
The RLC Meter
The ThieleSmall Control Window contains the RLC meter interface. The meter shows the measured impedance and phase of a
device connected to the Woofer Tester. The impedance and phase are broken down into a complex phasor consisting of a
real (resistive) and an imaginary (reactive) component connected in series. An equivalent inductance or capacitance is then
computed for the reactive part. For example, the DC resistance of an crossover inductor will be shown as the real or resistive
value, while the inductive reactance at the present test frequency will be shown as an imaginary reactance. The inductance
is also shown.
++
  Resistance (Real)
 oo/\/\+
 WT  
 ooXXXX+
  Inductance (phase>0)
++ Capacitive (phase<0)
Click on Image to Enlarge
Reactive components like inductors store energy within a magnetic field while capactors store energy electrostatically.
Inductors will have a positive phase angle while capacitors will have a negative polarity. The value of inductance or
capacitance is then calculated using the following equations.
Phase>0(Inductive): L = Imaginary/(2*PI*Frequency)
Phase<0(Capacitive): C = Imaginary*(2*PI*Frequency)
A sweep of resistance, inductance and capacitance can be automatically measured using an arbitrary sweep, or a particular
test frequency can be selected. If the arbitrary sweep method is used, the resulting data buffer can be exported to a file
or shown in the results window. Data is given in impedance, phase, real, imaginary, inductive and capacitive units.
The WT Output is a Current Source (not like an Amplifier or DVM)
The original and correct convention for testing and modeling drivers is to use a constant current source. The constant current,
multiplied by the motorconstant 'Bl' then produces a constant force from the voice coil. However, building a constant current
source is costly, and most traditional test rigs only approximate a constant current. A true current source will produce more
accurate ThieleSmall models.
When the tester is used to measure impedance values, the voltage at the terminals will be proportional to the impedance and the
selected current drive. The Woofer Tester will drive approximately 3.4 mArms when the output drive is set to maximum. The
output also saturates at around 750 mVrms, which sets the maximum testable impedance at around 220 ohms when the current
is at maximum. If the current is dropped to 1/10th of its maximum value, the testable impedance increases by a factor of ten.
Measuring a Capacitor
The impedance of a capacitor is inversely proportional to frequency and will be extremely high (only leakage current will exit)
at 0 hz. Any offset at the Woofer Tester's output, and there will always be some, will create a DC bias. Normally,
this bias will simply clamp to the supply rail with very little error. If extreme accuracy is desired, a high value parallel
resistor can be added (10 kohms usually works well) and its effects subtracted from the results by entering that value in
the options pulldown.
