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ENCYCLOPEDIA OF RADIO ELECTRONICS AND ELECTRICAL ENGINEERING
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Highly stable LC oscillator. Encyclopedia of radio electronics and electrical engineering

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Encyclopedia of radio electronics and electrical engineering / radio reception

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In transceiver equipment, oscillators based on a capacitive three-point are often used as master oscillators. A schematic diagram of such a generator is shown in Fig. 1.

High Stability LC Oscillator
Fig. 1

Like most other self-oscillators, the capacitive three-point contains a relatively large number of reactive elements (L1, C1, C2, C3 and C4), which not only affect the frequency of the generated oscillations, but also determine the conditions for the occurrence, and most importantly, the maintenance of a self-oscillatory process in the generator. For this reason, the implementation of a capacitive three-point, which provides the required frequency overlap, by experimental selection of the element values ​​is practically impossible.

In this regard, simple calculation methods are needed that are suitable for the entire family of LC oscillators based on a capacitive three-point. Earlier, in [1], general considerations were given on the methodology for calculating such circuits. As the author's experiments with various "three-point" generators have shown, the same calculated ratios can be used for all their varieties.

The scheme of the LC-oscillator with a capacitive three-point for a frequency of about 10 MHz is shown in fig. 2. If you need a generator operating at a frequency that is N times less, all the ratings of the frequency-setting elements (L1, C1 ... C6, C10) are increased by N times. Accordingly, vice versa. All other elements of the circuit have the same values ​​for frequencies from 1 to 50 MHz.

The cutoff frequency of the current transfer of all transistors used in the circuit should be 5 (and preferably 10) times higher than the generated frequency. Of course, the KT315A transistor used in the circuit is not the best option. To obtain stable generation (especially when using a relatively low-frequency transistor), it may be necessary to satisfy the condition

С5/С6=1,2...1,5 (1)

Required change in capacitance KPI (From C1min up to C1Max) necessary to obtain the desired frequency overlap (from fMax to fmin), is calculated by the formulas:

С1min = 1/(4*Pi2*L*fMax2) - 2,25*C3: (2)

С1Max = 1/(4*Pi2*L*fmin2) - 2,25*C3: (2)

at С2=С2Max/2 (in practice, this implies that the trimmer capacitor is in the middle position).

In formulas (2) and (3), the corresponding quantities are expressed in farads, henries and hertz. If the calculation results in too small values ​​of C1min and S1Max, or generally negative values, you can "borrow" a certain amount of capacitance (Cx) from the value of C3 and then add it to the value of C1. In this case we will have:

C3' = C3 - Cx, C1'min(C1'Max) = C1min(C1Max) + Cx. (4)

Example. Calculate the generator for fmin=14000 kHz, fMax=14350 kHz. In this case, for fmin the frequency increase factor is obtained (relative to 10 MHz)

Kf= 14000/10000= 1,4

Then

C2Max\u30d 1,4 / 22 \uXNUMXd XNUMX (pF);

C3 \u60d 1,4 / 43 \uXNUMXd XNUMX (pF);

C4 (C10) \u110d 1,4 / 75 \uXNUMXd XNUMX (pF);

C5 (C6) \u235d 1,4 / 160 \uXNUMXd XNUMX (pF);

L1 = 1,5/1,4 = 1,1 (µH).

Next, using formulas (2) and (3), we determine

С1min =1/(39,44*1,1*10-6*(14,35*106)2)-2,25*43*10-12= 1,12 * 10-10-9,67 * 10-11 = 1,53-10-11 (F)=15,3(pF);

C1Max=1/(39,44*1,1*10-6*(14,0*106)2)-2,25*43*10-12= 1,18 * 10-10-9,67 * 10-11 = 2,13 * 10-11 (F)=21,3 (pF);

When restructuring the calculated generator, the engine of the tuning capacitor C2 should be in the middle position (C2 \u2d CXNUMXMax/2). In practice, some adjustment of the loop capacitance, carried out with C2, may be required.

