AC2 Fundamentals Unit 6 – Bandpass and Bandstop Filters
Exercise 2 – BandStop 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 bandstop filters. You will verify your results with
an oscilloscope.
DISCUSSION
- The band stop filter consists of a series LC resonant network wired in series with a resistor.
Input voltage is applied across this voltage divider configuration and the output is taken
across the series LC circuit. - At resonant frequency, the output voltage is minimal since the impedance of the LC circuit is
small. This point on the response curve is the center frequency. - Center frequency of bandstop filters, series or parallel, is computed using this equation:
fc = 1/[2π(sqrt(LC))] - The series RLC bandstop filter has capacitive characteristics below resonance (Vo increases)
and inductive characteristics above resonance (Vo increases). - Bandwidth of a bandstop filter is the range of frequencies which the filter will attenuate, and
depends on the upper and lower cutoff frequencies. - Parallel LC networks connected to a series resistor create another bandstop filter
configuration. Input voltage is applied across this voltage divider and the output voltage is
taken across the resistor. - At resonance, the parallel tank network has a high reactance and the output voltage is
minimal. - The parallel bandstop filter acts inductively (Vo increases) at frequencies below resonance
and capacitively (Vo increases) at frequencies above resonance.