[Elecraft] 500 Hz filter measurement?

[Joe Subich, W4TV]

>  Is it likely my Xtal filter is as narrow as measurement 
> shows or is it more likely something is wrong with my testing? 

Your test data is probably correct.  My 500 Hz filter tests 
at 470 Hz.  The -6 dB points are 230 Hz below and 240 Hz above 
the "spot" (center) frequency.  

I used 800 Hz as the center frequency with DSP set to 2800 Hz for 
testing to make sure that there was no offset or DSP low cut to 
color the results.  I'll be making the same tests on two more 
500 Hz filters with my second K3/KRX3 as soon as I have time to 
finish that build. 

73, 

   ... Joe, W4TV 
 




> -----Original Message-----
> From: elecraft-bounces at mailman.qth.net 
> [mailto:elecraft-bounces at mailman.qth.net] On Behalf Of Paul Ferguson
> Sent: Tuesday, August 26, 2008 6:50 PM
> To: elecraft at mailman.qth.net
> Subject: [Elecraft] 500 Hz filter measurement?
> 
> 
> > 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
> 
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