PHY405F 2009
Due Date (NOTE CHANGE): Thursday, Oct. 22nd @ 5 pm; Late penalty in effect!
The sensitivity and precision of any physical measurement is ultimately limited by noise in the signals
being observed and in the equipment used to measure these signals. This lab illustrates some of the
aspects of noise that are present all around you! Be aware that some noise sources are actually real
signals transmitted for example by radio stations. Other noises are broad band caused for example by
flouorescent lighting or DC to DC switching power converters inside electronic equipment.
You need to become become familiar with various types of noise that present problems to the
experimenter and gain insight into precautions needed to reduce noise.
(1) Investigate the electronic noise sources all around you in room MP238. as seen on a CRO: Look at
the various noise signals around the room using your own CRO and the CRO which has a 100MHz
bandwidth equipped with Fourier Transform software.
Believe it or not, your hand is a good way to sense noise and inject it into the input of a CRO or even a
High Fidelity amplifier. Hold the CRO live (not ground) input lead with you hand and wave your other
hand near an insulated line power cord. Watch the power line pick up at the power line frequency and
maybe some of its harmonics.
Electric fields: Hang a wire a couple of feet long from the CRO input terminal and notice the pick up.
How does the frequency of the noise change if the input is also shunted to ground by a resistor of a few
Magnetic fields: Wind a loop of wire of maybe ten turns and connect the ends to the hot and ground pins
of the CRO. What do you observe at different frequencies? Replace the coil by an inductor with several
hundred turns. How does the character of the noise change?
(2) The laboratory possesses just ONE high quality – but old – spectrum analyser.
Based on your results in part (1), prepare a plan to look at the spectrum of the noises present
in the laboratory.
As a guide to the easier use of the spectrum analyzer, set the RF attenuation to 0 dB and
adjust the IF gain to keep the display on the screen. You may use sweep rates from 100 ms for fast
screen response to 2 s to see good averaging of the spectral trace. Adjust the resolution and the
sweep rate to enable the details of the peaks to be seen for each RF frequency (horizontal scale)
setting. Look at the following frequency ranges at the corresponding RF frequency setting:
0 → 25 kHz 1 kHz/div
0 → 2 MHz 50 kHz/div
0 → 200 MHz 5 MHz/div
0 → 1 GHz 20 MHz/div
Note that it appears that the spectrum analyzer has a filter at the front end to eliminate the ever present
power line harmonics. This is probably done to make it more useful. Note also that the analyzer seems to
produce its own noise from its switching power supply. This signal is best seen using the loop for
an antenna positioned near the cathode ray tube face, and appears as the harmonics of 19 kHz.
Observe what wire configurations and shielding configurations reduce the various components of
the noise picked up in the room.
In both parts (1) and (2), identify the various noise sources. You should see for example the VHF and
UHF FM signals transmitted from the CN tower and maybe the 1 MHz broadcast AM band.
(3) The final part of the lab involves using the Electronic Workbench. Use it to design a simple one pole
a high-pass filter (RC circuit) with a corner frequency fc = 100 Hz. Display the frequency response
(amplitude and phase). How would you change the circuit design to produce a more rapid change in
Amplitude with frequency through the corner frequency? (Hint look up multi-pole filter theory).
A.J. Diefenderfer and B.E. Holton, Principles of Electronic Instrumentation, 3rd Edition, Chapter 15.
P. Horowitz and W. Hill, The Art of Electronics, 2nd Edition, pages 428-466.
R.E. Simpson, Introductory Electronics for Scientists and Engineers, 2nd Edition, Chapter 8.
L.R. Fortney, Principles of Electronics, Analogue and Digital, pages 543-552.
G. Rizzoni, Principles and Applications of Electrical Engineering, 3rd Edition, pages 689-699.
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