Probably as you say that a tin box around the whole assembly would be an overkill. I was thinking that the crystals/caps in the filter could be spaced out more, and the whole area ground-planed top and bottom. I agree that coupling to the main board maybe a route for blow-by, and if space could be found a screen fitted to the underside of the board may bring some improvement. It should be possible to estimate the degree of leakage caused by P2/P3, and the signal diodes. Maybe PIN diodes would give higher isolation. With the excellent phase noise performance of the K2 it is a shame that some of this benefit may be being thrown away in the SSB filter. I am given to understand by people who know a lot more than me about ladder filters that there is more to be had.
73 Stewart G3RXQ On Wed, 2 Mar 2005 18:54:54 -0000, Geoffrey Mackenzie-Kennedy wrote: > I believe that you would be proven correct about the RF feedback. > > My thought about a tin box totally enclosing the PCB stems from an > observation that the skirts of the varicap "controlled" filter set for say > 700Hz bandwidth are a little wider with the KSB2 installed than when it it > is not, and noticeable from approx 40db below passband on down. I made no > measurements because I was Birdy hunting/killing at the time. If this > blow-by is due to leakage between P2 and P3 or through the switching diodes > (off) on the KSB2, then the tin box would have little if any effect. But the > KSB2 does come close to forming a bypass route just due to its proximity to > the post mixer amp, RF board filter etc, and my gut feeling is that a box > around the KSB2 or its replacement would be a good start. The box would also > help to keep out the RF from nearby oscillators. > > 73, > Geoff GM4ESD > > On Wed.2nd March 2005, Stewart G3RXQ wrote: > > Any form of increasing the mic gain internally (eg rf speech clipper) needs > to > be carefully thought out. The microphone signal routing and earthing from > the > front panel socket thru the RF board and into the KSB2 is far from ideal. > Common mode, series mode - the lot - RF feedback is the likely result. > > A groundplane board with the crystal filter laid out to reduce blow-by may > be > the way to go. A tin can over the whole assembly may help. The biggest > hurdle to > overcome in making changes to the KSB2 is interfacing with the existing > firmware > which is far from straightforward. > > 73 > Stewart G3RXQ > > On Wed, 2 Mar 2005 08:11:54 -0000, Geoffrey Mackenzie-Kennedy wrote: >> Intersting ideas. If you did enlarge the SSB board to get all the bits in, >> >> in which directions would you enlarge the exixting board? >> IMHO it would be essential to contain the "new" board in a 'tin' box in > turn >> mounted on metal, which raises the question of access to test points etc > on >> the RF board, but a solution might be available. _______________________________________________ Elecraft mailing list Post to: [email protected] 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
