Lab Procedures for Signal Analysis using Spectrum Analyzer and Oscilloscope, Cheat Sheet of Electrical and Electronics Engineering

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Department Of

Electrical And Electronics Engineering

Applied Microwave Engineering

Lab

Lab # 1

Frequency and Time Domain Representations

Prepared by

Sana Khan & Hassan Sajjad

LAB PROCEDURES

Note: Do not use the very short SMA cables for this lab such as for the connection between T-Junction and SA in- put. It is recommended that longer SMA or BNC cables be used for this lab. DO NOT FORGET TO USE DC BLOCK WITH SPECTRUM ANALYZER (SA).

Part I: SA Measurement of two signals close in frequency

(What you should see: The ability to clearly identify two signals that are close in frequency as separate signals depends strongly on the measurement settings on the spectrum analyzer. You will explore the effect of the span and resolution bandwidth settings on this type of measurement. The resolution bandwidth is really the key in this case, but the two set- tings are linked within the SA.)

We will use two SIGNAL GENERATORS in this lab and combine the signals using a combiner/splitter provided to you as shown in Fig. 1. The combined signal will be then observed in the SA to determine the frequency contents of the signal and to visualize the resolution bandwidth of two signals close in frequency.

Figure 1: SA Measurement of two signals close in frequency

1. Setup the Signal Generator at FREQ 300 MHz and power level -5dBm.
2. Setup the 2nd Signal Generator provided to you by the TA and set the FREQ to 301MHz and power level -5 dBm.
3. Using the two long SMA cables connect the output of the two SIGNAL GENERATORS to PORT 1 and Port 2 of the COMBINER/SPLITTER (SBCTJ-1W 1 to 750MHz) respectively.
4. Connect the Ouput Port of the Combiner to the SA with a DC BLOCK.
5. Set-up the SA for a 300MHz center frequency and a 20 MHz span. Push FREQUENCY 300 MHz and SPAN 20MHz. At this point, TURN RF ON for the SIGNAL GENERATOR 1. You should see a signal at 300 MHz.
6. Now Turn RF ON for the second SIGNAL GENERATOR and you should be able to see another signal at 301 MHz.
7. This will establish two signals at spectrum analyzer input that differ by 1MHz. With the specified SA settings you should be able to see two signal peaks on the SA. Record the frequency and amplitude of the two peaks corresponding to the two signal sources in Table 1.
8. Using the arrow keys, increase the SPAN while keeping the center frequency fixed at 300MHz. Note that the resolution bandwidth automatically changes with the SPAN setting to keep the sweep time approximately constant. This is what we want to happen for this part. Note that setting the span more than 100MHz will cause the SA to display frequencies below 0Hz. Hence, the maximum span should not be extended beyond these values.

Figure 2: Test configuration for measurement of SIGNAL GENERATOR as a single frequency source

5. Repeat steps 3 and 4 for varied test signals in order to make the measurements indicated in Table 2 shown below. To do this you will be changing the frequency, and in some cases power level on the SIGNAL GENERATOR control window, including the center frequency and span settings on the SA.

Table 2: Observed frequency and amplitudes for varied SIGNAL GENERATOR CW test signals

Source test Frequency (MHz)

Source Power Level Setting (dBm)

SA Measured Peak Frequency (MHz)

SA Measured Amplitude (dBm)

Comments

OBSERVATIONS:

By how much does the SA measured signal amplitude drop at 25 MHz and 300 MHz when the SIGNAL GENERATOR power level setting is reduced by 3 dB?

Other comments and observations:

Part III: Oscilloscope Measurement of the SIGNAL GENERATOR signal at varied frequency and amplitude

(What you should see: you will measure the same signals in the time domain on an Oscilloscope that you measured above on the SA. You should see agreement between the O-Scope and SA frequency measurements at low frequency, but will see differences at higher frequencies. Amplitude correlation between the two instruments requires you to properly understand and calculate powers in dBm from voltages across a 50 ohm load and vice-versa. The 1 MΩ vs 50 Ω coupling setting is critical to getting this right.)

The test configuration for this part is shown below. You should read the reference guide for the oscilloscope provided in handout material before completing this part.

1. Connect a cable directly between the SIGNAL GENERATOR output and the Ch1 input connector of the oscillo- scope.
2. Set-up the SIGNAL GENERATOR for a test frequency of 25 MHz, with a power level setting of +0 dBm.
3. Press the CH1 Push AUTOSET on the OSCOPE, then adjust the vertical and horizontal position and scale as needed to see a sinusoidal signal with a few signal periods in the display. Press the VERT MENU button and select DC coupling.

Figure 3: Configuration for time domain measure- ment of SIGNAL GENER- ATOR test signal using os- cilloscope

4. Use the vertical line cursors to determine the period T of the displayed signal and calculate the frequency as f= 1/T.

OBSERVATIONS: T= s f= Hz

5. Use the horizontal line cursors to determine the peak-to-peak voltage levels for both coupling impedance options: 1MΩ and 50 Ω. Pushing the VERTICAL MENU button and then selecting the coupling impedance soft key allows you to change the coupling impedance until the desired coupling impedance is highlighted.

