Microwave Metrology -ECE 684 Spring Lab Exercise I&Q.v3: I&Q Time and Frequency Domain Measurements

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1 Lab Exercise I&Q.v3: I&Q Time and Frequency Domain Measurements In this lab exercise you will perform measurements both in time and in frequency to establish the relationship between these two dimension realms. Students should keep on mind that required measurements are not given for the measurement sake; but they are rather given to introduce students to some underlying concepts. Student should expect that this lab will last around 4hours. Please read the instruction for the whole sub-exercise before performing the measurement itself. 1. Image rejection with I&Q modulator I&Q modulators are often used in RF/microwave links to suppress image products that result from the up-conversion or down-conversion process. This part of the lab exercise evaluates the given modulator as an image reject mixer, IRM. It also demonstrates the time and frequency domain relationship between the input and the output of an I&Q system system. Figure 1 depicts the measurement set up for this exercise. Set the both channels of ARB_I&Q ( arbitrary function generator connected to the transmitter I&Q ports) to output sinusoidal waveform at 1 MHz with 100mV p-p amplitude. Make sure that the output channels are set at 0º phase. Channel 1 of ARB_LO, arb used as a LO source for the I&Q modulator / demodulator pair, should output 70MHz sinusoid at 12dBm power level. Note the resolution bandwidth you used in this exercise and try to keep it consistent to avoid possible confusion in post data analysis Set the spectrum analyzer at 70MHz center frequency, 10MHz span and 3kHz resolution bandwidth to locate the output of modulator. What is the ratio (isolation) between the USB (upper side band) and LSB (lower side band) signals Change the phase of Q input to -90º and observe the spectrum and 1 st order modulation products. Then change the phase of Q input to +90º. What do you observe now? Explain the effect in your report. See TA if not clear what is going on because student should understand these effects clearly before proceeding any further Change the Q channel phase from 0º to -180º in 10º steps and record the isolation in db between the sidebands Using 1.3 as a starting point change the phase of Q channel in 1º to find the one that results in best LSB rejection. Record its value. Lab Exercise I&Q: I&Q Measurements - 1 -

2 1.5. Change the frequency (sweep) both I and Q channels (they should be at the same frequency) as follows: 1kHz, 10kHz, 100kHz, 1MHz, 2MHz, 5MHz and record the levels of both side bands Compare the result to the modulator data sheet Change the waveform on both channels to square wave and perform few measurements. What do you observe? Do you notice the difference between sinusoid and square wave spectra and signal levels? Can you explain why there is such difference? 1.8. Discuss how this I&Q modulator compares in performance with image reject filter IRF. For comparison use the 915MHZ BPF (K&L 4B /T68-0/DP) scaled down to the desired frequency. Explain the advantage/disadvantage of either image reject approach. Which approach is better in direct modulation or direct up conversion from baseband to RF in terms of image rejection. 2. I&Q modulation demodulation measurements In this part of the lab exercise you will measure some of the performance parameters of a system shown in Figure 2. This system can be referred as a RF modem (MOdulation/DEModulation) or coherent I&Q RF transceiver. This exercise relates the I&Q system driving parameters such as the coherence to the distortion of the received signals and isolation between the channels, both of which are important characteristic of many microwave and communications systems. Also, the recorded results of this exercise can be latter compared to the VNA measurement to evaluate the relationship between the time and frequency domains. Set the both outputs of ARB_LO as at 70MHz sinusoidal output with 12dBm power with channel 1, channel connected to the transmitter, at 0º phase offset and channel 2, which output is connected to the receiver, at 90º. Set the variable attenuator at 40dB. ARB_I&Q should be set to -5dBm (356mV p-p ) at 1 MHz sinusoidal outputs. Set the LO signal generator at 970 MHz with 15dBm power level. Set the oscilloscope connected to the output of the receiver so it correctly displays the I&Q outputs, which should be around ~100mV p-p Note that you can record the resulting waveforms onto USB stick; however this process would result in large amount of data that can be fairly tedious to analyze. Make sure that the oscilloscope channel inputs are band limited, set to 50ohm impedance and DC coupled. Lab Exercise I&Q: I&Q Measurements - 2 -

3 2.A. Coherence and Isolation 2.1. Record the waveforms. That is, record the peak to peak voltages and any visual deviation from an ideal sinusoid at the oscilloscope Turn off the Q output of ARB_I&Q. Are the I and Q signals present on both channels? Now turn of the ARB_I&Q I output off and turn the Q output on. Does this have the same effect? Record the peak to peak voltages in both instances 2.3. Change the output of ARB_LO channel 2, to 0 and repeat 2.2 again Make all necessary notes so you can discuss steps 2.2 and 2.3 in your report Turn the I channel on and the Q channel off. Change the phase ARB_LO channel connected to the receiver 0 to +180.in 10 and from 85 to 95 in 1 steps. For every phase step record the peak to peak voltages so you can deduce the power levels and channel isolation. That is the difference in power levels (V p-p2 /100) between I and Q outputs From 2.5 as a starting point choose the phase of ARB_LO channel connected to the receiver which results in maximum isolation both ways. That is, I input to Q output and Q input to I output. Change the LO frequency from 970MHz to 975MHz in 0.5MHz steps and record the peak to peak voltages measured at Q output (isolation for I-in to Q-out) Set the LO frequency back to 970MHz. Set the modulation frequency on I modulation inputs to 10 khz, 100kHz, 1MHz, 2.5 MHz and 5MHz and observe isolation. Does the isolation change with modulation frequency? Now, change the I modulation channel to 1MHz and then to 5 MHz square waveform. Compare the isolation with sinusoidal waveform case and for different modulation bandwidths Make all necessary notes and record data not only to answer given questions. Lab Exercise I&Q: I&Q Measurements - 3 -

