BioNB 440: Lab 8

Digital and analog noise sources.

Introduction.

This assignment will show one way you can generate band-limited noise. The digital approach will be to produce a stream of pulses from a digital shift register, then do a simple digital to analog conversion. See section 9.32-9.37 of Horowitz and Hill. The analog approach will use the amplified noise in a resistor to provide a noise source.

Download the file lab7noise1.ewb. The circuit consists of a 15-stage shift register, a level shifter, and a low-pass filter. There is a set of gates attached to the shift register to ensure that at least one bit is logic-high because the circuit can occasionally get stuck on all zeros when it is powered up.

Note that in this and every lab assignment the verb build means to construct a circuit on the protoype board whith actual parts. The verb simulate means use Electronics Workbench mathematically simulate the circuit.

1. Simulate the circuit given above. Use the FFT option of Electronics Workbench to measure the spectrum of the output signal.
2. Build a resistor noise source. Use a 1 Mohm resistor and an opamp connected to give a non-inverting gain of 100. You may want to add a second opamp for a total gain of 1000 to 10,000. Compare the amplitude of the noise with the formula shown below which estimates the noise from a resistor of R ohms.
   Vnoise(rms) = (4kTRB)1/2
   Where 4kT=1.6x10-20 V2/(ohmHz) and B is the bandwidth in Hz. Connect the noise source to the computer speakers and listen to it.

Your written lab report should include:

The results of the simulations and the actual circuits you built.