Nicolas Farah (3349738)- Foundation Engineering at University of Southampton

Introduction: The aim of this experiment is to build an RC high-pass filter circuit and to measure its frequency response over a range of frequencies around the cut-off frequency.

Very often, small AC signals of interest lying in one frequency range are obscured by additional, unwanted noise signals which lie in another frequency range. If such a mix of signals is connected to the input of a suitably designed AC filter circuit, then the output of the filter will enhance the wanted signals relative to the unwanted signals, thereby usefully enhancing the signal-to-noise ratio. As illustrated by the frequency response curves in Figure 4, there are three types of AC filter circuits: high-pass filters which pass high frequencies and reject low frequencies; low pass filters which pass low frequencies and reject high frequencies; band-pass filters which pass a range of frequencies and reject both higher and lower frequencies. The simplest form for a high-pass or low-pass filter consists of a capacitor in series with a resistor as shown in Figures 1 and 2. The signal to be filtered is applied to the 'input' terminals on the left and the filtered signal appears across the 'output' terminals on the right.

Figure 1 - high pass filter

Figure 1 - high pass filter

Figure 2 - low pass filter

Figure 2 - low pass filter

Apparatus:

  1. A plug-in electronics breadboard for constructing all electrical and electronic circuits. The electronics ‘breadboard’ allows quite complex circuits to be connected without the need to make solder joints.
  2. Hand-held digital multi-meters for routine measurements of potential difference, current and resistance.
  3. A power supply with fixed and variable voltage outputs.
  4. Resistors and cables.
  5. A signal generator and an oscilloscope.

Method:

  1. We will construct the high-pass filter circuit as shown in Figure 1, with R=680Ω and C=220nF. We will measure the values of these components with the hand-held lab instruments before assembling the circuit, to be sure they are the correct value.
  2. We will then connect the sine output of the signal generator to the circuit.
  3. Then, we will connect one input of the oscilloscope to the input of the filter (i.e. the voltage source), and the other oscilloscope input to the filter output. Due care must be taken so that the earth terminals of the oscilloscope's probes are connected to the same point.
  4. Then, adjust the amplitude of the signal generator so that the filter input voltage is 0.8V (pk-pk).
  5. Now, we will measure the amplitude and phase of $V_{out}$ relative to $V_{in}$ at 100, 300, 600, 1000, 2000, 5000 and 9000Hz. We will record the data in a table.
  6. Plot the amplitude and phase response to the filter (see Figure 3).