[Elecraft] 500 Hz filter measurement?

[Paul Ferguson]

> W4ZV wrote:
> You only need the two -6 dB points, which can be measured to a resolution of
> 1 Hz and 0.1 dB using the internal AFV/dBV function.  You should set AGC Off
> and ensure WIDTH (i.e. the DSP BW) is >> than the filter you are
> measuring...otherwise you're measuring the combination of XFIL plus DSP.  I
> do this by disabling all filters except the one under test and setting WIDTH
> >> XFIL BW.  Once you have the two -6 dB points, then you can figure how
> much offset to enter so these two points are equidistant about the center.


I used the above procedure to test my 500 Hz filter. The offset printed on the 
filter is -0.82 kHz. Testing shows the center frequency to be about 43 Hz low 
when using the above offset. I put -0.78 KHz in the filter configuration, and 
it tests centered now.

The -6 dB bandwidth measured about 480 Hz. I have seen the nominal -6dB 
bandwidth of the 500 Hz filter listed as 565 Hz. When I am measuring the 
bandwidth, I have the DSP bandwidth set to 2.8 KHz. Since the DSP filter is 
fairly wide, I would not expect the cascading to have much affect on the Xtal 
filter bandwidth measurement. Is it likely my Xtal filter is as narrow as  
measurement shows or is it more likely something is wrong with my testing? 

Here is info about the test setup:
  PITCH = 600
  AGC = off
  Mode = CW
  Only 500 Hz filter enabled
  500 HZ filter Offset set to -0.82 KHz
  DSP BW = 2.80
  RX EQ = reset to 0 for all EQ bands
  Center freq on K3 determined using CWT and Spot
  Sig source = HP sig generator at 50 microvolts


73,
Paul Ferguson
K5ESW
Raleigh, NC