Low and high pass filters - calculate the cutoff frequencies, Electrical Engineering

At the completion of this unit, you will be able to determine the cutoff frequencies and attenuations of RC and RL low- and high-pass filters by using test circuits.


A filter is a frequency-selective circuit that permits signals of certain frequencies to pass while it rejects signals at other frequencies.

1231_Low and High Pass Filters 1.png

A low-pass filter, as its name implies, passes low frequencies but rejects high frequencies.

1982_Low and High Pass Filters 2.png

The dividing line between the passing of low frequencies and the rejecting of high frequencies is the cutoff frequency (fc), or -3 dB point. In a low-pass filter, signals lower than the cutoff frequency pass essentially unmodified. Frequencies higher than the cutoff frequency are greatly attenuated, or reduced.

1595_Low and High Pass Filters 3.png

In a high-pass filter, signals higher than the cutoff frequency pass essentially unmodified. Signals lower than the cutoff frequency is greatly attenuated, or reduced.

The cutoff frequency (fc) is the point where the output voltage (Vo) drops to 70.7% of, or 3 dB down from, the input voltage.

1668_Low and High Pass Filters 4.png

Frequency response data may be expressed in terms of output voltage but is usually expressed in decibels (dB). Decibels are units that express or measure the gain or loss (attenuation) in a circuit. The decibel can be based on the ratio of the output voltage (Vo) to the input voltage (Vi).

2117_Low and High Pass Filters 5.png

NOTE: In the type of filters studied in this volume, the output voltage (Vo) is always less than the input voltage (Vi).

1191_Low and High Pass Filters 6.png

The rate of attenuation, or loss, beyond the cutoff frequency (fc) is highly predictable. This attenuation is 6 dB per octave or 20 dB per decade. An attenuation rate of 6 dB per octave is the same rate as 20 dB per decade.


band - a range of frequencies.

dB per octave - decibels per octave (dB/octave); a 1 dB increase or decrease over a two-to-one frequency range.

dB per decade - decibels per decade (dB/decade); a 1 dB increase or decrease over a ten-to-one frequency range.

octave - a two-to-one or one-to-two ratio; a frequency factor of two. One octave is the doubling or halving of a frequency.

decade - a ten-to-one or one-to-ten ratio; a frequency factor of ten.

rolled off - gradually attenuated, or decreased. A filter attenuates when its rejected frequencies are rolled off.


F.A.C.E.T. base unit

AC 2 FUNDAMENTALS circuit board

Oscilloscope, dual trace

Generator, sine wave

Exercise 1 - Low-Pass Filters  


When you have completed this exercise, you will be able to calculate the cutoff frequencies and attenuations of RC and RL low-pass filters. You will verify your results with an oscilloscope.


  • Several ways exist for the implementation of low-pass filters, each of which consist of a voltage-divider network containing a resistor and a frequency-varying component (inductor or capacitor).
  • Output voltage from the filters is "tapped off" the voltage divider.
  • Changes in the frequency of the supply voltage cause changes in the circuit reactance, resulting in output voltage variations.
  • In RC filters, the capacitive reactance is high at low frequencies compared to the resistance, causing most of the input voltage to appear across the output capacitor.
  • Capacitive reactance decreases as the generator frequency increases, causing larger voltage drops across the R and decreasing the voltage across the output capacitor.
  • Low-pass filters are designed so that frequencies below the cut-off frequency are passed while higher frequencies are attenuated.
  • In low-pass RL filters, the inductive reactance is small at low frequencies compared to the resistance, and most of the input voltage falls across the output resistor.
  • Inductive reactance increases as the generator frequency increases; therefore, more and more voltage is dropped across the inductor and less across the output resistor.
  • Cutoff frequency is defined as the frequency where the output signal is 3 dB down, or 0.707 x Vo.
  • For RC circuits: fc = 1/2πRC
  • For RL circuits: fc = R/2πL







Posted Date: 3/5/2013 4:44:57 AM | Location : United States

Related Discussions:- Low and high pass filters - calculate the cutoff frequencies, Assignment Help, Ask Question on Low and high pass filters - calculate the cutoff frequencies, Get Answer, Expert's Help, Low and high pass filters - calculate the cutoff frequencies Discussions

Write discussion on Low and high pass filters - calculate the cutoff frequencies
Your posts are moderated
Related Questions
star delta drawing with timer

Why memory decoding is required? To attach a memory device to the microprocessor, this is necessary to decode the address sent by the microprocessor. Decoding creates the memo

Capacitors : These are electrical condensers having a system of electrical conductors and insulators. The simplest form consists of two parallel metal plates separated by a layer

what is power factor of generator at overexcitation

Flow of electrons - Electrons flow down a wire that is what current, is. Good conductors like copper and silver and aluminum have free electrons in their outer shells, and they jum

Give a short description of soft ferrites . Soft ferrites and their applications: all non- metallic compounds are consisting of ferric oxide and one or two bivalent metal oxid

De multiplexing Address Data Bus (AD 7 - AD 0 ) As it is already discussed that  lower  order address bus (A 7 - A 0 )  is multiplexed  with data bus (D 7 - D 0 ). Hence  pins

Dielectric Heating Dielectric  losses  occur in  insulating materials. These are same  as hysteresis losses  in magnetic materials. When  an alternating  electric  field  is a

Q. Why do we cascade amplifiers? Why is RC coupling the most widely used coupling between 2 stages of a cascading amplifier? The voltage or power gain or frequency response obt

Q. Explain Machine-tool process to drill or punch holes? Figure depicts an elementary system, including the input, digital processor, drill-positioning mechanism, and sampled-d