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Objectives

The objective of this lab is to measure the standing waves on a segment of microstrip transmission line terminated with various loads (open, short, matched) using two different test & measurement setups:
  1. A near-field RF probe with a spectrum analyzer.
  2. An RF detector/demodulator probe with an oscilloscope.


Equipment

  1. One RF signal generator
  2. One spectrum analyzer (here we use the Agilent FieldFox)
  3. One RF detector/demodulator probe (here we use the BK Precision PR 32A 100 kHz - 950 MHz RF detector/demodulator probe kit).
  4. One set of RF passive probes for measuring E and H fields (here we use the Rhode & Schwarz HZ-15 Passive probe set)
  5. One oscilloscope
  6. A few short coaxial cables with SMA connectors and adaptors as needed
  7. A set of three microstrip transmission lines (terminated in an open, short, and matched load) with silkscreened rulers as shown in the figure below:

    (Click to enlarge)

    I created the board layout using KiCad . You can download my KiCad project files here. You can also download the Gerber and Drill files here. These Gerber/drill files can be sent to OSH Park for order, and they will fabricate three of these boards on FR4 board for around 10 dollars (See the two-layer fabrication specs here).

    For the open termination, leave the load end (the load end is at the reference "0" mark) open. For the short termination, solder a little piece of wire from the edge of the load end to the grounded pad. For the matched termination, solder a surface mount 50 Ohm resistor (SMD 0603) like the 0603 (1608 Metric) Vishay Thin Film SMD 50 Ohm resistor - Mfg. part number FC0603E50R0BTBST1 from the edge of the load end to the grounded pad. On the generator end of each line is soldered a SMA female PCB edge mount RF solder adaptor (0.062 inch thickness) like these adapters from Custom Cables Group LLC you can purchase from Amazon.


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.

Your presentation will have 27 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 still have exactly 27 slides total. Your first slide should be:

Measure standing waves with near-field RF probe and spectrum analyzer

  1. Connect an RF signal generator to the microstrip line board with an open load (the end of the line is open). It helps to tape your microstrip board down to prevent it from moving as shown in the picture below. Adjust the RF signal generator output frequency to 900 MHz and the RF output power to 10 dBm.
    (Click to enlarge)

  2. The Rhode & Schwarz HZ-15 Passive probe set contains five different probes. We will use the small H-field probe and E-field probe, as indicated in the image below. Connect the E-field probe to the spectrum analyzer. If using the FieldFox, press the "Mode" button and select "SA" to set the instrument in spectrum analyzer mode. Set the spectrum analyzer frequency range to be around 899 MHz to 901 MHz.

    (Click to enlarge)

  3. When using the E-field probe, hold the probe at a 90 degree angle with respect to the board as shown in the image below. Please be careful when probing your board. The probes are very expensive! Please do not drag the tip as you move the probe position. Lift the probe up and gently touch down on the position you want to measure.

    (Click to enlarge)

  4. When probing the opened load end of the board, you should see a large spike on the spectrum analyzer around 900 MHz, indicating a large received power at that frequency. Fine tune the frequency setting of the spectrum analyzer to center the frequency spike and "zoom in" to see more frequency details of the bandwidth of the spike. The reason for doing this is because you want to make sure you can get a good measurement of the spike's peak power.
  5. Measure the standing wave using the E-field probe. To do this, probe the line starting at the load (at reference 0) and work your way towards the generator, with a the finest resolution you can reasonably achieve (at least 5 mm). Record the peak value of the spike measured by the spectrum analyzer for each position. The deliverables are as follows:
    • Slide 2: Open load termination. Frequency = 900 Mhz. Plot standing wave using E-field probe.
    • Slide 3: Short load termination. Frequency = 900 Mhz. Plot standing wave using E-field probe.
    • Slide 4: Matched load termination. Frequency = 900 Mhz. Plot standing wave using E-field probe.
    Please present well-labeled plots that are easy to read.
  6. Disconnect the E-field probe from the spectrum analyzer and replace it with the H-field probe. When measuring with the H-field probe, be sure to hold the probe at a 90 degree angle with respect to the board. Make sure the probe is positioned such that the little white line marked on the probe tip is along the direction of the microstrip line, as shown in the image below.

    (Click to enlarge)

  7. Measure the standing wave using the H-field probe. To do this, probe the line starting at the load (at reference 0) and work your way towards the generator, with the finest resolution you can reasonably achieve (at least 5 mm). Record the peak value of the spike measured by the spectrum analyzer for each position. The deliverables are as follows:
    • Slide 5: Open load termination. Frequency = 900 MHz. Plot standing wave using H-field probe.
    • Slide 6: Short load termination. Frequency = 900 MHz. Plot standing wave using H-field probe.
    • Slide 7: Matched load termination. Frequency = 900 MHz. Plot standing wave using H-field probe.
    Please present well-labeled plots that are easy to read.
  8. Repeat the above experiments at 2000 MHz (2 GHz) using the E-field probe for open and matched lines. The deliverables are as follows:
    • Slide 8: Open load termination. Frequency = 2000 MHz. Plot standing wave using E-field probe.
    • Slide 9: Matched load termination. Frequency = 2000 MHz. Plot standing wave using E-field probe.
    Please present well-labeled plots that are easy to read.
  9. Based on the above measurements, please answer the following questions:
    • Slides 10 and 11: What is the measured wavelength, phase velocity, and attenuation constant at 900 MHz? Present your calculations. What is the measured VSWR for each load at 900 MHz?
    • Slides 12 and 13: What is the measured wavelength, phase velocity, and attenuation constant at 2000 MHz? Present your calculations. What is the measured VSWR for each load at 2000 MHz?

