Adding APF and Variable Bandwidth to the Sub Rx

Bringing the main RX DSP flexibility to the sub RX using modern active filter technology

Based on the testing of the main RX APF, and with the success of the sub RX 1K mod, I wanted to build up an external active filter that would provide the same overall AF response from the sub RX that we enjoy from the main RX. This page is dedicated to that project.



First Steps

We concluded from the examination of the main RX APF characteristic that the response was what we would expect from a 2-pole Q=8 bandpass filter.  To verify the response in reality is the same as the theory, a 2-pole Q=8 filter was built and plots taken from the main and sub RX to compare results. 

The match is excellent.  The response in the first 15db of the transition band is almost exactly the same as the first receiver. 












A Simple AFP Filter For the Sub Rx - Hardware Design Using TI's Filter Pro Software


Texas Instruments provides for a free active filter design program called Filter Pro.  Registration with TI is required but that just takes a minute.


Click HERE to download Filter Pro


An example of the kind of circuit that would provide the sub RX AF with an APF type filter response.  The design steps in the software are easy to follow. 

Select a 2nd order response - and pick about a 75 Hz passband bandwidth. If needed, adjust the passband bandwidth so that the "Q" of the filter is listed at 8 with a butterworth filter choice.

My basic circuit design summary from Filter Pro looked like this:



Click HERE for the 600 Hz Q=8 Filter Pro report


Selection of the op amp is not very critical. The components listed are of a 5% tolerance value - the capacitor selection would probably be best served with 5% poly types.


For a more extreme variation of an audio filter project, click HERE.


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