[Elecraft] Six meter birdies -- theory and mitigation

[wayne burdick]

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


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