Posted by
Geoffrey Mackenzie-Kennedy-2 on
Mar 18, 2006; 12:31pm
URL: http://elecraft.85.s1.nabble.com/Coax-Switch-between-two-rigs-tp387104p387108.html
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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