Home / Lab 4

Lab 4

The questions below are due on Friday September 27, 2019; 05:15:00 PM.
 
You are not logged in.

If you are a current student, please Log In for full access to the web site.
Note that this link will take you to an external site (https://shimmer.csail.mit.edu) to authenticate, and then you will be redirected back to this page.

Music for this Lab

Goals:In Lab 4 we're going to build and test the first stage of our doppler ultrasound system. This is the stage that will drive our sender ultrasound transmitter.

1) Preliminaries

Before starting this lab, make sure you have:

  1. Successfully finished the ultrasound transmitter design in Prelab4.

  2. Soldered up at least some of the probing ports, sockets, header strips, decoupling capacitors, connector, and ultrasound transducers onto your PCB in lab3. If have a few parts remaining to solder, that's fine. If you haven't done any of the soldering, you may not be able to finish the lab on time.

2) Doppler ultrasound system overview

We introduced the doppler ultrasound in class this week. The system block diagram is shown below:

3) Lab 4 overview

In Lab 4 we'll prototype and test the amplifier stage on the transmitter side. It is reproduced here from Prelab 4 for convenience:

Specifically, we'll:

  1. Build the rail-splitter circuit, which will be used to create that 2.5 V signal we see in the schematic above.

  2. Build the circuit designed in prelab 4 on the sockets that you soldered to the PCB.

  3. Test that circuit works as designed, i.e., that is it should amplify the input signal to an amplitude large enough to be able to drive the ultrasonic transducer.

Before starting this lab check that your function generator is in High-Z mode by pressing Shift, Enter, the right arrow three times, down arrow twice. If it shows 50 Ohm, use the left or right arrows and Enter to switch to High-Z.

4) Rail-splitter

At several places in the system we'll need 2.5 V, which is conveniently $\frac{1}{2}$ of the 5 V USB supply voltage. We could make 2.5 V using a resistive divider or a number of other topologies, but a very common way to make a well-defined voltage is using a circuit called a rail-splitter. One of the most common ones is the [TLE2426](http://www.ti.com/lit/ds/symlink/tle2426.pdf), which provides an output voltage that is one-half of the input voltage, hence the name. You can see this stage in the overall ultrasound system [schematic](COURSE/ultrasound/Ultrasonic Velocity Sensor Schematic V1_4_NV.pdf).

Obtain the TLE2426 IC from the staff table and put it into U8.

5) Transmitter amplifier

You've already designed the component values for this amplifier. Now let's add your [TLE2141](http://www.ti.com/lit/ds/symlink/tle2141.pdf) op-amp into the socket (socket U1 in your PCB). You can find the pinout of the op-amp reproduced below for convenience:

After adding the op-amp, add the passive components (R1, R2, R3) around the op-amp, including capacitor C1, which is 0.1 \muF.

Only take ONE op-amp, please.

6) Let's test it!

We need 0 V (ground), 5 V, and 30 V to test the circuit. Let's set that up.

In order to make powering our board as easy as possible, grab a power plug connector from up front and hook up three wires as shown below so we have convenient wires to grab on for our 0V, 5V, and 30V connections. You'll need to use a screwdriver to secure the wires in there. When all set, insert the plug into the green socket on the front like shown below:

Your station should have a CSI305 power supply, set it up as:

  • Set the supply labeled III to 5 V by setting the little indicator switch.

  • Set one of the other two supplies to 30 V.

  • Connect the two grounds from the 5 V and 30 V outputs to the ground wire on your socket's wires.

  • Connect the 5 V and 30 V leads to the respective pins on the connector as well!

To test your circuit:

  • Create a 40 kHz square wave with 3.3 Vpp and 1.65 V offset (so, the signal will go from 0 V to 3.3 V) using the function generator. Scope the signal generator output to make sure the signal is correct. Then apply the signal to the input of the amplifier (test port TP1).

  • Scope the input and output (test port TP2) and make sure that the gain is ~ 8, and that the signal at TP2 goes from the low value (0.2-1.5 V) to the high value (26-29 V) calculated in Prelab 4.

The signal may not look very much like a square wave anymore. That's ok, and something we'll discuss at the checkoff. The main thing to check is the amplitude and absolute min and max voltages.
  • Connect the receiver transducer to TP3 by adding a 2-pin header/jumper at J1 (slip the jumper over the 2 pins at J1).

  • When you put an object in front of the transmitter/receiver pair, some fraction of the transmitted ultrasound wave should get reflected from that object and excite the receiver transducer, and you should see a sine wave on the receiver if you scope TP3 (which is connected to the receiver). If you cover the receiver so that no ultrasound gets received, the sine wave should go away.

Checkoff 1:
Explain your circuit to the staff.

7) Cleanup

Before you leave, it's time to clean up again! Steps for cleanup:

  • Carefully pick up your system and place into its plastic case.
  • Throw away loose wires on your desk.
  • Throw away paper, food, etc. on your desk.

Checkoff 2:
Show your cleaned-up lab space to a staff member.