What Is a Low Frequency Module Oscillator?
A low frequency module oscillator is a device that generates a relatively low frequency signal.
The definition of the term “low frequency” itself varies depending on the field in which it is used. However, the specifications of low frequency module oscillators sold in the market range from a few Hz to several hundred kHz. Oscillator circuits can be used to generate AC signals of specific frequencies, and are generally used to generate reference signals for testing using frequency signals.
Signals in the range of approximately 20Hz to 20kHz that can be produced by oscillators are in the audible range and can be heard by humans as sound when amplified.
Uses of Low Frequency Module Oscillators
The most common use of low frequency module oscillators is to analyze the frequency response of audio equipment. Since most electronic devices that use frequency signals for communication use radio frequency (RF), and the familiar low frequency is sound waves, acoustic devices are the most common circuits that use low-frequency signals.
For example, an audio amplifier is used as a DUT (Device under test), and the input signal is generated by a low frequency module oscillator. By analyzing the output signal with an FFT analyzer, it is possible to assess the characteristics of the amplifier under test.
Principle of Low Frequency Module Oscillators
The principle of the low frequency module oscillator lies in the technology of generating a stable frequency signal based on an oscillator circuit. Since it is difficult to realize low-frequency waveforms with the well-known Hartley or Colpitts type oscillator circuits based on LC resonant circuits due to wavelength limitations, RC (or CR)-type oscillator circuits are used.
RC-type oscillators include Wien bridge circuits, bridged T-type circuits, and state-variable-type circuits, all of which can achieve oscillation by adjusting the amount of amplifier feedback. Of these, the Wien bridge circuit is the most well-known and widely used, partly due to its simplicity of having only one amplifier in the circuit.
On the other hand, the state-variable-type provides lower distortion and two-phase output. A variable resistor or a variable capacitor called a varicap may be used to control the frequency. Step-variable with capacitors and fixed resistors is another example of a frequency determination technique.
An RC oscillation circuit alone cannot produce a sine wave with a constant amplitude. Therefore, a device to control the amplitude is usually required. There are methods such as using an element whose resistance changes with the applied voltage, such as a light bulb or thermistor, to adjust the feedback amount, or assembling an electronic circuit that detects the voltage at the output to adjust the feedback amount.
Other Information on Low Frequency Module Oscillators
1. Self-Made Electronic Circuit Kits and PC Software
Low frequency module oscillators are relatively familiar electronic circuits, and the resulting devices can be used to create buzzers for home use, for example. For this reason, they are widely used in home-built electronic circuit kits and as educational materials for students learning electronic circuits.
Free software (e.g., WaveGene) that can generate low-frequency sound sources on Windows PCs is also widely available.
2. Accuracy and Calibration of Low Frequency Module Oscillators
In general, the frequency accuracy of RC oscillators is not as good as that of crystal (quartz) or ceramic oscillators. This is due to variations in resistance and capacitance values, as well as changes in the RC value depending on temperature.
However, compared to crystal (quartz) oscillators, RC oscillators are relatively inexpensive and easy to configure. As a result, they are widely used, and there have been some innovations to improve the accuracy of RC oscillators. A typical method for improving accuracy is calibration. Calibration is a widely used method that utilizes digital circuits, and frequency values are adjusted by calibration especially before shipment.
Calibration using digital circuits is based on a method that compares the frequency with an internal clock and corrects it. For example, a certain value of a memory address is commonly used for calibration purposes. This can be done by changing the value of a register called OSCCAL in the operation of the digital circuit, and automatically selecting the register value that will result in the desired oscillation frequency.