Electronics > RF, Microwave, Ham Radio

Low Pass Filter for 20 meters - determine the best cutoff frequency

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Jeff Weinmann:
Hello,

I'm looking to 3 design a 3 pole low pass filter with the aid of the following site:

FYI its Dale Weatherington's site, one of the co-founders of Hayes Modems FYI:

http://www.wa4dsy.net/filter/filterdesign.html

I'm looking into filtering for a 20 meter frequency of 14.090 Mhz and this is what I came up with:

http://www.wa4dsy.net/cgi-bin/lc_filter4?FilterResponse=Lowpass&poles=3&cutoff=25&funits=MHZ&Z=50

There are tons of sites showing HOW to calculate the -3db cut off frequency, but I'm looking to minimize the db loss at the working frequency of 14.090 Mhz.

I picked 25 Mhz because its below the 28 Mhz harmonic, but I'm also looking to filter out as much between 14.300 Mhz and above. I would like to do this for other bands but I would like to understand more why I should pick a particular cut off frequency instead of guessing.

The purpose of the filter is to clean up the square wave signal of an SI5351a clock.

Any help would be appreciated!  73 W8ZLW

bd139:
Start with how many dB you need to knock off the first harmonic, work out how many toroids you can be assed to wind, convert that to the filter order and work back to the cut off frequency. It will probably be well in the flat low insertion loss part of the filter.

If itâ€™s a low pass you probably need more than a third order filter though. 5th at the very least.

Edit: also dont assume the Si5351A is a 50 ohm match for the filter as well so measuring insertion loss will be difficult.

UR5FFR:
Try RFSimm99 software and use real Q for inductors

fourfathom:
What will you be driving with the output of your filter?  This really determines the level of filtering needed.  And what filter loss can you tolerate?  Sometimes accepting a higher loss can give you a sharper or more tolerant filter.  BTW, with the Si5351 output set for "high" drive, the output Z is supposed to be close to 50 Ohms.

You might try something like this filter.  It's not flat, but broadly peaked around 14 MHz.  The 2nd harmonic attenuation is around -50dB, and the third harmonic attenuation is close to -65dB.  I am assuming 1 Ohm DC resistance for the inductors, which are T37-6 Iron Powder, 14 turns.  This doesn't have the return loss of a nice Butterworth Pi filter, but it's decent.  The loaded Q for this circuit is low enough that reasonable component losses and tolerances aren't very critical.  Note the tapped capacitor input and output -- this allows the Q to be independently set, and also provides the necessary AC coupling for the Si5351 output.

I use this topology a lot, as I work with synthesized signals that may have significant sampling artifacts in addition to the textbook harmonics, both above and below the desired frequency.  The Si5351 is pretty clean in this regard though, so a flatter LPF is probably fine.
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