That's very interesting. I found that I can accomplish much the same
thing on CW by simply using a very narrow bandwidth (250 Hz in my case)
... the birdies tune so fast that they drop outside the passband with
just a minor shift in VFO frequency.
I don't use SSB as often, but there the notch filter has been
effectively plinking the birdies.
73,
Dave AB7E
wayne burdick wrote:
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
---
http://www.elecraft.com
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