synonymVCXO(VCXO) generally refers to voltage controlled oscillator
Voltage controlled oscillator refers tooutput frequency Oscillation circuit (VCO) corresponding to input control voltage, whose frequency isfunctionVCO, the working state of the oscillator or the component parameters of the oscillation circuit are controlled by the input control voltage, which can constitute a voltage controlled oscillator.
Chinese name
Voltage controlled oscillator
Foreign name
voltage-controlled oscillator
Field
Circuit
Substantive
electronic component
Type
LC voltage controlled oscillator, RC voltage controlled oscillator
Control characteristics of voltage controlled oscillator
Its characteristics are determined by the relationship between the output angular frequency ω 0 and the input control voltage uccurve(Figure 1).In the figure, the angular frequency ω 0,0 when uc is zero is calledFree oscillationAngular frequency;The slope K0 of the curve at ω 0,0 is called control sensitivity.In communication or measuring instruments, the input control voltage is the signal to be transmitted or measured (modulation signal).People usually call VCO frequency modulator to generate frequency modulation signal.stayAutomatic frequency controlLoop andPhase-locked loopIn the loop, the input control voltage is the error signal voltage, and the voltage controlled oscillator is a controlled component in the loop.
The types of VCOs include LC VCO, RC VCO and crystal VCO.Technical requirements for voltage controlled oscillator mainly include:Frequency stabilityGood, high control sensitivity, wide frequency modulation range, linear relationship between frequency offset and control voltage, and suitable for integration.The frequency stability of crystal VCO is high, but the frequency modulation range is narrow;RC VCO has low frequency stability and wide frequency modulation range, and LC VCO is in between.
LC voltage control type
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In any kind of LC oscillator, the LC voltage controlled oscillator can be formed by inserting the voltage controlled variable reactance element into the oscillation circuit.In the early days, the voltage controlled variable reactance element was a reactor tube. Later, most of them used varactor diodes.Figure 2 iscaratThe principle circuit of the splash type LC voltage controlled oscillator.In the figure, T is transistor, L is loop inductance, C1, C2, Cv are loopscapacitance, Cv is the capacity of varactor diode in reverse bias;C1 and C2 are usually much larger than Cv.When the input control voltage uc changes, Cv changes accordingly, thus changing the oscillation frequency.The relationship between the output frequency of this VCO and the input control voltage is
Relation between VCO output frequency and control voltage
Where, C0 is the capacitance of varactor diode under zero reverse bias;φ is the junction voltage of varactor diode;γ is the change index of junction capacitance.In order to obtain the linear control characteristics, various compensation measures can be taken.
RC voltage controlled oscillator atMonolithic integrated circuitRC voltage controlled multivibrator is commonly used in (see frequency modulator).
LC voltage controlled multivibrator
Crystal voltage control
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In usequartz crystalIn a frequency stabilized oscillator, a varactor diode is connected in series with a quartz crystal to form a crystal voltage controlled oscillator.In order to expand the frequency modulation range, the quartz crystal can be cut and taken from its base frequency by AT, and the circuit can also use the conversion network to expand the frequency modulation range.
staymicrowaveThe frequency band, the reflection governor oscillator whose frequency is controlled by the reflector voltage and the magnetron oscillator whose frequency is controlled by the anode voltage are also voltage controlled oscillators.Voltage controlled oscillators have a wide range of applications.Integration is an important development direction.The contradiction between frequency stability and frequency modulation range in quartz crystal VCO also needs to be solved.With the development of deep space communication, internalNoiseVery low level voltage controlled oscillator.
Practical application circuit of VCO
Some colortelevision receiverThe LO circuit of the 6-12 frequency bands in the VHF tuner is as shown in the figure. The control voltage VC is 0.5-30V. Changing this voltage will change the junction capacitance of the varactor to obtain the frequency change.It can be seen from Figure (3) that this is a typical Schiller oscillator circuit. The oscillator tube is in a common collector configuration, and the oscillation frequency is about 170-220MHz. This method of frequency adjustment by changing the DC voltage is usually called electrical tuning, which has great advantages over mechanical tuning.
performance index
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1. Center frequency
It refers to the middle value of the frequency adjustment range, that is, the middle value of the maximum and minimum values of the oscillator frequency. The size of the center frequency depends on the structure of the oscillator and the parameters of components, and also changes with the process and temperature;With the continuous development of science and technology and the improvement of product performance, the center frequency of CMOS voltage controlled oscillator can reach 10GHz today.