OBSERVATIONS: Vpp (1MΩ coupling) = mV Vpp(50Ω coupling) = mV

6. Use the automatic measurement features to take the same measurements made in parts 4 and 5. After pressing the “MEASURE” key, the measurements you want to select are: Frequency, and Pk-Pk. Tabulate your measurements, along with the results of the next step, in the table (Table 3) shown below.
7. Change the frequency and amplitude settings as indicated in the table shown below and use the automatic measure- ment features to complete the table.Use DC coupling with a coupling impedance of 50 ohms. You will need to hit AUTOSET on the OSCOPE each time frequency is changed. When making the frequency measurement you will find that the readout changes as you look at it, making it dif- ficult to specify a number. To estimate the precision of the measurement try to “eyeball” a nominal value and an uncertainty. For example if the 300MHz measurement is changing between 300.9 and 298.9, state the measured frequency as 299.9 +/- 1.0. You should notice more difficulties in making the measurements at 600MHz and 900MHz than at lower frequencies. Comment on this in the space provided in and/or below the table.

Table 3: Summary of Oscilloscope Amplitude and Frequency Measurements

Signal gen. Test Fre- quency (MHz)

Signal gen. Power Level (dBm)

OSCOPE Meas. Frequency (MHz) fo +/-∆

OSCOPE

Measured Vpp Z: 1MΩ

OSCOPE

Measured Vpp Z: 50Ω

Comments

Observations: What values do you estimate for A 0 and A 1 for your measurement configuration? A 0 = A 1 =

Enter these values into the Matlab file, and compare to the O-Scope display.

5. One aspect that is harder to quantify and remains an ambiguity in the comparison (Matlab vs. measurement) is the phase relationship between the two signals. Vary the phase of one of the two Matlab signals until you get a response that looks similar to what you have on the O-Scope display.

Observations: Printout the Matlab file with plots that are closest to what you can measure for the two signals on the O-scope.

(Optional) See if you can capture the digital waveform from the scope and obtain a plot to compare to your Matlab file.

Hint: You may have to vary the time scale (ns/division) on the scope and the total time span of the Matlab file in looking for similar waveform representations. By increasing the total time span of the screen you should definitely see a repetitive “beat signal” pattern that repeats on both the scope and on the Matlab representations. By varying the phase you should be able to get the O-scope screen view to look similar to the Matlab view.

Part V: Simulations and additional analysis

(What you should see: if done properly you should see reasonable agreement between SA and oscilloscope measure- ments of frequency and power – i.e. powers calculated from voltage for OSCOPE – for lower frequencies. You will see problems at higher frequencies with this agreement.)

Convert your peak-to-peak voltage values from part IV for the 50 ohm coupling case to power in dBm and compare to the results obtained with the spectrum analyzer in Table 4 below. Also summarize the frequency results from the SA and OSCOPE. The following equation may be used to convert the 50 ohm voltages to power in dBm.

P (dBm) = 10 log

(Vpp)^2 400 × 10 −^3

where Vpp is the peak-to-peak voltage in volts measured on the OSCOPE. Use of Matlab or a spreadsheet facilitates these calculations.

Table 4: : Comparison of Spectrum Analyzer and Oscilloscope Measurements

Signal gen. Test Freq. (MHz)

Signal gen. Power Level (dBm)

OSCOPE Meas. Freq. (MHz) fo+/

• ∆

OSCOPE

Measured Vpp Z: 1MΩ

OSCOPE

Measured Vpp Z: 50Ω

Comments

OBSERVATIONS:

How well do the SA and OSCOPE frequency measurements compare?

How well to the SA and OSCOPE amplitude measurements compare?

Other comments:

Part VII (Optional) : Oscilloscope Measurement of Noisy Signal.

(This part is required for graduate students, but optional for undergraduates.)

(What you should see: you will measure a very low level signal sent through a “wireless link” and observe the dif- ferent measurement requirements that such “noisy” signals may require. You will be receiving your signal as well some other signals.)

In this portion of the lab you will construct a simple wireless link and note the effect of link losses and noise on time- domain and frequency-domain measurements. The test configuration is shown below.

Figure 5: Configuration to observe the effect of link losses and noise on Time-Domain and Frequency - Domain mea- surements using OSCOPE and SA respectively.

1. Set the SIGNAL GENERATOR to a frequency of 300 MHz plus twice your roll numbers last digit (in MHz); e.g., if last digit of your roll number is #4, use 308 MHz. Make sure the SIGNAL GENERATOR RF power is off.
2. With the power supply off, build the system, and connect the output of the ZFL-1000LN amplifier to the SA. If you have any questions, especially in connecting the power supply to the amplifier, be sure to request assistance from the TA. You can leave the antenna on the bench top
3. Call for the TA. He/ She will check out your configuration, set the SIGNAL GENERATOR power to +7 dBm, and “power-up” the amplifier. Ask the TA or an instructor if you have any questions about the setup (e.g. selection of antenna).
4. Cycle the SIGNAL GENERATOR RF power on and off to verify that you are viewing your own signal. Do you see signals from any of the other benches? If so, what are their powers?
5. Carefully and slowly move the antenna around (a foot or two at most), and move your hand over it. Note the effect on the signal level in both cases. How much variation do you observe (in both dB and percent)?
6. Now connect the ZFL-1000LN/ (Any Amplifier provided to you by TA) output to the OSCOPE. Explore the effects of changing the Acquisition mode under the Acquire menu. Try each mode in sequence and notice the effects on the signal display. Record your observations below.