[Elecraft] K2 SSB flattening mod - does it compromise CW?

[Ron D'Eau Claire]

You are right Andy. I has not deleterious effect on the CW performance
at all. 

There are two filters in the receive path. The first one - ahead of the
IF amp - is the most complex one and the one for which many ops have
installed replacement crystals because Elecraft is offering a set of
crystals with tighter matching specifications. That first filter is the
one that really defines the selectivity of the K2. It is the one that
can be programmed for various CW bandwidths and the one that is replaced
by the OPT1 filter on the SSB board for SSB-equipped K2s. This filter is
NOT the one involved in the "filter flattening" mod. 

 There is a second filter AFTER the IF amp called the "2nd Xfil". This
is a very simple crystal filter with a rather broad frequency response.
It's only function is to "clean up" noise added to the receive path by
the IF amplifier itself. This is the filter that has the "tilt" in its
bandpass that the "filter flattening mod" straightens up. 

If you hear any difference in CW operation after doing the "flattening"
mod, it would be to IMPROVE the sound of the K2. That's because the 2nd
Xfil, as originally designed, tends to make higher-frequencies stronger
on one sideband and lower frequencies stronger on the other sideband.
Because of the way the mixing scheme in the K2 works, this means that
the effect of the filter will change depending upon what band your are
operating on. Depending upon what sidetone frequency you like, the
"tilt" in the bandpass of this filter might be fighting you on some
bands by increasing the gain on frequencies outside of your CW filter
bandpass. But if there is ANY audible difference at all in the receiver
performance using CW bandwidths, it is miniscule. Where the tilt becomes
a problem in the wider filter bandwidths (1.5 kHz or more) where you
will hear quite a difference it the sound of the background noise as you
switch from one side of the 'carrier' to the other. For most of us, that
wasn't really a big problem either. Where it become REALLY objectionable
was when listening to SSB. 

In some off-line correspondence with Wayne at Elecraft a while back he
reiterated that he recommends that the 2nd Xfil Filter Flattening Mod be
performed on EVERY K2. The only reason that it hasn't been incorporated
into the basic kit so far seems to be that it requires a change to the
r-f board layout. The r-f board is the biggest board in the K2. That's a
big project that has been pre-empted by work on other projects like the
KPA100 and KAT100 so far. 

Ron AC7AC
K2 # 1289


the filter flattening
mod: does installing this mod compromise the CW 
performance at all? 

-Andy