Capacitor three-point lc oscillator _ capacitor three-point LC oscillator experimental guidance

1. The knowledge points that should be possessed when doing this experiment:

1) Three-point LC oscillator 

2) Sile and Clap circuit

3) Influence of power supply voltage, coupling capacitance, feedback coefficient and equivalent Q value on oscillator operation

2. The instrument used in this experiment:

1) LC oscillator module

2) Double trace oscilloscope

3) Multimeter

1. Familiar with electronic components and high frequency electronic circuit experiment system;

2, master the basic principle of the capacitor three-point LC oscillator circuit, familiar with its various components;

3. Familiar with the effects of static working point, coupling capacitance, feedback coefficient and equivalent Q value on oscillator oscillation amplitude and frequency;

4. Familiar with the effect of load changes on the oscillation amplitude of the oscillator.

1 Overview

The lc oscillator is essentially a positive feedback amplifier that satisfies the oscillation conditions. The lc oscillator means that the oscillating circuit is composed of lc elements. It can be known from the AC equivalent circuit that three terminals are connected by the lc oscillation circuit, respectively connected to the three electrodes of the oscillation tube, and constitute a feedback type self-excited oscillator, which is also called a three-point oscillator. If the feedback voltage is taken from a voltage divider inductor, it is called an inductive feedback lc oscillator or an inductive three-point oscillator; if the feedback voltage is taken from a voltage divider capacitor, it is called a capacitive feedback lc oscillator or a capacitor three-point oscillator.

In several basic high-frequency oscillation loops, the capacitive feedback LC oscillator has better oscillation waveform and stability, and the circuit form is simple, suitable for working in a higher frequency band, especially the feedback branch formed by the distributed capacitance between the transistors. The oscillation frequency can be as high as several hundred MHZ to GHZ.

2, the starting condition of the lc oscillator

Whether an oscillator can oscillate depends mainly on two basic conditions of self-oscillation of the oscillation circuit, namely: amplitude start-up balance condition and phase balance condition.

3, LC oscillator frequency stability

The frequency stability indicates: the relative change degree of the oscillation frequency within a certain time or a certain range of temperature, voltage, etc., commonly expressed by Δf0/f0 (f0 is the selected test frequency; Δf0 is the frequency error of the oscillation frequency, Δf0=f02-f01; f02 and f01 are f0 at different times, and the smaller the relative change in frequency, the higher the stability of the oscillation frequency. Since the components of the oscillating circuit are the main factors determining the frequency, in order to improve the frequency stability, it is necessary to improve the standard of the oscillating circuit. In addition to the high stability and high Q circuit capacitance and inductance, the oscillating tube can be partially used. Access to reduce the influence of the inter-electrode capacitance and distributed capacitance on the oscillating circuit, and the temperature compensation can also be realized by the negative temperature coefficient component.

4, LC oscillator adjustment and parameter selection

Taking the improved capacitor three-point oscillating circuit (Siller circuit) as an example, the AC equivalent circuit is shown in Figure 2-1.

(1) Adjustment of static working point

Reasonable selection of the static working point of the oscillating tube has a certain influence on the stability of the oscillator operation and the quality of the waveform. The bias circuit generally adopts a voltage dividing circuit.

When the oscillator is stable, the oscillating tube operates in a non-linear state, usually relying on the nonlinearity of the transistor itself to achieve a stable amplitude. If the selection transistor enters the saturation region to achieve the amplitude stabilization, the equivalent Q value of the oscillation circuit will be lowered, the output waveform will be deteriorated, and the frequency stability will be lowered. Therefore, the quiescent operating point is always kept away from the saturation region in the low power oscillator, close to the cutoff region.

(2) Calculation of oscillation frequency f

Where CT is the series value of C1, C2 and C3, because C1(300p)》》C3(75p), C2(1000P)》C3(75p), CT≈C3, therefore, the oscillation frequency is mainly from L, C And C3 decided.

(3) Selection of feedback coefficient F

The feedback coefficient F should not be too large or too small. The general empirical data F≈0.1～0.5, this experiment takes

5, Clappo and Sile oscillator circuit

Figure 2-2 shows a series-modified capacitor three-point oscillating circuit, a clapper oscillating circuit. Figure 2-3 shows a parallel improved capacitor three-point oscillating circuit, the Sile oscillator circuit.

6. Capacitor three-point LC oscillator experimental circuit

The experimental circuit of the capacitor three-point LC oscillator is shown in Figure 2-4. In the figure, when the 3K05 hits the "S" position (left side), it is a modified clapper oscillation circuit. When it hits the "P" position (on the right side), it is a modified Sile oscillator circuit. 3K01, 3K02, 3K03, 3K04 control loop capacitance changes. Adjusting 3W01 changes the supply voltage of the oscillator transistor. 3Q02 is an emitter follower. 3TP02 is the output measurement point, and 3TP01 is the oscillator DC voltage measurement point. 3W02 is used to change the output amplitude.

1. Observe the oscillator output waveform with an oscilloscope, measure the oscillator voltage peak-to-peak VP-P, and measure the oscillation frequency with a frequency meter.

2. Measure the amplitude-frequency characteristics of the oscillator.

3. Measure the effect of power supply voltage variation on the oscillator frequency.

1. Experimental preparation

Insert the LC oscillator module and press the switch 3K1 to turn on the power to start the experiment.

2. Measurement of amplitude-frequency characteristics of Sileer oscillation circuit

The oscilloscope is connected to 3TP02, and the frequency meter is connected to the oscillator output port 3P01. Potentiometer 3W02 is turned counterclockwise to maximize the output. Switch 3K05 is turned to the right, and the oscillation circuit is a Sile circuit. 3K01, 3K02, 3K03, and 3K04 control whether 3C06 (10P), 3C07 (50P), 3C08 (100P), and 3C09 (200P) are connected to the circuit. The switch is turned on and turned on to be off. Different combinations of four switches can control the change in capacitance. For example, 3K01 and 3K02 are dialed up, and the capacitance of the access circuit is 10P+50P=60P. The oscillation frequency and output voltage (peak-to-peak VP-P) corresponding to the capacitance were measured in accordance with the change in capacitance of Table 3-1, and the measurement results are shown in Table 2-1 (A).

Note: If the oscillator is stopped and there is no output during the switching process, adjust 3W01 to restore oscillation. 3. Measurement of amplitude-frequency characteristics of clapper oscillation circuit

Turn the switch 3K05 to the left and the oscillation circuit to convert to a clap circuit. According to the above method, the oscillation frequency and the output voltage were measured, and the measurement results are shown in Table 2-1 (B).

4, measuring the impact of power supply voltage changes on the oscillator frequency

Switch 3K05 to the left (S) and right (P) positions respectively, change the power supply voltage EC, and measure the oscillation frequency under different ECs. The measurement results are recorded in Table 3-2.

The method is as follows: the frequency meter is connected to the oscillator output 3P01, and the potentiometer 3W02 is adjusted to the end, and the selected loop capacitor is 50P. That is, 3K02 is dialed up. Use the multimeter DC voltage file to measure the 3TP01 measurement point voltage, adjust the 3W01 potentiometer according to the voltage value Ec given in Table 3-2, and measure the frequency corresponding to the voltage. In the table, Δf is the shift of the oscillation frequency when Ec is changed. When Ec=10.5V, Δf=0, then Δf=f-f10.5V.

1. According to the test data, the amplitude-frequency characteristic curves of the Sile oscillator and the Clapper oscillator are respectively plotted and compared.

2. Analyze and judge the problems that occur in the experiment.

3. Summarize the experience gained from this experiment.