4 2.B Linearity In this part of the exercise you will evaluate the basic linearity performance of the I&Q system. Through two similar measurements, you ll be able to differentiate between the linearity effects of the transmitter and receiver Set the both channels of ARB_I&Q to 1MHz sinusoid at -12dBm and turn them both on. ARB_LO should be at 12dBm and 0 and 90 degrees phase on both channels respectively Increase the both ARB_I&Q channels amplitudes to 10dBm in 1dB increments and start measuring and recording the receiver outputs. As you increase the power to the transmitter inputs modulator will saturate. Calculate the IIP1[dB] from this measurement? Make a sketch (or save it to disk) record of observed saturated waveform on the oscilloscope Set the ARB_I&Q outputs at roughly 6dB bellow the IIP1[dB] compression point found in Note, 6dB in power is 3dB in voltage for fixed impedance system which translates in ½ of the peak to peak voltage of the IIP[1dB] voltage level. To get find the OIP1[dB] point of the receiver, change the variable attenuator from 40dB to 10dB. Also, at 10dB attenuator setting please sketch the observed waveform shape or record it. Is the received waveform shape same as the one found in step 2.10? Along with the received waveform shape, discuss the difference between IIP1 and OIP1 in your report. If the reason behind the waveform difference is not clear to you, you can observe the received signal at the I port with spectrum analyzer (10MHz span should suffice). Lab Exercise I&Q: I&Q Measurements - 4 -

5 3. Basic Evaluation of I&Q Mod/DeMod using VNA This part of the exercise introduces students to the capabilities of VNA instrument in complete end-to-end system measurements. It also relates the measurements in between the time and frequency domains. Perform the VNA S21 display calibration (mathematically null) from instrument s minimum frequency to 10MHz at the input and output of the given I&Q system at - 12dBm power level. Connect the VNA s port 1 the I input and port 2 to the I output of the I&Q system. ARB_I&Q outputs should be off and all other parameters should be as described as in part 3 of this exercise with signal generator set at 970MHz 3.1. Measure S21 in magnitude. Make note of its 3dB 10dB bandwidth cutoffs. It actually happens at 30MHz so skip it Vary the VNA output power level to determine the P1dB of the system. Is this P1dB same as the one found in part 2.11 or as depicted in data sheet? Explain / discuss Add the second S21 trace to measure phase. Mathematically null it from the display menu. Place marker at 1MHz. Decrease the signal generator LO in 0.25 MHz steps to 970MHz and monitor the change in phase and magnitude S21 response. Record the insertion loss and phase change at the 1MHz marker at 5 example points Connect the VNA s port 2 to the Q output and don t forget to terminate the I output. What does the S21 magnitude represents in this case? Record its value. Change the LO back to 975MHz and record the S21 magnitude at the example points you chose in Choose a couple of phase points for the ARB_LO channel connected to the receiver as in step 5 in part 2. Record the isolation. How does it compare to 2.5. Keep on mind that change in LO signal affects both insertion loss and isolation; however, to different degree. Lab Exercise I&Q: I&Q Measurements - 5 -

6 4. Equipment Used CSA Spectrum Analyzer HP8656 Signal Generator AFG3252 Arbitrary Function Generator DSO6054A Oscilloscope HP5062A Network Analyzer E4418B EPM Series Power Meter Component List: Function Manufacturer s part number I&Q Modulator MiniCircuits ZFMIQ-70ML I&Q De-Modulator MiniCircuits ZFMIQ-70MD BandPass Filter 60MHz MiniCircuits SIF-60 Power Splitter 2-way MiniCircuits ZFSC-2-11 Power Splitter 3-way MiniCircuits ZFSC-3-11 Low Noise amplifier MHz MiniCircuits ZFL-1000LN Variable Step Attenuator Weincschel AE 117A Band Pass filter 915 +/-34 MHz K&L 4B /T68-0/DP Mixer MiniCircuits ZFM-5X Band Pass Filter 60 +/-10 MHz MiniCircuits SIF-60 IF amplifiers MiniCircuits ZFL-500 Lab Exercise I&Q: I&Q Measurements - 6 -

7 Figure 1 Figure 2. 5 Report In your final report compare make sure that you address the questions and discussion points given in this document. Present only those values and plots that reinforce the provided answers / conclusions. Discuss the difference / similarity between the oscilloscope based and VNA based isolation and linearity measurements. Compare the linearity and isolation deduced form modulator / demodulator datasheets to the ones found through oscilloscope and VNA based measurements. Derive the theoretical relationship between gain, isolation and coherence (phase) and check how your measured values follow that. Lab Exercise I&Q: I&Q Measurements - 7 -