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AC bridges for measuring inductance - RF Cafe Forums

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oz7acs
Post subject: AC bridges for measuring inductance Posted: Thu Jul 26, 2007 9:42 am

Lieutenant


Joined: Thu Jul 26, 2007 9:25 am
Posts: 4
Location: Roskilde, Denmark
Hi fellow electronicians
I have some questions about AC bridges:
1) Does anyone know the upper practical frequency limit for measurement of inductance and Q on ferrite cores using one of the following AC bridges: The Maxwell-Wien bridge, The Hay bridge, The Radio Frequency bridge?
2) Also what will the particular bridge give as error limit per frequency with common available components? -Other advantages/drawbacks?
3) Is the Maxwell bridge most suited for coils with a low Q and why?
4) AC bridges are probably very accurate at low frequencies f<1MHz, but at higher frequencies f>1MHz wouldn't an LC-resonance circuit be a better choice in order to find L and Q from known C and ESR of C?

Proposed litterature:
1) "Student Reference Manual for Electronic Instrumentation Laboratories" by Stanley Wolf and Richard F.M. Smith at Prentice-Hall 1990.
2) "Electronic Instruments and Measurements" 2.ed. by Larry Jones and A. Foster Chin, Prentice-Hall 1991.
3) "Alternating Current Bridge Methods" by B. Hague, Sir Isaac Pitman & Sons, Ltd. 1959.
Other good litterature?

_________________
Best regards and
Vy 73 de OZ7ACS aka JPH


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IR
Post subject: Posted: Thu Jul 26, 2007 10:54 am

Site Admin


Joined: Mon Jun 27, 2005 2:02 pm
Posts: 373
Location: Germany
Hello oz7acs,

Here are some answers:

1. Maxwell-Wien bridge is not a freuqnecy dependant bridge while Hay bridge is frequency dependant.
2. Maxwell-Wien bridge is used to measure lossy inductors (Low Q) while Hay bridge is used to measure inductors with high Q.
3. Maxwell-Bridge can be used to measure a wider range of inductors compared to Hay bridge.
4. Hay Bridge will give a better accuracy (calibration error) as the Q factor of the measured inductor is higher.

Both of these bridges are using a capacitor which is fully characterized and known i.e.: Capacitance and ESR.


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oz7acs
Post subject: AC BridgesPosted: Thu Jul 26, 2007 8:45 pm

Lieutenant


Joined: Thu Jul 26, 2007 9:25 am
Posts: 4
Location: Roskilde, Denmark
Hi IR,
1) Yes I know that a Maxwell-Wien bridge is frequency independant, but what about stray capacitances? They must put a limit to the upper frequency at which the bridge is useful in practice or not...?
Then there is the wiring and shielding of the box containing the bridge.
2) Also the tolerances and temperature coefficients of normal available capacitors and resistors put a limit to the total accuracy of the bridge, right?
3) Don't you agree that an LC-resonance circuit would be simpler and more suited for say 5MHz measurements of L and its Q than a Maxwell-Wien bridge?
4) Last but not least: What is the upper limit to Q-measurement of coils in a Maxwell-Wien bridge and why?

_________________
Best regards and
Vy 73 de OZ7ACS aka JPH


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IR
Post subject: Posted: Fri Jul 27, 2007 12:11 am

Site Admin


Joined: Mon Jun 27, 2005 2:02 pm
Posts: 373
Location: Germany
Hello,

1. There is always stray capacitance which will limit the performance of the bridge accuracy, especially in high frequencies. As high as the frequency goes you should pay good attention to PCB layout and shielding of the bridge.

2. Also true, the tolerance and temperature coefficients of the capacitor should be better than those of the measured inductor.

3. I agree that LC resonance circuit is a simplar solution also for 100KHz measurements becuase it contains a lower number of components and requires less tuning to reach to the final value of the indcutor.

4. The calibration error goes lower as the Q of the measured inductor goes higher. I think that the upper limit of the Q is around 50 because then, as far as I have read, the measurement error is less than 0.1% and other parasitics interfere much as well.


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nubbage
Post subject: Posted: Fri Jul 27, 2007 8:44 am

General


Joined: Fri Feb 17, 2006 12:07 pm
Posts: 218
Location: London UK
IMO, at HF and VHF it will be more accurate to measure the resonant frequency of an inductor with reasoable Q (>20) by using a GDO (Gate Dip Oscillator) with the coil resonated with a precision 1% capacitor. The GDO frequency can be measured using a digital frequency counter to say 0.1%, and capacitors are available for HF/VHF with a tolerance of 0.5%
The end result should then be accurate to 0.6% or so. Measuring the inductor in situ, but disconnected from other circuit elements, automatically takes account of strays.
Another approach is use a commercial bridge for the non-professional market such as Autek, AIM, or MFJ-269. These should give an accuracy of around 5%, but perhaps worse at VHF than HF.




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