[Elecraft] K3 Birdies: firmware-based fix in progress

[wayne burdick]

The K3 is a down-conversion superhet that uses high-level signal 
injection to achieve its excellent dynamic range. Short of adding many 
more pounds of shielding and elaborate cable dressing, there's no way 
to completely eliminate the few spurious signals that rise above the 
noise floor.

These "Fast-tuning" birdies result from UHF harmonics of the signal 
sources that leak back into the main mixer. In some cases they combine 
and end up in either the I.F. or R.F./image range of the receiver. 
Typically, they involve 9th-order or higher harmonics of the VFO. 
That's what makes them "fast": if you move the VFO 100 Hz, the pitch of 
the birdie will shift on the order of 1 to 2 kHz.

While it is possible to attenuate some spurious responses by moving 
coax cables around, there is a firmware-based approach that we're 
working on. The general idea is to shift the 1st LO and BFO a small 
amount, simultaneously, when the VFO is tuned to specific frequencies. 
If the shift is small relative to the communications bandwidth in use, 
it will hardly be noticeable when the VFO is tuned over a "mapped out" 
spot in the tuning range.

I have this new firmware nearly completed, and in early tests, it 
appears to work very well. Fast-tuning birdies that are mapped out 
pretty much disappear as the VFO is tuned over them. The upshot is that 
you can have your cake (outstanding dynamic range in a rig that weighs 
less than 10 pounds) and eat it, too (no annoying birdies).

If you'd like to try a field-test version of K3 firmware that includes 
this new feature, please e-mail me directly.

73,
Wayne
N6KR

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