Basic Knowledge of LC Filters

LC filters refer to circuits consisting of a combination of inductors (L) and capacitors (C) to cut or pass specific frequency bands of an electric signal.
Capacitors block DC currents but pass AC more easily at higher frequencies. Conversely, inductors pass DC currents as they are, but pass AC less easily at higher frequencies.
In other words, capacitors and inductors are passive components with completely opposite properties. By combining these components with opposite properties, noise can be cut and specific signals can be identified.

Types of LC Filters

LC filters are broadly classified into three types.

1. Low-pass Filters (LPF)

   Low-pass filters are filter circuits that pass DC and low-frequency signals and cut high-frequency signals.
   They are the most widely used filter circuits and are mainly used to cut high-frequency noise.
   In audio, they are also used to cut treble/mid-range sound components of bass speakers.

   

2. High-pass Filters (HPF)

   High-pass filters are filter circuits that cut DC and low-frequency signals and pass high-frequency signals.
   They are used to cut low-frequency noise in the audible range, cut mid-range/bass sound components of treble speakers, etc.

   

3. Band-pass Filters (BPF)

   Band-pass filters are filter circuits that pass only signals at a specific frequency and cut signals at other frequencies.
   They are used for radio tuning (frequency adjustment) or for cutting the bass/treble sound components of mid-range speakers, etc.

   

Types of Low-pass Filters

Although capacitors and inductors each have noise removal capabilities on their own, combining these two components will achieve a significant level of noise removal. Inductors connected in series block high-frequency noises, whereas capacitors connected in parallel work to bypass high-frequency noises.
However, noise removal effects change depending on the magnitude of the external impedance on the input and output sides. For example, even if a low-impedance capacitor is used to bypass noise, the noise will flow to the load side if the output impedance is lower. Conversely, even if a high-impedance capacitor is used to block noise, the noise will flow to the load side if the output impedance is higher. Therefore, when the external impedance is high, capacitors should be placed nearby, and when it's low, inductors should be used.
The four types of low-pass filters shown below are used by taking external impedances into consideration

1. L-type Filter

   

   When the input impedance ⇒ High
   and the output impedance ⇒ Low

2. L-type Filter

   

   When the input impedance ⇒ Low
   and the output impedance ⇒ High

3. π-type Filter

   

   When the input impedance ⇒ High
   and the output impedance ⇒ High

4. T-type Filter

   

   When the input impedance ⇒ Low
   and the output impedance ⇒ Low

It should be noted that π- and T-type filters have better noise removal effects than the L-type, which should be taken into consideration when selecting circuits.

Component Selection for Low-pass Filters

To remove noise from a signal waveform in a signal circuit, it is necessary to select the constants of components that enable significant attenuation at the noise frequency rather than at the signal frequency. To remove noise from a DC voltage in a power supply circuit, it is necessary to consider only the attenuation amount at the noise frequency because the DC attenuation is zero.

The attenuation characteristics (change in attenuation amount depending on the frequency) of a filter can be calculated. However, because actual capacitors and inductors contain components that affect performance, in addition to pure capacitance and inductance, there is no simple method of calculating these characteristics.

Here is a circuit diagram of an L-type filter based on the actual equivalent circuits of a capacitor and an inductor.
The capacitor includes an equivalent series resistance (ESR) and an equivalent series inductance (ESL) in addition to a capacitance (C), whereas the inductor includes a DC resistance (DCR) and a parasitic capacitance (Cp) in addition to an inductance (L).

Therefore, simulation tools are often used to select LC filter components.
Simulation tools can usually calculate the accurate attenuation amount for each frequency using the S parameter and SPICE model provided by the component product number.

Example of Component Selection using a Simulation Tool

We will introduce an example of using the "Industrial & Automotive use LC filter simulator" available on the Panasonic website to select components for an LC filter intended to prevent the leakage of radio noise from an automotive ECU.

There is radio noise in the AM band (around 1 MHz) and FM band (around 80 MHz). Components that satisfy the attenuation amount criteria of -60 dB or more in these two frequency bands will be selected.
Please note that the prerequisite input/output impedance is 50 Ω.

• Target frequencies: 1 MHz, 80 MHz
• Target attenuation amount: -60 dB
• Input/output impedance: 50 Ω

1) Select the circuit

Select an L-type, π type, or T-type circuit
In this example, the π type is selected, and the input/output impedance is specified as 50 Ω.

2) Select the component

Select a capacitor part number and an inductor part number out of the registered candidate part numbers.
In this example, simulations were conducted with the following two conditions: (1) 100 μF capacitors and a 10 μH inductor; and (2) 10 μF capacitors and a 1 μH inductor.