Quartz oscillators on harmonics. Encyclopedia of radio electronics and electrical engineering

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Using the circuits of loopless quartz oscillators (CG) of the author [1, 2], it is possible to obtain generation not only at the first (fundamental) harmonic of quartz, but also at its third harmonic. At the same time, it is interesting to note that in these circuits, both quartz specially designed for generation on harmonics (the so-called harmonic ones) and ordinary ones work on the third harmonic.

However, the above circuits are far from exhausting the circuitry of circuitless overtone quartz oscillators. Another circuit from the family of such generators based on a bipolar transistor is shown in Fig. 1. Such a CH is simpler than the schemes from [1, 2].

At first glance, this circuit looks like the well-known capacitive "three-point" circuit, but it differs from the "classical" circuit. The generator lacks one of the feedback capacitors (between the base and emitter of the transistor) compared to the "classical" KG circuit.

In addition to reducing the number of elements, the circuit has other advantages. "Classic" KG generates exclusively on the first harmonic of quartz. In numerous experiments, the author has never been able to obtain generation at the third (mechanical) harmonic. The circuit shown in Fig. 1, with a sufficiently small capacitance C3 (usually several tens of picofarads), easily starts at the third harmonic of quartz. At the same time, as the capacitance C3 increases, the level of the output RF voltage of the KG gradually decreases (the frequency of the generated oscillations also slightly decreases by tens - hundreds of hertz). first harmonic. The amplitude of the generated oscillations in this case increases again.

Fig. 1

With an even greater increase in C3, a gradual decrease in the oscillation amplitude occurs, accompanied by a slight decrease in frequency, and finally, at a sufficiently large capacitance C3 (for example, several nanofarads for quartz at a frequency of 27 MHz), the KG oscillations break down. The picture of the occurring phenomena in the CG as the capacitance C3 increases is shown in fig. 2.

Fig. 2

The amplitude of the output voltage of the KG during generation at the first harmonic (for "harmonic" quartz) turns out to be greater than during generation at the third harmonic (for the same quartz). Thus, in fig. Figure 2 shows the most general case, when generation is possible for quartz both at the first and at the third mechanical harmonic.

Sometimes (very rarely) there are still quartzes that generate only at the first harmonic. In this case, in Fig. 2, only one peak (right) remains, while the left peak and the region of two-frequency oscillations disappear. To observe "jumps" in the KG frequency when capacitance C3 changes, it is necessary to connect an RF oscilloscope and a frequency meter to the KG through good buffer stages (with an input resistance of more than 10 kΩ and an input capacitance of no more than a few picofarads).

As C3, KPI (12 ... 495 pF) is used, which is included in the KG circuit either directly or through small capacitances (several tens of picofarads). The KPI is connected to the KG board with thick uninsulated wires of the minimum possible length.

However, from the point of view of practical use, the scheme shown in Fig. 3. In this case, the requirements for the buffer stage are significantly reduced. Nevertheless, even when such a KG circuit is used as part of a receiver or transceiver, a buffer stage (at least the simplest one) is still desirable. It is also necessary to stabilize the power supply of the above KG circuits. Particular attention should be paid to the resistor values ​​in the circuits (Fig. 1 and 3): they cannot be changed over a wide range. So, for the KG scheme according to Fig. 1 at a supply voltage of 9 ... 12 V, the following condition must be met:

R1=R2=20*R3;

R3 = 470...2000 ohm (1)

KG according to fig. 3 at the same supply voltage requires the following conditions:

R1 \u2d R20 \u4d 3 * R4 (at RXNUMX "RXNUMX);

R4 = 470.. 2000 Ohm, (2)

or

R1=20*R4;

R2 = 10*R4 (with R3 ~= R4);

R4 = 470...2000 Ohm;

R3 <= 1000 Ohm. (3)

Only when conditions (1), (2) or (3) are met, the CG schemes will behave as described above.

The choice of bias resistors is made using the recommendations from [3]. The output impedance of the KG (Fig. 3) is almost equal to R3.

Fig. 3

Buffer stages for such CGs can be used the same as in [2]. However, you should always remember that the buffer stage can differentiate (and in some cases integrate) the input signal, which leads to distortion of the waveform in the case of sinusoidal signals.

