[Elecraft] 250 Hz and 400 Hz Filter Measurements

[Jim Brown]

Crew,

I finally got around to dragging out my precision audio spectrum 
analzyer to check out the 250 Hz filters. I have two K3s, one 
with a single RX and one with two RS. I have 250 Hz and 400 Hz 
filters in all three RX. 

First, I tweaked the centering of each filter (that is, the 
offset), then I measured bandwidth at -6dB, -30dB, and -60dB with 
IF bandwidths of 1kHz and 250 Hz.  

Results: There was some variation from one filter to another, but 
trends are quite consistent. The only significant difference 
between the 250 and 400 Hz filters is at their -6dB points with a 
1kHz IF, where the average bandwidths were 311 and 412 Hz 
respectively. This is essentially the bandwidth of the roofing 
filters themselves. 

Once you crank the IF down to 250Hz, there is no significant 
difference between the two filters. The average -6dB bandwidths 
were 193 and 200 Hz; at -30 dB, both filters averaged 300 Hz; at 
-60dB, they averaged 381 and 397 Hz respectively. 

There IS one repeatable difference between the 250Hz and 400Hz 
filters -- their insertion loss, which is 3dB. That is, the 250Hz 
filters have 3dB more insertion loss. 

After doing these measurements, I firmly agree with W4ZV, W0YK, 
and others, who have noted that there is no good reason for 
having both of these filters in a radio. Indeed, there is no good 
reason for the EXISTENCE of this particular 250 Hz filter, 
primarily because it is NOT a 250Hz filter by any reasonable 
measure. 

It is long past time for Inrad to lean on their filter supplier 
and get them to ship 250 Hz filters. Failing that, I want my 
money back for three filters. BTW -- all of these were purchased 
from Inrad, not Elecraft. 

73, Jim Brown K9YC