[Elecraft] K-3 Roofing filter comment

[Bill W4ZV]

wa9fvp wrote:
> 
> Actually the AGC is generated by the DSP in software.  The signal VIFGAIN1
> coming from a D to A converter (AD5302A) if fed back to the IF amplifier
> to control the receiver's gain.  After the DSP filter reduces the
> bandwidth, the AGC is adjusted by the DSP.  It's all done using
> mathematical calculations.
> 

Not quite.  There are two separate AGC functions (analog and digital) in the
K3.  You're describing only the second stage which is digital AGC.  It
follows analog AGC which is after the roofing filter and before the DSP. 
Most modern hybrid heterodyne/DSP radios use this approach (e.g. IC-7xxx,
FT-5000/9000, Orion, Omni 7, Eagle TS-590S, etc) because DSPs currently only
have ~100 dB of dynamic range and 140 dB is required for good BDR
performance.  See Wayne's detailed explanation below.

73,  Bill

http://www.elecraft.com/K3/Roofing_Filters.htm

"In order to achieve the best K3 blocking dynamic range (desense) in the 140
dB+ range, you -must- use a narrow crystal filter (400 Hz 8-pole or 200 Hz
5-pole for closer interfering signal spacing) in front of the DSP. We use
hardware AGC after the narrow crystal filter and ahead of the DSP to protect
the DSP when signals inside the crystal filter exceed a 100 dB dynamic
range. If you only use the 2.7 kHz 5 pole stock filter for CW or data
operation you will be significantly desensed once signals within that
filter's bandwidth exceed about S9+25. This is before phase noise from the
transmitting station becomes a factor. Not uncommon on 40M at night, during
a contest or at a multi-op station -- Or every day in major cities.  As an
example, changing to a 400 Hz or 200 Hz filter reduces blocking from signals
1-5 kHz away.  I've personally confirmed this on the air with my K3 and the
other commercial rigs we have here. When I've operated with the K3, or
another DSP rig, on CW without using a narrow filter ahead of the DSP
filtering, I frequently experienced desense (BDR) from nearby signals.
Putting in the narrower crystal filter immediately cleaned it up. (While
both the 400 Hz and 500 Hz filters are excellent CW filters, the 400 Hz
8-pole filter performs slightly better than the 500 Hz 5 pole filter due to
its narrower shape factor. The 200 Hz 5 pole is even sharper.)"



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