The above KG schemes can be recommended for wide use in amateur radio designs. Particularly successful, according to the author, is the diagram in Fig. 3, which has a 50-ohm RF output (at ratings R1=R2=10 kOhm, R3=51 Ohm).

These KG schemes are, according to the classification [5], "two-point", capable of operating both on the first and on the third harmonic of quartz. For example, quartz RK-169 in the scheme according to fig. 3 (R3=51 Ohm) generated at a frequency of 27411 kHz at C3=51 pF, and at a frequency of 9142,42 kHz at C3=330 pF, while the frequency of 27,41 MHz was indicated on the quartz case.

Now consider the generators designed by the author on the basis of the prototype - the Pierce generator, which is a generator with capacitive coupling through capacitors C2 and C4 (Fig. 4).

Fig. 4

The quartz resonator, when operating in a Pierce oscillator, has an inductive reactance, so such an oscillator operates in the frequency range between the frequency of the series fs and parallel fp of the quartz resonance. According to [4], the quartz in this generator generates at a frequency close to fp, however, in [6] it is noted that the generation frequency is closer to fs than to fp. In this regard, the division of such CGs into series and parallel resonance generators is not entirely successful due to the dependence of the generated frequency on the reactivity values ​​included in the circuit (for example, in Fig. 5 these are C2 and C4).

Fig. 5

On fig. 4 resistors R1 and R2 form a voltage divider to create the necessary bias voltage of the base of the transistor VT1. To obtain a high temperature stability of the operating point, an OOS circuit for direct current R3-C3 is used.

Capacitors C1 and C3 are blocking capacitors; with sufficient capacity, they do not affect the frequency of the KG. At the same time, capacitors C2 and C4 are directly involved in the generation of oscillations, and the frequency depends on their capacitance. The reactive (inductive) resistance of the inductor L1 is very large (much more than the reactances of the capacitors C2, C4 and quartz ZQ1), so the role of the inductor L1 in the Pierce KG circuit is reduced solely to the separation of DC and RF currents. For this reason, L1 can be replaced by some other current source (even a resistor). It should be especially noted that the use of such chokes (especially with a high quality factor Q) in some cases can lead to excitation of the generator not at all at quartz frequencies. The introduction of a throttle reduces the reliability of the CG, so it is better to abandon it if possible.

The working diagram of the CG is shown in fig. 5. Choosing the capacitances of the capacitors C2=C3 sufficiently small, we obtain generation at the third harmonic of quartz. As these capacitances increase, the pattern shown in Fig. 2, and for sufficiently large values ​​of these capacitances, we obtain generation at the first harmonic of quartz.

On transistors VT2 and VT3, a buffer stage is made, which is an emitter follower connected one after another. Resistors R3 and R7 are antiparasitic, they serve to increase the stability of the buffer stage. If we accept that С2=С3, then when the KG operates at the third harmonic, these capacitances can be determined from the expression

C2 \u3d CXNUMX \uXNUMXd L, (pF)

where L is the wavelength for the third harmonic, m.

For reliable operation at the first harmonic, these capacitances must be chosen 3, and preferably 5 times large.

On fig. Figure 6 shows a diagram of an RF attachment to a voltmeter with a high input resistance, with the help of which, and using a calibration curve, the RF voltage at the VT1 collector was determined (Fig. 5). The prefix is ​​connected to a high-resistance (RBX> 1 MΩ) voltmeter in the DC voltage measurement mode.

Fig. 6

The data obtained for one of the harmonic quartz (46,516 MHz) are presented in Table 1. As can be seen from the table, for quartz at a frequency of about 50 MHz, those capacities that the circuit board and the transistor itself have are enough. For quartz at 27 MHz, generation at the third harmonic in the absence of C2 and C3 is not observed.

The bipolar transistors (BT) used to build quartz oscillators (CG) are characterized by sufficiently large capacitances between the electrodes (Cbe, CKg, Cke), inherent in the transistor itself. We will call them the internal capacitances of the transistor. Due to the significant internal capacitances of the BT, the operation of the KG on these transistors is already determined not only by the capacitances of the capacitors, but also by the internal capacitances of the BT. Microwave field-effect transistors (FETs) with one or two insulated gates have very small internal capacitances, which are an order of magnitude (or even more) less than the internal capacitances of RF BTs. Therefore, the work of the KG on the microwave FET will be determined mainly only by the capacitances of the capacitors, as well as by the parasitic capacitances of the installation.

