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Prelab 9

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1) Overview

In this lab, you will make a card reader that causes your MIT ID card to transmit its internal code, including your personal ID number!

Your MIT ID is a proximity card, or “prox card”, containing an integrated circuit (IC) that is powered by picking up nearby oscillating electromagnetic fields. The card readers at doorways produce 125-kHz oscillating electromagnetic fields. When powered, the IC, through an antenna in the prox card, transmits a signal at 0.5 and 1.5 times the incoming signal frequency. Thus, for an incoming signal at 125 kHz, the prox card will transmit at 62.5 kHz and 187.5 kHz.

The IC on the prox card encodes digital data onto its transmitted signal. It does so in a very clever way, known as phase-shift keying, which we will analyze in more detail in the lab. In quick summary, every 16 cycles of the 62.5-kHz signal, the IC phase shifts the sine wave by 180 degrees to flip the value of the next digital data bit; otherwise, with no phase shift, the value of the next bit remains the same as the previous bit. To better decode the data bits, we must first improve the ratio of the amplitude of the 62.5-kHz signal to the much bigger 125-kHz signal; the 187.5-kHz signal is small enough to ignore. We will thus build a notch filter, a filter that removes only one narrow frequency band from the input signal, in this case to remove the 125-kHz signal.. There are many filter design considerations we must examine before building our MIT ID card reader.

The diagram below shows a high level overview of the necessary steps to replicate a card reader in our lab. For the prelab, we will focus on the filter and amplifier design.

2) Filter Design

The basic topology of the filter implements what is known as a "notch", which is a filter that "notches" out, or removes, a narrow band of frequencies around a center value. In our case we want to design the notch to remove the 125-kHz signal from the receiving (RX) coil before sending that signal along for processing. That way the 125-kHz transmit signal is still present to power the prox card, but doesn't interfere as much with the data being read out.

To help you visualize the frequency response of a real notch filter, the frequency response of one is shown below. The center frequency in the figure happens to be about 128 kHz because nominal values were used. Notice that all frequencies pass through except those right around the notch.

The schematic of a simple series RLC notch filter is shown below, with the output measured across the inductor and capacitor. Although we want to design the filter to remove the 125 kHz signal, in practice the frequency of the filter is set by the precise values of the inductor and capacitor, which vary a fair amount due to manufacturing tolerances. We are also not taking any parasitics or other non-idealities into account at this stage of the analysis. In practice, you may find that your filter actually produces a notch anywhere from 100 to 150 kHz. That's fine, and it is to be expected. Even though prox cards are designed to work at 125 kHz, at least the low-frequency kind that MIT uses can actually operate across a broad frequency range. So, you can tune the transmitted signal frequency to whatever frequency your filter happens to notch.

When building the actual filter, we are limited to available component values, which we must keep in mind during our design; it also helps constrain the problem. For this lab we will use: * an inductor with a nominal value of L = 47 mH * a resistor with a nominal value of R = 51 kOHms

All that's left is to select a reasonable capacitor value to put the resonance/notch frequency near 125 kHz. You can determine the resonance/notch frequency of the filter using any of the methods developed in class. Remember to convert this frequency into radians per second (factor of 2\pi) for your calculations!(Also note this checker does not know what "pi" is so please use numbers)

Enter your design value for the capacitor:

Keep in mind, for the lab you may need to adjust this value a bit based on what parts you find and the exact value of your inductor.

You may have noticed that we could also use an RLC bandpass filter to allow only the 62.5-kHz signal through, since that is the primary band that we care about. That's another perfectly fine approach, though not one we'll use here.

3) Gain Stage

Although we will drive the transmitter coil with the maximum amplitude that the signal generator can provide, around 20 Vpp, the amplitude of the data signals transmitted by the IC in the prox card will be small. Thus, the desired signal at 62.5 kHz must be amplified in order to decode the data it contains. A gain of 20 should be adequate. We will implement this gain with a broadband op-amp amplifier.

We must also consider some practical design constraints. One important parameter is the input resistance of the amplifier. The amplifier should generally not load filter output because doing so could significantly change its resonance frequency, the depth of its notch, and/or its Q. This observation dictates the ideal op-amp amplifier topology. If you are unsure, look back at your notes on the two ideal topologies (inverting and non-inverting) and determine their input resistances, that is, their effective resistances between the input and ground. Then consider which topology would load the filter the least.

Which op-amp amplifier topology should be used to provide a high input resistance when looking into the amplifier stage?

Another design consideration is that the signal input into the op-amp will oscillate around 0 V, ranging from positive to negative voltages. Although it may be better to bias the op-amp or use a dual power supply to recover the full positive and negative swing of the signal, for our purposes it is acceptable to clip the negative half of the signal. The positive half of the signal already contains the phase shifts that encode the information we are trying to extract.

Thus, the op-amp amplifier can be powered between 5 V and 0 V. When making such decisions to simplify the amplifier design, we should proceed cautiously and keep this in mind when testing and debugging the card reader!

Draw up your final filter and amplifier circuit, with all component values labelled. Remember to design the op-amp for a gain of 20. Additional specs are:

  • Select reasonable resistor values for the amplifier; near 10’s to 200 k\Omega is good.

  • We will be using the TLV2371 for the amplifier. Refer to the datasheet (search for it online or in a previous lab) to determine the pinout, and how you will actually be able to build an amplifier with it on a breadboard!

Bring your design to lab to help guide the build.

And, of course, don’t forget your MIT ID card!

During the lab, we'll perform four major steps:

  1. Build and test the coils. In the process we will ensure good inductive coupling to the prox card, and that the prox card transmits a signal.
  2. Build the notch filter and tune the prox card transmit frequency to best match the filter notch.
  3. Add in the op-amp amplifier.
  4. Add in a comparator to turn the processed analog signal into a digital signal that can be read and interpreted by the Teensy. The Teensy will then display the card information on the OLED screen!