What is the best equipment to obtain the correct I and V of an RC Circ 2

[pybb3ezv]

This is an ongoing question I have for which I still have not found a satisfactory answer. This concerns measurements with oscilloscopes in general and to understand it I am considering one of the simplest model circuits -- the RC circuit. Suppose you have a 10Ohm resistor and a 100pF capacitor in this RC circuit and you apply 800kHz of 8ppV amplitude. I have asked a question regarding this circuit earlier in this forum and I remained with the impression that studying such model circuit even with the state of the art equipment is so unreliable that the I and V data obtained hardly reflect the real I and V that would be expected theoretically. Is that the case? Is it really true that there is no equipment at present that would provide true I and V? If it isn't then what exact equipment (oscilloscope and probes) would you recommend that would yield true current and voltage of the said sine curves measured across the capacitor and the resistor, with all parasitic effects (capacitances and inductances) at a negigible level?

 

[VEBill]

Yeah, Google terms like:
4-wire OR 5-wire measurement technique

I even see 6-wire technique mentioned in the results, something I've not done.

We have 4-wire Wayne Kerr RCL meter here that's fantastic. It can reliably measure components with very small values. I think it even gives the equivalent circuit for parallel or series mixed circuits (R and C, or R and L).

 

[pybb3ezv]

@MikeHalloran, so how does this book answer the problem at hand, regarding correctly measuring the current and voltage of a 100pF-10Ohm RC circuit?

 

[pybb3ezv]

@VE1BLL, you're suggesting methods to measure the values of individual components, right? This is not the question at hand. The question at hand is how do you correctly determine the values of current and voltage when you apply 800kHz signal on resistors and caps with relatively small known values, stated above. So nether the book cited above no the multiple wire techniques help answer that paticular question.

 

[btrueblood]

Mike,

You _would_ pick the book we used as a text for Aero lab class! Though I'm pretty sure it would tell how to build an op amp circuit to pick up the signal desired. I could even sell my copy to the OP, but that would just guarantee that I'd need to wire an op-amp the day after I posted it.

 

[pybb3ezv]

Maybe I should put the question differently. It has already been established that there is no known system capable of correctly measuring the voltage and current of an RC group consisting of a 10Ohm resistor and a 100pF cap when a signal of 800kHz is applied to it. What are the lowest values of the capacitance and resistance that would allow a correct measurement of I and V at 800kHz with an 8-bit scope and a garden variety of a passive probe (meaning a passive probe having 100pF capacitance and 1KOhm resistance)?

 

[Thealanator]

Is there a reason for picking 800kHz? Why not 20kHz, 100 kHz or 20 MHz? Holding the voltage constant and sweeping through various frequencies may enlighten.

Is it safe to assume that the test cap is capacitance, the whole capacitance and nothing but capacitance? Same goes for the test resistor.

 

[VEBill]

May I turn the question around?


How do you proposed to directly measure the current?

...And do so without affecting the voltage?

 

[pybb3ezv]

@Thealanator, like I said, the phase shift between I and V should be as large as possible.That' the main requirement, I guess. Also, the measured current and voltage should be within reasonable bounds to maintain the accuracy with the 8-bit scope. Why 800kHz? Just because it happened I started the question that way. Could be any hundreds of kHz. Let's stay with 800kHz for now, though.

 

[MikeHalloran]

I was hoping the book would educate the OP about the existence of parasitic components even in test gear, and some of the techniques used to compensate for their presence.

I wasn't going to attempt elucidation re Heisenberg.

pybb3ezv, if you wish for someone to point you to a particular line of a particular paragraph of a particular book to answer your particular question as phrased, how much money are you willing to pay for that information?



Mike Halloran
Pembroke Pines, FL, USA

 

[pybb3ezv]

Mike, a simple suggestion will do. Money is not under discussion in this forum. Probably you should look for a job-related forum.

 

[MikeHalloran]

The simple suggestion that comes to mind would violate local rules for decorum, even on Usenet.



Mike Halloran
Pembroke Pines, FL, USA

 

[pybb3ezv]

No, it won't, as long as it pertains to the question. Suggestions outside of what pertains to the question are not asked for and, like I said, you may address them in other forums to spare the bandwidth here.

 

[davidbeach]

I find it interesting how some people want to tell the old timers how the site ought to be used. Why the heck do they come here if they won't accept any answers? If they have all the answers already, what's the point.

 

[pybb3ezv]

David, I do accept the answers but where are they? The only thing I learned so far is that there is a lower limi, never mentioned in standard texts,t concering reasonable values of capacitance and resistance beyond which the electrical theory hasn't been and cannot be tested experimentally. If that's a nutty statement prove me wrong, cite a peer-reviewed paper that shows evidence to the contrary or make a suggestion as to how this difficulty can be overcome. I asked a question about the lower limits of C and R which would allow correct measurement with the available apparatus. Don't see an answer to that question. Do you?

 

[VEBill]

The limit is mostly about money, time and skill. That's why it's fuzzy.

"...cite a peer-reviewed paper..." Such a demand implies that it matters to the regulars on this forum. It doesn't.

 

[Skogsgurra]

You guys obviously find this "scientist" entertaining. So I refrain from RF.

Why is he discussing ohms and pF? There are techniques to measure microohms and femtofarads - and less than that. I see no point whatsoever in discussing a trivial case like his.

Gunnar Englund

http://www.gke.org

--------------------------------------
Half full - Half empty? I don't mind. It's what in it that counts.

 

[pybb3ezv]

@Skog, like I said, the question is not about measuring precisely the values of resistors or caps. These are known with the highest precision (or accuracy, if you wish). The question is how to measure correctly the current and voltage of an RC circuit consisting of these elements with precisely known small but not unusual values, when a 800kHz signal is applied to the circuit.

 

[patprimmer]

I am concerned about my own mortality. I demand a precise and accurate answer as to how I can overcome this problem.

Oh this is a work related engineering problem. I intend manufacturing and marketing the solution and I am not a student nor am I preparing a thesis about a theoretical problem that in practice does not really exist as workarounds are common practice and effective.

I reject the forum rules about not hijacking threads so don't bother criticising me in that regard either.

Oh I have been kicked off here before for cheating on my homework, so now I make sure my homework umm job is work related before I post.

Regards
Pat
See FAQ731-376 for tips on use of eng-tips by professional engineers &

http://eng-tips.com/market.cfm

for site rules

 

[Thealanator]

At these frequencies nothing is simple. I doubt that you have a pure 10 nF or a pure 10 ohm device.

 

[waross]

When the value of any parameter of a test device becomes significant in relation to the same parameter of the device under test, significant errors are often the result.
Take it to the extreme. I suspect that internal corrosion may be affecting the resistance of a piece of 500 MCM cable ten feet long.
I have been testing it with my $25 multimeter but have not been able to determine whether the cable is good or faulty.
Similar problem at the other end of the scale.
Why am I wasting my time?

Bill
--------------------
"Why not the best?"
Jimmy Carter