The proposed KG circuit on the FET (Fig. 7) is made on the basis of a source follower. Since at present the most widely used microwave FET with two insulated gates, and to compare the operation of the KG on bipolar and field-effect transistors, a single-gate FET is needed, such a FET is obtained from a double-gate FET by connecting its gates together. Given that the used microwave FETs operate in the frequency range up to a few gigahertz, they are very prone to self-excitation (printed tracks on the board "work" as a kind of microwave circuit).

Fig. 7

To eliminate self-excitation, the author used antiparasitic SMD resistors with low resistance, the value of which was selected empirically (in Fig. 7, these are R3 and R4). Such SMD resistors are soldered to the FET terminals shortened to the minimum possible length for installation. To eliminate the deviation of the KG frequency during measurements, a buffer stage of source and emitter followers connected in series is connected to it. The complete scheme of the investigated CG on the microwave FET is shown in Fig. 8. This buffer stage has much better properties than the buffer stage on the HF BT (Fig. 5).

Rice. 8 (click to enlarge)

At first glance, the CG circuits for BT and PT are the same in principle of operation (both circuits are based on broadband voltage followers), but experiments have shown that they behave differently. In the CG on the BT (Fig. 1), with a certain (small) capacitance of the capacitor in the emitter circuit of the transistor, generation occurs at the third harmonic. As the capacitance of the capacitor increases, the generation still occurs at the same harmonic of the quartz. And only with a further increase in the capacitance of the specified capacitor, the generator passes into the region of complex oscillations. The zone of complex oscillations is usually observed in a rather narrow range of changes in the capacitance of the capacitor (fractions ... units of picofarads). In the same area, there is a peak (maximum) of the output voltage. A further increase in the capacitance of the capacitor leads to generation at the first mechanical harmonic of quartz.

In a CG on a microwave FET, when using sufficiently low-frequency quartz (for example, with the first mechanical harmonic of about 9 MHz), the change of states described above is not observed at all, which can be explained in a first approximation by the very small internal capacitances of the FET.

To test this assumption with the help of a specially included capacitor (6,8 pF), indicated in Fig. 7 and 8 as Szi, the corresponding capacitance of the transistor was artificially increased, which makes the operation of the KG on BT and PT comparable. Data for KG on FET (frequency and output voltage) without a capacitor are presented in Table 2. In table. 3 shows the data for the case when an additional capacitor with a capacitance of 6,8 pF was installed. In this case, the same quartz (27668 kHz) was used, as well as resistors R1=R2=20 kOhm. After installing an additional capacitor Czi, the KG under consideration began to behave similarly to the KG on the BT.

If KG on FETs work with high-frequency quartz (for example, quartz with the first mechanical harmonic of about 15 MHz), then the internal capacitance of the FET (Czi) itself is already quite enough for normal operation of the KG. Data for CG with high-frequency quartz are presented in Table. 4 (at 46,516 MHz). In this case, R1 \u2d R20 \uXNUMXd XNUMX kOhm.

The dependence of the frequency and output voltage on the value of C3 from the table. 2 and 3 are presented graphically in fig. 9 and 10, and from the table. 4 - in fig. eleven.

Fig. 9

Fig. 10

Fig. 11

Notes:

1 At C3=20 pf there is a zone of two-frequency oscillations.

2 If R1=R2=1 MΩ, generation occurs only at a frequency of 15,52 MHz

The transistors of the generator and the buffer stage of all the considered KG circuits operate at significant levels of RF signals, and therefore introduce significant non-linear distortions. At the output of the KG, there are also electrical harmonics of the signal with a significant level. The frequency of these harmonics is an integer number of times greater than the fundamental frequency (i.e. the first harmonic). When quartz is operating, for example, at a frequency of 9 MHz, frequencies of 18, 27, 36, 45 MHz, etc. will also be present at the output of the KG. However, as a rule, these higher harmonics are an order of magnitude or weaker than the first harmonic. The mechanical harmonics of quartz are not exactly an integer number of times greater than one another. Therefore, the first and third mechanical harmonics of quartz will differ in frequency by a factor not equal to three.

