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Hans Polak
diyAudio Member

Join Date: Mar 2005
Location: Blaricum
Quote:
 Originally Posted by scott wurcer We need to look seriously at the sensitivity to model fit. Here is -10% on L and +10% on C. This would be buried in the noise that has been shown so far.
In the table below, one can see what 100% variation of the Cap and 10% variation in L does to the phase at 3kHz and at 10kHz.
But because phase is close to 90 degrees, a bit of noise on the phase doesn't hurt to calculate the imaginary from the complex modulus |Z|.

Maybe it would therefore be even better to measure at 10kHz instead of 3kHz, because at this frequency phase is closest to 90 degrees, giving probably the highest accuracy in calculating L.

Hans
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scott wurcer
diyAudio Member

Join Date: Jan 2004
Location: Belmont MA
Quote:
 Originally Posted by Hans Polak phase is close to 90 degrees, a bit of noise on the phase doesn't hurt to calculate the imaginary from the complex modulus |Z|.
Hans with all due respect this statement is nonsense, the modulus is real not complex and you don't calculate the imaginary from something, you measure magnitude and phase Euler's equation gives you the direct correspondence to real and imaginary part.
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"The question of who is right and who is wrong has seemed to me always too small to be worth a moment's thought, while the question of what is right and what is wrong has seemed all-important."

luckythedog
diyAudio Member

Join Date: Nov 2009
Location: London, UK
Quote:
 Originally Posted by scott wurcer At least we only need 200kHz or so, HP's accessories will all be intended for the full instrument BW.
This is true. It's well within DIY grasp.

LD

Hans Polak
diyAudio Member

Join Date: Mar 2005
Location: Blaricum
Quote:
 Originally Posted by scott wurcer Hans with all due respect this statement is nonsense, the modulus is real not complex and you don't calculate the imaginary from something, you measure magnitude and phase Euler's equation gives you the direct correspondence to real and imaginary part.
Scott,
And with much respect from my side, the magnitude that is measured is |Z| and is real, being by definition the complex modulus of Z.
And yes, with the phase you have to calculate from |Z| the re and the im magnitude.
But the magnitude from im1=|Z|sin(88 degrees) differs only 0.04% from im2=|Z|sin(89 degrees).
That's why I meant to take a frequency where the phase is closest to 90 degrees as possible, which is the case at 10kHz, getting L with the highest accuracy.

Hans

scott wurcer
diyAudio Member

Join Date: Jan 2004
Location: Belmont MA
Quote:
 Originally Posted by Hans Polak Scott, And with much respect from my side, the magnitude that is measured is |Z| and is real, being by definition the complex modulus of Z.
Sorry Hans we just have some kind of terminology problem, complex modulus makes no sense (to me). The modulus (magnitude) is a real number there is no imaginary part. It just makes a layer of translation for me to see what you mean.
__________________
"The question of who is right and who is wrong has seemed to me always too small to be worth a moment's thought, while the question of what is right and what is wrong has seemed all-important."

 12th February 2019, 12:52 AM #416 jackinnj   diyAudio Member     Join Date: Apr 2002 Location: Llanddewi Brefi, NJ The ratio "A/R" turns out to be "problematic" at very low levels for the HP3577 -- may be owing to the way this 1984 instrument performs the calculation, noise, etc. (I'm guessing you don't have enough "bits" in the low range level.)
luckythedog
diyAudio Member

Join Date: Nov 2009
Location: London, UK
Quote:
 Originally Posted by scott wurcer Sorry Hans we just have some kind of terminology problem, complex modulus makes no sense (to me). The modulus (magnitude) is a real number there is no imaginary part. It just makes a layer of translation for me to see what you mean.
Hi Scott what Hans is saying is that the real part of Z at 10kHz is << imaginary part. So magnitude of Z tells us what we wish to know ie magnitude of imaginary part of Z.

That's true until we add the loss mechanism we are looking for. Then vector isn't 90 anymore so all bets off. I think we do need to measure phase unfortunately.

LD

Hans Polak
diyAudio Member

Join Date: Mar 2005
Location: Blaricum
Quote:
 Originally Posted by scott wurcer Sorry Hans we just have some kind of terminology problem, complex modulus makes no sense (to me). The modulus (magnitude) is a real number there is no imaginary part. It just makes a layer of translation for me to see what you mean.
Hi Scott, no problem, but obviously I'm not the only one using this terminology Complex Modulus

Complex modulus - Wikipedia

Hans

Hans Polak
diyAudio Member

Join Date: Mar 2005
Location: Blaricum
Quote:
 Originally Posted by jackinnj The ratio "A/R" turns out to be "problematic" at very low levels for the HP3577 -- may be owing to the way this 1984 instrument performs the calculation, noise, etc. (I'm guessing you don't have enough "bits" in the low range level.)
Hi Jack,

That would be a shame, when resolution turns out to be the deal breaker.
The HP3577 spec's are a bit vague on that, but they mention that at levels between -90 dB to -100dB below Full Scale Input, resolution with 10Hz BW is still +/-0.75dB.
When taking +15dBm or 1.26V as Full Scale input, -100dB would be 12.6 uV.
That is still- above the 22uV that I was suggesting with the -40dBm setting + 40dB attenuation with 5K - 50 Ohm.
So do I make a wrong interpretation of the spec's ?

Hans

scott wurcer
diyAudio Member

Join Date: Jan 2004
Location: Belmont MA
Quote:
 Originally Posted by luckythedog Hi Scott what Hans is saying is that the real part of Z at 10kHz is << imaginary part. So magnitude of Z tells us what we wish to know ie magnitude of imaginary part of Z. That's true until we add the loss mechanism we are looking for. Then vector isn't 90 anymore so all bets off. I think we do need to measure phase unfortunately.
Yes I apologize again for mis-understanding. The observed measurements presented sometimes show 10 or more degrees of phase error so direct application of minimum phase theory would possibly miss what we are looking for.
__________________
"The question of who is right and who is wrong has seemed to me always too small to be worth a moment's thought, while the question of what is right and what is wrong has seemed all-important."

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