[Elecraft] K3 2 tone 3rd order dynamic range at 2 KHz tone spacing

[Dave G4AON]

I've been looking at the performance of my K3 using a buffered fixed 
crystal oscillator, hybrid coupler and Marconi 2018 signal generator... 
I've noticed a strange phenomenon where the two tone dynamic range 
appears to be approximately 10 dB worse at 2 KHz spacing with the 400 Hz 
8 pole filter than with the 2.8 KHz 8 pole filter (90 dB and 100 dB 
respectively). At 5 KHz the range is 100 dB with either filter. Altering 
the width control from 400 Hz to 450 Hz and back again switches the 
filters, the audible effect of the spurious signal being there or not, 
is quite striking. Both upper and lower 3rd order products behave the 
same, I also get the same results with the Marconi signal generator on 
the high side or low side of the crystal oscillator.

Anyone with a pair of signal generators and a hybrid coupler care to 
check their K3 and see if they obtain the same results? I'm assuming the 
reason is ripples in the passband of the 400 Hz filter many dBs down the 
passband.

73 Dave, G4AON
K3/100 #80