Martin,
On paper it would not be too difficult to "build" an IF crystal ladder
filter for the K2 whose stopband attenuation is an honest 100db or better,
with a passband ripple of something less than 0.5db - probably less than
0.25db. But the filter would require good quality high Q crystals,
reasonably well matched.
In practice to use such a filter in the K2 would, I believe, require quite a
lot of surgery so that the filter's performance is not ruined by unwanted
coupling between the input and output, "blow-by" is one example of this.
Consider all those control lines as a first thought! If a KSB2 is used, it
would probably be necessary to reposition it, and as a guess some shielding
might be required in other areas as well.
If different and selectable IF bandwidths are required, then I am certain
from past work that a filter for each bandwidth would be required, selected
by high quality relays that have the necessary inter-contact isolation.
There is no place for diodes of any sort in this 100db+ playground.
However, even if a better filter was installed and worked properly, the very
close in and far out phase noise of the K2's LO seen at the3 TUF-3 mixer LO
port would have to be reduced, if anything is to be gained from the new
filter's stopband attenuation. Kevin, ZL1UJG noted this problem. I cannot
comment on the effect of the existing BFO phase noise at this time.
73,
Geoff
GM4ESD
----------------------------------------------------------------------
Martin, OK1RR, wrote:
Looking at the Rob Sherwood's receiver data table
http://www.sherweng.com/table.html
I discovered a remarkable fact: the K2's weakest point is the FILTER
ULTIMATE REJECTION. Sherwood says it is 80 dB only (and there are two K2s)
and it was confirmed by one of our top grade contesters and DXpeditioners.
A closer look onto schematic awake an idea to reuse KSB2 crystal filter
for receiving on both CW and SSB. No doubt the additional 7 crystals will
improve this figure significantly. I had no opportunity to track the
rather complex diode switching scheme but Sherwood's figures saying it
clearly – there are 5+2 crystals in CW receive path only. So my question:
is there any mod to improve the ultimate rejection using KSB2 crystal
filter? (of course, G3RXQ's filter mods are here but I don't plan to
destroy a KSB2 board and I am not interested in SSB). Would any
experienced K2 builder willing to play around this?
And a QSK improvement to listen between elements up to 40 wpm would made
from the K2 a dream radio :-)
I don't own my K2 yet but a 100 dB ultimate rejection would ensure me that
I should't lose my time with thinking if to buy or not :-)
73,
Martin, OK1RR
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