anyone have strong feelings about filter topology and order for a 6m preselector? I lean towards a second order Legendre but I'm not terribly familiar with the intricacies of that topology. My understanding is that it's pretty flat in the passband and has the best rolloff for a particular order. I think a second order should get the attenuation down to -40 dB in the 36MHz/76MHz range and have nearly flat response in 50-54MHz. There are some smallish cap and inductor values in the filter although I can probably get SMD inductors in the 8 nH range that should do the job. And single digit pF caps should be doable too. This is a 4 element filter and my opinion has always been that the fewer parts the better just off the antenna as that leads to less loss and noise. -- Trevor R.H. Clarke Computer Science House Rochester Institute of Technology retrev@csh.rit.edu http://www.csh.rit.edu/~retrev/

First question is do you have specific sources of interference you are trying to suppress, and how loud are they? 2nd question is do you need the full 4 MHz BW, or are you just looking to work the weak signal end of the band? There are a few repeaters up at the high end. I'm not familiar with the Legendre topology, at least by that name. I'm guessing you want to use a WB front end, possibly an SDR? Filter loss isn't too critical on 6m if you have a reasonable amount of antenna gain. We don't bother with mast mounted preamps at our contest site because the antennas have around 10 dB gain, so even the feedline losses are not critical on receive. We would certainly improve the system noise figure with a preamp but haven't felt the need. Which gets back to the preselector question. If you have specific problem signals, maybe notch filter or two would be more effective and less loss in band? Tom Holmes, N8ZM -----Original Message----- From: mvus-list <mvus-list-bounces@lists.febo.com> On Behalf Of Trevor Clarke via mvus-list Sent: Wednesday, May 13, 2020 12:30 PM To: Midwest VHF/UHF Society Mailing List <mvus-list@lists.febo.com> Cc: Trevor Clarke <retrev@csh.rit.edu> Subject: [mvus-list] preselection filter topology anyone have strong feelings about filter topology and order for a 6m preselector? I lean towards a second order Legendre but I'm not terribly familiar with the intricacies of that topology. My understanding is that it's pretty flat in the passband and has the best rolloff for a particular order. I think a second order should get the attenuation down to -40 dB in the 36MHz/76MHz range and have nearly flat response in 50-54MHz. There are some smallish cap and inductor values in the filter although I can probably get SMD inductors in the 8 nH range that should do the job. And single digit pF caps should be doable too. This is a 4 element filter and my opinion has always been that the fewer parts the better just off the antenna as that leads to less loss and noise. -- Trevor R.H. Clarke Computer Science House Rochester Institute of Technology retrev@csh.rit.edu http://www.csh.rit.edu/~retrev/ _______________________________________________ mvus-list mailing list mvus-list@lists.febo.com http://lists.febo.com/mailman/listinfo/mvus-list_lists.febo.com

On Wed, May 13, 2020 at 1:16 PM Tom Holmes via mvus-list < mvus-list@lists.febo.com> wrote: > First question is do you have specific sources of interference you are > trying to suppress, and how loud are they? > No specific point sources of interference, just a general band filter to increase dynamic range and lower overall noise. > > 2nd question is do you need the full 4 MHz BW, or are you just looking to > work the weak signal end of the band? There are a few repeaters up at the > high end. > I'm planning an SSB radio for voice (when available) and FT8, so mostly small signal. I might eventually use the front end and split the rest of the chain for FM but that's not my priority as I don't think there's a ton of 6m FM traffic around here? I didn't think 7.5% bandwidth was too huge a block to cover with a preselector so I was going to include the whole band but I'm open to input from those more knowledgeable. > > I'm not familiar with the Legendre topology, at least by that name. I'm > guessing you want to use a WB front end, possibly an SDR? > It's often call an optimum "L" filter. Pretty flat similar to a Chebyshev but steeper fall off like a Butterworth. I'm not sure the specific disadvantages as I wasn't very familiar with the topology either until I came across some usages. > > Filter loss isn't too critical on 6m if you have a reasonable amount of > antenna gain. We don't bother with mast mounted preamps at our contest site > because the antennas have around 10 dB gain, so even the feedline losses > are > not critical on receive. We would certainly improve the system noise figure > with a preamp but haven't felt the need. > > Which gets back to the preselector question. If you have specific problem > signals, maybe notch filter or two would be more effective and less loss in > band? > > Tom Holmes, N8ZM > > -----Original Message----- > From: mvus-list <mvus-list-bounces@lists.febo.com> On Behalf Of Trevor > Clarke via mvus-list > Sent: Wednesday, May 13, 2020 12:30 PM > To: Midwest VHF/UHF Society Mailing List <mvus-list@lists.febo.com> > Cc: Trevor Clarke <retrev@csh.rit.edu> > Subject: [mvus-list] preselection filter topology > > anyone have strong feelings about filter topology and order for a 6m > preselector? I lean towards a second order Legendre but I'm not terribly > familiar with the intricacies of that topology. My understanding is that > it's pretty flat in the passband and has the best rolloff for a particular > order. I think a second order should get the attenuation down to -40 dB in > the 36MHz/76MHz range and have nearly flat response in 50-54MHz. There are > some smallish cap and inductor values in the filter although I can probably > get SMD inductors in the 8 nH range that should do the job. And single > digit pF caps should be doable too. This is a 4 element filter and my > opinion has always been that the fewer parts the better just off the > antenna as that leads to less loss and noise. > > -- > Trevor R.H. Clarke > Computer Science House > Rochester Institute of Technology > retrev@csh.rit.edu > http://www.csh.rit.edu/~retrev/ > _______________________________________________ > mvus-list mailing list > mvus-list@lists.febo.com > http://lists.febo.com/mailman/listinfo/mvus-list_lists.febo.com > > > > _______________________________________________ > mvus-list mailing list > mvus-list@lists.febo.com > http://lists.febo.com/mailman/listinfo/mvus-list_lists.febo.com > -- Trevor R.H. Clarke Computer Science House Rochester Institute of Technology retrev@csh.rit.edu http://www.csh.rit.edu/~retrev/