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Jeremy Crook - 1st Place (Newport, United Kingdom)
Frequency Response Sweep Oscillator
Downloads for this project:
All files and information © 2006 Jeremy Crook.
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Click on Image for a close-up
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Project Description:
I developed this project as a means of displaying the frequency response of a circuit on a standard oscilloscope. The standard way of measuring frequency response is to use a frequency generator with an oscilloscope or ac voltmeter and then plot the results on logarithmic graph paper. This is time consuming especially when dealing with voltage controlled filters which vary their frequency response with a control voltage. After a lot of trial and error with Microchip and Atmel microcontrollers I discovered the powerful capabilities of the Parallax SX family of microcontrollers. The speed and flexibility of these microcontrollers enabled me to add more features than I originally intended to the design.
There are a number of benefits to using this design:
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Click on Image for a close-up
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- As component values in the circuit under test are varied the change in frequency response is instantly observable on an oscilloscope screen. This makes it easier to show the difference in frequency response between various types of filter such as low pass, high pass, band pass and notch filters. You can even see the difference between filters with Butterworth and Chebychev responses.
- The sweep oscillator can be switched between two frequency ranges:
- 100Hz to 100K Hz. This allows for the testing of audio, ultrasonic and infra red communication circuits.
- 50 Hz to 15 KHz. Primarily for higher resolution audio testing.
- There is a facility to show a frequency marker on a second channel of the oscilloscope. A frequency counter (a digital multimeter with a frequency range is adequate) connected to the “Marker Frequency Output” will then show the frequency at that point, the marker can be moved to any point on the display. This enables the easy measurement of the -3db cut off points and Q of any filter.
- The display on the scope shows a true logarithmic response and, as such, volts per decade/octave measurement can be taken directly from the display.
- There is a facility for altering the amplitude of the output signal. This means that both passive and active (amplifying) circuits can be tested. The output of the circuit is buffered so that it won’t affect the response of the circuit under test.
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