Quartz generators. Encyclopedia of radio electronics and electrical engineering

Encyclopedia of radio electronics and electrical engineering / Knots of amateur radio equipment. Generators, heterodynes

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The oscillator, whose frequency is stabilized by a quartz resonator, is a mandatory unit for most modern receivers and transceivers, as well as for measuring instruments. This review presents options for the possible execution of such generators for frequencies from units to tens of megahertz.

Before moving on to practical circuits, we note that for widespread quartz, the main operating frequency usually does not exceed 10 ... 15 MHz. This is due to the difficulties in manufacturing (in serial production) very thin quartz plates with a high degree of parallelism of the working sides. The latter, in particular, strongly affects the monofrequency of the resonator (the absence of parasitic resonances, especially near the main operating frequency).

With regard to generators, the presence of such resonances can lead to excitation of the resonator at a different frequency than indicated on its case, or to a jump in the frequency of the generator when external conditions change (temperature, load resistance, etc.). If the frequency indicated on the body of the quartz resonator is higher than 15 MHz, then with a high degree of probability this resonator is harmonic, and its fundamental frequency is three or even five times lower than the "nominal".

In the generator, the circuit of which is shown in Fig. 1, the quartz resonator is excited at the fundamental frequency [1]. For its stable operation, the load resistance (input resistance of the next stage) must be at least 1 kOhm. In this case, the high-frequency voltage at the generator output will be at least 0,5 V (hereinafter - the effective value).

Ris.1

The values ​​of capacitors C3, C4 and resistor R4 depend on the operating frequency of the quartz resonator. For a frequency band of 1 ... 3 MHz, they should be 270 pF, 180 pF and 3,3 kOhm, respectively; for 3 ... 6 MHz - 180 pF, 120 pF and 3,3 kOhm; for 6 ... 10 MHz - 180 pF, 120 pF and 2,2 kOhm; for 10 ... 18 MHz - 150 pF, 68 pF and 1,2 kOhm; for 18...21 MHz - 68 pF, 33 pF and 680 Ohm.

As they say in such cases, with serviceable parts and error-free installation, the generator does not require tuning (with the possible exception of some correction of the operating frequency by adjusting the capacitor C2). If, when the two conditions mentioned above are met, the generator still does not work, then the only reason for this may be the low activity of the quartz resonator. In this case, it should either be replaced with another one, or try to "play around" with the values ​​​​of capacitors C3 and C4. In particular, a change in one direction or another of the ratio of their capacities can help.

On fig. Figure 2 shows a diagram of an oscillator in which a quartz resonator is excited at the odd harmonics of its fundamental operating frequency [2]. As in the previous version, the input impedance of the next stage must be at least 1 kOhm. The output voltage is approximately 0,5 V. For a frequency band of 15 ... 25 MHz, the capacitances of capacitors C2, C3 and C4 should be 100, 100 and 68 pF, respectively; for 25...55 MHz - 100, 68 and 47 pF; for 50...65 MHz - 68, 33 and 15 pF.

Fig. 2

Coil L1 is wound with a wire with a diameter of 0,3 mm on a frame with a diameter of 5 mm. It has a carbonyl iron trimmer (diameter - 4 mm). For the above three operating frequency bands, the number of turns should be 15, 10 and 7, respectively.

The generator is adjusted by tuning the coil 1.1 but stable generation at the third harmonic of the fundamental frequency of the quartz resonator. If this does not happen at any position of the trimmer, then you should select the number of turns of the coil or try to carry out this operation by installing capacitor C2 with a higher or lower rating. If this operation does not help, then most likely the reason is the low activity of the quartz resonator (see above). It should be noted that not all resonators that stably generate at the fundamental frequency also work stably at harmonics.

Such a generator can provide a voltage of about 2 V at a high-resistance load (for example, a mixing stage on an insulated gate transistor) at a higher frequency if a band-pass filter is introduced into the collector circuit of transistor VT1, tuned, for example, to the second harmonic of the operating frequency of the generator (i.e. i.e. it will be a generator - a frequency doubler on one transistor). The inductors L2 and L3 of such a filter are wound with a wire with a diameter of 0,6 mm on a frame with a diameter of 5 mm with two trimmers made of carbonyl iron (diameter 4 mm). Distance between coils - 5 mm. For a frequency band of 60 ... 90 MHz, the number of turns should be 9, and for 90 ... 130 MHz - 6. The values ​​\u6b\u7bof the capacitors C33, C22 of the filter are XNUMX and XNUMX pF, respectively.

The generator, the circuit of which is shown in Fig. 3 [3], is a little more complicated - it contains an oscillatory circuit. This will provide two benefits at once. First, it has a higher spectral purity of the output signal. Secondly, it provides a wider output level (about 1V into a 100 ohm load).

Ris.3

For a frequency band of 1 ... 3 MHz, the capacitances of capacitors C2, C5 and C6, respectively, are 470, 270 and 2000 pF; for Z ... 10 MHz - 330. 150 and 1500 pF; for 10 ... 30 MHz - 180. 47 and 330 pF. Coil L1 should have such an inductance at the middle position of the trimmer. to ensure resonance with the capacitor C5 at the operating frequency. This generator is adjusted for stable generation at the fundamental frequency of the quartz resonator or at its third harmonic.

Literature

1. Detlef Lechner, Peter Fink. Kurzwellen sender. - Militarverlag der DDR, 1979.
2. G. R. Jessop. VHF/UHF manual. - RSGB, 1983.
3. J-Pavlovec, J. Samur. Krystalove jednotky a oscilatory. - Amaterske Radio (B), 1987. No. 2, p. 42-61.
4. KV-magazine №1, 1992

Publication: N. Bolshakov, rf.atnn.ru

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