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ECE 3300 Lab 6
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FSK Communication System
Located in MEB 2275
Overview: This document is the lab 6 procedure. In this lab a frequency shift keyed
(FSK) communication system for a cardiac pacemaker will be tested, and the link budget
will be verified by checking how far the system transmits
Equipment: Network Analyzer HP 8720D; E4404B Spectrum Analyzer; E4438C Signal
Generator; and other circuit and materials referred to in this lab.
For more info: Contact the lab TA or class instructor. See: www.ece.utah.edu/~ece3300
Objectives:
1. Understand a simplex frequency shift keyed (FSK) communication system.
2. Compute the link budget for the communication system considering transmit power,
receiver sensitivity, etc.
3. Do a 2 port calibration on the Network Analyzer in preparation for 2 port analysis.
4. Learn to use the E4404B Spectrum Analyzer, and the E4438C Signal Generator from
Agilent. (See lab web sight for additional information on instrument use.)
5. Send a simple Morse Code FSK message.
Prelab:
1. Read the Introduction below, and the information on FSK given on the lab website.
2. Write a brief description of each component to be used in this lab. Do this by looking
up the data sheets for the following components found at www.minicircuits.com:
ZOS-535, ZFSC-2-11, ZFL-500LN.
Also, look up the data sheet for the helical filters at www.toko.com:
493S-1067A, 493S-1075A
3. Create a Matlab spreadsheet that calculates the power received including the effects
of all of the major losses in the system (see power link budget on the website, and
lecture notes if needed).
Introduction and Class Discussion
A simplex (single direction) FSK system consists of a transmitter and a receiver. The
transmitter is shown in Figure 1. It consists of control circuitry that specifies to a voltage
controlled oscillator (VCO) the frequency to generate. The VCO connects directly to the
transmitter folded dipole antenna. The control circuitry is shown in Figure 2.
1
UNIVERSITY OF UTAH DEPARTMENT OF ELECTRICAL AND COMPUTER ENGINEERING
50 S. Central Campus Dr | Salt Lake City, UT 84112-9206 | Phone: (801) 581-6941 | Fax: (801) 581-5281 | www.ece.utah.edu
ECE 3300 Lab 6
Figure 1- Transmitter
Figure 2. Transmitter Control Circuitry
The receiver circuit is somewhat more complex as shown in Figure 3. It consists of the
receiving monopole antenna and matching network (built during a previous lab),
followed by a radio frequency (RF) amplifier. A splitter then evenly splits the input
power to two outputs. Each output is then fed into identical legs of the receiver circuit.
A bandpass filter passes only the desired frequency. An RF diode passes the signal and
rectifies it. A capacitor holds the rectified signal, where it is fed to a decision making
circuit that determines which frequency is being sent. The receiver decision circuit is
shown in Figure 4.
2
UNIVERSITY OF UTAH DEPARTMENT OF ELECTRICAL AND COMPUTER ENGINEERING
50 S. Central Campus Dr | Salt Lake City, UT 84112-9206 | Phone: (801) 581-6941 | Fax: (801) 581-5281 | www.ece.utah.edu
ECE 3300 Lab 6
Figure 3. Receiver
Figure 4. Receiver Decision Circuit
In this lab, you will gain a better understanding of how each of the components works,
and how to use the test equipment to measure the components.
Procedure
1. Measure the receiver sensitivity. To measure the sensitivity, connect the signal
generator to the input of the splitter. The splitter should be connected to the receiver box.
Generate 420 MHz and 460 MHz signals and decrease the signal power until the receiver
can no longer differentiate between the signals.
2. Connect the VCO to a DC power supply, and adjust the power supply until it provides
either 420 MHz or 460 MHz signals. Measure the VCO output power and frequency. To
measure the output power, connect the VCO output to the spectrum analyzer, and observe
the peak. Put a marker on the peak for better measurements. Adjust the power supply
until the correct frequencies are obtained, and note the DC voltage required for each
frequency. Measure the power at each frequency.
