Morning Merlin, >From an old guy (hmmmpf) who has messed around with RF circuits for ages, for what they are worth a few comments. Your idea to keep RF away from where it should'nt be is a basic rule in good RF design, and will always be I would think. Some digital designs get hung up because this rule is not observed, and there is a lot of RF about in some digital circuits.
By adding your fences, houses, barriers and getting rid of little "antennas" is rocket science, and certainly not "dumb" !! I have built many many HF crystal filters for the homebrew gear, 10, 12 and 14 pole, and the isolation of various parts from other parts is critical if a stopband attenuation around 100db is to got, together with a good 6db / 80db shape factor and very small passband ripple. In this playground, 6db / 80db is used more often than 6db / 60db in design work. With VHF crystal filters using crystals in an overtone mode (I have hombrew filters at 130Mhz and lower) life really gets interesting - even the filter's box wants to reduce input / output isolation. The "box" problem can be there at HF, so both the layout and the enclosure of the filter must be thought about. But modifying an existing filter's mechanical / electrical design can be more difficult than starting from scratch. I have not played with K2's filter, but apart from the input / output isolation etc, I would suspect that the Varicaps and their bias circuitry could also spoil "isolation" especially as pcb traces are used, but by how much ? I have not done the sums, but I also have a hunch that the varicaps could be one of the things that limits K2's strong signal performance. But to be FAIR, it is my understanding that K2 started life as a QRP rig, highly portable, and it is very nice to have the variable bandwidth filter in a single conversion receiver. The KSB2's filter layout is compromised by lack of space - it is much better to have the crystals in a straight line. All said IMHO you have done the right sort of things. RF circuits that are supposed to be separate, must not gossip. 73, Geoff GM4ESD ----- Original Message ----- From: <[EMAIL PROTECTED]> To: <elecraft@mailman.qth.net> Sent: Tuesday, December 07, 2004 4:57 AM Subject: [Elecraft] ultimate rejection > > Going first for the simple, dumb approach I began putting little RF > "barriers" across the middle of the filter. They consisted of a piece of printed > circuit material, connected to ground and soldered all along its length. It > worked great! Everything about the filter got better: opposite sideband > > > Enough reminiscing; I just spent about a hour with my KSB2, making sure > that the solder connections near the 7 pole filter were filed down, or clipped > as much as possible, so that they don't act as little "antennas", thus messing > up the characteristics of the filter. It worked! The "audio image" that > one hears when tuning past zero beat got a lot weaker! > > Next, I went to local hobby shop and bought some tin. I made little tin > "houses" which I placed around the matching toroids, ground them carefully. It > worked too. Next I added a little tin "fence" which went on the bottom the > board, shielding the input of the filter from the output. It worked. Three > times is a charm! > > My point: it ain't rocket science, but the old ideas of keeping RF away > from places where it shouldn't be work just as well as they ever did. > > _______________________________________________ Elecraft mailing list Post to: Elecraft@mailman.qth.net You must be a subscriber to post to the list. Subscriber Info (Addr. Change, sub, unsub etc.): http://mailman.qth.net/mailman/listinfo/elecraft Help: http://mailman.qth.net/subscribers.htm Elecraft web page: http://www.elecraft.com
