[Elecraft] K3 roofing filter choices

[Dave G4AON]

Having spent a few hours conducting measurements of the 3rd order mixing 
products with my K3, I'm not convinced of the benefit of several roofing 
filters for the K3. If other owners with access to two signal generators 
and a hybrid coupler would like to try their K3 with whatever filters 
they have I would be most interested in the results.

Try as I might with very strong dual signals into the K3 from two signal 
generators and a hybrid coupler, changing the bandwidth by altering the 
width control between 400 Hz and 450 Hz there is mostly no difference in 
the 3rd order products except for an anomaly at 2 KHz signal spacing 
where my 400 Hz 8 pole filter seems to be about 10 dB worse!

I've tried the shift control, very careful measurements at higher signal 
levels with spacings from 1 KHz to 20 KHz, etc. There is a difference of 
perhaps 3 or 4 dB where the narrower filter has the advantage with 3rd 
order products in the spacing range 5 to 20 KHz but it's so small as to 
be insignificant and gives no measurable difference near the noise 
floor, only being apparent with signals some 10 dB higher than the 
standard ARRL test levels to measure the 3rd order products. The levels 
required to produce the small difference are hitting the end stop of the 
K3 and are - 20 dBm, levels not likely to be encountered in practice for 
most of us. The only filters I have at the moment are 2.8 KHz 8 pole and 
400 Hz 8 pole.

I also noticed the dynamic range appears better by a small margin with 
the pre-amp turned on... Although it should be pointed out that using a 
pre-amp improves the MDS (minimum discernible signal) and raises all 
signals by around 10 dB, so it's not increasing "signal handling".

My measurements give a 3rd order dynamic range of around 100 dB at 2 KHz 
signal spacing with a 400 Hz 8 pole filter and slightly better with a 
2.8 KHz 8 pole filter (around 105 dB).

73 Dave, G4AON