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Hf band pass filters head scratcher?

Discussion in 'General Ham Radio Discussion' started by fodendaf, Mar 25, 2015.

  1. fodendaf

    fodendaf Active Member

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    I was looking at different radio designs on the net and the amateur band only radios have a single bandpass filter switched in and out for each ham band. But I have used many general coverage radios for example the kenwood ts850 which as you tune you hear a relay switch at every half megahertz, inside and outside any ham band. That would mean the radio would need 56 band pass filters which is clearly overkill. So can someone explain what is happening here, i.e are bandpass filters combined in series like gearsticks on a truck to lower the number needed or what?


     

  2. Captain Kilowatt

    Captain Kilowatt Professional Amateur
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    Not sure how the TS850 does it but I have never seen a radio that switches every half MHz.Perhaps the relay switching you hear is not switching bandpass filters but rather something else. Usually the radio has either amateur band filters or usually a broader bandpass filter that switches. My IC735 switched filters at approximately 2,4,8, 12,18 and 24 MHz or somewhere close to that. Each filter covered one or more bands. Bandpass filters cannot be switched in series because one would be tuned to pass a signal while the other would be tuned to reject it.
     
  3. fodendaf

    fodendaf Active Member

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    Thanks for answering my question CK. As the 850 died with me and was sold on I cannot check it again but I distinctly remember a relay clicking in it and the receive dropping out momentarily on 7, 7.5, 27, 27.5 .Perhaps I am forgetful of other bands but truth is I didn't spend much time listening to them. While I would expect a dedicated filter for 40 I always found it unusual to hear the clunk on 11. I just checked my ts480sat and found a click on 7.030 but that disappears with the tuner out so cannot be a bandpass. I found another on 14.500 but that is to be expected. The only other explanation I can think of is how the 850 uses a pll for less spurs plus a dds whereas the 480 just uses the dds. Reminds me of some old hybrid radios I saw on the net possibly modded where the vfo covered so much and a switch was fitted to cover twice that amount. Perhaps the clunk was to expand the range of the pll like bandswitching on a cb. But I am guessing too much. It would be helpful if someone else familiar with the radio chimed in.
     

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