Radio Transmitter

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Wednesday, April 1, 2009

Low-distortion Audio-range Oscillator

Generates very low-distortion sine waves up to 1V RMS
No thermistors required - No settling time

Producing low-distortion sine waves, this oscillator operates over the range 16 to 22000 Hz.
The circuit is based on two articles that have appeared earlier in Wireless World - Roger Rosens' "Phase -Shifting Oscillator", February 1982 pp. 38-41, and J. L. Linsley Hood's "Wien-Bridge Oscillator with low harmonic distortion" from May 1981 pp. 51-53.

This design features the simplicity of the Rosens' circuit but avoids the use of a thermistor. Instead, oscillator stability is controlled by means of a common photo-resistor driven by a LED, as suggested in the Linsley Hood article. There is no settling time when the oscillator's frequency is changed and no bouncing of the output waveform. Use of an expensive and sometimes difficult to obtain thermistor is avoided.

  • P1_____________10K Log. Potentiometer (Dual-ganged)
  • P2______________2K2 Linear Potentiometer
  • R1,R2,R4,R5_____3K3 1/4W Resistors
  • R3,R6_________820R 1/4W Resistors
  • R7_____________10K 1/2W Trimmer Cermet
  • R8_____________22K 1/4W Resistor
  • R9_____________Photo resistor (any type)
  • R10_____________8K2 1/4W Resistor
  • R11,R12,R14,R15_3K3 1/4W Resistors
  • R13_____________2K7 1/4W Resistor
  • R16--R20________3K3 1/4W Resistors
  • R21____________56K 1/4W Resistor
  • R22____________68K 1/4W Resistor
  • R23_____________1K 1/4W Resistor
  • C1,C6_________220pF 63V Polystyrene Capacitors
  • C2,C7___________8n2 63V Polyester Capacitors
  • C3,C8__________82nF 63V Polyester Capacitors
  • C4,C9_________150nF 63V Polyester Capacitors
  • C5,C10________680nF 63V Polyester Capacitors
  • D1--D4______1N4148 75V 150mA Diodes
  • D5_____________LED 5mm. Red
  • IC1,IC2_____NE5532 Low noise Dual Op-amps
  • IC3__________TL084 Quad BIFET Op-Amp
  • SW1__________2 poles 3 ways rotary switch

Technical data:

Output voltage: Sine wave, 1V RMS max.
Total harmonic distortion @ 1V RMS output:

Frequency Reading
100Hz = 0.0035%
300Hz = 0.0028%
1kHz = 0.002 %
3kHz = 0.002 %
10kHz = 0.001 %

  • Any common photo-resistor and 5mm. red LED can be used, provided they are in close contact and enclosed in a light-proof small box. I used the metal screen of a small IF transformer for AM transistor radios sealed with black insulating tape.
  • The 10K trimmer must be set to obtain a 1V RMS output.
  • The circuit must be supplied by a + and - 15V dual regulated supply. Common 7815 and 7915 regulator ICs should be used for this purpose.

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