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- Signal Interference and Shielding

When doing measurements, in particular of low-level signals, you will notice that the signal carries often a lot of noise .This is usually due to interference from electric and magnetic sources which are present in the laboratory or in nearby spaces. This can include 50 Hz from the fluorescent lights, switching power supplies, motors, a radio transmitter, or even faulted grounding (ground-loop interference; see also section on "Circuit Ground and Grounding Practice". Here are a few precautions that can be taken to reduce the effect of interference.

- Capacitive interference is the result of wires at different voltage placed closed to each other. When the voltage changes in one wire, a voltage can be induced in the neighboring one through capacitive coupling. Examples are wires coming from equipment such as function generators, oscillators, and wires from the outlet (60 Hz). The higher the frequency of the interfering source the larger the interference will be. Another source of capacitive interference are fluorescent lamps. The best way to prevent capacitive interference is to use shielded cable or coaxial cable. The shield around the cable is usually connected to the ground and protects the inner signal carrying conductor from the interference.

- Inductive interference is the result of time varying currents in a conductor such as coils. It is not always easy to prevent this interference. If possible one should work far away from such sources or shield the source with a ferromagnetic shield. Also, keeping large current carrying wires and small signal carrying wires fare apart; and keeping current carrying wires perpendicular to one another.

- Radio frequency (RF) interference is the result of radio transmitters and arcing in motors. To reduce the interference one can shield the low signal carrying wire with a well-conducting shield connected to the ground.

- Ground loops are the results of voltage differences which often exists in the ground plane to which the circuits are connected. This can be eliminated by connecting all ground points in the circuit to only one point which is connected to the earth ground.

Another good practice is to use short cables which carry small signals and keep them fixed. If necessary fix them to the table with tape.

-Tools

Besides the instruments and electronic components you will be using a variety of tools and cables. Each student group should have their own toolset
Click here to learn how to purchase the tool set. .

A protoboard is a tool for constructing and testing circuits. It has several strips with lots of holes in them which conduct in one direction. The thin long strips on the sides and in the middle conduct along the long direction while the fat strips conduct along the short direction as shown in the figure below. Protoboards have also posts which facilitates the connection to the power supply and ground. You should follow the following convention: use the black pole for the ground or reference, the red one for the positive voltage and the yellow one for the negative voltage or signal line.

-Sources of Errors

Doing a lab is usually a pleasant and insightful experience. However, often it can be frustrating, if you get unexpected results and don't know how to solve the problem. The best way to prevent such time consuming errors, is to follow good lab practice and prepare yourself before doing the experiments. One of the common sources of errors in the lab are:

1. Improper grounding (make sure you read the section on Grounding Practices on the EE Lab homepage).

2. Sloppy protoboard organization. Use short and straight wires of different color. The layout of the protobaord should follow close the circuit.

3.Lack of understanding of the equipment.

4. Not reading the instructions or schematics carefully.

5. Not doing the pre-lab.

6. Rushing

7. Never connect a power supply to the output of a function generator. This will damage the function generator.

8.When doing current or resistance measurements with the multimeter, do not put a voltage over the current or resistance input terminal of the multimeter.

-References:

1. "Guide to Electronic Measurements and Laboratory Practice", S. Wolf, Prentice-Hall.

2. "Lab Manual - The Analysis and Design of Linear Systems", J. Getty, Prentice-Hall, 1994.

3. "Basic Engineering Circuit Analysis", J. D. Irwin, Prentice-Hall, 1996.

4. "Electric Shock Hazard", C.F. Dalziel, IEEE Spectrum, Feb. 1972, pp. 41-50.