Author Topic: 20pf testing meter  (Read 2680 times)

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Offline gkmaia

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20pf testing meter
« on: April 21, 2019, 07:42:58 pm »
Is there any test gear capable of testing ceramic capacitors at the 2pf or 20pf range?

Lowest i could find so far were 200pf range.
 

Offline nctnico

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Re: 20pf testing meter
« Reply #1 on: April 21, 2019, 07:53:28 pm »
The very cheap 'xjw01 lcr bridge' from Aliexpress or Ebay has no problems measuring capacitances this low. Either way you will need an LCR meter with a 4 wire lead set.
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Offline TurboTom

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Re: 20pf testing meter
« Reply #2 on: April 21, 2019, 08:27:10 pm »
A kelvin (four wire) test lead set is not required for measuring low capacitances since their impedance at the typical test frequencies is rather high, so residual resitances don't contribute much to the measurement. What's more important is a rigid test set (i.e. a PCB with pogo pins or the like) that's going to be plugged directly into the LCR meter's sockets so the stray capacitance doesn't change when handling the DUT and it can be nulled out reliably. A set of kelvin leads with shielded wires but the typical gold clips will easily have a tolerance of the null reading of +-5pF, depending on the distance and orientation towards each other. Similar is valid for measuring low inductances but in this case, it's recommendble to use kelvin connection(s) right to the DUT.

Cheers,
Thomas
 
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Offline Mechatrommer

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Re: 20pf testing meter
« Reply #3 on: April 21, 2019, 11:14:20 pm »
$30 LC200A L/C Meter can go down to 2pF or lower easy.
Nature: Evolution and the Illusion of Randomness (Stephen L. Talbott): Its now indisputable that... organisms “expertise” contextualizes its genome, and its nonsense to say that these powers are under the control of the genome being contextualized - Barbara McClintock
 
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Offline bob91343

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Re: 20pf testing meter
« Reply #4 on: April 22, 2019, 03:57:59 am »
I have used a Q meter for this.  It resolves less than 0.1 pF.  My GR 1658 bridge resolves much less but the readings aren't so reliable.

The problem is that the capacitance of the fixture is important.  And simply putting the DUT into the fixture affects the strays.  That is where the Q meter shines, in that you can watch the effect of position of the DUT and decide just what it is you want to measure.  Also, it can measure over a very wide frequency range.

Some like to connect one end of the unknown, make a measurement, then connect the second lead and subtract the two readings.  Not theoretically correct but still close enough for some purposes.

There are excellent papers written on this, many of them going back nearly a century.  GR made some capacitance bridges.  Later, HP did also.
 
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Offline TiN

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Re: 20pf testing meter
« Reply #5 on: April 22, 2019, 04:07:33 am »
HP 4263 LCR also have no problem with these capacitances.
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Offline BravoV

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Re: 20pf testing meter
« Reply #6 on: April 22, 2019, 04:22:20 am »
Most bench top LCR meters capable of resolving that range. Old Fluke PM6306 has resolution down to 0.01pF, but usually to get down to this resolution, attention is needed for the measurement techniques and also accompanied by the right supporting probes/cable/etc.
 
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Online kripton2035

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Re: 20pf testing meter
« Reply #7 on: April 22, 2019, 07:33:38 am »
romanblack made a special device for that purpose.
https://www.romanblack.com/onesec/CapMeter.htm
 

Offline WhichEnt2

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Re: 20pf testing meter
« Reply #8 on: April 22, 2019, 12:51:29 pm »
Once I used a usual avr transistortester to check some 5 to 27 pF ceramic and mica caps. Works well IMO.

Either way you will need an LCR meter with a 4 wire lead set.
I'd rather prefer very short leads connected to 2 pin socket for that purpose.
Short pieces, high value, small period, huge amount, long delay.
 

Offline Mechatrommer

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Re: 20pf testing meter
« Reply #9 on: April 22, 2019, 01:16:37 pm »
Either way you will need an LCR meter with a 4 wire lead set.
I'd rather prefer very short leads connected to 2 pin socket for that purpose.
the LC200A unit i mentioned can do calibration to zero capacitance with whatever setup used. so i guess longer cable can also be zeroed out, but i havent tried except with the 10cm leads that come with it.
Nature: Evolution and the Illusion of Randomness (Stephen L. Talbott): Its now indisputable that... organisms “expertise” contextualizes its genome, and its nonsense to say that these powers are under the control of the genome being contextualized - Barbara McClintock
 

Offline trobbins

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Re: 20pf testing meter
« Reply #10 on: April 23, 2019, 05:58:43 am »
Some general meter don't accurately measure below 100-200pF, but have resolution down to pF level.

A cheap and cheerful work around is to set up a 100-200pF capacitor as a continuous load, and add your small value cap in parallel, and use the difference as the small cap capacitance.  Helps if you have a rigid setup - perhaps with fixed alligator clips for the DUT.

