Impedance Analyzer (LCR Meter)

Overview

The Impedance Analyzer is a tool for precisely measuring the "Impedance ($Z$)" of speakers, electronic components, and more.

It has the same functions as instruments generally called "LCR meters" and can measure the following parameters:

• Resistance ($R$)
• Reactance ($X$)
• Inductance ($L$) - Component of coils
• Capacitance ($C$) - Component of capacitors
• Quality Factor ($Q$)
• Dissipation Factor ($D$)

Basic Knowledge for Beginners

What is Impedance?

In DC circuits, "Resistance ($R$)" represents the difficulty of current flow, but in AC circuits, capacitors and coils also obstruct or advance the flow of electricity. Together, these are called Impedance ($Z$).

Impedance includes not only "Magnitude ($|Z|$)" but also "Phase ($\theta$)," which is information about delaying or advancing the electrical waves.

• Resistance ($R$): Does not delay waves ($\theta = 0^\circ$)
• Coil ($L$): Delays waves (current lags behind voltage, $\theta > 0^\circ$)
• Capacitor ($C$): Advances waves (current leads voltage, $\theta < 0^\circ$)

This analyzer precisely separates and measures this "magnitude" and "offset (phase)" using lock-in detection technology.

Measurement Setup

Impedance measurement requires simple wiring using a separate "Shunt Resistor (Ref Resistor)." This is called the "I-V method (Current-Voltage method)."

Requirements

• Audio interface (2-in/2-out recommended)
• Reference Resistor: A precision resistor of about $10 \Omega$ to $1 k\Omega$ (e.g., $100 \Omega$).
  • Choosing a value close to the impedance of the target you want to measure will increase accuracy. Generally, $100 \Omega$ is versatile.

Wiring Diagram

1. Send the signal from Output L to the device under test (DUT).
2. Connect it so that after passing through the DUT, it passes through the Ref Resistor to ground (GND).
3. Input L (Voltage Ch): Connect between the DUT and the resistor (the voltage divider point). This measures the "voltage across the resistor," but since the resistance value is known, the "current" can be calculated.
   • Note: In the default settings, Input L is for voltage and Input R is for current measurement, but since current is actually calculated from the voltage difference across the shunt resistor, check the settings according to your circuit configuration.
   • Standard Settings for this App:
     • Voltage Ch (Input L): Measures the voltage $V$ across the DUT.
     • Current Ch (Input R): Measures the voltage $V_{shunt}$ across the shunt resistor. The current is $I = V_{shunt} / R_{shunt}$.

Click the "Show Connection Diagram" button at the bottom left to view this diagram in a separate window.

Operation Procedures

Initial Setup

1. Confirm that the audio device is correctly selected in Audio Settings.
2. Open Measurements > Impedance Analyzer.
3. Configure the Settings panel.
   • Voltage Ch: Left (Ch 1) (upstream of the DUT)
   • Current Ch: Right (Ch 2) (shunt resistor side)
   • Ref Resistor: Enter the value of the resistor being used (e.g., 100.0 $\Omega$). If this number is incorrect, all calculations will be wrong.

Calibration

A process to cancel the resistance and capacitance of the measurement cables and connectors themselves to increase accuracy.

• Open Cal: Leave the measurement terminals unconnected (open) and press Run Open Cal.
• Short Cal: Connect the measurement terminals directly to each other (short circuit) and press Run Short Cal.
• Load Cal (Optional): Connect an accurate reference resistor and perform Run Load Cal (usually can be omitted).

Manual Measurement

Continuously measures values at a specific frequency.

1. Open the Manual Control tab.
2. Frequency: Set the frequency you want to measure (e.g., 1000 Hz).
3. Press the Start Measurement button.
4. Measurement values are displayed in the Result panel on the right.
   • |Z| (Magnitude): Absolute value of impedance.
   • θ (Phase): Phase angle.
   • Ls / Cs: Equivalent inductance or capacitance value at that frequency.

Note: Check "Time Series (Manual)" below the plot to graph impedance changes over time instead of frequency.

FRA Sweep

Graphs changes in impedance while changing the frequency. Ideal for measuring the $f_0$ (resonant frequency) of speakers, etc.

1. Open the Frequency Response (FRA) tab.
2. Start Freq / End Freq: Determine the measurement range (e.g., 20 Hz to 20,000 Hz).
3. Sweep Mode: Log (logarithmic) is common.
4. Press Start Sweep.
5. A graph is drawn on the Bode Plot.
   • At the resonance point of a speaker, a mountain-like peak appears.

Results and Modes

Series vs Parallel

LCR meters have two display modes: "Series equivalent circuit" and "Parallel equivalent circuit." These can be switched via buttons or settings.

• Series (Ls, Cs, Rs):
  • Used when measuring components with low impedance (a few ohms or less).
  • Examples: Low-resistance coils, large-capacity capacitors.
• Parallel (Lp, Cp, Rp):
  • Used when measuring components with high impedance (tens of kΩ or more).
  • Examples: Small-capacity capacitors, high-resistance.

Note: If unsure, for intermediate values (hundreds of ohms to several kΩ), either mode will yield almost the same value.

Parameter Descriptions

• Rs (Equiv. Series Resistance): The "pure resistance component (loss)" of a capacitor or coil. The smaller this value, the more ideal the component is.
• Q factor (Quality Factor): The sharpness of resonance or the quality of a component.
  • For coils: A higher value means less loss and better performance.
  • For speakers: An index indicating the damping of resonance.
• D factor (Dissipation Factor): The loss tangent ($D = 1/Q$). Often used for evaluating capacitor performance. A smaller value indicates better performance.

Application Examples

Speaker Unit Measurement (TS Parameters)

By connecting to the terminals of a single speaker unit and performing a sweep measurement, you can observe a "peak" where the impedance rises sharply at $f_0$ (lowest resonant frequency). From the height ($Z_{max}$) and width of this peak, parameters necessary for enclosure design can be calculated.

Cable Capacitance Measurement

Connect between the core wire and shield of an RCA cable or similar to measure. Looking at Cs (Capacitance) at a frequency of about 1 kHz, you can measure cable capacitance, such as 120 pF. A lower value means the cable has less attenuation in the high-frequency range.