Some birdies are unavoidable in a high-dynamic-range, down-conversion design. We worked very hard to keep them out of the ham bands, and in the case of 6 m, we focused on keeping them out of the portion of the band typically used for weak-signal work. (The Orion has a similar conversion scheme to the K3's, but doesn't provide 6 meter coverage. That simplifies things ;)
Up-conversion transceivers generally have fewer spurious signals on 6 m, but they also typically have very weak receivers, due to the use of wide first I.F. filters, as well as very high-frequency 1st LOs with much worse minimum phase noise than the K3. Birdies usually result from VHF or UHF harmonics of the K3's high-level signal sources, mixed with another signal source, internal or external. In both cases, they may not actually be on the band you think they are; they could be on an image band, circumventing the filters. External VHF or UHF signals could be getting in through other I/O besides the antenna jack, and every receiver will respond differently to them. So, while a given VHF receiver with a whip doesn't pick them up, that doesn't mean they aren't there--they could just be well outside the 6-m band, and mixing to produce a response in the K3. The only way to tell for sure is to put the unit under test in a screen room. It's possible to eliminate every last portal for such signals, but to do so would take many pound's worth (and $ worth) of additional shielding, bypassing, and filtered connectors. That's just not possible within the K3's budget or package size, so we aimed for the best possible compromise. All that said, there are things we can do on an individual K3: 1. Make sure that the screw from the left side panel into the front shield is tight. If a birdie drops a lot in amplitude when you wedge a knife blade between these two panels, near the screw, then the panels aren't making good electrical contact. We're considering whether to add a small piece of copper EMI tape here so that the screw tension isn't so critical. 2. EMI tape can be applied in various places within the cabinet, typically along seams where panels join. This tape is very expensive. Some experimentation may be in order here to see were we can get the most bang for the buck (dB for the inch?). 3. Specific birdies due to VHF harmonics can sometimes be shifted out of harm's way just by shifting the passband (SHIFT control) or selecting a different crystal filter. The reason for this is that it moves the synthesizer (1st LO). 4. If all else fails, try turning on the notch filter. You can use auto-notch in voice modes, but in CW mode you're limited to manual notch. If you still feel that your K3 has excessive 6-m birdies, or that they're causing specific operating problems, please contact me directly and I'll try to help work out a solution. 73, Wayne N6KR --- http://www.elecraft.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
