Projects

Project 2, Spring '98

L and C values

The inductors and capacitors in the lumped parameter model are 1 microHenry and 390 picoFarads respectively.

lumped parameter circuits

The LC circuits that we used in class 27 are on the shelves behind one of the screens. We're using them again for the project.

HP Benchlink

In part B.1., I made a comment that you could probably only display 2 traces. That isn't true. I must have had something set up incorrectly. You can get an image of V1, V2, and V3 all simultaneously.

Review classes 27 and 28

The project is based on classes 27 and 28.

Project 1, Spring '98

Scope probe capacitance

The HP oscilloscope probes have a capacitance of 15 pF. (The gray cables with the nice connections for looking at signals). This is substantially less than the equivalent length of the standard coaxial cable.

Sensors

The original assingment sheet stated that you had to turn in the sensor with the project. That is optional.

Grounding

When making measurements, keep in mind that one of the two coax connections to the oscilloscope will be grounded.

Function generator amplitude

The function generator amplitude setting is the voltage output if it sees a 50 ohm load. Since the internal resistance is 50 ohms, we model the function generator as a Thevenin equivalent circuit with a source that is twice the value on the readout and a 50 ohm resistor in series.

Report requirements

As part of the report, you should do an analytical circuit analysis that predicts the voltage you measure. You should do 2 cases. One with the metal far away and one with it nearby. Ideally, you should use the circuit value you calculated for your sensor in the second case. However, for some of the circuits, it is hard to calculate the sensor's effect on inductance or capacitance directly. In that case, you should at least use the experimental results to estimate the change in properties.

For those of you doing resonant circuits, it is best to do your calculation for a range of frequencies. In order to compare to experiment including slight errors, you need to know the width of the resonance as well as the location of the peak.

Parts

You are allowed to use parts from the studio for the project. However, in order to keep the cabinet areas from becoming a mess, you must have one of the course staff get the items for you. All items are to be turned with your project. At least 20 protoboards are available. Since they do not belong to the studio room, please sign up when you take one.

Inductors

There are 3 types of inductors in stock. I measured one sample of each with a high quality bridge and obtained the values below.

frequency

		120 Hz	1 kHz	10 kHz	100 kHz
Type 1	L	82 mH	81 mH	81 mH	110 mH
	R	180 ohms	180 ohms	187 ohms	1050 ohms
Type 2	L	136 microH	136 microH	135 microH	125 microH
	R	0.06 ohms	0.07 ohms	0.23 ohms	5.3 ohms
Type 3	L	1.13 microH	1.14 microH	1.14 microH	1.12 microH
	R	0.04 ohms	0.04 ohms	0.04 ohms	0.07 ohms

Coaxial cable circuit model

See Cheng Figure 9-5 for a circuit model of a coaxial cable. For short cable lengths, the model can be used for the entire cable, not just a differential length. At the frequencies available with our function generators, cable lengths less than a couple of meters long meet this criteria.

Initial comments, Feb. 20

One approach to the project is to build a resonant LC circuit. The sensor will be part of the L or C (possibly all of it). As you bring the sensor near the metal, it changes the L or C. If you are near resonance, changing L or C by a small percentage can change the voltages at various nodes substantially. An example of a capacitive sensor is a piece of metal. The sensor's metal and what you want to detect form two plates of a capacitor, and therefore, become part of your general LC circuit. An inductive example is that a piece of iron near an inductor changes its inductance value.

The Theremin demonstrated in class works on a capacitive pickup. Note that you do not need complicated circuitry for our project to get a signal.

Class periods on March 4 and 6 will be open shop to work on the project. Some additional open shop hours (mostly evenings) will also be posted later.