Capacitance Meter or LCR

Hello every one, Can i measure the continuous variation(with frequency above 10 kHz) in the capacitance and dielectric constant of a fine capacitor with capacitance meters or LCR's? Please introduce me a web link for these instruments.

frequency

Hi. First off, I believe you're possibly talking about D (dissipation factor) instead of "dielectric constant".

Second, your question provides insufficient data to expect an intelligent answer. I mean, this is the nuculer age, and these are the internets, but honestly, you might want to put a little more effort into it.

Please introduce us a question (all your base are belong to us?) which includes:

• What, specifically, are you doing? Trimming a cap with a laser, tweaking a variable cap with a screwdriver, motion control from determining the variable capacitance of two sliding conductor plates with an air dielectric, or what? There is actually more than one reason to continuously measure a variable capacitance. I've run across these and more in my day.

• What kind of measurement accuracy do you need?

• What is the "fine capacitor"? They're all fine. What capacitance value or range? In pF, nF, or uF please.

• What does "above 10 KHz" mean? Capacitance can be measured at many frequencies, and you'll have to be more specific. What's the frequency, Shayan? (Dan Rather's going to have a lot of time to ponder that one ;-)

• Do you have a requirement for a test voltage? How about a DC bias? If so, what are they?

• What do you mean by "continuous"? Once a second? Twenty times a second? A purely analog measurement which gives a continuous analog output without any updating, like some of the old General Radio meters? This goes back to the question of what you're doing here.

• What kind of output or interface do you need? A 4-digit LCD on a handheld LRC meter? How about GPIB, or do you require a handler interface or an analog output voltage? Some combination of these?

• What kind of fixturing do you have, or are you building it? Do you have an idea what the impedance of your fixturing is at the specified frequency? (If you think this is picky, you should know that for low pF measurements, the fixturing is sometimes the part of the project that requires the most engineering work, and if there's automation involved, you can easily be pulling your hair out trying to get good measurements.)

• Last, but not least, how about a price range? If you want something for less than .95 USD, set up a 555 with a 9V battery, and infer capacitance from frequency. Prices for real solutions aren't cheap. Whether it's possible in your price range is dependent on answers to the above questions.

I'm not trying to be harsh, because you may just need a little more help than most OPs. But the solution to your problem might be fairly expensive, and you wouldn't want to waste time on an instrument that won't do the job for you. If you don't know the answers to these questions, please ask around and find out.

Look forward to hearing from you. Chris

google make them, for example

martin

"An eye for an eye makes the whole world blind" Gandhi

An equally vague answer to your vague question is us a network analyzer with an impedance function if you need to measure impedance over a frequency range. Hewlett Packard (Agilent) makes these. HP4195 and Agilent 4395 come to mind. This will allow you to measure complex impedance from 10 Hz to 500 MHz with the proper fixturing. Although the spec sheet says it will measure impedance above 100 kHz, the analyzer will do impedance measurements below 100 kHz if you use DC coupled fixtures - at least in the HP4195.

Mark

I have an hp 4275A LCR meter, which measures from 10kHz to 10MHz with 5.5-digit resolution. You will find, as I have, that most capacitors will not have a significant capacitance variation over most of that frequency range. If you have in mind a specific part or type of capacitor, I would be happy to measure it for you.

OTH, there is the issue of dielectric absorption, which affects the observed value of capacitors at lower frequencies. If the polarizing voltage is present long enough, "remnant polarization trapped on dielectric interfaces" adds to the capacitor's value, see AoE pages 220-221. The hp 4274A 5.5-digit LCR meter measures from 100Hz to 100kHz, and is a good instrument to observe this effect.

Both of these hp instruments appear on eBay from time to time, e.g.

For frequencies below 100kHz, capacitance bridges are better suited for high resolution measurements. For example, the General Radio 1615-A and 1616 bridges have 6 and 12-digit! readout, respectively. They can be used down to 10Hz, but the maximum bridge-excitation voltage must be reduced, lowering the achievable resolution. I have both of these fine, elegant instruments. :) A google search will turn up used-instrument dealers, and they're supposedly still being produced by IET,

At least the old GR datasheets are available there for you to read. Search on 1620 and 1621 precision capacitance-measurement systems.

--
 Thanks,
    - Win

Yes, of course, especially if the capacitor's leads are long enough, but this is easily corrected for. Actually, my hp 4191A RF impedance analyzer is better suited for complex-impedance measurements; It goes from 1 to 1000MHz, and has 4.5-digit readout resolution. I'm looking for a used 4192A (that I can afford) for more flexible measurements below 1MHz. And a 4194A would be very nice to have as well.

