The test beeper
generates a sinusoidal signal with a frequency of 1,000 Hz, a common
test frequency for audio amplifiers. It consists of a classical
Wien-Bridge oscillator (also known as a Wien-Robinson oscillator). The
network that determines the frequency consists here of a series
connection of a resistor and capacitor (R1/C1) and a parallel connection
(R2/C2), where the values of the resistors and capacitors are equal to
each other. This network behaves, at the oscillator frequency (1 kHz in
this case), as two pure resistors. The opamp (IC1) ensures that the
attenuation of the network (3 times) is compensated for.
In principle a gain of 3 times should have been sufficient to
sustain the oscillation, but that is in theory. Because of tolerances in
the values, the amplification needs to be (automatically) adjusted.
Instead of an intelligent amplitude controller we chose for a somewhat
simpler solution. With P1, R3 and R4 you can adjust the gain to the
point that oscillation takes place. The range of P1 (±10%) is large
enough the cover the tolerance range. To sustain the oscillation, a gain
of slightly more than 3 times is required, which would, however, cause
the amplifier to clip (the ‘round-trip’ signal becomes increasingly
larger, after all).
To prevent this from happening, a resistor in series with two
anti-parallel diodes (D1 and D2) are connected in parallel with the
feedback (P1 and R3). If the voltage increases to the point that the
threshold voltage of the diodes is exceeded, then these will slowly
start to conduct. The consequence of this is that the total resistance
of the feedback is reduced and with that also the amplitude of the
signal. So D1 and D2 provide a stabilizing function. The distortion of
this simple oscillator, after adjustment of P1 and an output voltage of
100 mV (P2 to maximum) is around 0,1%. You can adjust the amplitude of
the output signal with P2 as required for the application. The circuit
is powered from a 9-V battery. Because of the low current consumption of
only 2 mA the circuit will provide many hours of service.
Author: Ton Giesberts – Copyright: Elektor Electronics 2007