Introduction to measuring and testing RF/wireless function blocks

Part 2: Amplifiers, Voltage Controlled Oscillators, and Mixers

Prepared by Dr. Aaron Scher
[email protected]
Oregon Institute of Technology

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1. Objectives

To measure and test basic parameters of RF/wireless function blocks. These blocks include a coupler, a mixer, an amplifier, a splitter, a voltage controlled oscillator, a couple bandpass filters, and a couple attenuators. For part 2, we will focus solely on amplifiers, voltage controlled oscillators, and mixers.

2. Equipment

1. Mini-Circuits mixer (ZX05-43MH-S+). Here is the datasheet.
2. Mini-Circuits amplifier (ZX60-2522M-S+). Here is the datasheet.
3. Mini-Circuits voltage controlled oscillator (ZX95-2500). Here is the datasheet.
4. Mini-Circuits 12dB Attenuator.
5. 50 Ohm coaxial cable assemblies for testing along with an assortment of adapters/connectors (N-type and SMA type).
6. Agilent E3630A tripple output DC power supply
7. RF signal generator with a max frequency of at least 3 GHz.
8. Spectrum analyzer with a max frequency of at least 3 GHz.

3. Report guidelines

For the lab report, you will create a PowerPoint presentation (or use a similar presentation program), save it as a PDF, and submit it on-line according to the instructions given in class. The presentation should be tutorial in nature; your target audiences are other engineers and scientists who are interested in learning more about circuits and electromagnetism.

Your presentation will have 10 slides. Please include a slide number in the footer of each slide. To earn full credit your presentation must contain the slides in the order asked for in this lab. If you miss a slide, please leave a blank slide in its place so that you have still have exactly 16 slides total. Your first two slides should be:

• Slide 1: Title slide with our name, student ID number, date, lab name, class number/title.
• Slide 2: A team picture or insignia with the names of your teammates.

4. Amplifier

The Mini-Circuits components you are going to measure in this lab are shown in Figure 1.

For this part, you are going to use a signal generator with spectrum analyzer to measure the gain and 1dB compression point of a ZX60-2522M-S+ Mini-Circuits RF amplifier. Here is the amplifier's datasheet. To perform this measurement, you are going to need a spectrum analyzer and a signal generator.

1. Set the spectrum analyzer for a frequency range of 775 MHz to 825 MHz.
2. Setup the spectrum analyzer to measure power.
• If you are using the Rigol DSA1030 spectrum analyzer, this means press the Detector button, and select the RMS average option. Then press the Trace button. Set the trace type to power average and the average time to 5.
• If you are using FieldFox N9912A as your spectrum analyzer, set the mode to SA (spectrum analyzer), press the measurement setup button (button 4), and select power as the average type and set the average count as 5. Then press the trace button (button 6), and select average as the state option. Select average as the detector option.
3. On the spectrum analyzer, set the resolution bandwidth = video bandwidth = 1 MHz.
4. Set the frequency of the RF signal generator to be 800 MHz and the output power to be -20 dBm. Make sure all the modulation options are off, so that you are just generating a single tone.
5. Connect the signal generator directly to the spectrum analyzer as indicated in Figure 2. Measure the power received (put marker on peak of received signal to get accurate measurement). We shall call this P₁. Write this value down.

6. Setup the DC power supply to output 3 Volts DC, and connect amplifier power the amplifier with the supply. Connect output of amplifier to spectrum analyzer and the input to the RF signal generator. It's very easy to damage the amplifier so please confirm your measurement setup with your instructor before making major connections. The measurement setup is illustrated in Figure 3 below.
7. Measure the power received by spectrum analyzer now with the amplifier. Let us call this value P₂. Given P₁ and P₂, compute the gain G of the antenna. Present your values of P₁, P₂, and Gain in Slide 3. Does your measured gain agree with that of the datasheet for the amplifier?
8. Slightly increase the voltage supply from 3 V to 3.5 V. What does this do to your gain? Note your observations in slide 3 also. Bring the DC voltage supply back to 3 V as before.
9. Connect a 12 dB attenuator to the output of the amplifier as shown in Figure 4. The purpose of the attenuator is to protect the spectrum analyzer from large input power levels. This represents a common application of attenuators.
10. You are now going to see how the gain of the amplifier changes with input power. For signals with low input power, the gain should remain constant. For higher input levels, the amplifier goes into saturation and gain decreases. By definition, the 1 dB compression point (P1dB) indicates the power level that causes the gain to drop by 1 dB from its small signal value. Set the signal generator for an RF power level of -25 dBm and record the output power to find the gain. Complete the table below by increasing the signal generator power by 1 dBm increments and repeating the measurement until the signal generator power level is 0 dBm. Present this table in Slide 4.
11. Graphically plot the Power Out (dBm) versus Power In (dBm) and present this plot with your measured 1 dB compression point in Slide 5. Does your result agree with the data sheet's 1dB compression point?

