AC2 Fundamentals Unit 6 – Bandpass and Bandstop Filters
Exercise 1 – BandPass Filters
EXERCISE OBJECTIVE
When you have completed this exercise, you will be able to calculate and measure the center
frequency and bandwidth of series and parallel bandpass filters. You will verify your results with
an oscilloscope.
DISCUSSION
- The bandpass filter consists of a series LC resonant network wired in series with an output
resistor. Input voltage is applied across this voltage divider configuration. - Inductive and capacitive reactance cancel one another at the resonant frequency resulting in a
circuit with resistive characteristics. Under these conditions circuit current is maximum. - At resonant frequency the output voltage is maximum. This point on the response curve is
called the bandpass filter center frequency. - Center frequency of bandpass filters, series or parallel, is computed using this equation:
fc = 1/[2π(sqrt(LC))] - The series RLC band pass filter has capacitive characteristics below resonance (Vo decreases)
and inductive characteristics above resonance (Vo increases). - Bandwidth of a bandpass filter is the range of frequencies the filter will pass, and depends on
the upper and lower cutoff frequencies. - Parallel LC networks connected to a series resistor create another bandpass filter
configuration. Input voltage is applied across this voltage divider and the output voltage is
taken across the parallel LC tank circuit. - At resonance, the parallel tank network has a high impedance and the output voltage is
maximum. - The parallel bandpass filter acts inductively (Vo decreases) at frequencies below resonance
and capacitively (Vo increases) at frequencies above resonance.