Hello, this is not a problem but simply a mystery for me. After having successfully finished my K2-#5776 (Gary and Scott, thanks a lot for your fast help!), I built up the KSB2 board - it works basically fine. However, when I studied the diagram of the KSB2 and the RF board, I stumbled over the fact that T1 and T2 on the KSB2 have very different turns ratios (7:22 vs. 22:4). Because the ladder filter is symmetric, it requires the same termination on both sides (here, ~1500 Ohms). Coming from a source impedance of 150 Ohms, T1 is duly transforming it to 1500 Ohms.
However, at the filter output, T2 transforms the 1500 Ohms down to only 50 Ohms. (BTW, this down-transformation could very well be the reason for the unusually high insertion loss of >7db as listed in the "SSBCAPKT" document. I measured abt. 3db with an external setup, using the original xtals, caps, and FT37-43 transformers with identical turns ratios). So far so good - but this is NOT looking into 50 Ohms on the RF board (T7) as one would expect. T7 has a turns ratio of 5:20. Looking backwards from the input of the IF amp MC1350 with an input impedance of ~3000 Ohms (according to the datasheet), this generates an impedance of over 180 Ohms at the 1&2 terminals of T7 - a significant mismatch for the filter. Does anybody have a clue what the reason for this arrangement is? Because of time constraints, I could not yet take RF measurements of the filter/IF path (I dont't trust Spectrogram measurements - too many unknown variables :-)) ) . 73, Horst, dj6ev P.S. I am not using the original 7-pole filter but a homebrew 8-pole ladder on a tiny subboard with SMD caps. The transformers are on BN43-2402 cores instead of FT37-43, with identical ratios on both sides (6:17 / 17:6 turns - because of lower xtal inductances / filter impedance of approx. 1200 Ohms). Works nicely on the KSB2. -- View this message in context: http://n2.nabble.com/-K2--KSB2-filter%3A-unequal-terminations---why--tp588100p588100.html Sent from the Elecraft mailing list archive at Nabble.com. _______________________________________________ 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
