Measurement of Frequency by Wien’s Bridge:
Some bridges have balance equations which involve frequency directly even if the performance of individual bridge elements is independent of frequency. Wien’s bridge may be used for determination of frequency in terms of values of various bridge elements.
We shall describe here the measurement of Frequency by Wien’s Bridge, which is the most important, one for determination of frequency.
The Wien’s bridge is primarily known as a frequency determining bridge and is described here not only for its use as an ac bridge to measure frequency but also for its application in various other useful circuits.
A Wien’s bridge, for example, may be employed in a harmonic distortion analyzer, where it is used as a notch filter, discriminating against one specific frequency.
The Wien’s bridge also finds applications in audio and HF oscillators as the frequency determining device. The below figure shows a Wien’s bridge under balance conditions.
Wien’s Bridge Circuit Diagram:
|Wien’s Bridge Circuit Diagram|
In most Wien bridges, the components are so chosen that
R1 = R2 = R and C1 = C2 = C
Then first equation reduces to,
and second equation reduces to
Switches for resistors R1 and R2 are mechanically linked so as to fulfil the condition R1 = R2.
As long as C1 and C2 are fixed capacitors equal in value and R4 =2 R3; the Wien’s bridge may be used as a frequency determining device, balanced by a single control.This control may be directly calibrated in terms of frequency as is evident from the above equation.
Wien’s bridge is suitable for measurement of frequencies from 100 Hz to 100 kHz. It is possible to obtain an accuracy of 0.1 to 0.5 percent.
Because of its frequency sensitivity, the Wien’s bridge may be difficult to balance unless the waveform of the applied voltage is sinusoidal.Wien’s bridge is not balanced for any harmonics present in the applied voltage so that these harmonics will sometimes produce an output voltage marking the true balance point.
This difficulty can be overcome by connecting a filter in series with the null detector.A Wien’s bridge may be used for measurement of capacitance also.