The Aneng AN8009 has 1pF resolution, and is not too far off when used in above jig to confirm pF level cap values, especially if you have known comparison caps.
 

Offline joeqsmith

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Re: 20pf testing meter
« Reply #11 on: April 23, 2019, 01:42:54 pm »
Is there any test gear capable of testing ceramic capacitors at the 2pf or 20pf range?

Lowest i could find so far were 200pf range.

Normally if I am using small values (<300pf) it's for RF.  I will want to know more about how the part behaves over a wider range of frequencies.   So I normally will mount them to a test board that I can decouple and use the VNA to measure them. 
How electrically robust is your meter?? https://www.youtube.com/channel/UCsK99WXk9VhcghnAauTBsbg
Software, documentation and test reports for the low cost NanoVNA, V2+4 and LiteVNA may be found here:
https://github.com/joeqsmith
 

Offline bob91343

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Re: 20pf testing meter
« Reply #12 on: April 23, 2019, 04:42:28 pm »
Tell about this test board and how you use the VNA.
 

Offline PedroDaGr8

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Re: 20pf testing meter
« Reply #13 on: April 23, 2019, 04:47:06 pm »
Is there any test gear capable of testing ceramic capacitors at the 2pf or 20pf range?

Lowest i could find so far were 200pf range.

Normally if I am using small values (<300pf) it's for RF.  I will want to know more about how the part behaves over a wider range of frequencies.   So I normally will mount them to a test board that I can decouple and use the VNA to measure them.
Agreed! At these ranges you really want a picture of how it behaves over a spread of frequencies.

Even inspite of that, there is way too little info here. No mention of test/use frequencies, resolution, or accuracy desired. All of these would need to be provided before offering up suggestions.

Saying measuring at the 2pF or 20pF range actually tells us very little. For example, a 2.00pF range with a resolution of 0.01pF offers no additional resolution compared to a 200.00pF range. In fact, it likely sacrifices resolution at higher values when it range switches. Similarly, I've seen some meters that will claim down to a single digit pF range but the listed accuracy is +/- 10% +/- 50 counts.

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Offline joeqsmith

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Re: 20pf testing meter
« Reply #14 on: April 23, 2019, 05:05:31 pm »
Tell about this test board and how you use the VNA.

No problem Bob.   Just go to the following link and then let me know if you have any specific questions. 

https://www.eevblog.com/forum/testgear/very-low-and-accurate-inducdance-and-capactiance-meter/
How electrically robust is your meter?? https://www.youtube.com/channel/UCsK99WXk9VhcghnAauTBsbg
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https://github.com/joeqsmith
 

Offline bob91343

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Re: 20pf testing meter
« Reply #15 on: April 23, 2019, 05:19:38 pm »
I want to know more, such as how you set up the fixture, the VNA settings, and so on.  I am a newcomer to VNAs and have a lot to learn.  I want to measure components with it but don't know what to do.

A capacitor will generally show a reflection factor of unity until you get into the higher frequencies.  A resistor will show zero if it's 50 Ohms, again until high freq.

Can you enlighten me as to procedure, and details of test fixture?
 

Offline Mechatrommer

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Re: 20pf testing meter
« Reply #16 on: April 23, 2019, 05:51:25 pm »
no capacitor behaves the same across frequencies however big or small, what you are measuring in RF are parasitics. if you want to go down that VNA road, carry on then you'll see even a resistor is not a resistor,a wire or trace is not zero ohm etc. btw the simplest form in capacitance is RC time constant over R, ymmv.
Nature: Evolution and the Illusion of Randomness (Stephen L. Talbott): Its now indisputable that... organisms “expertise” contextualizes its genome, and its nonsense to say that these powers are under the control of the genome being contextualized - Barbara McClintock
 

Offline joeqsmith

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Re: 20pf testing meter
« Reply #17 on: April 23, 2019, 07:38:20 pm »
.... what you are measuring in RF are parasitics.

Actually, parasitics or stray, is main reason we de embed the fixture.   We want to know how the component behaves.   

if you want to go down that VNA road, carry on then you'll see even a resistor is not a resistor,a wire or trace is not zero ohm etc. btw the simplest form in capacitance is RC time constant over R, ymmv.

The models for each component can be complex, sure.  My VNA is too old to have ANY math, so I wrote the SOLT and models and run them on the PC.   

I want to know more, such as how you set up the fixture, the VNA settings, and so on.  I am a newcomer to VNAs and have a lot to learn.  I want to measure components with it but don't know what to do.

