K2 - 2nd Xtal Filter

[Stewart Baker]

Anyone any experience of, or thoughts about adding another 3 crystals to the K2
2nd crystal filter ?

73
Stewart G3RXQ

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[Geoffrey Mackenzie-Kennedy-2]

Hello Stewart,

Tail end IF Filter:  Given that the prime purpose of the 2nd IF Crystal
filter is to reduce wideband noise created by the MC1350 IF amplifier, which
comes out as audio "hiss", yes a better filter would do a better job
provided that it is wider than the SSB filter. BUT I suspect that the MC1350
would not be happy driving such a filter with its varying input impedance.
More distortion / intermod would result than when driving the two crystal
filter. The other possibility is that the drive to the AGC system could
"bounce" due to these impedance variations, and depending on time constants
etc oscillation at some frequency high or low could result. An Image Reject
Demodulator can eliminate the need for any "tail end" filter, but no room,

AGC Take-off point. Here I am guessing. It could be 1) To reduce the BFO
signal leaking through the demodulator that might drive the AGC. The filter
chops this down. 2) A simple matter of drive level  3) To reduce AGC Attack
delay.

73.
Geoff,
GM4ESD

----- Original Message -----
From: "Stewart Baker" <[hidden email]>
To: "Elecraft Reflector" <[hidden email]>
Sent: Saturday, March 05, 2005 11:27 AM
Subject: [Elecraft] K2 - 2nd Xtal Filter


Anyone any experience of, or thoughts about adding another 3 crystals to the
K2
2nd crystal filter ?

73
Stewart G3RXQ

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[wayne burdick]

On Mar 5, 2005, at 3:27 AM, Stewart Baker wrote:

> Anyone any experience of, or thoughts about adding another 3 crystals
> to the K2
> 2nd crystal filter ?

Hi Stewart,

The present 2-crystal filter (as on newer units, or older units with
the 2nd XFIL mod) is very carefully designed to provide different
bandwidths on CW and SSB. Equally important, this filter must be
centered at the same frequency as the main filter over the full
controllable bandwidth range. L34 and related circuitry, along with the
varactor diode, accomplish these tasks.

If you wanted to add more crystals here for the purpose of improving
ultimate attenuation, two crystal filters switched by pin diodes would
be needed, along with a complete re-design of all associated circuitry
(I.F. amp output, product detector input, CW/SSB control, bandwidth
control in the CW case). This would be very difficult and
time-consuming, and the resulting circuit would not fit in the present
location. That would require the use of coax cables to a separate
module, along with other potential issues such as RF pickup. Chances
are the result would not be much of an improvement.

If your goal is to improve ultimate attenuation, the KDSP2 is a much
saner route.

73,
Wayne
N6KR


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