Thoughts on the NM0S Hi Pass Filter


NM0S has provided a very nice Hi Pass Filter Kit,, for 4SQRP to include in it's stable of kits. It is the same form factor as his earlier low pass filter, and there are no toroids to wind. Spiral coils on the pcb provide the needed inductance. IMHO this nice little filter hasn't received the attention that it deserves.

Why a Hi Pass?
Anyone using  a direct conversion receiver probably need one to prevent overload from local AM broadcast stations, unless that rcvr has a built in HPF.

My QTH is abt two miles from two kilowatt BCB stations, 1240kc and 1480 kc, and they have been a problem for every direct conversion receiver I've ever built - homebrew or commercial. Several years ago I added a very simple HPF to a project and it helped but didnt do the job completely.  So I added caps in series with the shunt inductors, to trap the BCB signals and that worked fb.

While my version is ok, Dave's rendition is superior (to be expected), and the kit's pcbs make it extremely easy to build and it looks great in it's case.  If you have an offending BCB station or two, make them disappear by changing C4 and C5 so as to make two series resonant traps.  There is a nice calculator here

Attached are schematics and performance graphs for your review, all are screenshots from Tonnesoft's very excellent filter design program LC.  Thanks you Jim for a fantastic program and your willingness to share a free version.

Observe the awesome attenuation Dave's design has right off the bat. 1M attenuation is -96db!!!  My measly effort only attenuates 1Mc 36 dB..

If you need to use a high pass filter, the NM0S/4SQRP will do the job, and can't be beat for simplicity and effectiveness.  I just bought one !

This Saturday musing is brought to you buy the weather, is raining and cold here.  Questions and comments are welcome.

I love this radio stuff.

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