In transceiver equipment, oscillators based on a capacitive three-point are often used as master oscillators. A schematic diagram of such a generator is shown in Fig. 1. Like most other self-oscillators, a capacitive three-point contains a relatively large number of reactive elements (L1, C1, C2, C3 and C4), not only affecting the frequency of the generated oscillations, but also determining the conditions for the occurrence, and most importantly, maintaining the self-oscillatory process in the generator . For this reason, the implementation of a capacitive three-point, which provides the required frequency overlap, by experimental selection of the element values ​​is practically impossible.

In this regard, simple calculation methods are needed that are suitable for the entire family of LC oscillators based on a capacitive three-point. Earlier, in [1], general considerations were given on the methodology for calculating such circuits. As the author's experiments with various "three-point" generators have shown, the same calculated ratios can be used for all their varieties.

The scheme of the LC-oscillator with a capacitive three-point for a frequency of about 10 MHz is shown in fig. 2. If you need a generator operating at a frequency that is N times less, all the ratings of the frequency-setting elements (L1, C1 ... C6, C10) are increased by N times. Accordingly, vice versa. All other elements of the circuit have the same values ​​for frequencies from 1 to 50 MHz.

High Stability LC Oscillator
Rice. 2 (click to enlarge)

The cutoff frequency of the current transfer of all transistors used in the circuit should be 5 (and preferably 10) times higher than the generated frequency. Of course, the KT315A transistor used in the circuit is not the best option. To obtain stable generation (especially when using a relatively low-frequency transistor), it may be necessary to satisfy the condition

С5/С6=1,2...1,5 (1)

Required change in capacitance KPI (From C1min up to C1Max) necessary to obtain the desired frequency overlap (from fMax to fmin), is calculated by the formulas:

С1min = 1/(4*Pi2*L*fMax2) - 2,25*C3: (2)

С1Max = 1/(4*Pi2*L*fmin2) - 2,25*C3: (2)

at С2=С2Max/2 (in practice, this implies that the trimmer capacitor is in the middle position).

In formulas (2) and (3), the corresponding quantities are expressed in farads, henries and hertz. If the calculation results in too small values ​​of C1min and S1Max, or generally negative values, you can "borrow" a certain amount of capacitance (Cx) from the value of C3 and then add it to the value of C1. In this case we will have:

C3' = C3 - Cx, C1'min(C1'Max) = C1min(C1Max) + Cx. (4)

Example. Calculate the generator for fmin=14000 kHz, fMax=14350 kHz. In this case, for fmin the frequency increase factor is obtained (relative to 10 MHz)

Kf= 14000/10000= 1,4

Then

C2Max\u30d 1,4 / 22 \uXNUMXd XNUMX (pF);

C3 \u60d 1,4 / 43 \uXNUMXd XNUMX (pF);

C4 (C10) \u110d 1,4 / 75 \uXNUMXd XNUMX (pF);

C5 (C6) \u235d 1,4 / 160 \uXNUMXd XNUMX (pF);

L1 = 1,5/1,4 = 1,1 (µH).

Next, using formulas (2) and (3), we determine

С1min =1/(39,44*1,1*10-6*(14,35*106)2)-2,25*43*10-12= 1,12 * 10-10-9,67 * 10-11 = 1,53-10-11 (F)=15,3(pF);

C1Max=1/(39,44*1,1*10-6*(14,0*106)2)-2,25*43*10-12= 1,18 * 10-10-9,67 * 10-11 = 2,13 * 10-11 (F)=21,3 (pF);

When restructuring the calculated generator, the engine of the tuning capacitor C2 should be in the middle position (C2 \u2d CXNUMXMax/2). In practice, some adjustment of the loop capacitance, carried out with C2, may be required.

Literature

  1. Red E. Reference manual for high-frequency circuitry. - M.: Mir, 1990.

Author: V.Fhntvtyrj, UT5UDJ, Kiev

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