Take it all in, then filter out

what you don’t need or want

An oversimplified view of roofing filters

 By  Jim Pickett - K5LAD

NOTE:  This article was originally written for inclusion in the TARC (Tulsa Amateur Radio Club) Newsletter


There have been millions of words written about the subject of filters used in typical ham applications.  A quick Google search for filters in ham radio would probably offer millions of hits and many would be quite involved, complete with formulas, charts, and diagrams explaining them in great detail.  This article is an oversimplified look at filters to, perhaps, help a newcomer to the hobby.

 The value of a filter in a receiver has always been known. Filters can be made of tuned circuits using inductors and capacitors, with crystals, and by mechanical means.  Even newer is the use of electronic circuitry to convert an analog signal to a digital one and apply “electronics magic” to deal with the signal.  Sometimes it is desirable to have a very broadly tuned filter that covers a wide range of frequencies and sometimes it is preferred for them to be very narrow.  For instance, a 2 meter VHF receiver front end might want all or most of the frequencies within that band to pass but to exclude, or at least attenuate, the frequencies in the VHF commercial bands several MHz away.  A very tight filter in the IF of a receiver would want to pass a SSB audio signal of approximately 300-3000 Hz but exclude a similar signal located just a few KHzs away, and a CW filter might want to exclude a closeby signal as near as 100-200 Hz away.   The advantage of tighter filters is the exclusion of very closeby signals, the disadvantage is, the tighter the filter, the more difficult it is to tune and, in the case of voice signals, the less understandable is the person speaking.

 Imagine being crowded into a room with many, many other people.  Ideally you want to build walls around yourself to define "your space" from all others so you could concentrate on the one person you were speaking with.  If you build your walls out too far, so that you have plenty of space to move around in, you'll be into someone else's space.  When those others talk, their sound is a bother to you.  Should you build your walls too closely, you might exclude many of those others but you feel "jammed up."

 With the advent of new DSP (digital signal processing) circuitry it's now possible to build higher walls but still make your space of a comfortable size.  The DSP circuits can look out over the sea of faces and identify just the one face it wants to see (or actually, to hear).  If only the DSP could see fewer faces originally, the easier it would be to zero in on just the one they want.

 Enter, the roofing filter to this picture and example.  The current roofing filters being used are made with crystals cut to essentially the same frequencies that allow signals within that small area to pass to the exclusion of all others around it.   It says to the DSP, "here, just look at these few signals and do your processing on them.  Let's forget about all those others around the one you want."  The roofing filter limits the range of signals fed to the DSP for processing.

 An additional benefit of the roofing filter is to protect or, perhaps a better term would be, to ‘shelter’ the AGC of the receiver.  The AGC (Automatic Gain Control) is the circuit that protects your ears when a strong station or stations suddenly appears.  The AGC is ‘looking’ at the entire receiver IF passband and if a strong station appears in the passband, it knocks the gain down and in doing this it protects your ears from the blasting audio.  A fringe benefit (and a distinct disadvantage) is that it also lowers the gain on the weak station you’re really trying to hear.  The roofing filter makes the passband seen by the AGC much less (narrower) and in doing so, the weak station is not minimized because of the presence of the strong station nearby.  If the AGC cannot see (or hear) signals nearby in the passband it does not need to use part of its resources to attempt to deal with it.  As far as the AGC is concerned, if it’s outside of the roofing filter’s bandwidth, it does not exist.

 Imagine being in a room with quite a few people and trying to listen to one person but the piano is playing loudly across the room.  If you could pull the walls of the room in but the piano would remain in its original location then the piano is on the outside of the wall and does not bother you.  In fact, the new wall might even make it so you don’t hear the piano at all.  The roofing filter isolates the sounds coming from nearby signal and makes it like the signals are not even there, just as though the piano is not even in the room.

 Several of the older commercial transceivers are being modified to include a roofing filter in their IF string to improve their selectivity.  There are also a few of the newer rigs being built "from scratch" with roofing filters as a part of their design.  My favorite is the K3 transceiver from Elecraft, shown in the picture below.


This transceiver has now been available for about 2 years and has received rave reviews.  It comes with a 2.7 KHz roofing filter which is very good for all bands and modes.  In addition, the transceiver has provisions for adding up to 4 additional crystal roofing filters in widths like:  200 Hz, 400 Hz, 500 Hz, 1.8 MHz, 2.1 MHz, 6 MHz, and 13 MHz.  The narrowest filters are typically for CW and other digital modes, the widest for AM and FM mode and the ones in the middle are more desirable for SSB.  Remember that the narrower the filter, the fewer interfering station you'll be hearing. 

 It might seem then, that if you always used a very narrow, i.e., 200 Hz filter then you could easily exclude all the interfering stations leaving only the one currently holding your interest.  Unfortunately, that is not always the case.  Listening to a SSB signal with a very narrow roofing filter would make it impossible to understand.  I picture a person in an elevator whose lips have somehow gotten squeezed together by the closing elevator doors.  They try to speak but they are not understandable due to the severe restriction.

 To quickly demonstrate the value of a roofing filter, I used my own K3 with the 2.7 KHz filter, and using an LP-PAN panadapter to demonstrate the signals spread within a band, I made the following screen shot of the PowerSDR software.   

 The receiver is tuned to 14.192 MHz and there were several signals on the band.  I was listening to the one on 14.192 but there was another, even stronger signal at about 14.195 Mhz.  Ordinarily, on a typical receiver it would have been very difficult, if not impossible for me to hear the station on .192 due to the stronger station nearby.  This other annotated version of the same screenshot, explains the value of the roofing filter in my K3.  The picture should explain it better graphically than I could in words.


 Whether you’re operating during a contest, where the panadapter would have vertical blips of signals strung all the way across on both the left and right side of the green bar above, or during a normal time with fewer signals, the receiver is only responding to the signals within that green bar.  It doesn’t ever know (or care) that others exist.

 If you’re in the market for a new radio, you would be ahead of the game to look at one with a roofing filter.  Roofing filters are the wave of the future and are a highly recommended feature.  Why should we listen to everybody when you only want to hear just the one? 


Created August 23, 2007   


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