Design Projects

Project 2, Fall '97

12-3-97 correction, part 2

In part 1b of the lumped parameter section, it asks for the current phase and magnitude at each node. It should read the current phase and magnitude through the inductors.

  • Since this is unclear, it will not be required to calculate the current in parts 1b. and 1c. to get full credit. If you do calculate the current correctly, a few extra points will be given.

    • Transmission line link

    Darren Charles has prepared some web pages on lumped parameter models of transmission lines. It includes some examples of Spice modeling and data taken.

    12-3-97 clarifications

    1. You may substitute similar component values, (e.g. 91 ohms instead of 93 ohms)

    2. For part 1c. of the lumped parameter section: In class a few weeks ago, we calculated the equivalent length of an LC section. Take that length times the number of LC sections in your circuit.

    3. For part b., Zin is complex. Therefore, you should either plot its real and imaginary parts or plot the magnitude and phase.

    12-2-97 clarifications

    1. You may take data with another group. However, each group of 1-3 people must turn in a separate report

    2. For part 1b. of the lumped parameter section, the easiest thing to use is a Spice analysis of the lumped model.

    3. Part 1c. of the lumped parameter section is referring to a coaxial cable type transmission line.

    Design Problem 1, Fall '97

    Some lessons learned - Oct 31, 1997
  • Watch ground connections when making measurements. There were quite a few times where someone measuring the voltage across a capacitor or inductor connected part of their circuit to ground through the scope. Consequently, the circuit did not behave as designed.
  • Cables have capacitance. A typical coaxial cable has a capacitance of 100 pF/m. They also have 0.25 microH/m inductance, but that wasn't critical. If you designed a small capacitance into your circuit, cable capacitance affected things by increasing the total C in the circuit and therefore lowered the resonant frequency.
  • Dimension were critical and often hard to measure. A related issue is that the d in the C = epsilon *A/d formula is the distance between the electrodes, which was not necessarily the thickness of the dielectric since air gaps were often present.
  • Inductance and capacitance are functions of frequency. In this case, the inductance probably varied because of the material properties. Many high mu materials do not respond well at frequencies in the 200 kHz range and therefore have a lower relative permeability at higher frequency. Its worth noting that in other situations, the inductance can vary with frequency because the current distribution in the wire changes.
  • Materials properties were also critical. Most people did a reasonable job of guessing dielectric properties within a factor of 2. Much to my surprise, the small toroids had a relative permeability of about 70, which was much lower than I suspected.
  • When the theory and experiment differed by factors of 3 or less, the difference could generally be attributed to uncertainties in dimensions or material properties. However, there were many cases where the reports had factor of 10 or more discrepancies between theory and experiment. Except for the small toroid permeability, the errors were usually in the calculation. CHECK YOUR CALCULATION WHEN THERE IS AN ORDER OF MAGNITUDE DISCREPANCY.
  • If the meter says -393 pF, something is wrong. This does not mean you have 393 pF. It probably means you have a short.
  • A crack in the toroid has a big effect. I think this only happened to one group, though.
  • Pushing a wire against aluminum foil is not a very good connection. I think the extra contact resistance lowered the Q of some of the circuits.
  • The early bird gets the better toroid. As always, getting an early start makes the job easier.

    • October 21, 1997 - Miscellaneous comments

  • OPEN SHOP HOURS - There will be an open shop hour from 10-11 on Wednesday. It should also be possible to work in 4107 during the E&I lab from 6-9 PM, but you should check with the instructors of that section. You can also work on the project during the other Fields and Waves sections.

  • TOROIDS - We have run out of toroids. If anyone has one they are not using, please bring it back. Some people have been getting strange results withthe toroids. If your toroid is cracked, it may be possible that an effective air gap has formed and it no longer acts like an ideal toroid.

  • OTHER HIGH MU CORES - Note that the toroids I purchased are designed to work at high frequency. Some other high mu materials may behave differently at high and low frequencies. One difference is an extra loss that is similar to an extra resistance. (You don't have to calculate this resistance). The permeability, mu, may also vary with frequency.

  • POOR CONNECTIONS - Several people have gotten strange results because they do not have good connections or they are shorting out electrodes. This has been the cause of some of the negative readings with the inductance bridge.

  • WED. CLASS - We will leave time during Wednesday's class to work on the project.

    • CORRECTION PREVIOUSLY POSTED ON ANNOUNCEMENTS

    There is a mistake in the 1st sentence of the grading criteria section (on the back side). It should say 200 +- 20 kHz; not 15 +- 1.5 kHz. Shortly after that, it should also say "Achieving a resonant frequency very close to 200 kHz.