Features
The relationship curve (Fig. 1) between the characteristics of the output angular frequency ω0 and the input control voltage UC is represented. In the figure, the angular frequency ω0, 0 is called a free oscillating angle frequency; the slope K0 of the curve is referred to as a control sensitivity. In a communication or measuring instrument, the input control voltage is a signal (modulation signal) that wants to transmit or want to measure. People usually refer to the voltage controlled oscillator as a frequency converter to generate a frequency modulation signal. In the automatic frequency control loop and the phase-locked loop loop, the input control voltage is an error signal voltage, and the voltage controlled oscillator is a controlled component in the loop.
The type of voltage controlled oscillator has an LC voltage controlled oscillator, an RC voltage controlled oscillator and a crystal voltage controlled oscillator. The technical requirements for voltage controlled oscillators are mainly: the frequency stability is good, the control is high, the frequency range is wide, and the frequency offset and the control voltage are linearly related to the integration. The frequency stability of the crystal voltage controlled oscillator is high, but the frequency range is narrow; the frequency stability of the RC voltage controlled oscillator is low and the frequency range is wide, and the LC voltage controlled oscillator is between them.
LC voltage control type
In any LC oscillator, an LC voltage control oscillator can be formed by inserting a voltage-controlled variable electrical antibody into the oscillation circuit. Early voltage-controlled variable power resistance elements are electric resistance, which later uses variable diodes. Figure 2 is a principle circuit of a Craglet type LC voltage controlled oscillator. In the figure, t is the transistor, L is a circuit inductor, C1, C2, and CV are circuit capacitance, and CV is a capacity presentation when the vane diode is reversed; C1, C2 is usually much larger than CV. When the input control voltage UC changes, the CV changes, thereby changing the oscillation frequency. The relationship between the output frequency and the input control voltage of the voltage controlled oscillator is
C0 is a zero-reverse biased capacitor; φ is the junction voltage of the beam diode Γ is a cosmetic change index. In order to obtain linear control characteristics, various compensation measures can be taken.
RC voltage controlled oscillator often uses the RC voltage controlled multi-resonator (see the array) in a single-chip integrated circuit.
Crystal voltage control
In an oscillator with a quartz crystal, the variable diode and the quartz crystal can be formed into the crystal voltage controlled oscillator. In order to expand the frequency modulation range, the quartz crystal can be cut and take the quartz crystal of its fundamental frequency, and the exhibition network can also be used on the circuit.
in the microwave band, the reflection regulator oscillator with the reflective voltage control frequency and the magnetron oscillator of the anode voltage control frequency also belong to the nature of the voltage controlled oscillator. The application range of the voltage controlled oscillator is wide. Integration is an important development direction. The contradiction between frequency stability and frequency modulation range between quartz crystal voltage controlled oscillators is also presented to be resolved. With the development of deep-air communication, a voltage-controlled oscillator that is extremely low in internal noise levels will be required.
VCO's actual application circuit
The local oscillator circuit of the 6-12 frequency band in a color TV receiver VHF tuner is shown in the circuit. The control voltage Vc is 0.5-30 V, changing this voltage, changes the tongue capacitance of the beam tube to obtain a frequency of frequency. As can be seen from Fig. (3), this is a typical Seller oscillation circuit, and the oscillating tube is a co-collector configuration. The oscillation frequency is about 170-220 MHz, which is often referred to by changing the DC voltage to achieve frequency adjustment. Electric tuning, compared to mechanical tuning compared to it has great advantages.
Performance Index
1. Center frequency
is the intermediate value of the frequency adjustment range, that is, the maximum value of the oscillator frequency and the minimum value, The size of the center frequency depends on the structure of the oscillator and the components parameters, and also changes accordingly as the process and temperature; with the continuous development of science and technology, the central frequency of the CMOS voltage control oscillator today can now Reached 10GHz.
2. Tuning range
refers to the range of changes in the output frequency, that is, the maximum tuning frequency of the oscillator and the difference of the minimum tuning frequency
3. Tuning gain
The sensitivity of the voltage controlled oscillator means the change in the input voltage and output frequency of the unit, generally in KV, in the unit is Hz / V, In practice, the higher the sensitivity of the voltage controller, the stronger the noise response on the control line, the greater the interference output frequency, the noise performance of the voltage controlled oscillator is lowered. So you need to find a balance of gain and noise performance of VCO.
4. Output amplitude
is the peak of VCO output spectrum. By optimizing phase noise, it is necessary to increase the amplitude of the output voltage as much as possible, so that the voltage control gain
5. Tuning linearity
is the pressure control gain
6. Phase Noise
The oscillator enters the steady state, the noise interference circuit in the circuit works, which is phase noise. The unit is DBC / Hz.
7. Power consumption
in work, noise in the circuit, reducing power consumption is the main research direction of CMOS voltage control oscillator, oscillator The frequency of power consumption is closely related to the frequency of the work, the frequency of the input voltage and the output of the output. The oscillator power consumption can reach one to dozens of MW.
8. Other performance indicators
The spectral density of the output frequency, due to other influences such as noise, the output waveform is not ideal waveform, in order to achieve the ideal waveform, design circuit To suppress the existence of harmonics; power supply and common mode suppression, power supply noise has a large influence on the voltage controlled oscillator, in order to achieve better common mode suppression, the differential line or other route should be taken as the situation is designed.
actual application
The voltage controlled oscillator is often used in:
1, the signal generator.
2, the electronic music is used to make changes.
3, lock circuit.
4, the frequency synthesizer in the communication device.
Effect
Using voltage control oscillators to control frequency
High-frequency voltage controlled frequency control frequency part, It is usually used to be used as an inductance diode C and an inductance L, and the connected LC resonance circuit is used. Improve the reverse bias of the beam diode, the airproces in the diode will increase, the distance between the two conductor surfaces, the capacitance is reduced, the resonant frequency of this LC circuit will be improved. Conversely, reduce the reverse bias When the capacitance within the diode is large, the frequency will be reduced.
and the low frequency voltage controlled oscillator selects different methods according to different frequencies, for example by changing the charging rate of the capacitor to get one Voltage control of current source. See the wave type generator.
Voltage controlled crystal
A "Voltage-Controlled Crystal Oscillator, VCXO) is usually used in the following occasions: When the frequency needs to be adjusted within a small range, when the correct frequency or phase is very important for the oscillator, use different voltages as the oscillator of the control source to disperse interference within a certain frequency range. This band is not affected by too much. The typical frequency of the pressure-controlled quartz oscillator is between dozens of PPM because the quartz oscillator of the Quality Factor (or Q Factor) only produces a small amount of frequency range displacement.
When the radio frequency circuit is transmitted, there is a frequency of heat generation, and "Temperature-Compensated VCXo, TCVCXO" is widely used because of Tcvcxo. It is not subject to temperature to change its piezoelectric characteristics.