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[vna] Capacitor measurement

Koji Terai kj_terai at ybb.ne.jp
Fri Mar 31 06:49:14 UTC 2006


KONNICHWA, John-san!

Thank you very much for your advice.
I visited Agilent's H.P. and found the technical notes
that you recommended.

I find VNA is not adequate for measuring very low or high
impedance,  VNA suit for measuring impedance about a range of
10 to few 100 ohms.

I just measured the capacitor assuming antenna tuning on my desk top.
Any way, I have to learn more about VNA and measuring technique.

73 N SAYOHNARA,

Koji/ JA3XNO



----- Original Message ----- 
From: "John, KI6WX" <ki6wx at pacbell.net>
To: "TAPR/Ten-Tec VNA Users List" <vna at lists.tapr.org>
Sent: Friday, March 31, 2006 11:05 AM
Subject: Re: [vna] Capacitor measurement


> Koji-san;
> Using S11 to measure component impedances is extremely difficult.  Almost 
> the entire wave is reflected by the capacitor, so you are trying to 
> accurately determine a ratio between the transmitted wave and the 
> reflected wave that is nearly 1.  This would be a difficult measurement 
> even if you were using a top of line Agilent VNA.
>
> One indication of this difficulty is that you are seeing a series 
> resistance of 19 ohms at 10 MHz.  That value should be less than an ohm at 
> this frequency.
>
> Agilent has a series of application notes on their web site that explain 
> problems with the various methods to measure component impedance.  You can 
> download their PDF files.  To start, do a search on VNA on their web site.
>
> The resonant frequency of the capacitor is likely to be in the range of 
> several hundred MHz.  It is no surprise to me that you are seeing an 
> increased value of the equivalent capacitance as you go up in frequency.
>
> The best way to measure the impedance of components with near unity 
> reflection is to use S21.  The errors for this type of measurement are 
> significantly reduced when measuring capacitors, inductors, or crystals. 
> For the frequency range of 1 to 100 MHz, you can generally use an 
> equivalent circuit model for the component.  The typical model for a 
> capacitor is a series R, L, and C.  If it has a lossy dielectric, you 
> would also want to use a parallel R across the capacitor in addition to 
> the other three terms.
> -John
> KI6WX
>
> ----- Original Message ----- 
> From: "Koji Terai" <kj_terai at ybb.ne.jp>
> To: "TAPR/Ten-Tec VNA Users List" <vna at lists.tapr.org>
> Sent: Thursday, March 30, 2006 9:21 AM
> Subject: [vna] Capacitor measurement
>
>
>> Hi Tom/N5EG,
>>
>> I made a measurement of a capacitor by the VNA and
>> report you the result as follows.
>> ---------------------------------------------------------------------
>> 1. measurement procedure
>> (1) Measured a 100 pF ceramic chip capacitor with my capacitance meter
>>      and readout was '97.6 pF'.
>> (2) Made detector calibration on the VNA (for scrupulous care).
>> (3) Done a fixture calibration. the fixture consists of  a 0.5 m RG-174/U
>>     w/ 2pcs of SMA(P) and a SMA(F) which is made for PCB use.
>>     For a 'short' and a 'open' standards used during the calibration,
>>     I prepared the same type of SMA(F)s as the fixture, I soldered and
>>     shorted the center conductor and the outer conductor of the SMA(F)
>>     with a small round copper foil for the 'short' , as for the 'open', I
>> cut the
>>     center conductor of another SMA(F) from the same surface of the
>> insulator
>>     of the connector.
>> (4) Soldered a pair of  5m/m leads to the chip cap, then soldered them
>>     to the SMA(F) fixture .
>> (5) Connected the cable w/ the SMA(F) connector fixture loaded the DUT
>>     to TX port of the VNA.
>> (5) Set the VNA 'start frequency' and 'stop frequency' as 1 MHz and 100 
>> MHz
>>     respectively. 'Frequency glid' setting was 1020 points.
>> (6) Made sweep.
>>
>> 2. measurement result
>> (1) S11 readout VS frequency
>>      19.2 ohms + 99.7 pF @10.909715 MHz
>>      2.5 ohms + 111.2 pF @30.729146 MHz
>>      1.8 ohms + 130.7 pF @40.930323 MHz
>>      -1.0 ohms + 136.5 pF @70.270853 MHz
>>      -0.4 ohms + 174.8 pF @99.902845 MHz
>> (2) The slope of S11 (phase) changes at around 40 MHz.
>> ----------------------------------------------------------------------
>>
>> I am sorry but I think the measurement result is not good for specifying 
>> a
>> complex
>> characteristic of DUT.
>> I would like to hear your opinion on my measurement result.
>>
>> I have installed a pdf printer on my PC and  I tried to print the curve 
>> out
>> in pdf format
>> directly from the application, but I couldn't. On last version (Ver1.4), 
>> I
>> was able to print
>> curves into pdf from 'File'-'Print'.  What's wrong ??
>>
>> Best Regards,
>>
>> Koji/ JA3XNO
>>
>>
>>
>>
>>
>>
>>
>>
>>
>>
>
>
> --------------------------------------------------------------------------------
>
>
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>
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