Measure standing waves with RF detector probe and oscilloscope

  1. Hook up the BNC end of the RF detector/demodulator probe to Channel 1 of the oscilloscope, and hook up a short length of coax to the RF signal generator. Connect the probe to the end of the coaxial cable. To do this, use the sprung hook probe (mini grabber hook) for the RF detector/demodulator probe tip, and use a home-brew SMA to wire lead adaptor, as shown in the image below.

    Using the measurement features of the oscilloscope, set the oscilloscope to measure the average voltage. Set the RF signal generator to output a continuous 900 MHz sine wave (single tone) at 0 dBm. You should see a voltage on the oscilloscope with an average around 0.6 V to 1 V (this is a rough estimate!). While observing the oscilloscope display, gently move the detector probe and handle it at different positions - you should see the voltage changing on the display as you disturb the probe. This gives you an idea of how sensitive the detector probe is to it's orientation, cable position, and environment. In Slide 14 explain the observed effect of handling the probe at different positions and moving the cable, and comment on how this effects measurement accuracy, precision, and sensitivity.



    2. (Click to enlarge)

  2. As discussed in Design FAQs: RF Detectors for Wireless Devies, there are two basic types of RF detectors - the logarithmic type and the rms type (please read this document for more information as needed). To find out what type is our detector probe, I would like you to manually sweep the output power of the RF signal generator (Pgen) in 1 dB steps from -15 dB to 6 dB, and plot the average voltage measured with the oscilloscope (Vout). You should generate a table like that shown below. Present your table in Slide 15. Plot Vout (V) versus Pgen (dB) and present your plot in Slide 16 (in this plot Vout is the y-axis and Pgen is the x-axis). Plot 10log10(Vout) versus Pgen (dBm) and present your plot in Slide 17. Based on your plots, what type of detector is this (a logarithmic type or an rms type?). Present your answer and your reasoning in Slide 18.

    Pgen (dBm) Vout (V) 10log10(Vout)
    -15 ... ...
    -14 ... ...
    ⋮ ⋮ ⋮
    +6 dBm ... ...

  3. Remove the home-brew SMA to wire lead adaptor, and connect the RF signal generator to the microstrip line board with the open load (the end of the line is open). It helps to tape your microstrip board down to prevent it from moving as shown in the picture below. Adjust the RF signal generator output frequency to 900 MHz and the RF output power to 0 dBm (the absolute power level is not important in our next experiments, so you can change this later as needed to keep the measured voltage within the input range of the oscilloscope).


    (Click to enlarge)

    Remove the sprung hook probe (mini grabber hook) from the RF detector/demodulator probe tip, so that the probe tip is just the simple metal needle. Probe a point on the microstrip near the end of the line and you should see a voltage reading on the oscilloscope display. As shown in the figure above, hold the probe at a non-right angle to the surface so that you avoid digging the sharp metal point into the line and scratching the metal trace. Please do not hold the probe at a right angle to the surface, like the disaster shown in the figure below - that could scratch the metal trace and damage the probe tip. Also, please do not drag the tip as you move the probe position along the line. Instead, lift the probe up and gently touch down on the position you want to measure.

    Please do not hold probe at at a right angle to the surface because it can scratch the metal trace and damage the probe tip (Click to enlarge)

  4. Measure the standing wave using the detector probe. To do this, probe the line starting at the load (at reference 0) and work your way towards the generator, with a the finest resolution you can reasonably achieve (at least 5 mm). Record the average voltage measured by the oscilloscope for each position. To make your measurements as accurate as possible, it is important to be consistent when holding the probe and taking measurements (i.e. try to keep a steady hand on the same position of the probe and minimize movement of the probe's cable when probing different positions along the line). The deliverables are as follows:
    • Slide 19: Open load termination. Plot standing wave using detector/demodulator probe.
    • Slide 20: Short load termination. Plot standing wave using detector/demodulator probe.
    • Slide 21: Matched load termination. Plot standing wave using detector/demodulator probe.
    Please present well-labeled plots that are easy to read.
  5. Based on the above measurements, please answer the following questions:
    • Slides 22 and 23: What is the measured wavelength, phase velocity, and attenuation constant at 900 MHz? Present your calculations. What is the measured VSWR for each load at 900 MHz?

Summary and conclusions

  1. In Slides 24 and 25, compare and comment on how well the measurements conducted with the E and H field probes and the detector/demodulator probe agreed (in your comparison, please present side-by-side plots or tables).
  2. In Slides 26 and 27, present your conclusions, summary and any extra information you would like to present for this lab. In these slides you can include additional tables or plots or comments. For example, did you learn anything interesting in this lab? Did you have any particular issues or challenges that you met? How did you meet them, etc.? I welcome your constructive feedback and suggestions that could improve this lab experience in the future.