The audio oscillator is a piece of equipment that was bought for £4 from the S. Gloucestershire Radio rally, 2009. The original intention was to remove the circuitry and use the case and front panel controls for an RF oscillator. After tracing the circuitry and determining that the instrument is of good quality, however, it has been decided to keep the device in its current working condition. It is not known what it was originally used for, or where it came from. On applying power and connecting an oscilloscope to the output, the instrument worked first time.
A little bit of information about the device: It is powered by an external supply, and this is rather unconventionally a negative supply of either -24V or -50V. This makes no practical difference, but if used with a 0V and +24V supply it must be noted that the case and chassis of this device will be at +24V and not 0V. The frequency of oscillation is from 20Hz to 20kHz in three ranges, 20Hz to 200Hz, 200Hz to 2kHz, and 2kHz to 20kHz. The output is a sine wave. The output amplitude can be switched using an attenuator with these settings: +10dB, 0dB, -4dB, -10dB, -20dB, -30dB, -40dB. The attenuator circuitry is of 75ohms impedance design, but the output from the device is isolated using a step-up transformer, so the output impedance is higher at 600 ohms.
The audio oscillator is based on a Wien Bridge design. A Wien Bridge uses two resistors of equal value, two capacitors of equal value, and a non-inverting amplifier with a theoretical voltage gain that must be exactly x3.0 in a circuit as shown.
The diagram shows two versions of the Wien Bridge, the circuit on the right is identical except for R1 and C1 being interchanged. The operation is identical. The audio oscillator described on this page uses the circuit on the right.
The gain of the amplifier stage is critical. If there is too little gain (<3.0), the oscillations quickly collapse (or never get going in the first place). If there is too much gain (>3), the oscillations grow until the amplifier starts to clip the sine wave, which introduces distortion. Simple designs rely on an amplifier with just a little too much gain, so that the oscillator starts reliably and the sine wave is only very slightly clipped once running. Better designs (such as this one) rely on some form of gain control to dynamically adjust the gain of the amplifier to exactly the correct value. This design of this instrument uses a thermistor to achieve this.
The functional blocks of the device are shown in the diagram below.
The negative supply is first regulated down by a resistor and zener (the 'Low-tech voltage Regulator'). This block supplies the various circuits. The range-switch and frequency control are used to set the frequency of the Wien-Bridge Oscillator. The output from this goes through a level adjustment potentiometer before being amplified. The amplifier block output is rectified to supply the level meter. Following the amplifier is the 75-ohm attenuator, followed by a transformer which converts the impedance to 600-ohms for the output terminals.
The instrument is housed in a steel case with an aluminium front panel. The steel casing is removed from the rear, with all components being attached to the front panel by a frame.
A backplane printed circuit board at the bottom holds two vertically mounted PCBs: the 'rear' PCB is the power regulation (zener and resistors), amplifier, and rectification circuit to show the output level; the 'front' PCB is the Wien Bridge oscillator circuit.
The meter, level adjustment potentiometer, frequency adjustment potentiometer, frequency range switch, attenuator & switch and impedance transformer are all mounted on the front panel.
A simplified circuit diagram for the voltage regulator, oscillator and amplifier (omitting connection details etc.) is shown below.
Components R37, R38 and Z1 regulate the input voltage of either -24V or -50V.
Transistors VT1, VT2 and VT3 form the non-inverting amplifier of exactly x 3 voltage gain necessary for the Wien Bridge. VT1 forms a high input-impedance inverting amplifier with a less than unity gain (so, it is in effect an attenuator). It's voltage gain is -0.375. The less than unity gain is caused by the feedback from the output of the amplifier via C9, RY1 and C13. Transistor VT2 forms an inverting amplifier with a voltage gain of -8.0. Transistor VT3 forms an emitter follower output stage of unity voltage gain. The gains of these stages combine as follows:
-0.375 x -8.0 x 1.0 = 3.0
Thermistor RY1 ensures that the combined voltage gain of the amplifier is exactly 3.0. If the gain rises, the output becomes larger and the resistance of RY1 will change to bring it back to 3. The capacitors C1-3 and C5-7, together with the range switch and frequency potentiometer form the other components of the Wien Bridge. In this design, the "C" part of the Wien Bridge circuit is switched based on the range. One C is from C5 (x10 range) ,C6 (x100 range) or C7 (x1000 range). The other C is from C1 (x10 range), C2 (x100 range) or C3 (x1000 range).
It is assumed that R1a, R2a, R2b, R3a, R3b and R4 are used to either limit the range or to ensure accurate calibration.
Transistors VT4-VT8 form a higher power amplifier circuit. The output of this amplifier is controlled by the input 5k adjust level potentiometer, together with R17, RY2 and feedback resistor R25. The output at C17 is therefore a variable frequency sine wave of constant amplitude.
Resistors R27-34 together with MR2-MR5, the panel meter, C18 and a 200pF capacitor form the level circuit. The user adjusts the level potentiometer for the correct reading on the meter.
The output at R26 is sent to the attenuator circuits.
The attenuator consists of a 4dB, 10dB and two 20dB attenuators. The attenuator components are soldered directly to the tags of the rotary 'Wafer' type switch. Each 'wafer' of the switch was numbered with 1 being nearest the front panel. The table below details each attenuator section and how it corresponds to the front panel setting:
Whilst studying this circuit, it became apparent that some improvements could be made to the device. The improvements would make the device more versatile as a piece of modern test equipment. As some of the components are no longer commercially available, the circuit could also be modernised.
