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# Oscilloscope Ultrasound Question watch

1. My physics textbook (AQA Physics A Level Year 1 Student Book, Page 65) has a section on measuring the speed of ultrasound using an oscilloscope as a part of the waves topic, but the wording is very confusing.

The textbook says: "By measuring the horizontal distance on the screen from the leading edge of the pulse to the start of the spot's sweep, the travel time of the pulse from the transmitter to the receiver can be determined."

Not even my teachers know what 'spot's sweep' means. Please could someone translate this?

2. (Original post by Hughcifer)
My physics textbook (AQA Physics A Level Year 1 Student Book, Page 65) has a section on measuring the speed of ultrasound using an oscilloscope as a part of the waves topic, but the wording is very confusing.

The textbook says: "By measuring the horizontal distance on the screen from the leading edge of the pulse to the start of the spot's sweep, the travel time of the pulse from the transmitter to the receiver can be determined."

Not even my teachers know what 'spot's sweep' means. Please could someone translate this?

Hello.

Can you scan and post the relevant pages and diagram?

I don't have a copy of that book, but it sounds very much like the 'spot' is the cursor of a delayed time-base function for the oscilloscope, which is placed on the start of the return echo pulse.
3. (Original post by uberteknik)
Hello.

Can you scan and post the relevant pages and diagram?

I don't have a copy of that book, but it sounds very much like the 'spot' is the cursor of a delayed time-base function for the oscilloscope, which is placed on the start of the return echo pulse.
I'd guess* the scope was being triggered on the outgoing pulse... i.e. the spot begins moving from the left edge of the screen at the instant the pulse was transmitted and the horizontal distance gives you the elapsed time.

TBH I don't think there's any teaching about scope cursors at A level cos it's an advanced feature that certainly wasn't on cheap (i.e. school) scopes when I was at school... but I think every scope has trigger controls.

* of course if the OP had included the entire question rather than an excerpt we wouldn't have to guess
4. (Original post by Joinedup)
I'd guess* the scope was being triggered on the outgoing pulse... i.e. the spot begins moving from the left edge of the screen at the instant the pulse was transmitted and the horizontal distance gives you the elapsed time.

TBH I don't think there's any teaching about scope cursors at A level cos it's an advanced feature that certainly wasn't on cheap (i.e. school) scopes when I was at school... but I think every scope has trigger controls.

* of course if the OP had included the entire question rather than an excerpt we wouldn't have to guess
That would be the most logical answer, I agree.

I hesitated with that as my response to the OP, since the trigger and return would be fractions of a millisecond apart. Meaning, that unless the 'scope could store the trace, a 'single shot' would not be seen by the naked eye, neither would a repetitive trace (at the sweep speed necessary to capture sub millisecond measurements) appear as a 'spot'.

5. Here is the whole page.

6. (Original post by Hughcifer)

Here is the whole page.
Thanks for that.

The text wording could be a bit better.

It's the same as saying "by measuring the distance from the start of the spot sweep to the leading edge of the pulse".

i.e. the trace sweep is started simultaneously with sending a trigger signal to the ultrasound transducer to transmit the ultrasound pulse. The return echo signal is then displayed by the spot sweeping left to right across the screen.
7. (Original post by uberteknik)
Thanks for that.

The text wording could be a bit better.

It's the same as saying "by measuring the distance from the start of the spot sweep to the leading edge of the pulse".

i.e. the trace sweep is started simultaneously with sending a trigger signal to the ultrasound transducer to transmit the ultrasound pulse. The return echo signal is then displayed by the spot sweeping left to right across the screen.
So in terms of the 'bump' on the oscilloscope, this is from the bottom of the curve to its peak?
8. (Original post by Hughcifer)
So in terms of the 'bump' on the oscilloscope, this is from the bottom of the curve to its peak?
the spot will start from the left hand side of the screen (y=0) the moment the pulse is sent so you're measuring from there to the pulse - you're interested in the flight time of the pulse... the amount of time after the pulse is sent that passes before the receiver detects it...
IMO you'd measure to the bottom of the peak as that's the time the front of the first waves that travelled in a straight line are beginning to arrive - waves travelling by reflected paths off walls and floors will take slightly longer because they have greater path lengths than the straight line from transmitter to reciever.

you could also measure the rise and fall times of the pulse by looking at the time taken to get from zero to peak but that's beside the point for this example.

btw oscilloscopes are great - probably get another warning for this but scope manufacturer Keysight is doing giveaways next month so if you want to win a scope and be the envy of your mates (or flog it on ebay I guess) www dot scopemonth dot com

not on a kickback - in fact I'll be trying to win one too so I'm making it slightly worse for myself by mentioning it.
9. (Original post by Joinedup)
the spot will start from the left hand side of the screen (y=0) the moment the pulse is sent so you're measuring from there to the pulse - you're interested in the flight time of the pulse... the amount of time after the pulse is sent that passes before the receiver detects it...
IMO you'd measure to the bottom of the peak as that's the time the front of the first waves that travelled in a straight line are beginning to arrive - waves travelling by reflected paths off walls and floors will take slightly longer because they have greater path lengths than the straight line from transmitter to reciever.

you could also measure the rise and fall times of the pulse by looking at the time taken to get from zero to peak but that's beside the point for this example.

btw oscilloscopes are great - probably get another warning for this but scope manufacturer Keysight is doing giveaways next month so if you want to win a scope and be the envy of your mates (or flog it on ebay I guess) www dot scopemonth dot com

not on a kickback - in fact I'll be trying to win one too so I'm making it slightly worse for myself by mentioning it.
Aaaahhhh, I see! Now it seems obvious lol. Thanks for the help!

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