, 49.2 feet of antenna max, counts length of coax too. I should just use David Curry's circuit as it has been already tested and tried. I hope it ok for me to give you a link to the tranceiver and my story. link Thanks for the reply.
     
  6. KL7AJ

    AAC Fanatic!

    Nov 4, 2008
    2,040
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    Greetings, FREE!

    Glad to run across another LOWFER here. I spent quite a long time down on 1750, and have JUST received my 500KC experimental license.

    I'll tell you a deep dark secret about the selectivity you want for 1750. You want a DIRECT CONVERSION receiver! That way, you use a simple LOW pass filter for your selectivity...which can be made arbitrarily narrow. This is the basis of the Lock-In amplfier. You will need a stable 160 khz generator for a local oscillator, but that's the only tricky item.

    Trust me on this one...it will blow your socks off. :)

    eric
     
  7. yourownfree

    Thread Starter Active Member

    Jul 16, 2008
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    0
    I was wondering about that. direct conversion. I guess that is why David Curry did it that way in his design. ok enough of that I guess thanks to all who replied . Is there a section here for 500 kHz band?Tell me, how do you apply and get a 500 kHz license? I did not know that was available in the U.S. I thought It was all 100 mw. That's sounds interesting too. Can we use the 138 kHz band here in the U.S. yet? As you probably figured out I love to build transmitters. If there is a topic in this forum I would love to read about it, or if you can set me in the right direction that would be great. i would also love to see If i can receive your station. I am getting ready to put up antennas as I have been lazy for over 8 years and done nothing to speak of. I have 10 acres of land here to play with and an old windmill tower at about 40 foot or so. cant wait to load it up for fun.
     
  8. KL7AJ

    AAC Fanatic!

    Nov 4, 2008
    2,040
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    Here's the site for the 500kc experiment www.500kc.com

    There are only a limited number of licenses granted; I just managed to squeak through on the last extension...it took about three years and an act of congress (almost) :) . I don't know if or when they're going to have another extension. In the meantime, you can certainly try receiving any of the existing stations. Most of the participants are using digital processing such as used for PSK31. You can get on the mailing list and see what everyone's doing.

    Anyway....no question about it, the direct conversion receiver is the only way to go.

    Eric
     
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