ultrasonic transducer signal measurement

Thread Starter

Catriona82

Joined Feb 15, 2017
40
Hi folks,

I have a query which i'm sure is a stupid one but I am having trouble measuring the signal seen at my receiver ultrasonic transducer. I few days ago I tested my transmitter circuit by monitoring the signal seen at the receiver and all was good. I had a nice clean stable signal while the scope (i'm using a picoscope) and ground reference clip was connected between the sensor pins.

Well today I had connected up my amplifier and charge pump, ready to move into the signal conditioning section. I connected the receiver to the input of the amplifier and turned it on. I had one probe connected to the receiver legs as before and a second probe connected to the output of my amplifier. The scope was set up to trigger on the receiver signal but the signal was so erratic it was going out of range and constantly triggering the scope. the "0V" signal was basically all over the place.

I first thought I must have done something with the amplifer circuit so I disconnected the receiver from the breadboard however the "0V" was still floating all over the place. I removed the wires from the sensor and this improved the stability but it was still floating around when the receiver was close to the transmitter. As I moved it further away the signal stablised so it looked like it did when I tested it a few days ago.

I played around with it and i found if anything metal touched the pins the signal became erratic, even the end of an insulated screwdriver.

The thing is, this did not happen at all when I tested it previously. Even when the sensors were practically touching the "0V" was completely steady and I got good clean waveforms as I was expecting.

Can anyone suggest what I might be doing wrong? I know the signal is not references to anything so it does technically float but if you are measuring between the pins I would expect the signal to be stable. If I put wires back onto the sensor and re-connect it to my amplifier I have no way to properly trouble shoot if the circuit doesn't work so I'm hoping what is happening has a "fix" that I can implement before moving on.

Thanks in advance.
 

OBW0549

Joined Mar 2, 2015
3,300
It's hard to visualize what you've got there, especially the bit about "the signal is not references to anything so it does technically float" which is confusing me.

Perhaps if you could post a schematic of the amplifier and its connections to the sensor, it would clear things up and open a path to finding a fix.
 

MrChips

Joined Oct 2, 2009
20,876
A common problem with regular oscilloscope probes is a broken ground connection.
The flexible wire used in the grounding clip is very fragile and breaks easily after many hours of usage.
Check that your grounding clip is still good.
 

Thread Starter

Catriona82

Joined Feb 15, 2017
40
I think I might have made my post too long and confusing. I have a receiver sensor that is not connected to anything. My scope probe is connected to one leg and the ground clip connected to the other leg. The voltage measurement is therefore "floating" and I'm trying to measure the differential voltage across the terminals however the no voltage signal is drifting drastically when I have fly wires connected to the legs, even though they are not connected to anything at the other end. Without wires the signal is still drifting just not as drastically.

Hope this makes my situation clearer...
 

OBW0549

Joined Mar 2, 2015
3,300
When you say the voltage across the sensor is "drifting," do you mean it has a DC component that slowly changes over time? To me, that's what "drifting" implies. Or, do you mean there is some other signal (like mains frequency "hum") superimposed on the sensor signal?
 

MrChips

Joined Oct 2, 2009
20,876
I think I might have made my post too long and confusing. I have a receiver sensor that is not connected to anything. My scope probe is connected to one leg and the ground clip connected to the other leg. The voltage measurement is therefore "floating" and I'm trying to measure the differential voltage across the terminals however the no voltage signal is drifting drastically when I have fly wires connected to the legs, even though they are not connected to anything at the other end. Without wires the signal is still drifting just not as drastically.

Hope this makes my situation clearer...
Your transducer is like a capacitor with high impedance. Your measurement would be similar to having a floating probe.
Connect a 100kΩ resistor across the transducer and see if that makes any difference.
 
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