Thanks to those who replied to my post. I've now reached the point where the level of the spurious tone is low enough. Here's what I did.
Following Don's suggestion I isolated the KSSB2 speech processor from the balanced modulator by removing C20 and grounding the free end of R2 via a 100nF capacitor. Unfortunately the spurious was unchanged; a problem in the speech processor could have relatively easy to fix. I followed up on Steve's suggestion with some careful measurements on the spacing between spurious tone and K2 carrier in LSB and USB compared with the difference in BFO frequency for the two. This confirmed that the 4MHz oscillator in the PIC U1 was coupling into the KSB2 output. Some years ago I had made a modification to the KSB2 between D2 and Q2 to reduce the peak SSB amplitude so it matched the Tune level. I reverted to the published circuit and the level of the spurious dropped to about -45dB ref the SSB output which suggested that this signal path is sensitive to pickup. Next I moved C31, C40 and C43 from the top to the bottom of the PCB and constructed a 1/2" high screen connected to their Gnd pads to shield the PIC, the resonator and U2 from the o/p circuits (D1 ... Q2). It made a marginal improvement, not really worthwhile but I shan't undo it Next I increased the output to 2 watts. The spurious tone's amplitude is not affected by the power setting so this should win me 3dB. I also lifted the base of Q2 off the PCB and joined it to D2 and R8 with a piece of wire 1/16" off the top of the PCB. That way the PCB track that connects to Q2's base is no longer used in case it's picking up the 4MHz. Finally I retuned the 28MHz BPF higher in frequency to try and move the spurious tone towards the filter stopband. The spurious is now 50dB below the SSB output and I can't see how to make any further improvement. There are other spurious outputs at that level so there's little to be gained in any case. I don't believe I have found the main mechanism for the coupling but as they say "sometimes good is good enough" 73 Chris G3WIE -- Sent from: http://elecraft.365791.n2.nabble.com/ ______________________________________________________________ Elecraft mailing list Home: http://mailman.qth.net/mailman/listinfo/elecraft Help: http://mailman.qth.net/mmfaq.htm Post: mailto:Elecraft@mailman.qth.net This list hosted by: http://www.qsl.net Please help support this email list: http://www.qsl.net/donate.html Message delivered to arch...@mail-archive.com
