Project Yamhill Active Audio Filter
For Pleasant Listening
As Project Yamhill nears actual physical implementation, it’s time to start building some more real-time, two-way communication. If you’d like to participate in Project Yamhill chat, please click on the link below. Thank you!
Nearly every receiver, other than the most very basic, will need to have some form of audio filtering in the audio chain, to eliminate the unwanted signals in direct conversion receivers, and to keep audio hiss from early AF gain stages from being amplified in all types of receiver systems. There are a few different ways of accomplishing audio filtering (as I documented in a recent post), but one tried-and-true method that is common in modern ham receivers is the op-amp active audio filter. All it takes is an op-amp with a decent GBW rating of at least a few MHz, and a handful of resistors and quality audio capacitors.
Since these types of filters are well-known and characterized, it’s almost trivially easy to design one using equations or web tools. The filter response that I chose was the classic Chebyshev, which gives a nice steep rolloff compared to most other choices. A quad op-amp seemed like the right choice, as each op-amp filter stage in the chain can give a rolloff of 40 dB per decade, for a total of 160 dB/decade when cascaded, which will aggressively cut off the unwanted high audio components.
I used the popular TI Webbench Filter Design Tool in order to get the values for two different cut-off frequencies: 800 Hz and 2.7 kHz, which will make for appropriate CW and SSB filters. If an end-user wanted a filter with a different cut-off, all they would have to do is specify the same type of filter in the TI tool and then enter their own desired cut-off frequency.
The filter topology itself is multiple-feedback Sallen-Key, and the op-amp is biased to virtual ground at 1/2 VCC with a simple voltage divider network on the input to allow single supply voltage usage. All very bog-standard stuff.
Because I’ve used this particular circuit multiple times successfully, I’m not too concerned about it being wrong (famous last words, eh?) so I’m going to order these circuit boards soon, along with all of the rest of the completed Project Yamhill boards. I do have some prototypes started on copper clad, but I don’t have all of the requisite capacitors on hand, so the actual circuit verification will have to wait a bit until I make a big order of prototype components from Mouser soon.
What’s Next
I’ve got a large stockpile of various Project Yamhill PCB designs on hand, just about ready for fabrication. Plus I finished the layout of the new 80 meter CW QRP transceiver board as well, and I’m itching to get that one too. I realized that I had forgotten one additional PCB design that’s necessary for an initial build: a board that gives a BNC antenna connector. This should be an easy one to knock out in an afternoon. After that, I will go back through each board design for one last review, and then should be ready to export the Gerbers and send them off to fab. Exciting times!
Getting interesting!