Using this feature of the mechanical harmonics of quartz, one can distinguish between mechanical harmonics proper and electrical harmonics. For example, using the data from Table 1, we obtain the frequency ratio

f3/f1 = (46518,46+46518,15)*2/(2*(15516,82+15513,54))=46518,3/15515,18=2,998 (4)

The frequency of resonators based on mechanical harmonics is determined, according to [9], by the expression

fn = n(1 -Yn)*f1, (5)

where fn is the frequency of the nth mechanical harmonic of quartz, n is the number of the corresponding harmonic (in this case, an odd integer), f1 is the frequency of the first mechanical harmonic of quartz, Yn is a correction factor depending on the harmonic number. For example, Y3=0,001 [9] Thus, expression (5) for the third mechanical harmonic takes the form:

f3=3*(1-0,001)*f1, (6)

whence

f3/f1 = 3*(1 -0,001) = 2,997 (7)

Since the numerical values ​​of expressions (4) and (7) practically coincide, we can say that generation is possible in the generator both at the first and third mechanical harmonics of quartz.

The region of complex oscillations (Fig. 2) exists in all the KG circuits considered above. It can be detected by connecting an oscilloscope to the KG output. A complex picture is observed on the screen, far from the usual sinusoid. In the zone of complex oscillations, oscillations of both the first and third mechanical harmonics coexist. An increase in the capacitance of the corresponding capacitor (C3) leads to a decrease in the amplitude of the third harmonic and an increase in the amplitude of the first.

In all the considered CGs, when generating at the first mechanical harmonic, the output voltage turns out to be somewhat higher than when generating at the third. Oscillations with the frequency of the first mechanical harmonic are always "stronger" than oscillations with the frequency of the third, therefore, the output voltage of the KG increases in the region of two-frequency oscillations with an increase in the capacitance of the "control" capacitor (C3). An increase in the capacitance of the "control" capacitor outside the zone of two-frequency oscillations, on the contrary, leads to a decrease in the output voltage of the generator.

The observed differences in the operation of the CG on the BT and the FET, as well as the anomalous operation of the CG on the PT in the case of using sufficiently low-frequency quartz, are due to the difference in the values ​​of Cbe for the BT and Czi for the PT (Cbe "Czi). If we compare Cbe and Czi by connecting an additional capacitance Cdop (Cdop ~= Szi) between the gate and the source of the FET, the KG on the BT and the FET begin to behave approximately the same.Since all the KG circuits discussed above operate both on the first and third mechanical harmonics of quartz, an equivalent quartz circuit can be used for analysis, shown in Fig. 12.

Fig. 12

Using such a quartz circuit, it is possible to represent the equivalent circuit of a FET oscillator according to Fig. 13.

Fig. 13

All the considered KG schemes do not contain any oscillatory (resonant) circuits, except for the quartz itself. This greatly simplifies the manufacture and tuning of such harmonic CGs by selecting basically only the capacitance of the "control" capacitor.

Literature

1. Artemenko V. Circuitless crystal oscillator. - Radio amateur, 2000, No. 8, p.27.
2. Artemenko V. Quartz overtone circuitless oscillator. - Radiomir KB and VHF, 2002, No. 1, pp. 27-29.
3. Horowitz P., Hill W. The art of circuitry. Volume 1. - M.: Mir, 1984.
4. Khablovski I., Skulimovski V. Electronics in questions and answers. - M.: Radio and communication, 1984.
5. Belousov O. Two-point quartz oscillator. - Radiomir, 2001, N7, p.33.
6. Mason W. Piezoelectric crystals and their applications in ultraacoustics.- M.: IL, 1952.
7. Belousov O. Quartz oscillator - Radioamator, 2002, No. 6, p.25.
8. Artemenko V. Barrier high-frequency generators on bipolar transistors. - Radiohobby, 2000, No. 2, p.35.
9. Labutin L. Quartz resonators - Radio. 1975, N3, pp. 13-16.

Author: V.Artemenko, UT5UDJ, Kiev

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