2. Tuning range
It refers to the change range of the output frequency, that is, the difference between the maximum tuning frequency and the minimum tuning frequency of the oscillator
The VCO must have a large tuning range to meet the output frequency to reach the required value.
3. Tuning gain
That is, the sensitivity of the voltage controlled oscillator refers to the change of the unit input voltage and output frequency, which is generally expressed in Kv, and the unit is Hz/V. In practical applications, the higher the sensitivity of the voltage controller, the stronger the noise response on the control line, and the larger the interference output frequency, which will reduce the noise performance of the voltage controlled oscillator.Therefore, we need to find a balance between VCO gain and noise performance.
4. Output amplitude
The peak value of VCO output spectrum.By optimizing the phase noise, it is necessary to increase the amplitude of the output voltage as much as possible, thus making the voltage control gain
Lower.Increasing the output amplitude is especially important with the continuous development of CMOS technology and the decreasing input voltage. Increasing the output amplitude is particularly important.
5. Tuning linearity
It refers to voltage control gain
The ideal VCO is a constant. In practice, the performance of the VCO is nonlinear
。Is a constant, so as to minimize the variation within the tuning range.
6. Phase noise
When the oscillator enters a stable state, the noise in the circuit interferes with the circuit operation, which is called phase noise.The unit is dBc/Hz
7. Power consumption
In operation, noise and power reduction in the circuit are the main research directions of CMOS voltage controlled oscillator. The power consumption of the oscillator is closely related to the operating frequency, input voltage and output frequency.The power consumption of the oscillator can reach one to dozens of mW.
8. Other performance indicators
The frequency spectrum density of the output frequency. Due to noise and other influences, the output waveform is not ideal. In order to achieve the ideal waveform as far as possible, the existence of harmonics should be suppressed when designing the circuit;Power supply and common mode suppression, power noise also has a greater impact on the voltage controlled oscillator. In order to achieve better common mode suppression, differential lines or other lines should be adopted as appropriate in the design.[1]
practical application
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Voltage controlled oscillators are often used in:
1. Signal generator.
2. In electronic music, it is used to create tone sandhi.
3. Phase locked loop.
4. Frequency synthesizer in communication equipment.
effect
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Use voltage controlled oscillator to control frequency
High frequency voltage controlled oscillatorVoltageControl frequency part, usually with varactordiodeLC resonant circuit formed by C and inductance L.When the reverse bias voltage of the varactor diode is increased, the empty area in the diode will increase. As the distance between the two conductor surfaces becomes longer, the capacitance will decrease, and the resonant frequency of the LC circuit will be increasedConversely, when the reverse bias voltage is reduced, the capacitance in the diode becomes larger, and the frequency will be reduced
The low-frequency voltage controlled oscillator selects different methods according to different frequencies, such as changing thecapacitanceThe charging rate is the means to obtain a voltage controlled current source.See wave pattern generator.
Voltage controlled crystal oscillator
A "voltage controlled crystal oscillator (VCXO)" is usually used in the following situations:frequencyWhen it is necessary to adjust in a small range, when the correct frequency or phase is very important for the oscillator, the oscillator uses different voltages as the control source, and the interference used to disperse in a certain frequency range will not affect the frequency band too much.Voltage controlquartzThe typical frequency variation of the oscillator is between tens of ppm, because the quartz oscillator with high quality factor (Quality Factor, or Q Factor) will only produce a small amount of frequency range displacement.
Whenradio frequencyWhen the circuit transmits electric waves, there will be heat generation and frequency drift, which makes the "temperature compensated VCXO (TCVCXO)" widely used, because TCVCXO will not be affected by temperature to change its piezoelectric characteristics.[2]
