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--- In LTspice@..., "Helmut" <helmutsennewald@> wrote:

--- In LTspice@..., legg@ wrote:

Given a plot of a pink or white noise voltage, and a synchronized plot of the current induced in a partially reactive load -

Will the 'normalized' ratio of the FFT plots be an indication of Z?

I'm looking at the low frequency end of the FFT, to avoid sampling issues. By normalized, I mean that 1 volt would produce 1 amp with both plots resolving to 0dB at the minimum sampled frequency (the load being partially inductive in this example).

It seems much too simple......

RL

Hello RL,

It will only work when you filter the FFT-output, but the
the FFT-results can't be filtered in LTspice. Thus you have
to export the FFT-data and process them in an external program.
This method only work with a linear system and it's precision
may be somewhat limited due to group delay variation.

Best regards,
Helmut

Filter the FFT results? The source is, ideally, uniform in amplitude across the band; so ideally the FFT would be a flat (or at least a straight) line.....What does filtering an FFT output entail?

I am looking at a pink noise source that has been prefiltered to include only a decade or two, so a few assumptions can be made about any FFT output registering outside this region.

With both V and I being monitored synchronously, there are a hell of a lot of variables being weeded out, but the sample duration is limited and the resolution is fairly course (~8bit 2500 data points per variable). So even if the calculated Z were valid, it'd be a crude approximation at best. I'll post a few plots in temp to show what the results actually look like.

The Z in this case is a loudspeaker transducer coil, so it's static characteristics are fairly easily obtainable using a simple swept tone. Given the signal processing capabilities that are falling into our laps with LTspice and even the most modest digital scopes these days, some questions go begging for an answer.

I was also concerned about phase relationships and delays, hence the curiosity about the actual spot 'Z' produced by the calculation. If R is known, then the phase could be intuited. If L is independent of temperature, then a new R value could theoretically be winkled. If other things are known, then phantom R/L/C quantities, or functional shifts in the same could also be evaluated. It's probably already standard practice, somewhere, DSP101 or something FAIK.

As with any 'new' tool, there are applications that don't jump out at you, or get their own chapter in the manual. Knowing the limits could save some time fiddling about - so a reference or pointer could be worth a thousand words.

Doing this again, with a source that had an unpredictable frequency content......could be a very non-invasive sensing method that costs only software, which, as everyone knows, is free......(insert smiley face here).

RL

Hello RL,

Please try my example to see and understand my point.

Files > Temp > z_from_FFT_with_noise.asc

Run the TRAN simulation.
Plot V(z2) and I(V2)
FFT of V(z2) and I(V2) with 1048576 points
-> plot FFT of V(z2)/FFT of I(V2)

Best regards,
Helmut


--- In LTspice@..., legg@ wrote:

--- In LTspice@..., "Helmut" <helmutsennewald@> wrote:

--- In LTspice@..., legg@ wrote:

Given a plot of a pink or white noise voltage, and a synchronized plot of the current induced in a partially reactive load -

Will the 'normalized' ratio of the FFT plots be an indication of Z?

I'm looking at the low frequency end of the FFT, to avoid sampling issues. By normalized, I mean that 1 volt would produce 1 amp with both plots resolving to 0dB at the minimum sampled frequency (the load being partially inductive in this example).

It seems much too simple......

RL

Hello RL,

It will only work when you filter the FFT-output, but the
the FFT-results can't be filtered in LTspice. Thus you have
to export the FFT-data and process them in an external program.
This method only work with a linear system and it's precision
may be somewhat limited due to group delay variation.

Best regards,
Helmut

Filter the FFT results? The source is, ideally, uniform in amplitude across the band; so ideally the FFT would be a flat (or at least a straight) line.....What does filtering an FFT output entail?

I am looking at a pink noise source that has been prefiltered to include only a decade or two, so a few assumptions can be made about any FFT output registering outside this region.

With both V and I being monitored synchronously, there are a hell of a lot of variables being weeded out, but the sample duration is limited and the resolution is fairly course (~8bit 2500 data points per variable). So even if the calculated Z were valid, it'd be a crude approximation at best. I'll post a few plots in temp to show what the results actually look like.

The Z in this case is a loudspeaker transducer coil, so it's static characteristics are fairly easily obtainable using a simple swept tone. Given the signal processing capabilities that are falling into our laps with LTspice and even the most modest digital scopes these days, some questions go begging for an answer.

I was also concerned about phase relationships and delays, hence the curiosity about the actual spot 'Z' produced by the calculation. If R is known, then the phase could be intuited. If L is independent of temperature, then a new R value could theoretically be winkled. If other things are known, then phantom R/L/C quantities, or functional shifts in the same could also be evaluated. It's probably already standard practice, somewhere, DSP101 or something FAIK.

As with any 'new' tool, there are applications that don't jump out at you, or get their own chapter in the manual. Knowing the limits could save some time fiddling about - so a reference or pointer could be worth a thousand words.

Doing this again, with a source that had an unpredictable frequency content......could be a very non-invasive sensing method that costs only software, which, as everyone knows, is free......(insert smiley face here).

RL