When it comes to direct conversion receivers, to get good selectivity you need to use good audio filters. There is no IF so you cannot use narrow crystal or collins type mechanical filters to narrow up the bandpass and filter out all those close by signals. For CW use this is important, because you might have 20 different signals in the typical 3K bandpass of most receivers, and listening to those sorts of crouded conditions, sounds like machine gun fire.
So for my direct conversion CW rig, I had decided to experiment with a number of different filter types and find the one that i find works the best. The first one to roll off the production line is an active filter using OP Amps.
Analog devices have a really nice online Filter Wizard that takes all the drudgery and though out of designing filters. As you can see in the above image, the filter is designed with a 600hz centre frequency, this is where I prefer to listen to CW and find it a pleasant listening experience. The filter width is ultra narrow. being only 200hz wide, so 100hz either side of the centrer frequency i should hear a fair bit of attenuation. This is a contest type filter, for very crowded bands. I will also build a 400hz filter for more casual listening or band searching and a 2K SSB filter for SWL’ing.
The schematic above has been optimized for low noise, one of the neat outcomes of doing this is all the capacitors become 100nf. And it becomes so much easier to buy a bulk of 100nf 5% caps than have to make sure you have a number of different values on hand. All resistors are 1% types of which some values might need to be rounded off. I am not sure yet on weather rounding up or down makes a big difference, but this is something i will have to determine experimentally at some point.
Something i did learn is that OP Amps need to have both positive and negative supply voltages for them to work right. After showing my work to VK2DDS , who pointed out I had things all wrong with power, I was sent on my way to learn about OP Amps and virtual ground. Now there are a number of ways to do this correctly, supply the OP Amp with negative volts, or use a Virtual Ground. By using another general purpose OP Amp configured as above feeding the negative supply line of the OP Amp with 1/2 the voltage of the positive supply line, you and up with the situation of the voltage being able to swing 12v <-> 6v <-> 0v. This is the same as 6v <-> 0v <-> -6v because the voltage of ground is relative, it can be whatever you want it to be.
So now the final piece of the puzzle is the design a board, etch it and test it out to see how the thing actually performs. For this I am using a TL074 Quad OP Amp. The design work was performed initially in KiCad, a Linux circuit board design program. It has a funky way of working and one that I am not all that fond of, but, it is also extremely powerful and is software i will persist with as it will be handy when i design something a little more complex than this filter.
Now, that’s not to say i did it all in KiCad, in the end i swapped out to a windows program called Sprint Layout. This is more a drop and join visual sort of method i am more comfortable with and for small jobs very easy for complete beginners to use. So the board layout is all done and all that is left now is to etch the board, I use toner transfer for this and then build and test the filter and see if it worked up to expectations and design parameters.