Introduction.

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 this wien bridge audio oscillator device follows.

It is powered from an external supply, and this was rather unconventionally a negative supply of either -24V or -50V. The earth was connected to the device chassis, so this had to be remembered if powering the device from a conventional negative earth power supply. The device circuitry has now been modified to work from a +12V supply (with the more usual negative earth).

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.


Wien Bridge Background Information.

Wien Bridge.

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.

 


Block Diagram.

The functional blocks of the device are shown in the diagram below.

Block Diagram.

 

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.


Physical Layout.

Front Panel.

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.

 


Voltage Regulator, Oscillator And Amplifier Circuit Diagram.

A simplified circuit diagram for the voltage regulator, oscillator and amplifier (omitting connection details etc.) is shown below.

Oscillator Circuit Diagram.

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.


Rear Oscillator Board BOM.

Capacitors

C1 500000pF +-1% 125VDC
C2 50000pF
C3 5000pF +-1%
C4 100uF 25V
C5 500000pF +-1% 125VDC
C6 ?
C7 5000pF +-1%
C8 100uF 25V
C9 220uF 16V
C10 ? electrolytic
C11 100uF 10V
C12 393 poly
C13 100uF 25V
C14 100uF 25V
C19 100uF 10V
RY1 thermistor ??

Semiconductors

VT1 OC170
VT2 AF117
VT3 NKT 213

Resistors

R1a 1k5 brown green red red
R1b none 1 16 2
R2a 1k5 brown green red red
R2b 56k green blue orange grey
R3a 1k5 brown green red red
R3b 15k brown green orange grey
R4 1k3 brown orange red red
R5 15k brown green orange red
R6 6k8 blue grey red red
R7 10k brown black orange red
R8 5k6 green blue red red
R9 1k brown black red red
R10 330R orange orange brown red
R11 56k green blue orange red
R12 12k brown red orange red
R13 4k3 yellow orange red red
R14 1k brown black red red
R15 330R orange orange brown red
R16 390R orange white brown red
R46 none 7 30 8
R47 none 11 30 12
R48 none 9 30 10

Rear Amplifier Board BOM.

Resistors

R17 4k3 yellow orange red red
R18 15k brown green orange red
R19 220R red red brown red
R20 3k orange black red red
R21 4k7 yellow violet red red
R22a 1k5 brown green red red
R22b 15k brown green orange red
R23 220R red red brown red
R24 220R red red brown red
R25 9k1 white brown red red
R26 15R brown green black red
R27 16k brown blue orange red
R28 10k brown black orange red
R29 5k1 green brown red red
R30 2k4 red yellow red red
R31 1k2 brown red red red
R32 620R blue red brown red
R33 300R orange black brown red
R34 150R brown green brown red
R36 ?? w/w
R37 100R 5% w/w

Thermistors ????

RY2

Capacitors

C15 100u 25V
C16 470u 10V (corroded)
C17a 220u 16V
C17b 220u 16V
C17c 220u 16V
C17d 220u 16V
C18 STCPMA 1.5M 100
C20 1000u 63V

Transistors

VT4 ASY 26 1F50
VT5 ASY 26 1F50
VT6 ASY 26 1F50
VT7 ASY 28 2S50
VT8 ASY 26 1F50

Zener Diodes ????

Z1
96-P-PB00-221A00

Attenuator Desription and Circuit Diagram.

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:

Front Panel Setting Effective Attenuation 4dB Attenuator 10dB Attenuator First 20dB Attenuator Second 20dB Attenuator
    Wafer 1 Wafer 2 Wafer 4 Wafer 5
+10 0 out out out out
0 -10 out in out out
-4 -14 in in out out
-10 -20 out out out in
-20 -30 out in out in
-30 -40 out out in in
-40 -50 out in in in

Attenuator Circuit Diagram.


Ideas for Improvement.

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.

  • Add an internal mains and\or battery-based power supply.
  • Replace R36, R37 and Z1 with a voltage regulator.
  • Change all PNP transistors to modern NPN equivalents (and VT7 to a modern PNP), and reverse all polarity-sensitive components so that the device is powered by a positive supply rail.
  • Add a 75 ohm output (and make the output transformer switched)
  • Add a 50 ohm output
  • Add switched internal dummy loads of 50ohm, 75ohm and 600ohm.
  • Determine how to measure and assess linearity, and if needed improve.
  • Make the attenuator an external unit, add ranges.
  • Add a very low impedance output with short-circuit protection.
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