[Elecraft] K2 filters charactoristics

[Ron D'Eau Claire]

Neil, if your "spot" frequency - the place in the bandpass at which your BFO
produces the desired frequency sidetone - is always at the center of the
filter passband, then it must change, or in some cases you'll hear signal on
both sides of zero beat.

For example, using a 700 Hz beat frequency and a 250 Hz filter. If it were
centered in the 250 Hz bandpass, the filter passband would run from 575 Hz
to 825 Hz. (700+125Hz and 700-125 Hz). 700 Hz would be in the center. 

But, if you centered the 700 beat frequency in a 2,000 Hz filter bandpass,
the passband would go from a high frequency of 1700 Hz on the high end, and
then you'd go clear through zero beat to 700 Hz on the wrong sideband!  That
is, a 700 Hz signal would appear twice on your tuning - 700 Hz on either
side of zero beat! 

When the filter bandwidth exceeds twice the beat frequency you are using,
you MUST set the frequency off-center if you want to have "single-signal"
(single-sideband) reception. 

The K2 will allow you to set the BFO wherever you want, so you can center it
on any bandwidth you want, but most of us would rather have single-signal
reception on the wide bandwidths so we purposely off-set the center point on
the wider filter settings so the opposite sideband is always suppressed. 

Ron AC7AC



-----Original Message-----
From: elecraft-bounces at mailman.qth.net
[mailto:elecraft-bounces at mailman.qth.net] On Behalf Of NHarvey113 at aol.com
Sent: Friday, May 21, 2004 9:54 PM
To: elecraft at mailman.qth.net
Subject: [Elecraft] K2 filters charactoristics


I have a Yaesu FT-990 with four filters in it.  They are 250  500 2K & 2.4k.

In CW mode the
SPOT frequency is centered in each one know matter which one I'm in.  
Hamscope, which has
a frequency presentation, easily shows this.  The SPOT frequency will vary
in 
frequency as you 
tune within the frequency band width of the filters and can be heard until
it 
rolls off either edge 
of the filter it is in.

I have done the filter alignment on my K2  S/N 4215 as per the procedure in 
the manual and
my numbers match the manuals numbers.  When I look at the filter
presentation 
on Hamscope
I notice that all the four filters have the correct band width, but as you
go 
from wide to narrow
the change in band width is only changed at the high frequency end.  So if 
you have a SPOT
frequency of say 700 cps. the tone would be fine in the 1.5K B/W filter,  on

the edge and weak
in the 700cps. filter, and not heard on the remaining narrow filters.  If
you 
change the SPOT
frequency to 400 cps. it can be heard on all of the filters.  

What this says also is the SPOT frequency can never be higher than the 
narrowest filter if you
want to hear it.

Is this the way it is using the K2?

Neil Harvey   AC6YO
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