Having had to today off work because of the snow, I had a chance to look at my KSB2 SSB filter characteristics. My biggest interest was whether the filter had blow-by and if so, how much.
First I modeled the filter input and output switching networks using 150 ohms as the source and filter load impedance. I expected more leakage from the switching, and was pleasantly surprised when I got 110 dB of isolation. Of course the real thing will have greater leakage, but at least it is in the Ball Park. To get a rough idea of the crosstalk between the input and output connectors on the RF board and also the tracking I used the CW filter. Using a signal generator I injected a low level signal and peaked it in the filter. I then removed the CW filter and increased the signal generator level until I got the same S meter reading. Although rather crude this measurement gave me a measurement of around -95dB. Again not bad. Using Spectrum Lab set to 24bit sampling I got a K2 OFF on screen noise floor of about -115dB. Switching the K2 ON raised this to -95dB. I connected a noise generator and set my K2 to 28MHz, AGC OFF, and PreAmp ON. I made the following measurements:- 0dB 1kHz -6dB 2.7kHz -60dB 3.5kHz -90dB 3.9kHz Shape factor 1.3 The noise floor was intercepted at about 4.5kHz. Both sidebands were nearly identical. Switching off the PreAmp improved the reading slightly. This performance was very much better than I expected. The passband has, however about a 2dB ripple in it. Any suggestions on a cure for this ? I tried to improve the filter response by adding brass shim as screening on the top and bottom of the board, but it made no noticeable difference. No blow-by here... I think that the filter is working just about as well as the design permits and I am very pleased with the results. 73 Stewart G3RXQ _______________________________________________ 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
