Crystal parallel resonant oscillator composed of gate circuit

Crystal parallel resonant oscillator composed of gate circuit

Under the action of an external voltage, the quartz crystal will produce a piezoelectric effect, and the quartz crystal will produce mechanical vibration. When the frequency of the external voltage is the same as the natural oscillation frequency of the crystal, the mechanical amplitude of the crystal will be the largest, and the alternating electric field generated will be the largest. , forming piezoelectric resonance.
It can be seen from the reactance frequency characteristics of the quartz crystal that it has two very close resonance rates, a series resonance frequency and a parallel resonance frequency. When the quartz crystal is in series resonance, the reactance is minimum, when it is in parallel resonance, the reactance is maximum, and when it is in parallel resonance, the reactance is the smallest. Between these two frequency ranges, the quartz crystal is inductive. When it is outside these two frequencies, the quartz crystal is capacitive.

Figure A is a TTL gate oscillator working in a series resonance state (taken from the example attached to Protel99SE). When the circuit frequency is the series resonance frequency, the equivalent reactance of the crystal is close to zero (series resonance occurs), and the series resonance frequency signal is most likely to pass through. N1, N2 closed loop, this frequency signal forms a feedback oscillation after two stages of inversion, and the crystal also plays the role of frequency selection. That is to say, in an oscillation circuit working in a series resonance state, its frequency depends on the frequency parameters of the crystal itself.

Figure B is a CMOS gate oscillator working in a parallel resonance state. The crystal is equivalent to an inductor (when the crystal works between the series resonance frequency and the parallel resonance frequency, the crystal is inductive) and an external capacitor form a three-point LC oscillator. The frequency can be fine-tuned through an external capacitor.
The resistor R is connected to the input and output terminals of the inverter N3, and its purpose is to bias N3 in the linear amplification area to form an amplifier.
Looking at C1 and C2 from both ends of crystal It forms a parallel resonance circuit (for convenience, I simply equate the crystal to inductance here). From the circuit shape of the capacitor split into two, the crystal and capacitors C1 and C2 also form a π-type frequency selection network feedback channel (also called π-type resonant circuit, see Figures B2 and 3).
The output signal of the N3 amplifier returns to the input end of the N3 amplifier through the π-type resonant circuit composed of determined by the crystal).
It is also because the output end of N3 is connected to the X, C1, C2π type resonant circuit, and the output signal is similar to a sine wave. In order to prevent the load circuit from interfering with the oscillation circuit and improve the load capacity, the N3 output signal needs to pass through the buffer of N4. The amplified shaping is connected to the load.
In a parallel resonance circuit composed of crystal , but the resistor R plays a larger role. Usually, when sufficient excitation is provided, the resistance value of R is increased as much as possible or a resistor is inserted in series between the N3 output terminal and the frequency selection network (i.e. between BC), from C2 The impedance also increases. Generally, the value of resistor R is 1M ~ 30M.
In addition, attention should also be paid to the connection between C1 and C2. The connection wire is thick and short, which can not only reduce the loss, but also prevent interference sources from being mixed in and interfere with the normal operation of the oscillator.
The frequency marked on the crystal shell is neither the series resonant frequency nor the parallel resonant frequency, but the frequency measured when an external load capacitor is connected. The value ranges between the series resonant frequency and the parallel resonant frequency.
This means that when we apply the crystal, the value of the load capacitance (Cx) is directly provided by the manufacturer, and we do not need to calculate it.
In practical applications with low requirements, for the convenience of design, we can generally split the load capacitance Cx into 1:1, that is, C1 = C2 (see the formula above). In the case of higher requirements, this convenience is obviously Unreasonable. First, C1 should subtract the input average capacitance of the gate circuit and the discrete capacitance generated by various factors (estimate). Similarly, C2 should also subtract the discrete capacitance generated by various factors (estimate). However, due to There are deviations in the discreteness and estimation of components, and the frequency is still not very accurate. We can appropriately reduce the value of C1 or C2 and add a trimming capacitor to adjust it.
To obtain a more accurate frequency, in addition to selecting capacitors with low loss and good characteristics, the temperature coefficient of the PCB layout and each component is also very important.
The above is my understanding and some tips. If there is anything wrong, please correct me.