Lab Report 5

Experiment

Basically, the purpose of this experiment was to show how diodes affect AC signals. This was achieved by placing a diode within our audio circuit in a few different arrangements, and using oscilloscopes to visualize the signals. 

Here is the first circuit we made, which was simply running our audio signal through the diode, and the diode was input with it's bias facing the output jack. The oscilloscope was connected both before and after the signal went through the diode.

Here's a picture of the oscilloscope results. The diode basically truncated the negative portion of the signal, and reduced the peak voltage a little bit. This was the test at 200 Hz, and at 1000 Hz and 10 kHz, the slope of the truncation was different. At 1000 Hz, the diode signal didn't follow the shape of the unaffected signal, and at 10 kHz, the voltage level the signal was cut off at seemed to be higher.

200 Hz

10 kHz

The next circuit we used was essentially the same circuit, but the diode's bias was reversed. The effects were very similar to the forward biased diode, the main difference being that the positive portion of the signal was truncated, instead of the negative side.

200 Hz

10 kHz

The next circuit we built connected the diode to ground, instead of having it run directly to the audio output. Here is a picture of this circuit. A resistor was also added to the circuit. 

The oscilloscope showed that the signal was cutoff at a certain positive voltage, and the voltage it cut off at depended on the volume of the signal. There also seemed to be a threshold volume where the signal began to be cutoff, and any volume below this threshold showed no change in signal. The effect was basically the same at all frequencies I tested at. When I reversed the bias of the diode, it had the same effect, but the signal was cutoff at a negative voltage. Unfortunately, I forgot to take a picture of the reverse biased diode, but I think you can get a pretty accurate idea of what it looked like from the forward biased diode. 

Forward Biased Diode to Ground

The last circuit we constructed was called a peak follower, which is a forward biased diode connected to a capacitor. The capacitor is then bypassed to ground.

The peak follower did its job and output a signal that was relatively flat at the peak voltage of the input signal. As frequency increased, the peak follower line became straighter and more uniform. Also, when I took out the capacitance, the base line of the output voltage went back to 0, and was basically the same output as the forward biased diode circuit from earlier in the lab. I also tried a few different values of capacitance, and found that higher values of capacitance will make a straighter output line.

Capacitance: .1 microFarad

Capacitance: 1 microFarad

Questions

1. The peak follower could essentially act like a vocoder, because it will output the peak voltage of an input signal. The output signal could then be used to control the frequency, cutoff frequency, or amplitude of a VCO or VCF etc. There are a ton of musical applications for this sort of set up.

2. To isolate the bassiest frequencies of an input signal, you would need to run it through lowpass filter first, then connect the filter to the peak follower circuit. 

The values of the capacitors would probably be changed, and a variable resistor would work better because you would be able to pinpoint the corner frequency that isolates the bass drum.

Final Project

To be completely honest, I haven't been giving my final project a whole lot of thought recently. However, I'm fairly committed to this ResoDrum idea. I think a peak follower would be very useful actually, because I need to somehow convert the pressure on the bowl into some sort of control voltage. If I can use piezo disks to convert the amount of pressure on the bowl, I think this will be not too difficult to pull of. I'm mostly worried about figuring out the oscillator/filter set up I would need to make the drums actually interesting to use.