PicoScope 7 Software
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Loudspeaker Impedance Measurement
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dunstanlangrish
- Newbie
- Posts: 1
- Joined: Fri Aug 24, 2018 1:42 pm
Loudspeaker Impedance Measurement
Hi, I'm new to using oscilloscopes, and I just want to double check my proposed measurement setup, as during my first try I learned the hard way with regard to the common ground leads by short circuiting across my amp terminals.
Fig 2 below shows the test setup I'm trying to build. The idea is to measure voltage across the loudspeaker terminals, voltage across the resistor, and then calculate magnitude and phase of the loudspeaker impedance using the frequency response program for pico that outputs bode plots.
Fig 3 shows my first attempt, the spark symbol indicating the final, unsuccessful connection. This completes the short circuit via the ground leads on the probes.
Thankfully, nothing broke.
Fig 4 shows my proposed solution. Before I try again, can anyone see anything wrong with it? I guess it would not actually be necessary to connect both ground leads, and probably make more sense to put the resistor on the -ve side of the speaker lead, although of course we can just change the sign of the data.
Fig 2 below shows the test setup I'm trying to build. The idea is to measure voltage across the loudspeaker terminals, voltage across the resistor, and then calculate magnitude and phase of the loudspeaker impedance using the frequency response program for pico that outputs bode plots.
Fig 4 shows my proposed solution. Before I try again, can anyone see anything wrong with it? I guess it would not actually be necessary to connect both ground leads, and probably make more sense to put the resistor on the -ve side of the speaker lead, although of course we can just change the sign of the data.
Re: Loudspeaker Impedance Measurement
Hi dunstanlangrish,
I assume that you're looking for a plot of the waveform lag for the current going through the speaker coils w.r.t. the voltage. The main problem with your circuit is that you need to make a measurement of at least one voltage that is not referenced to ground. In this case you will need a differential probe to give you the capability to perform the required measurement. There are 2 scenarios based upon whether or not the return from the speaker is directly connected to ground.
If the return is connected to ground then the only possible way that you could connect 2 single ended probes to get the measurements you need would be as in the attached circuit (Math Channel circuit).
The current waveform will be the voltage across the shunt resistor divided by the resistance, and the shunt resistor voltage would need the creation of a Math Channel calculating 'channel 2 - channel 1'. The FRAScope (which what I assume is the frequency response program that you're referring to) needs live data to perform the plot (i.e. it can't plot a math channel against a live data channel). So you wouldn't be able to use the software with FRAScope software to perform an automatic plot, unless you get a differential probe to measure the voltage across the shunt resistor (as shown in the attached Diff Probe circuit).
If the return isn't directly connected to ground (which is often the case), then you would need 2 differential probes to place across the speaker and across the resistor.
Regards,
Gerry
I assume that you're looking for a plot of the waveform lag for the current going through the speaker coils w.r.t. the voltage. The main problem with your circuit is that you need to make a measurement of at least one voltage that is not referenced to ground. In this case you will need a differential probe to give you the capability to perform the required measurement. There are 2 scenarios based upon whether or not the return from the speaker is directly connected to ground.
If the return is connected to ground then the only possible way that you could connect 2 single ended probes to get the measurements you need would be as in the attached circuit (Math Channel circuit).
Regards,
Gerry
Gerry
Technical Specialist
Technical Specialist