That board I show has the SOLT built onto it so I do the normal steps but using that board.  The board also has areas where I can add the parts I want to test.   There are several good, free papers on-line.   Maybe start with this:

http://literature.cdn.keysight.com/litweb/pdf/5989-5765EN.pdf
How electrically robust is your meter?? https://www.youtube.com/channel/UCsK99WXk9VhcghnAauTBsbg
Software, documentation and test reports for the low cost NanoVNA, V2+4 and LiteVNA may be found here:
https://github.com/joeqsmith
 
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Offline bob91343

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Re: 20pf testing meter
« Reply #18 on: April 23, 2019, 09:57:34 pm »
That Agilent paper starts off innocently but by the time I get through several pages I am lost.  I ignored the details of the math but the thing is becoming incomprehensible.

Added to the confusion is the poor grammar of the paper.  And the introduction of terms and parameters without proper explanation.

So I quit reading before I got halfway through.  I do intend to go back and try again.
 

Offline Mechatrommer

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Re: 20pf testing meter
« Reply #19 on: April 23, 2019, 11:15:58 pm »
Added to the confusion is the poor grammar of the paper.  And the introduction of terms and parameters without proper explanation.
i found this guy has videos explaining it from basic in clearly and orderly manner. but i myself haven't got into great mathematical details of it, esp when it comes to de-embedding. fwiw...
Nature: Evolution and the Illusion of Randomness (Stephen L. Talbott): Its now indisputable that... organisms “expertise” contextualizes its genome, and its nonsense to say that these powers are under the control of the genome being contextualized - Barbara McClintock
 

Offline bob91343

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Re: 20pf testing meter
« Reply #20 on: April 24, 2019, 12:00:01 am »
I checked out that video and found that I could understand less than half of his talk.  Between talking too fast, an Asian accent, and excessive audio reverberation it was a lot of work to decipher his speech.

Obstacles abound.
 

Offline joeqsmith

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Re: 20pf testing meter
« Reply #21 on: April 24, 2019, 02:12:53 am »
That Agilent paper starts off innocently but by the time I get through several pages I am lost.  I ignored the details of the math but the thing is becoming incomprehensible.

Added to the confusion is the poor grammar of the paper.  And the introduction of terms and parameters without proper explanation.

So I quit reading before I got halfway through.  I do intend to go back and try again.

If you want to understand the VNA, there is a bit of math involved.   

To work out the equations for the SOLT, two of us worked them out independently and then we compared the results as a check.  I remember that one part being 3 - 4 pages of equations and a day to work through.   We had different results, and I had to find my mistake.  Once that was all sorted out, I wrote the code to align it.   

I don't remember the component models or TDR being as complex.           

It was a fun project and I did learn a lot by working through all the math and coding it up.  I doubt the VNA's design team would have ever envisioned their instrument being used this way.   :-DD

I'm afraid most papers I have collected are even more math heavy.   I tried  VNA for dummies on Google and this came up:
https://www.tek.com/dokument/primer/what-vector-network-analyzer-and-how-does-it-work
How electrically robust is your meter?? https://www.youtube.com/channel/UCsK99WXk9VhcghnAauTBsbg
Software, documentation and test reports for the low cost NanoVNA, V2+4 and LiteVNA may be found here:
https://github.com/joeqsmith
 

Offline bob91343

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Re: 20pf testing meter
« Reply #22 on: April 24, 2019, 03:40:55 am »
Don't get me wrong; I am not afraid of the math.  I just skim over it when it seems that I'd like to move on, and trust the author has done it correctly.

I want the big picture first.  Then I may go back and dig in.

I am conversant (albeit very rusty) with matrix algebra.  I follow the uncomfortable notation swap in order to make the math less confusing.  But there are statements and assumptions that leave me scratching my head.  Maybe I am just too aware of inconsistencies and incomplete sentences to pay attention to the subject.
 

Offline joeqsmith

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Re: 20pf testing meter
« Reply #23 on: April 24, 2019, 04:15:12 am »
"... incomplete sentences ... "  Good luck trying to decode any of my posts!!    :-DD

You may find the Tektronix documents easier to read.  If not, I would try similar searches and see what you come up with.   

There isn't much to the big picture IMO depending how deep you want to dive in.
How electrically robust is your meter?? https://www.youtube.com/channel/UCsK99WXk9VhcghnAauTBsbg
Software, documentation and test reports for the low cost NanoVNA, V2+4 and LiteVNA may be found here:
https://github.com/joeqsmith
 

Offline bob91343

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Re: 20pf testing meter
« Reply #24 on: April 24, 2019, 06:03:51 pm »
Well I don't want to burden myself with abstract issues.  I mainly want to be comfortable using a VNA to measure quantities at various frequencies, e.g. vector impedance, transfer quantities, etc.  I am pretty good with a spectrum analyzer and have been reasonably happy with one that includes a tracking generator (I guess that's called an SNA) but I want more complete information about antennas and discrete components and cables.

It seems that I run into difficulties and there is little practical help on the web to get me moving.
 


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