[Elecraft] KS 2nd Xfil SSB Mod

Sat Apr 20 10:48:00 2002

Folk:

You may have missed John (KI6WX's) recent post regarding fruther flattening 
of the K2 SSB filter response. I am including it below.

NOTE that John assumes that you have ALREADY made the previouly-posted "2nd 
Xfil SSB Mod", which also REQUIRES that RF-C180 be removed as well. So, if 
you are just now going to perform the mod, download the docs for the 
original mod from the Elecraft web site, follow them, as shown in that 
document, BUT use 100pF IN PLACE OF the 120pF cap originally specified and 
follow the rest of John's instructions.

73,

Tom  N0SS
__________________

From: "John Grebenkemper, KI6WX" <[email protected]>
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Subject: [Elecraft] Improvements To The K2 SSB 2nd XFil Mod
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X-Original-Date: Sat, 06 Apr 2002 13:22:26 -0800
Date: Sat, 06 Apr 2002 13:22:26 -0800

The Elecraft web site contains a modification to improve the SSB response of
the K2 in receive by changes to the Second Xtal Filter on the K2 RF Board.
While this change is much better than the original filter response, there is
still some difference between the LSB and USB.  Using some simulation
software to guide me, I was able to reduce the measured SSB ripple of the
Second Xtal Filter to 1 dB peak-to-peak from 4914 to 4916 kHz.  Now, I can
not hear any difference when switching between LSB and USB.

If you want to try this change, the steps for doing it are listed below.
This change has been applied to two K2's, but it has not been checked or
approved by Elecraft.  The change will increase the Second Xtal Filter
insertion loss by 2 dB, but it will not have any effect on receiver
sensitivity since this filter is at the end of the IF amplifier.

The change may be installed with the following steps:

1.  Install the 2nd Xfil Mod described in
http://www.elecraft.com/Apps/new_fil_docs/k2_2nd_xfil_ssb_mod.pdf

2.  Replace the 120pF capacitor in the above mod with a 100pF capacitor (10%
tolerance or better).

3.  Install a 5.6k resistor from pin 1 of U11 to pin 2 of U11 on the RF
board.

4.  Check the adjustment of L34.  In CW mode, tune to the weak birdie heard
at 7 MHz.  Set the filter bandwidth to 0.1 kHz and tune L34 for maximum
signal relative to the background noise.

Your K2 now has a much flatter response when receiving SSB signals through
the OP1 filter.  A plot of the K2's Second Xtal Filter response in the OP1
setting is available on my home page at
http://home.pacbell.net/johngreb/KI6WX.html . The plots were done with
original filter configuration, the 2nd Xfil Mod as described on the Elecraft
web site, and my change to this mod with the 100pF capacitor and 5.6k
resistor.

You may want to check your LSB and USB response with Spectrogram before and
after the change.  With my K2, the Spectrogram LSB and USB curves fall
nearly on top of each other.

-John
  KI6WX