About bandpass filtering and filters to improve noise?

Discussion in 'General Technical Questions and Answers' started by KE0EYJ, Jul 24, 2017.

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  1. KE0EYJ

    KE0EYJ Ham Member QRZ Page

    I live at a very noisy QTH in central Seoul, Korea -- a city of over 12 million. Lots of lighting, wifi, broadcast radio, and whatnot, all around me.

    I currently own an ICOM 7300 and recently purchased a cheap little Yaesu FT-891. Low and behold, the FT-891 consistently beats my 7300 on receive, when I utilize the DSP noise reduction. This seems to go against all wisdom of tests, as I understand them (big point), about these two radios. Shouldn't the 7300 be blowing the 891 out of the water?

    Now, I must also say that I am employing every single known trick of the 7300 -- adjusting the Twin PBT in every way, adjusting the internal EQ, and setting the RX bandwidth filtering (which I must have mistaken equaled a bandpass filter, of sorts). and releasing the ALC to boost the snot out of the signal (at my QTH, overloaded with noise). I am even using the notch filter on voice signals, to some success.... I do know how to adjust every nuance of the 7300, and spend a lot of time tweaking. The 891 STILL beats it -- mostly because the 7300 seems to not handle the noise as well.

    I posted about this fact, and someone told me it may have something to do with the 891 having a 3khz roofing filter, and the 7300 not having one. I have very little understanding of roofing filters. My experience with a roofing filter started with a Yaesu FT-950 (which, by all accounts I've read, has a borderline useless set of roofing filters). The roofing filter on that rig only ever seemed to change the EQ of my rig, but didn't help the signal much. Perhaps this skewed my understanding of what the roofing filter can do?


    What I'm curious about now... can I buy/add some kind of external filter, at the antenna, to help my Icom 7300 shine better? Something that will remove all of those extra noises that may be overloading my 7300's front end, so that I can take advantage of the sensitivity of this rig, in this noisy environment? Or maybe I should look into a better home rig with a solid roofing filter?

    Maybe I'm still not understanding what roofing filters do... please school me, because it's still a shaky concept, for me.
    Last edited: Jul 24, 2017
  2. KB1YFD

    KB1YFD Ham Member QRZ Page

    The 7300 can easily overload it's A/D front end. Try turning the RF gain down. Front panel control, no menus to deal with.

    Best, John KB1YFD
    KK5JY likes this.
  3. AF6LJ

    AF6LJ Ham Member QRZ Page

    You have to operate the radio a little differently from an old school super-het...
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  4. VA3VF

    VA3VF Guest

    Turn the RF gain down. The IC-7300 is more sensitive than it needs to be in most situations. Don't activate the RF pre-amps ever, well, in very rare ocasions only.

    As for the roofing filters, the easiest analogy is to think of a single or multi pane main entrance door. The 3kHz roofing filter is a single pane door. The IC-7300 has an 8 pane door. Guess which one lets more people in simultaneously. :D

    Yes, bandpass filtering will improve the situation, but it can be costly, depending on the number of filters you want to use. One per operating band would be needed for maximum effectiveness. They should handle TX power as well, unless you use an RX/TX switch.

    73 de Vince, VA3VF
    Last edited by a moderator: Jul 25, 2017
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  5. KE0EYJ

    KE0EYJ Ham Member QRZ Page

    I have also turned the RF Gain down. That was one of the first things I did, of course!

    I find it works better to turn the AGC off entirely, and ride it myself, however, the 7300 still does a poor job when noise in present, compared to my 891 with DSP and that bandpass filter.
  6. VA3VF

    VA3VF Guest

    This is the ideal setting for any data/digital decoding, but can be dangerous on SSB and CW, specially when using headphones. Be careful!

    73 de Vince, VA3VF
  7. AA7EJ

    AA7EJ Ham Member QRZ Page

    I do not wish to hijack this thread so I take anybody answers in private, if OK.

    "Don't activate the RF pre-amps ever, well, in very rare ocasions only."

    If my failing memories serves me right - RF preamps purpose is to narrow the inbound signals range.
    Not to purposely amplify such range, but to serve as active / tunable bandpass filter(s).

    Just the opposite what was suggested.

    73 Shirley
  8. VA3VF

    VA3VF Guest

    You maybe thinking RF preselector, both passive and active (amp included). An RF amp will amplify everything that is going through the 'door', and after recoverable/audible signal is present, it becomes useless or even counterproductive, as the noise is amplified by the same amount as the signal, keeping the SNR unchanged.

    Vince, VA3VF
    Last edited by a moderator: Jul 25, 2017
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  9. WB2WIK

    WB2WIK Platinum Subscriber Platinum Subscriber QRZ Page

    Direct conversion receivers can't have a roofing filter, since they convert RF to audio all in one shot. All the filtering is done via DSP at low frequencies.

    Roofing filters are for receivers having an I.F.

    However, I "doubt" the 891's receiver is actually better; if the 7300 noise blanking doesn't work well, whatever algorithm they're using isn't useful for your particular noise signature. I don't have a 7300 so can't make any measurements on it, but "noise blanking" is serious business and some systems work much better than others.

    I have four fairly "modern" transceivers (well, the newest one is from about 2010) with DSP noise blanking, and one quite old transceiver without DSP (the 1978-vintage TR-7) and the TR-7's noise blanker is much better than the more modern rigs. It hasn't any adjustments, just a "button." You push the button to engage the NB, or push it again to disengage it. It takes S9 noise down to S0 or S1 with many types of noise, or at least all the types of noise I encounter. Just push the button and watch it drop, and you can easily hear the difference; on weak signals they can go from "impossible copy" to "solid copy." Pushing one button.

    It's an "I.F. noise blanker" and is an extra I.F. stage (optional plug-in, the NB-7).
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  10. WB2WIK

    WB2WIK Platinum Subscriber Platinum Subscriber QRZ Page

    ^I agree.

    A preselector might help sometimes with various types of noise, if the sources are way out of band. But if they're in-band, how can they help?

    Most of the preamps don't even include preselection, they're just wideband amplifiers.

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