Table 1: Measured Output Power versus Input Power of Amplifier

Input Power [dBm]	Measured Output Power [dBm]	Measured Output Power + 12dBm [dBm]	Gain [dB]
-25	___________	____________
-24
-23
-22
-21
-20
-19
-18
-17
-16
-15
-14
-13
-12
-11
-10
-9
-8
-7
-6
-5
-4
-3
-2
-1
0

12. In slide 6 comment on how this experiment be improved. To help anser this question, think about the following questions: Does the output power you set on the RF signal generator represent an accurate representation of the true output power? Is the spectrum analyzer as accurate as a calibrated power meter for measuring RF power levels? How could you use a precision variable attenuator to improve the experiment?

5. Voltage controlled oscillator (VCO)

1. Disconnect and remove the amplifier. You will not be using the amplifier for the rest of the lab.
2. Keep the same setting on your spectrum analyzer as in the previous section, but change the frequency span to be 1 GHz to 2 GHz.
3. You will power the Mini-Circuits ZX95-2500 VCO (datasheet) with an Agilent E3630A tripple output DC power supply. Connect the "COM" port of the power supply to the ground pin of the VCO. Adjust the output labeled "+20V" on the power supply to be +12V and connect this to the Vcc pin on the VCO. Adjust the output labeled "+6V" on the power supply to be 0V and connect this to the Vtune pin on the VCO. It's very easy to damage the VCO so please confirm your measurement setup with your instructor before making major connections.
4. Connect the RF OUT of the VCO to the RF input of the spectrum analyzer. Figure 5 shows the VCO setup.
5. If everything is setup correctly, you should see a tone on the spectrum analyzer at a frequency around 1.4 GHz. Use a marker to determine the output power. Record these values in the corresponding row in Table 2. Increase the voltage on the Vtune pin from 0 to 6 V in steps of 1 V and complete the rest of Table 2. Present your table in slide 7 . Do your results agree with those on the datasheet?

Table 2: Measured Output Power and frequency versus voltage of VCO

Vtune [V]	Frequency [GHz]	Output Power [dBm]
0	___________	____________
1
2
3
4
5
6

6. Change Vtune back to 0V. Change the frequency span on the spectrum analyzer to view 1 GHz to 3 GHz. You should see a tone at the fundamental frequency and a second tone at the first harmonic. Place markers at the peak of these two tones to measure their frequencies and amplitudes. In slide 8 present a screenshot of the spectrum analyzer and address the following questions: What is the fundamental frequency? What is frequency of the first harmonic? How many dB down is the harmonic frequency from the fundamental?
7. Change Vtune on the VCO until the output frequency is 1.5 GHz. Don't put away the VCO yet, since you will be using the VCO in the next section.

6. Mixer

1. In this part we will explore the function of a Mini-Circuits ZX05-43MH-S+ frequency mixer (datasheet).
2. Keep the same setting on your spectrum analyzer as in the previous section, but change the frequency span to be 50 MHz to 2 GHz.
3. Set up an RF signal generator to generate a single tone at 400 MHz with 13 dBm output power.
4. Connect the output of the RF signal generator to the LO port of the mixer. Connect the RF output of the VCO to the RF port of the mixer. Connect the IF port of the mixer to the RF input port of the spectrum analyzer. Figures 6 and 7 illustrate the measurement setup.
5. The spectrum analyzer should display a number of tones that correspond to the various mixer products. You should see a strong tone at 1.5 GHz that corresponds to the VCO input frequency. If this tone is offset in frequency from 1.5 GHz, you may need to adjust the VCO to keep it at 1.5 GHz (the output of the VCO drifts with temperature and load).
6. In slide 9 present a screenshot of the spectrum analyzer showing the various tones (i.e. harmonics) .
7. Using markers, measure the frequencies and power of the harmonics. What harmonic corresponds to the upconverted signal? What harmonic corresponds to the down converted signal? Present your results in slide 10 .