Re: VK4PP uBitx LPF BANK add-on Board

Warren Allgyer <allgyer@...>
 

Suggestions based on experience and good practice are all good. It is my experience however that some fairly simple measurements can and need to be made in order to confirm a layout.

First, simply connect up the input and output on a completely unpopulated board and measure the attenuation. If you find less than 80 dB of attenuation at any frequency below 100 MHz, you are probably not going to like the end result. I would then add the relays and check it again. At this point your attenuation needs to be at least 70 dB. Add all remaining components except the filters themselves and check it again. Finally, one at a time, add the filters and check after each one is added.

If, at any step in the process, your input to output crosstalk gets higher than -60 dB, you are not going to be very happy with the outcome. Note all of these measurements are made with the relays deactivated so there is no actual intended path between input and output.

When working on a similar project with Tony Parks for an all-band Softrocks RXTX our first two prototype filter boards measured less than 30 dB isolation between input and output. And we thought we were following good layout and RF principles.

BTW, on the Softrocks project we finally got there........ only to realize that the radio would never make compliance goals without a similarly switched interstage filter prior to the RF drive chain. At that point we had pushed the cost of the radio beyond the target and the project was scrapped.

My bottom line reaction is: layout as best you know how but test, test, test.

WA8TOD

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