Thursday, August 29, 2013

Low pass filter and VCO update

VCO update

I added an LED to indicate the oscillation frequency of the VCO.  I also found an old power amplifier in the electronics junk room in the basement. From that I harvested a whole bunch of RCA jacks and a couple of very nice potentiometers. The jacks will be great for connecting modules, since RCA cables are shielded and easy to find/cheap. I disconnected the voltage source for the VCO, so now you have a choice of connecting either the voltage divider or another external voltage source to control the VCO.

I also discovered that when connecting the triangle wave output to a speaker (+ amplifier) i got some high frequency oscillations superimposed upon the triangle wave. I think these oscillations arise due to driving a capacitive load which is in the cabling or perhaps a DC-block capacitor in the amplifier. I'm going to deal with this later, but the TLC721 is a programmable opamp and is currently programmed as low biased. This is the slowest and most energy effective mode with a phase margin of 34°. The high bias mode has a phase margin of 47°, which might make the difference in this case. A first I thought that the output needed to be buffered, so I added an emitter follower opamp. This removed the oscillations from the original output opamp, but transferred the problem to the buffer output instead.

Superimposed oscillations on sawtooth wave. Upper waveform is from stage connected to amplifier. Lower waveform is from stage before output buffer.

Active Low Pass Filter

From The Art of Electronics, I took the simplest active filter circuit i could find. It's a two pole low pass Butterworth filter, which consists of two identical capacitors and two identical resistors, making up the filter, one opamp and two resistors for gain control.
Having two identical resistors that make up the filter gives a problem when making a variable filter, since you will need a double gang potentiometer. Lucky enough I found a couple of those in the power amplifier I mentioned. I took one that goes to 50k\(\Omega\). The -3dB frequency of the filter is given by
\[f_0 = \frac{1}{2 \pi RC}\]
with the lowest frequency being 20Hz, that gives me:
\[C = \frac{1}{2\pi 50000\Omega 20\text{Hz}} = 150 \text{nF}\]
with higher cutoff frequency with lower resistance in the potentiometer.

I added RCA jacks and the whole thing looks like this:
Active low pass filter schematic

Active low pass filter to the right

works like expected as well...

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