Filter Response

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wsprowls
Posts: 141
Joined: Sat Apr 28, 2018 4:05 pm

Filter Response

Post by wsprowls » Wed Jun 02, 2010 2:21 pm

Hi All,

The other day, Neil had some concerns about his 3 KHz Crystal Ladder Filter he had built using the 10.11 MHz crystals. I responded that his skirt slope was not as sharp using 5 crystals, compared to the 8 pole monolithic crystal filter I was using.

As a comparison, I have taken two MSA plots (SA Mode) and put them on the Photos page at:

http://groups.yahoo.com/group/spectruma ... 9/pic/list



Both are "zero sweeps" showing the upper side band phase noise.(no input to MSA, only a 50 ohm load)

One is the 8 pole with a 2.2 KHz bandwidth. The other is the 5 pole, 10.11 MHz, 900 Hz bandwidth. Comparing the two photos, you can see the skirt responses. Even though the 900 Hz filter is much narrower in bandwidth than the 2.2 KHz, its skirts are not nearly as sharp. This is not really a big factor in the MSA. It would be in an expensive, ultra low phase noise, commercial spectrum analyzer, where phase noise measurements are critical.



In both sweeps, the self-measured, close-in phase noise is limited by the skirt of the filter. For the 900 Hz filter, the lowest frequency noise that can be measured is about 3 KHz. For the 2 KHz filter, the lowest frequency noise that can be measured is about 2.5 KHz.

Scotty

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