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Before starting this lab, make sure you have:
2) RecapFirst, let's recall again what we're trying to do with the Doppler ultrasound system, and what we've done so far. Our overall system block diagram looks like:
We create ultrasound using the transmit stage. In this stage, we first use the Teensy to create a square wave, and then we use an amplifier to amplify the voltage.
That ultrasound bounces off an object. If the object is moving, it changes the ultrasound wave frequency by a little bit. We are trying to discern that frequency change.
To do that, we first amplify the received signal approximately 10\times using our receive amp.
Then we want to turn the signal into a square wave to make it easier to manipulate. The comparator does this. Remember, we can do this because the information we care about is the frequency of the signal, which remains unchanged when we turn it into a square wave.
Then we want to measure the frequency of that digitized signal. Rather than measuring the frequency of the digitized signal directly, we first multiply the received signal with the original transmit square wave using an XOR gate.
You'll recall from lab7 that this created a signal with two main frequency components, one at \delta f, the doppler shift frequency, and one at ~ 80 kHz. We want to remove the component at 80 kHz. We'll do that in this lab with a low-pass filter.
Populate the PCB. Take a look at the [full system schematic](COURSE/ultrasound/Ultrasonic Velocity Sensor Schematic V1_4_NV.pdf) to see where these parts fit in.
We will use a TLV2371 op-amp to build our Sallen-Key filter. You can find the pinout of this op-amp in Section 4 of lab6.
We will perform one primary test on our actual system: an end-to-end test.
Power your board with 5 V and 30 V (and gnd, of course) as before.
Set up a reflector about a foot in front of your ultrasound transducers.
Apply a square wave of 3.3 Vpp with 1.65 V offset at 40 kHz to TP1. Scope TP6, which is the output of the XNOR. This should look like a square wave with two frequencies (~ 80 kHz and \delta f).
Add a second scope probe at TP7, which is the output of the filter.
You should also see that a signal arises at TP7 when the reflector moves. This is the wave at the Doppler frequency!
Show your working amplifier stage to the staff.
5) One Last Comparator for the Road
For this second comparator, we don't have much work to do, since our signal is pretty square to begin with. We just want to clean it up a bit. So alot of hysteresis is fine. In fact, let's just go for 2.5 V of hysteresis.
Choose R12 and R13 such that there is 2.5 V of hysteresis in this comparator stage. As before, choose resistors in the 1 k\Omega to 100 k\Omega range.
Let's build it. Grab the following components:
- MAX941 comparator
- two resistors for comparator
And assemble the circuit.
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.
Show your cleaned-up lab space to a staff member.