Low cost and low distortion controlled oscillator circuit

Function generators are often designed, tested, and used with encoders (encoders that encode or convert signals (such as bitstreams) or data into devices that can be used to communicate, transmit, and store signals. The displacement or linear displacement is converted into an electrical signal, the former becomes the code wheel, and the latter is called the code wheel. According to the readout mode, the encoder can be divided into two types: contact type and non-contact type. Contact type uses brush output, and one brush contacts conductive a region or an insulating region to indicate whether the state of the code is "1" or "0"; the non-contact receiving sensitive component is a photosensitive component or a magnetic sensing component, and when the photosensitive component is used, the code is represented by a light transmitting region and an opaque region. Whether the status is "1" or "0", the collected physical signals are converted into machine-readable electrical signals for communication, transmission, and storage by binary encoding of "1" and "0".) The process of demodulators and measuring instruments plays a crucial role. This article describes how to construct a sinusoidal oscillator with little distortion and controlled by the bus in a low-cost way.
The circuit produces a sinusoidal output with typical second and third harmonics in the full output range of 10 Hz to 10 kHz, which are -76.1 dB and -74.2 dB lower than the fundamental signal, respectively. The performance of this sine wave oscillator is 40 dB higher than that of a conventional diode-shaped sinusoidal generator that converts a square wave into a sine wave using diode shaping. Typically, the second or third harmonic of a diode-shaped sinusoidal generator is -35dB and -25.5dB lower than the fundamental frequency, respectively.
This circuit consists of four parts (as shown). The core of the design, the first part is the oscillator, including the dual filter building block IC (U1), the second-order clock filter (the bandpass filter section sets the frequency of the oscillator), and the comparator (U2A). The bandpass filter determines the frequency of the oscillator by allowing only signals near the center frequency of the oscillator to pass. Equation 1 gives the frequency of the oscillator, and Equation 2 shows the Q value of the filter.

The second circuit portion is a tracking notch filter that is set and tracks the third harmonic of the oscillator, which is a higher amplitude harmonic. The tracking filter synchronizes with the oscillator's frequency setting filter to provide a lock-step oscillator tracking and filter response characteristics.
Equation 3 gives the tracking filter -3dB cutoff point, and Equation 4 represents the notch filter frequency.
The third part has a -1 gain buffer amplifier (U3A). This section includes a 13.3 kHz low pass filter that is used to reduce the high frequency components produced by the clock steps in the output waveform.
The fourth part is a bus control clock generator, which mainly includes an IC serial port programmable oscillator (U4), which can be the LTC6903 for Serial Peripheral Interface (SPI), or it can be used for LTC6904 with internal IC (I2C) interface. Some pull-up resistors, decoupling capacitors (the decoupling capacitors function as a battery to satisfy the change in the drive circuit current, avoiding mutual coupling interference.), and a resistor connected in series at the output is only required for several external Device.
In addition, the circuit can be easily adjusted to produce an integrated, sine/cosine waveform output. Just add a second output op amp and accept its input from the bandpass output of U1 (BPB at pin 11).