Today I am experimenting with a single chip Universal Active Filter, in this case I made a small PCB for the UAF-42 from Texas Instruments. I chose this part in particular as it facilitates setting the filter frequency by changing just a pair of resistors and the somewhat critical values that are contained on the chip have been laser trimmed for accuracy. This type of active filter includes Operational Amplifiers to supply gain and it supports various configurations including simultaneous operating modes such as Band Pass, Low Pass and High Pass make it “Universal”.
Looking at the block diagram you can see where I have inserted a dual-ganged potentiometer to change both resistors simultaneously which should allow a straight forward adjustment for our purposes here.
Looking into the components of a simple RC filter which can easily implement a simple Low Pass or High Pass filter, we see that the math is fairly straight forward and swapping the components with each other is all that is needed to change the type of filter.
To tell the difference between the high and low pass examine the circuit shown where the capacitor is in series with the signal. Simply put, no Direct Current (DC) can make its way through a capacitor in series, so DC (0Hz) and other low frequencies are rejected or attenuated. Likewise when the capacitor sits across the signal (from the signal to ground) it can act like a battery; it stores energy and resists a change in voltage allowing slower, lower changes or frequencies to get through while resisting faster changes in voltage.
Whats Your Frequency Response
To demonstrate the frequency response of a system a Sweep Frequency Generator can be used. Here my old and venerable HP 3314A Function Generator is set up to sweep from 80Hz up to 5kz and pass it through the filter circuit. Looking at the input you can see the sweep start at a visibly low frequency and then sweep to a high frequency. Looking at the output one can see the amplitude of the different outputs and due to the way the oscilloscope is set up the output is synchronized with the start of the frequency sweep. This means that we can estimate the signal frequency based on its location on the scope.
When a Picture is Worth a 1,000 Hertz
Looking at the image we can see that the blue trace of the High Pass has an initial “blip” (a nonlinearity, possibly of the “ripple” variety) and that the amplitude of the signal on the right shows that it’s passing the higher frequencies. Likewise the green trace is passing the lower frequencies on the left and the pink trace a pass region in the middle.
Due to the adjustability provided by the dual resistors, I can adjust the filter frequency easily from low to high using my potentiometer. And if you watch the video you can see the effects of these adjustments, I tend to think of an analog synthesizer when I think of interacting filters sweeping the audio band, perhaps I can automate the adjustment of these filters in a following video.
Check back soon to catch Part 2 where I test some of what we talked about in earlier episodes about Square waves and harmonics. I also talk a little bit about the real-life math involved.