The TDC1000-TDC7200EVMTDC1000-TDC7200EVM evaluation module has 4 main parts The TDC1000 analog front-end and a pre-programmed MSP430 microcontroller on the board, a 1MHz Piezo Ceramic sensor that plugs into the board and a GUI client for on your PC.
In this blog I measure the start and stop pulse, and show how you can display 3 signals on a 2 channel oscilloscope.
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What are we Measuring
The evaluation kit has 3 test points that are of particular interest:
- the start pulse - the moment that the board sends the ultrasonic signal to the piezo transducer
- the compin signal - the echo signal returned by the piezo element
- the stop signal - the TDC1000 sends this pulse after it has detected the same number of zero crossings in the compin signal as it has sent out.
Some Theory
I've configured the evaluation board to send 5 pulses. The TDC1000 creates a 1MHz sinusoidal signal with 5 cycles and agitates the piezo transducer with that signal.
The signal travels through the water, and bounces back when it hits the surface (the boundary between water and air is a reflector for 1 MHz signals).
The reflection of that signal is picked up by the piezo transducer and sent back to the board.
As soon as the returned signal exceeds a threshold, the TDC1000 starts counting how many times the returned signal crosses the zero, on the positive edge.
After 5 crossings the TDC1000 generates the stop signal.
image source: TDC1000 data sheet
The Expected Result
We're expecting to see these 3 correlated signals on our oscilloscope (this image is from the User's Guide, made with a "more than 2 channel" scope):
image source tdc1000-tdc7200evm User's Guide
The yellow line is the start pulse, the purple one is the echo of the transducer. The stop pulse, generated after 5 zero crossings, is green.
We're interested in the time between the rising edge of yellow and green. That's an indicator of the back-and-forth time of our signal through the liquid.
note: take care that you enable continuous triggering on the TDC1000 tab of the evaluation board's GUI.
My Scope Only Has 2 Channels
Sometimes it's a blessing to have limited resources. It forces you to be inventive. You'll have to find solutions - and you learn more about your test equipment.
It is true that my scope is 2 channel. That would mean that I'd have to ignore one of the 3 signals when probing. And that's OK. You can analyze the functionality by looking at two signals each time.
- probe start + compin, and see how long it takes for the echo to return (trigger on start rising edge)
- probe start + stop, and measure the time between both rising edges (trigger on start rising edge). That's the travel time of the signal from sensor to surface and back
- probe stop and compin (trigger on stop rising edge - put the trigger point near the right end of the scope screen). The trigger point is the nth zero crossing of the composite signal.
Rigol DS1052E REF function
But it would be more intuitive to show the 3 signals in one picture. And that's possible, because we have one signal that's always fixed: the start signal. And it's our trigger point - so this signal will always have a fixed position on our screen.
It's a signal with fixed frequency, length and amplitude. A good candidate to store the start pulse as a Reference Signal, using your scope's REF memory.
The REF function (search the Rigol's User's Guide for "Using REF") allows you to capture a waveform and freeze it on the display. The DS1052E shows it as a white waveform.
You can remove the channel A probe from the START test point. The reference signal stays on the screen. You can use the vertical position button to move it out of the way.
But Wait, I Was Triggering on the Start Pulse
... so when I take away the probe A from the START test point, I loose my trigger base, don't I?
External trigger to the rescue. The DS1052E allows you to trigger on a signal that's not channel A or B.
We can connect the EXT TRIG input to the START test point, and set up external triggering.
Open the trigger menu, set the source to EXT, and play with the level button until the scope triggers reliably. You can see this when the T'D label appears, and the orange T icon appears above the rising edge of the reference signal.
tip: it's easier to do this exercise when both EXT TRIG and channel A are connected to the START signal of the evaluation board. You have a stable external trigger when the yellow signal shows stable on the screen, with it's rising edge exactly at the same horizontal position as the one of the white reference waveform. You can safely remove the channel A test probe from the START test point when this is achieved. |
Once you've established a stable external trigger on the start signal, you can connect channel A to the compin test point and channel B to the STOP test point.
You now have the 3 signals on the screen, and can use the scope's cursors to analyse the display.
In my test setup, the signal has traveled 37.4 µs through the fluid. That is 18.7 up and 18.7 back down. When you know that the speed of sound through water is approx. 1480 m/s, you can do the math.
I got a bit sidetracked in this blog post. My aim was to talk about measuring the analog, digital and acoustic signals flying through board and liquid-under-test. It turned into a scope probing technique story.
Maybe you find this useful in your own measuring activities...
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