Design and fabrication of Hartley type LC oscillator circuit

In high-frequency circuits, oscillation circuits composed of L and C are often used. Here, the working principle of the LC oscillator is explained. The first one introduced is the oscillation circuit called Hartley type. Its oscillation frequency is 10M~20MHz.

Figure 2 shows the block diagram of the oscillator produced this time. Tr1 is a transistor for oscillation, and Tr2 is a buffer. The buffer circuit is mainly between the oscillation circuit and the load, so that the working principle of the oscillation circuit is not affected by the load. Figure 3 shows the circuit diagram of the oscillator produced. In order to make the oscillation frequency variable, a variable capacitance diode (varicap) is used. The buffer is an emitter follower amplifier with high input and low output impedance.

Schematic diagram of Hartley oscillator circuit 4 shows the schematic diagram of Hartley oscillator circuit. This is composed of an amplifier circuit composed of transistors and a feedback circuit composed of LC. As shown in the figure, the Hartley oscillator circuit divides the coil into L1 and L2 to meet the oscillation conditions.

When the mutual inductance between L1 and L2 in Figure 4 is M, the synthesized inductance L becomes L= L1+ L2+2M. In this way, the oscillation frequency f is determined by the oscillation frequency. Here, to meet the oscillation condition, the phase of the feedback signal must be consistent with the phase of the signal. Assume that the voltage generated by the synthesized inductance L is e, the left coil of the middle junction E is L1, and the right coil is L2. At this time, although the voltages generated by L1 and L2 are in the same direction, if the voltages of L1 and L2 are taken into account with point E as the reference, the voltage generated by L1 becomes anti-phase relative to the generated voltage e. Therefore, with point E as the reference, the voltage Vbe and Vce are anti-phase, that is, the phase difference is 180°. Vbe is the input signal of the transistor amplifier, and the phase difference with the output signal Vce is 180°. As a result, the total phase difference is 360°, making the feedback signal in phase and meeting the conditions for oscillation.

Determination of Oscillation Frequency Since the designed oscillation frequency is 10M~20MHz, the oscillation coil L is one of the HAM Band coils (FCZ Research Institute) shown in Figure 5.

The data of the coil type 10S used in the amateur radio band of the FCZ Research Institute can be calculated. It can also be replaced with a 0.7S (7mm square) type. Here, FCZ21-10S is used. The inductance L between the terminals ① to ③ of this coil is 1.45 μH. The electrostatic capacitance connected in parallel is the variable capacitance diode (varicap) 1SV149 used for AM electronic tuning, and its electrostatic capacitance value will change with the voltage added. Here, lSV100 with the same characteristics can also be used. The characteristics of the variable capacitance diode lSV149 are shown in Figure 6. From the voltage-capacitance characteristic (VR to C characteristic), it can be known that the capacitance changes by 500pF~20pF when the reverse voltage 1~9V is added. Therefore, in the LC oscillation circuit, as shown in Figure 7, the variable capacitance diode is connected in series with the 680pF capacitor Cs. When the reverse voltage VR applied to the variable capacitance diode is 2 V, its capacitance is 300pF, and the combined capacitance becomes 280pF, so the resonant frequency fmin becomes

Next, as shown in Figure (b), a reverse voltage VR = 9V is applied to the variable capacitance diode, and its combined capacitance becomes 19.4pF.

Therefore, the variable range of the oscillation frequency is 9.16MHz~30.0MHz.

1SV149 varactor diode maximum limit parameters (Ta=25℃)

[From Toshiba product manual] (This is in an AM electronic tuner, and its capacity variation ratio C1V/C8V is close to 20).

Figure 7 Method for calculating the range of circuit oscillation frequency