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Lab 6

The questions below are due on Friday March 13, 2020; 05:15:00 PM.
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Music for this Lab

Goals:In Lab 6 we're going to design, build and test the receiver amplifier stage of our doppler ultrasound system.

1) Preliminaries

Before starting this lab, make sure you have:

  1. Successfully finished the ultrasound receiver design in prelab6.

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.

2) Lab 6 overview

The amplifier stage on the receiver side is reproduced here from prelab6 for convenience:

You can see from the [full schematic](COURSE/ultrasound/Ultrasonic Velocity Sensor Schematic V1_4_NV.pdf) where this part fits in.

Specifically, in this lab we'll:

  1. Test our existing circuit to see how large the received signals are and therefore how much gain we need.

  2. Build the circuit designed in prelab6 (and maybe adapted by our measurements) on the sockets that you previously soldered to the PCB.

  3. Test that circuit works as designed. That is, it should amplify the received signal to an amplitude large enough to be able to be digitized by 3.3V logic.

  4. Finally, we'll discuss some system-level design choices in the two stages we have created thus far.

3) Measure your existing device

First, let's figure out how much gain we need.
  • Set up your ultrasound board approximately 2 feet in front of a reflective surface, such as one of the chipboards that we used as a backer for Lab 5. Make sure to also have a sheet in-between the two transducers to minimize direct coupling of the transmitter signal into the receiver.

  • Power the circuit with 5 V and 30 V as you did in Lab4.

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

  • Measure the receive transducer output voltage at TP3. Roughly how large is the voltage, in volts?

What we want is to turn this sinusoidal signal into digital HI and digital LO. The next stage (the comparator) will do that. Thus, we want the signal to get well above and below 2.5 V, say 2.0 V - 3.0 V, or 1 Vpp.

Use your measurements from the transducer and your analysis to estimate how much gain you need.

Checkoff 1:
Explain your necessary gain estimate to the staff.

4) Receiver amplifier

You've already designed the component values for this amplifier. If your gain estimate is close to what you designed, then you're all set. Else, adjust your resistor values accordingly.

Now let's add your TLV2371 op-amp into the socket (socket U2 in your PCB). You can find the pinout of the op-amp reproduced below for convenience (note "NC" means no contact):

This part is similarly number BUT DIFFERENT than the IC we used in Lab 4, which was a TLE2141. Don't mix them up! Get the correct part.

Use caution and make sure you insert the chip in the right orientation!

Why the TLV2371? Because it has rail-to-rail output and rail-to-rail input, which is important when you only have 5 V of supply voltage, and it is single-supply. And not too expensive, about $1 ea. And finally, it is available in a DIP package, which is increasingly uncommon these days.

After adding the op-amp, add the passive components (R4, R5) around the op-amp.

5) Let's test it!

To test your circuit, it's much the same setup as above, but now:

  • Scope the input (TP3) and output (TP4) of the receiver stage and make sure that the gain is about what you expect.

6) System-level questions

We now have a transmit-receive pair with two amplifiers. Before asking for a checkoff, be prepared to discuss the following questions:

  • What type of topology did we choose for the transmit amplifier? Could we have performed the same function with a different topology? What are some of the pros/cons of the different approaches?

  • What type of topology did we choose for the receive amplifier? Could we have performed the same function with a different topology? What are some of the pros/cons of the different approaches?

  • If you want to detect an object far away, how would you change the gains of the two amplifiers? What sets the upper limit of the distance of the object?

  • What about if you want to detect an object very close in?

  • What is the purpose of the 2.5 V offset on the two amplifiers?

Checkoff 2:
Explain your circuit and it's behavior 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 3:
Show your cleaned-up lab space to a staff member.