--
 Thanks,
    - Win

I read in sci.electronics.design that Winfield Hill wrote (in ) about 'Capacitance Meter or LCR', on Sat, 26 Feb 2005:

Do you not find any self-resonance effects, e.g. with a 1 uF, well below

10 MHz?

--
Regards, John Woodgate, OOO - Own Opinions Only. 
The good news is that nothing is compulsory.
The bad news is that everything is prohibited.
http://www.jmwa.demon.co.uk Also see http://www.isce.org.uk

Hi Chris. Tank you for your kindness and answering to my request. Indeed I'm a chemical engineer and I want to use a special capacitor for measuring the variation of dielectric constant in a fluidized bed. A fluidized bed is column in which air or another fluid is entering from bottom and fluidized the fine particles (5 to 500 micrometer) exist in the column. By variation in the particle concentration in the capacitor volume its dielectric also changed. If we could measure this changing in dielectric with a high sampling rate, we could calculate the particle concentration using convenient equations. But the traditional LCR's have not enough measurement speed (minimum 2 ms) and we want the sampling frequencies above 5 kHz (measurement speed below 0.2 ms). By which type of capacitance meters can I do this?

Hi Win. Tank you for your kindness and answering to my request. Indeed I'm a chemical engineer and I want to use a special capacitor for measuring the variation of dielectric constant in a fluidized bed. A fluidized bed is column in which air or another fluid is entering from bottom and fluidized the fine particles (5 to 500 micrometer) exist in the column. By variation in the particle concentration in the capacitor volume its dielectric also changed. If we could measure this changing in dielectric with a high sampling rate, we could calculate the particle concentration using convenient equations. But the traditional LCR's have not enough measurement speed (minimum 2 ms) and we want the sampling frequencies above 5 kHz (measurement speed below 0.2 ms). By which type of capacitance meters can I do this?

In theory the 4275A is a bridge, but quite unlike a conventional bridge such as the GR instruments. The latter use ratio transformers, and all four arms of the bridge are apparent, including the reference capacitors. HP calls the 4275 circuit a "voltage-vector current measurement" method. A detailed look at the circuit schematic reveals many other components, although there is a balance resistor and null point. I haven't learned much about the nature of this balance resistor, which I imagine must not have any self capacitance. The schematics show five or six components making up each "resistor." I haven't torn my 4275A apart to examine it.

I haven't been following your posts, what were some subject headings?

As for the measurements, yes it's easy to get into the fF territory. The 4275A measures 0.01fF in its slower high-resolution mode, the same capability as the GR 1615-A bridge (the GR bridge has the benefit of allowing one to use an external variable-frequency oscillator). My own hybrid distance-meter design (ratio transformer + multiplier fine balance) had even better capability. But GR's 1616 was much better, with a 0.1aF most-sensitive measuring digit. They literally have a 12-digit readout! I don't think that can be achieved without using precision low-impedance ratio transformers in an honest-to-god bridge.

It would be nice if I understood your circuit.

In the case of the 4275A, highly-linear RF phase-detector multipliers are necessary, which they implement with JFETs driven from an unusual staircase waveform circuit that suppresses the 3rd and 5th harmonics.

--
 Thanks,
    - Win

Ah, but what you find easy, the OP (and many others) would find a mite (indeed a 'might') difficult.

--
Regards, John Woodgate, OOO - Own Opinions Only. 
The good news is that nothing is compulsory.
The bad news is that everything is prohibited.
http://www.jmwa.demon.co.uk Also see http://www.isce.org.uk

bed.

its

Hi, Shayan. Glad to hear from you again. Looking back on my previous post, I think I was a little impatient with your very brief problem description, and wasn't sure if it was a legitimate post. Sorry.

But I believe I got the drift of where you're headed. I'm hearing you saying that you've got two plates suspended in a fluidized bed, and you're trying to find particle density by measuring capacitance to find dielectric constant of the fluidized powder between the plates. A couple of questions come to mind:

• I'm going to assume you've got a fluidized bed with a powder being fluidized by air (again, your description of your problem leaves something to be desired -- what's the powder, what's the medium?). If so, you're depending on the difference between the dielectric constant of the air and that of the powder to provide you with something you can measure to infer powder density. Note that unless there's a lot of difference in the dielectric constant of the air and the powder ('tain't necessarily so), it might be that you are trying to infer more accuracy than you can get with that type of measurement. If the dielectric constants are close, the most accurate capacitance measurement isn't going to help, except in a very rough way. If there's a more dramatic difference in masses per given volume for powder and medium than their respective dielectric densities, might it be better to use a stirrer and measure torque?

