I am suppose to build Impedance Meter for my Senior Design with the following specifications:

1. Freq 20KHz to 500KHz
2. 1ohms to 1000ohms
3.-180deg-180deg

I have read a lot of confusing literature about this. Does any body have an idea on how to implement both the magnitude and phase measurement circuit on the same circuit.

 

I dont think it is too difficult to do both amplitude and phase in the same measurements. Is it supposed to be all analog (i.e. outputs a voltage/current that represents the phase/impedance) or do you have some type of analog-to-digital conversion and then use a micro to handle the rest of the system?

Since you are setting the frequency yourself, if you are supposed to do the work in the digital domain, you can do a very simplistic DFT that would just need 1 multiplication per sample. You would then have both the amplitude and phase of the output signal. If you also capture the input signal, then the change in phase/amplitude is easily measured.

In the analog domain, you would build a phase detector circuit that would give you a voltage that is linear with respect to the phase difference between the input/output signal. The RMS amplitude difference in conjunction with the known source impedance could then be output.

 

Thanks for your assistance Expert Crust

Crust:
"Since you are setting the frequency yourself, if you are supposed to do the work in the digital domain, you can do a very simplistic DFT that would just need 1 multiplication per sample. You would then have both the amplitude and phase of the output signal. If you also capture the input signal, then the change in phase/amplitude is easily measured. "


Can you please explain more about DFT(above paragraph) and the digital procedure in general. Also a circuit diagram (if available) will be apprecitated).

 

do a search on DFT (discreet fourier transform) on google. You'll find several good descriptions on how to do it.

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To measure the phase, use a small resistor in series. It should be small compared to 1 ohm. Comparing the voltage across the current sense resistor with the voltage across the DUT (device under test) in a X-OR gate will give the phase angle. plus or minus 90 degrees is doable, but I don't know about 180 degrees.

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