[Elecraft] K3 2.7kHz filter DSP FC

[Paul Christensen]

> "If the center frequency was set to 1.225 KHz (or close) as you suggest, 
> then
the 2.7 KHz filter would let though too much opposite-sideband energy. We
want some stopping power down close to zero Hertz, so the filter edge is
kept 250 Hz above zero."

Bruce, as Barry points out, the K3's audio response really needs the ability 
to extend down significantly lower than the current ~ 250 Hz high-pass 
cut-off, especially when the 6 kHz filter is employed.  I would recommend 50 
Hz, although 70 Hz would be fine if there is some inherent limitation that 
precludes a lower value on Rx and Tx.  Since we're not concerned with the 
broadcast of music in our service, then the male voice (lowest relevant 
fundamental of 70 Hz) and certain digital modes in SSB mode (e.g., PSK31) 
may be used to establish the lower response threshold.

Transceivers such as the Orion, Omni VII, Icom Pro Series, Yaesu FT-2K/950 
series, IC-7800...all provide reasonably good Rx and TX response to at least 
100 Hz, some of these mentioned go even lower.  However, the benchmark of 
all time goes to the Kenwood TS-870S, where I've measured flat low-end Rx 
and Tx response down to 10 Hz (@ -3dB) and the opposite sideband remains 
completely filtered out.   For example, if you tune into a carrier in SSB 
mode, you will hear that carrier down to near DC, then when you approach the 
other side of zero, absolutely nothing is heard.  That's one Helluva' design 
accomplishment.  But obviously, that kind of low-end response is overkill. 
On Tx, I've actually heard room rumble coming from my modified D-104.

So, to be competitive with these other radios, this is an area that needs 
further refinement.  And, it is really my only significant adverse issue 
with the K3 at the moment (okay, other than placement of the REV button). 
Once you've listened to SSB or CW on a clear, open band, the audible 
difference between a lower limit of 250 Hz and 70 Hz is absolutely amazing. 
It really does offer a whole new dimension to the listening experience.

Paul, W9AC