High frequency circuit that can oscillate in the low frequency ~5MHZ range

Function of the circuit

The emitter-coupled multivibrator can be used as a voltage-controlled oscillator with a large range of variation. By controlling the emitter current, FM modulation can be directly achieved. By changing the capacitor C2, the frequency range can be from a few hertz to 5MHZ. In addition, when this circuit is used as a VCO, a control voltage of 0~5V can be used to obtain a variation range of about 15 times more.

How the Circuit Works

TT2 and TT3 are emitter-coupled multi-stage oscillator circuits. A coupling resistor is connected between the emitter and the ground. Like this circuit, a constant current circuit composed of TT4 and TT5 is added to control the current with the base voltage. Diodes D2 and D3 are ground clamping diodes that can increase the switching speed.

Using the parameters in the figure, when the bias voltage is 5V, the oscillation frequency can be calculated by the following formula, that is, FO≈3.3*103/C2. The temperature coefficient is positive for ambient temperature changes. This is because the VBE temperature coefficient of the transistor is negative, so the emitter current is positive. For this reason, the PNP transistor TT1 is used as a compensation circuit to compensate for the VBE of the diode D1 between the base and the collector. At the same time, TT1 also completes the VBE level shift of TT4 and TT5.

The collector of TT3 outputs an oscillating signal with an amplitude of +VCC-0.6V. The capacitor C3 is used to block the DC and then outputs it through the emitter follower.

Component Selection

Although the stability of the oscillation frequency is not very good, the temperature coefficient of the circuit itself is positive, so the capacitor C2 can be a polyester film capacitor with a positive coefficient. The choice of transistor is related to the frequency, and small signal transistors can basically meet the requirements.

Adjustment

When a stable voltage is input from the bias input terminal and the voltage range is 0~5V, the oscillation frequency range should be 10~150KHZ according to the parameters of this circuit. Because the bias voltage that makes the oscillation frequency FO=100K can be set or the stable power supply voltage can be used to connect VR1 as shown by the dotted line in the figure to adjust the oscillation frequency.

When used as a VCO in a PLL circuit, remove R1 and D1, increase the input resistance, and connect the comparator output to the bias input.

Photo A shows the FM spectrum when the bias voltage is 3.3V (FO=100KHZ) and a 1KHZ, -20DBM modulation signal is input from the modulation input terminal. Because direct modulation is used, a large frequency deviation can be obtained.

Application Notes

The parameters of this circuit are designed based on an oscillation frequency of 100KHZ, but the oscillation frequency of the emitter-coupled multivibrator can be above 10MHZ. In order to work at a higher frequency, the emitter current of transistors TT2 and TT3 can be increased, the resistance of R4 and R5 is below hundreds of ohms, and R6 and R7 are less than 100 ohms. Of course, the capacity of C2, which determines the oscillation frequency, should also be small.