Effect of Pitch on Filter Measurements

pOn February 5, 2009 in an email to the Elecraft List, Joe Subich, W4TV wrote: "The DSP begins to roll off at 200 Hz even if LO = 0.00. Any measurement where FC (Pitch) is less than [BW60/2 + 200] will be inaccurate on the skirts and any measurement where FC is less than BW6/2 + 200 will have essentially identical lower frequency responses."

This suggests that Pitch has a significant effect on filter skirt response and low frequency response. For example, if we are measuring a 500 Hz filter in CW mode with a -60dB bandwidth of 1,640 Hz, Joe's formula BW60/2 + 200 would indicate a Pitch less than 1,020 Hz (1640/2 + 200) gives inaccurate results on the filter skirts. Further, if the -6dB bandwidth of this filter is 530 Hz, Joe's formula BW6/2 + 200 indicates a Pitch less than 465Hz (530/2 + 200) will have inaccurate low frequency response.

To test these theories we ran a series of tests using K3 Filter Tools on a 500 Hz filter in CW mode with different Pitch settings. In CW mode, Pitch can be varied from 300 Hz to 800 Hz. So, we ran tests at Pitch settings of 300, 400, 500, 600, 700, and 800 Hz. The first set of tests used a 500Hz Xtal filter with DSP bandwidth set at 500Hz, CW mode, -55dBm injected signal, and RF gain at maximum. The second set of tests had everything the same as the first test except DSP bandwidth was set at 2,800Hz.

In the first graph below with DSP bandwidth set at 500Hz, you can see the measurements are definitely offset to the right for 300 and 400Hz pitch settings. The plots for pitch of 500, 600, 700, and 800 Hz are virtually identical.

In the second graph with DSP bandwidth set at 2800Hz, the 300 and 400Hz plots are offset as before. Also note how at the lower frequencies the skirts move to the left with higher pitch. At 800Hz Pitch we measured the 6dB BW as 481Hz and the 60dB BW as 1426Hz. These numbers are pretty close to lab measurements for the 500Hz Xtal of 530 and 1640Hz, however they are narrower than they should be.

In conclusion, it is clear that Pitch does have an effect on filter measurements in the K3 and the recommended Pitch to use in CW mode is 800 Hz with DSP bandwidth set to the maximum 2,800 Hz available in CW mode. However, even at a Pitch of 800 Hz the measurements on the filter skirts are narrower than they should be just as Joe predicted. Is there a way around this dilemma? My suggestion is to use Data A mode rather than CW mode to run tests of CW filters. See Effect of Operating Mode on Filter Response for details.

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