AC Theory: Resonance and Filters

A low-pass filter

Attenuates high frequency energy but passes low frequency energy

high pass filter

Attenuates low-frequency energy but passes high-frequency energy

C is the symbol for

a capacitor

L is the symbol for

an inductor

In a series ac circuit, inductive reactance leads by 90°, compared with the zero reference angle of the resistance,

and capacitive reactance lags by 90°.

You may choose to use a series resisitance instead of a choke

because it is a cheaper and smaller filter.

The symbol for resonant frequency is

fr

fc is the symbol for

frequency cutoff

A narrower BW signifies

higher selectivity

The high-pass filter passes to the load all frequencies ...

higher than the cutoff frequency: fc.

The width of the resonant band of frequencies centered around fr

is called the bandwidth of the tuned circuit.

If dB value is negative,

it indicates a loss and is referred to as an attenuation.

If dB value is positive,

it indicates an increase in power.

A wider BW signifies

lower selectivity.

The low-pass filter passes to the load all frquencies

lower than the cutoff frequency: fc.

Output power equals to half of the

max output power at cutoff frequency

The higher the ratio of the reactance at resonance to the series resistance,

the higher the Q and the sharper the resonance effect.

At the resonant frequency,

the inductive reactance and capacitive reactance are equal.

At cutoff frequency fc,

the output voltage or current is 70.7% of its maximum value

3dB equals

½ power

attenuation

The ability of a filter to reduce the amplitude of undesired frequencies is called ....