3
UNIVERSITY OF UTAH DEPARTMENT OF ELECTRICAL AND COMPUTER ENGINEERING
50 S. Central Campus Dr | Salt Lake City, UT 84112-9206 | Phone: (801) 581-6941 | Fax: (801) 581-5281 | www.ece.utah.edu
ECE 3300 Lab 6
3. Calibrate the network analyzer using a full 2-port calibration.
4. Measure the reflection coefficient (Γ=S11) of the folded dipole shown below on the
network analyzer. Record values at 420, 440, and 460 MHz when the antenna is in
air (where it was not designed properly) and tissue simulant material (where it is well
designed).
Figure 5. Folded dipole antenna is built with one end soldered to and SMA feed, and the
other end soldered to ground (the outside of the SMA connector). This antenna is 125
mm long from tip to tip, and was designed to be resonant at 440 MHz in tissue simulant
material.
Figure 6. Real and imaginary impedance of the folded dipole antenna in Figure 5 as
simulated in XFDTD (3D FDTD software, see Remcom.com) and measured on a
network analyzer. Solid lines are real, dotted lines are imaginary.
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UNIVERSITY OF UTAH DEPARTMENT OF ELECTRICAL AND COMPUTER ENGINEERING
50 S. Central Campus Dr | Salt Lake City, UT 84112-9206 | Phone: (801) 581-6941 | Fax: (801) 581-5281 | www.ece.utah.edu
ECE 3300 Lab 6
5. Connect the transmitting antenna to the network analyzer port 1 and the receiving
antenna (in tissue simulant material) to port 2. Measure S21 from the transmitting
antenna to the receiving antenna 1 meter away using the network analyzer.
6. Measure all of the distances and related values that are needed in order to compute
the power link budget for your system. Plug these values into your spreadsheet, and
determine the expected received power. Consider the cases where the receiving
antenna is 1 to 10 meters from the body surface. Given the sensitivity that you
measured in part 1, at approximately what distance is there enough power for the
receiver to measure?
7. Connect the complete communication system as shown in Figures 1-4. The
transmitter control circuitry and the decision circuit have already been built for you.
Verify that the system works by sending a short Morse Code message.
Figure 7 - Morse Code
8. Observe the effect of multipath propagation on your received power. Choose at least
one distance where you receive a good signal. Move the antenna in air in 1”
increments for at least 1 foot. Plot the received power as a function of distance. For
these closely spaced measurements, you should see what looks like a lot of noise,
typically 10-30 dB of variation. Observe one point as a function of time. Does it
change as you move around?
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UNIVERSITY OF UTAH DEPARTMENT OF ELECTRICAL AND COMPUTER ENGINEERING
50 S. Central Campus Dr | Salt Lake City, UT 84112-9206 | Phone: (801) 581-6941 | Fax: (801) 581-5281 | www.ece.utah.edu
ECE 3300 Lab 6
9. Now measure the maximum transmission distance by moving the transmit and
receive antennas apart until the receiver can no longer reliably receive a signal. Be
aware that the multipath ‘noise’ is present in these measurements as well.
10. Determine the effect of an amplifier. Connect the receiving antenna to the amplifier.
Connect the output of the amplifier to the spectrum analyzer. Make sure that the
power input to the amplifiers is below 5 dBm, or else they will saturate. What is the
gain of the amplifier? What is the maximum distance that the receiver can receive a
signal?
Add the second amplifier in series with the receiving antenna. Make sure that the
power input to the amplifiers is below 5 dBm. What is the new calculated maximum
distance that the receiver and receive a signal?
6
UNIVERSITY OF UTAH DEPARTMENT OF ELECTRICAL AND COMPUTER ENGINEERING
50 S. Central Campus Dr | Salt Lake City, UT 84112-9206 | Phone: (801) 581-6941 | Fax: (801) 581-5281 | www.ece.utah.edu