• Fluidization usually occurs with mechanical vibration of the bed. In a system with air bubbles, the mechanical shaking makes the air bubbles progressively smaller, until the powder/air mix acts like a liquid. I'm not sure why, in a mechanical system like a fluidized bed,

5,000 measurements per second are desirable. Usually it takes from tenths of seconds to seconds for the powder and the meduim to homogenize. Again, a better description might help. This might be an esoteric application we don't know about.

• This sounds like a fairly small (low pF range) capacitance. From a production/lab standpoint, if "real-time control" required such speeds, I might be tempted to cobble together a fairly high speed oscillator using the capacitance of the plates in the bed (possibly even suspending the rest of the oscillator in the bed itself), divide it down to an appropriate range with a comparator and some logic ICs, and after optocoupling, use a high speed counter board in a PC to collect and process period data. Either that, or you can use the divided down optocoupled signal to gate a counter, which would drive a D-to-A converter. You would infer capacitance from period or analog voltage, and then infer powder density from the inferred dielectric constant of the fluidized powder -- after all, your plate size remains the same, right? This has several advantages. And you'll have your 5KHz data stream.

I believe that, with reasonable care, you can get much better than 1% accuracy with this type of system, which might be enough if the differences in dielectric constants are great enough. It would require some cobbling, but might cost significantly less than your instrument, and might be a lot more reliable in a production/lab environment.

By the way, you should watch for the development of static charge in the fluidized bed, and see if that's going to interfere with your measurements. You might be able to detect this by putting a high impedance voltmeter on your measurement plates with nothing attached, and monitoring to see if any charge is developed. Also, if you're using an air bed, you want to get a good handle on humidity issues, because that will seriously affect your measurements, as well as possibly affecting your process. Fluidized beds are a battle to get running well, and usually something of a mess. By the way, I'm sure it really isn't necessary to mention this, but from my experience, make sure everyone's read the MSDS and uses adequate breathing protection where necessary. Lab/machine operators sometimes don't follow even the best instructions.

Thanks for posting again, and putting up with some unjustified impatience on my earlier post. I'd like to hear any additional details or project requirements.

Chris

IIRC, the hp4275A is a bridge (more precisely an auto balanced bridge), and I really don't see what could prevent such bridges to give really high resolution measurements. I even see more reasons for an ABB to be more accurate than an "ordinary" bridge (note also that I don't know precisely how the Genrad are build, so... But I guess there's nothing very special about them. In case yes, I'd be curious if you had a schematics).

For the ABB, as you might recall from some of my previous posts, I'm in the process of building a VNA that can also be used as an ABB (and also some other functions that are only pertinent to my use). I've made some small tests on those ugly solderless breadboards for the detector with my hp3456A for the measurement section and got really amazing results, given the ugliness level. I could easily measure the 0.13pf parasitics that's between two series of contacts separated by a guarding one for example, and also could easily resolve the fF with good stability. The expected linearity is on the ppm range and only depends on DC amplifiers, so can be improved at will (almost). The final product is expected to be largely as good.

I really can't see what makes you saying this.

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Thanks,
Fred.

"shayan" a écrit dans le message de news: snipped-for-privacy@g14g2000cwa.googlegroups.com...

What is your measurement frequency range? What is your estimated capacitance range? What is you budget?

Without those values, nobody can help you more than some vague considerations. Even orders or magnitude will help.

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Thanks,
Fred.

I'm not aware of any that any that fast, although that's likely my own ignorance. I'm puzzled by your need for a 5kHz sampling rate, are you trying to measure turbulence dissipation or something, with a very small cell? You can make an LC oscillator that's running at several MHz (so its frequency changes within 200us, keeping a high resonance Q in mind), and take rapid measurements with a vernier- interpolation period counter. Period counting is normally limited in resolution, but the vernier-interpolation technique solves this problem for short measurements. See AoE pages 1020 to 1024, where we mention the hp5370B counter and describe how it works. According to the datasheet, the 5370B can take 8000 sustained measurements/sec. The 5370B went for $12,100 in my 1989 catalog, but I got one on eBay at a much lower price. Another counter with high-speed capability is the Stanford Research SR620, which can do 1400 measurements/sec.

Costs about $5000.

--
 Thanks,
    - Win

Measurement frequency: minimum 5kHz

Estimated capacitance: .001pF - 1mF

Budget: maximum 5k$

Does this mean 5000 meas/sec, or does it mean a 5kHz ac measurement frequency?

--
 Thanks,
    - Win

We need the sampling frequency( =measurement frequency) above 5000 sample per second

Regards Shayan

Why?

--
 Thanks,
    - Win