Simple Audio Filter Analyzer Circuit

Posted on Feb 7, 2014

When this circuit is connected to a filter and an oscilloscope, the scope displays the filter`s frequency response. A frequency that sweeps from low to high is applied to a filter. An oscilloscope is triggered by* the start of the sweep and ends its trace at the highest frequency of the sweep. The filter output goes to the vertical amplifier of th

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e oscilloscope. Using bandpass filters as an example, as the bandpass frequency is approached, reached, and passed, the scope follows the peaking output and draws the response curve. A neat effect! The 566 VCO (Ul) produces a VLF triangle wave to frequency modulate the next stage. It also produces a square wave to externally trigger the scope. Op amp U2 (a 741 unit) optimizes the amplitude and the dc component. Another VCO (U3) produces the actual sweeping triangle wave. Its frequency is selectable via SI. Op amp U4 (another 741 op amp) is set up as a bandpass filter and has been included as an example filter. Finally, diode D1 chops off the bottom half of the output, and leaves a nice bell curve. lb set up and operate, power-up the circuit and scope. Set the scope`s TIME/CM to 50 ms/cm. Set the VOLTS/CM control to 2 V. Attach a probe from the circuit`s trigger to the scope`s external trigger input. Set the triggering mode to normal, external. Attach a probe from the vertical amplifier to TP1. You`ll see a diagonal line that runs across the CRT. Input coupling should be set to dc. Adjust the triggering level until the diagonal runs from the upper left to the lower right of the CRT to ensure a displayed sweep from low to high. Now, disconnect the probe from TP1 and attach it to the filter output past the diode.

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