AC bridges for measuring inductance - RF Cafe Forums
Because of the high maintenance needed to monitor and filter spammers from the RF Cafe Forums, I decided that it would
be best to just archive the pages to make all the good information posted in the past available for review. It is unfortunate
that the scumbags of the world ruin an otherwise useful venue for people wanting to exchanged useful ideas and views.
It seems that the more formal social media like Facebook pretty much dominate this kind of venue anymore anyway, so if
you would like to post something on RF Cafe's
page, please do.
Below are all of the forum threads, including all
the responses to the original posts.
Post subject: AC bridges for measuring inductance Posted:
Thu Jul 26, 2007 9:42 am
Joined: Thu Jul
26, 2007 9:25 am
Location: Roskilde, Denmark
I have some questions about AC bridges:
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?
1) "Student Reference Manual for Electronic Instrumentation
Laboratories" by Stanley Wolf and Richard F.M. Smith at Prentice-Hall
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?
Vy 73 de OZ7ACS aka JPH
Posted: Thu Jul 26, 2007 10:54 am
Mon Jun 27, 2005 2:02 pm
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.
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.
AC BridgesPosted: Thu Jul 26, 2007 8:45 pm
Joined: Thu Jul 26, 2007 9:25 am
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
Then there is the wiring and shielding of the box containing
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
Best regards and
Vy 73 de OZ7ACS
Post subject: Posted: Fri Jul
27, 2007 12:11 am
Joined: Mon Jun 27,
2005 2:02 pm
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.
Post subject: Posted: Fri Jul 27, 2007 8:44 am
Joined: Fri Feb 17, 2006 12:07 pm
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.
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.