[1000mp] 4 kHz roofing filter

[Bill Tippett]

K4IK wrote:

 >Performance specs (Dynamic Range) are:

             Signal spacing            Dynamic Range
                10 KHz                    107   dB
                 5 KHz                    100.5 dB
                 3 KHz                     97.5 dB

 >This compares to an unmodified 5 KHz dynamic range in the
mid-80 dB range (from memory).

         Joe, do you know where the 107 dB DR could have come from?
In looking over Sherwood Engineering's list shown at Dayton, the
maximum I see for ANY radio at ***20 kHz*** spacing is "100+ ?"
listed for the IC-7800 (which is probably based on Icom's unqualified
advertising claim of 110 dB).  Also, the original MP was measured by
ARRL* to be 97 dB at 20 kHz.  I simply don't understand how a 4 kHz
roofing filter could suddenly improve the MP's performance at 10 kHz
spacings well beyond what has been measured previously at 20 kHz.
After all, both 10 and 20 kHz interfering signals fall well outside
the current +/- 6 kHz BW of the MP's 12 kHz roofing filter.  There is
something in the above specs that doesn't compute for me.  Either there
are some significant differences in measurement technique or something
else I don't understand.  Any ideas?

                                                 73,  Bill  W4ZV

* Summary of ARRL measurements (20 kHz IMD is 4th column from right):

http://www.elecraft.com/K2_perf.htm#5%20kHz%20numbers