ECE Undergraduate Laboratory
ECE 469 - RF/Microwave and Optics Lab

ECE 469 - RF/Microwave and Optics Lab

Lab 9: Introduction to Spectrum Analyzers and Shielding

What is the spectrum Analyzer? Unlike the vector network analyzer (VNA) that can measure magnitude and phase of the signal, the spectrum analyzer measures only the signal power. The spectrum analyzer can be thought of as a band pass filter whose center frequency is varied linearly over a range of frequencies. The center frequency of the filter is plotted on the horizontal axis and the output of the filter is plotted on the vertical axis. The filter must be swept over the frequency range slowly enough so we obtain the steady state filter output. The internal circuitry of the analyzer automatically changes the sweep rate as the filters bandwidth is changed to insure this condition is met. There is a default setting for the filter bandwidth, but it can be changed using the front panel controls. A wide bandwidth allows a more rapid sweep, but a narrow bandwidth is necessary to resolve closely spaced signals. Suppose for example that we wanted to observe a signal that consisted of two sine waves, one at 500 kHz and one at 505 kHz. If we had a filter bandwidth of 30 kHz we would not be able to resolve them as two separate signals, they would appear as one. However, if we reduced the bandwidth to 1 KHz we would be able to resolve them into two distinct signals, and be able to measure their frequency separation and their individual amplitudes.

In operating the spectrum analyzer there are three principal parameters that the operator should set. These are, center frequency, frequency span and the signal's amplitude. The center frequency setting determines the frequency that corresponds to the middle of the screen. The calibration of this setting is not very accurate on the older analyzers, but this is not a serious problem in using the analyzer as we are usually interested in frequency differences. The span determines the range of frequencies from one side of the screen to the other. For example, if we set the center frequency to 10MHz and the span to 2 MHz, the screen would cover the range 9 MHz to 11 MHz and any signals in that frequency range applied to the input would be seen on the screen. The amplitude can be set to either the log or linear mode. The log mode is useful for looking at .two or more signals that have large differences in amplitude. In this mode the scale is calibrated in dB/division, with a given reference value corresponding to the top line. In the linear mode, which is good for observing small differences in amplitude, the top line can be set to correspond to a given voltage.

To get some experience in working with the spectrum analyzer, the first thing to observe is a single sine wave. Connect the signal generator both to the scope and the spectrum analyzer using a T connector. Adjust the generator so the signal frequency is 10 MHz and the peak to peak amplitude is 0.2 volts. Adjust the center frequency of the spectrum analyzer to 10 MHz, the span to 10 MHz and the signal amplitude so the reference level is 0 dBm and the vertical calibration is 10 dB/div. You should see a line approximately in the middle of the screen. Vary the frequency of the generator and check that the line moves 1 box / MHz.

It is worthwhile to note that the spectrum analyzer is calibrated in dBm. The dBm is a unit of power which is defined as,

$$P_{dBm} = 10log_{10}(P_{signal}/1 milliwatt)$$

The input resistance of the spectrum analyzer is 50 Ohms. A signal that has an amplitude of 0.1 volt has an rms value of 0.0707 V and a peak to peak voltage of 0.2 volts. Its power is, Psignal=(0.0707 V)2/50 Ohms=0.0001 W=0.1 milliWatts

When this power is referred to 1 mW we get, PdBm=10log10(0.1 mW/1 mW)=-10 dBm

With respect to a reference of 0 dBm and power scale of 10 dB/div, then the height of sinusoidal signal on the screen should be one box below the reference. If you turn the attenuator knob on the generator, the height of the line on the screen should decrease one box for each additional 10 dB of power attenuation. If you change the amplitude mode to linear, then you should be able to measure the rms value of the signal. In this case, the original 0.2 volt peak-to-peak signal should measure 70.7 millivolts.

Modulation: a large signal-to-noise ratio is achieved when the signal, say 10 KHz as a result of speaking to a cell-phone, is superimposed on a carrier frequency, say 5 MHz. For example we may use a functional generator (Wavetek generator in this example) with an internal AM modulation of 10 KHz to produce modulation of a 5 MHz carrier frequency. The modulation index may be chosen as 0.5. The equation of this signal is,

$$ x(t)=(1+0.5cos2π*10^{4}*t)cos(2π*5*10^{6}*t) $$

On the spectrum analyzer you should see 3 lines separated by 10 KHz, with the outer two lines having an amplitude 1/4 of the amplitude of the center line. On the log scale this corresponds to a 12 dB difference in amplitude.


Objectives

To become familiar with spectrum analyzers and shielding properties of various materials


Laboratory Procedure


Section 1

  1. Watch an introduction video. For example
  2. Connect a function generator to both oscilloscope and a spectrum analyzer via a T-coupler and using a coaxial cable,
  3. Set functional generator to 25 MHz with AM and FM modulation. Record your finding and explain the various frequency lines.

Section 2

  1. Using two make-shift dipole antennas (with banana couplers and a set of two wires) measure the transmission between the antennas as a function of distance. Use a frequency of f=30 MHz. What is the optimal wire length? Why does the antenna work at non-optimal length? Why the signal deteriorates as the distance increases? Corroborate your explanation with simulation of the transmission between two dipole antennas.
  2. Now use a high-frequency functional generator and repeat (1) at f=1 GHz.
  3. Now repeat the measurements with either functional generators such that a shield is placed in-between the antennas: (1) a bench top; (2) A metal enclosure. Why do we measure a signal behind the metal enclosure? It the metal enclosure work better at high, or low frequencies?

Report

  1. One report per group. No timeline for the lab's final report until, of course, the end of the semester; however, if the report is submitted early, comments may be provided and the final grade may be improved.
  2. Follow the template that is provided in the Welcome announcement In addition:
    • Provide all files and pictures with explanations
    • Answer all questions
    • - Provide simulations with either, HFSS, Comsol or AWR or MultiSim to support your findings.