[Elecraft] [K3] Elecraft roofing filters

[ni0c]

W4TV wrote:
"The difference between the 250 and 400 Hz filters is not 
enough to be worthwhile (approximately 375 Hz vs. 430 Hz). "

Joe, what is your source for these numbers?  I've seen numbers 
like this mentioned previously on the reflector.

I recently tested my K3 CW roofing filters using the XG-2 generator 
at 7040 KHz (50 microvolts input) and KS7D's nice software package, 
"K3 Filter Tools."  

Here's what I came up with, with AGC off and with the DSP 
bandwidth set at least as wide as 900 Hz for all tests (to isolate 
the effects of just the crystal roofing filter):

200 Hz, 5-pole: -6 dB BW = 210 Hz; -30 dB BW = 430 Hz

250 Hz, 8-pole: -6 dB BW = 260 Hz; -30 dB BW = 500 Hz

400 Hz, 8-pole: -6 dB BW = 380 Hz; -30 dB BW = 580 Hz

Note the uncertainty in each of the bandwidths above is 
plus or minus 20 Hz, because I ran these sweeps in 10 Hz 
increments to save time. (I'll repeat these tests using a 
1 or 2 Hz increment, when I have some spare time.) 

These tests on my filters (as well as my experience by ear) 
indicate there is an appreciable difference in the 250 and 
400 Hz 8-pole filters.  I've always wondered, too, about 
manufacturing tolerances on narrow crystal filters.  

73, 
Chuck Guenther  NI0C


   

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