Saturday, December 9, 2017

Doing a little filter eval

I played with the filter programs a little and here is what I found to be one possibility. The main problem with the direct conversion receiver is the LO signal goes in the antenna and overloads the front end. A dual conversion would eliminate this problem. I have a box full of 25.125Mhz crystals so that is what I'll be playing with. The receiver will be from 0 - 20 Mhz. First a filter for the front end.
I told Elsie what I wanted and presses the calculate button and here we are. Click another button and see the frequency response.
So it is down 20 db at 20Mhz and I'm looking at a 25Mhz IF. Think that should keep my LO out of the frontend? Now for the IF filter.
I told the ladder Filter Program (LFP) I wanted to use 4 Xtals and an little about them. Now for the filter.
My post from yesterday had a page of possible filters to design. I copied this from that page. N=4 four Xtals. The component labels ID their location.
The print out list the components values. So moving the values to the schematic gives this.
The values are in picofarad. Now it would be interesting to put the circuit together and test it. I think I have some 1nf and 900pf caps so I may go with 2nf and 1.9nf for a test.

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