[Elecraft] The Myth of Lying to Your K3

[Ed Muns]

Setting the crystal filter engagement bandwidth is sometimes described as
"lying to your K3" when the number is different than the number on the
filter.  While this phrasing is cute and descriptive, it can be very
misleading.  There is no lie, but in fact a very precise setting that
determines the DSP frequency at which the crystal filter engages.  If
anything, the user is lying to themselves.  Some facts:

1.  The bandwidth in the filter's marketing name is not the actual bandwidth
of the filter.  Some filters are significantly different than their name.
For example, the "KFL3A-250  250 Hz, narrow 8-pole  filter" is nominally
370Hz wide at the -6dB points.  The "KFL3A-400  400 Hz, narrow 8-pole
filter" is 435Hz.

2.  The actual IF bandwidth of the K3 is determined by the cascade effect of
the DSP and crystal filter bandwidths.  When those bandwidths are close to
one another, the actual IF bandwidth is significantly narrower.  So, if you
configure your K3 to engage the 250Hz filter (actually 370Hz) at 350Hz or
400Hz, the resultant bandwidth will be closer to 300Hz.

3.  If you engage a crystal filter at a DSP bandwidth much greater than its
actual bandwidth, then the K3 IF bandwidth will decrease immediately to the
crystal filter bandwidth and the passband shape will be governed by the
"rounder" shape of the crystal filter.  Some users do this because they
"like the sound" of the crystal filter passband shape compared to the
sharper DSP passband.  Keep in mind, though, that the bandwidth step at the
crystal filter engagement point is much larger than the normal DSP steps.
For example, if you have the 1.8kHz filter engage at 2.2kHz, you give up any
IF bandwidth between 2.3kHz and 1.8kHz.  The K3 WIDTH control shows the DSP
bandwidth at values between 2.3-1.8kHz, but the actual K3 IF bandwidth is
governed by the narrower 1.8kHz crystal filter and fixed at that bandwidth
for these intermediate DSP bandwidths.  Failure to recognize this fact is
lying to yourself.

4.  The crystal filter engagement point can alternatively be set at a DSP
bandwidth much less than the actual crystal filter.  In this case, the DSP
bandwidth steps are consistent and represent the true IF bandwidth of the K3
at any time.  The crystal filter is always wider than the DSP bandwidth and
therefore does not cause the IF bandwidth to be narrower due to cascading.
In this case, the K3 IF passband shape is governed by the sharper DSP shape
and the crystal filter is only providing roofing filter protection for the
DSP, which happens to be the primary function of the crystal filter.

Any value you choose for a crystal filter to engage is valid.  There is no
right or wrong.  There is no lie.  The engagement point you choose is a
simple fact.  The important thing is to fully understand the implication of
your choice so that you accomplish what you intend to accomplish.

Ed - W0YK
-----------------------------------------------
Ed Muns
Muns Vineyard - www.munsvineyard.com 
FaceBook - www.facebook.com/munsvineyard