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ENCYCLOPEDIA OF RADIO ELECTRONICS AND ELECTRICAL ENGINEERING Circuitless crystal oscillator. Encyclopedia of radio electronics and electrical engineering
Encyclopedia of radio electronics and electrical engineering / Knots of amateur radio equipment. Generators, heterodynes In amateur radio practice, the problem of obtaining highly stable oscillations in frequency is relevant. Usually, crystal oscillators are used for these purposes. The industry produces quartz up to frequencies of at least 100 MHz. If a radio amateur has quartz for a frequency, for example, 27 MHz or 45 MHz, this does not at all guarantee that such a generation frequency will be obtained. In most cases, quartz at frequencies above 20 ... 25 MHz is harmonic (most often it is the 3rd harmonic). This means that a quartz that has an inscription of 27 MHz will actually generate at a frequency of 9 MHz, and a quartz with an inscription of 45 MHz will actually generate at a frequency of 15 MHz. Therefore, in many circuits discussed in the literature, a resonant LC circuit is used, tuned to a frequency of 27 or 45 MHz. Typically, such an LC circuit is included in the collector (or drain for a field-effect) transistor. In addition to the complexity of tuning the LC circuit itself, in this case it must be shielded, since at such frequencies it is a source of interference. In addition, when operating an LC circuit for a low-resistance load, a good buffer stage is also needed. As a result, in [1] it was proposed to dispense with the LC circuit when working with harmonic quartz. However, checking the performance of the circuit [1] showed that none of the tested quartz (more than a dozen different harmonic quartz were tested) was excited at the 3rd harmonic. Moreover, even those quartzes (on the 1st harmonic) that work reliably in other circuits do not work in this circuit. In this regard, the author does not recommend using the circuit [1] in amateur radio practice. At the same time, analyzing in detail the numerous circuits of 27 MHz portable radio stations, you can see that when using the K174PS1 (K174PS4) chip and 27 MHz quartz, you can do without an LC circuit. The author effectively used this important conclusion when developing his oscillator circuit, which operates on the same principle, but on discrete elements, since it is rather inconvenient to use these microcircuits due to the impossibility of obtaining a 50-ohm output without the use of buffer stages. In the proposed circuit, the output impedance is approximately 50 ohms.
The operation of ZQ1 quartz in the circuit is possible both on the main and on the 3rd harmonic - depending on the capacitance of the capacitor between the emitters of the transistors (C4). With a capacitance of the order of 100 pF (the capacitance should be selected), most quartz works on the fundamental harmonic, i.e. quartz, on the body of which it is written, for example, 27 MHz, generates at a frequency of 9 MHz. However, at a capacitance of about 10 pF, generation is observed directly at the 3rd harmonic, i.e. we get the frequency that is written on the case of this quartz. In the proposed scheme, with such a small capacitance of C4, even non-harmonic quartz is generated at the 3rd harmonic, i.e. designed to work only on the 1st harmonic. This is especially true for quartz with frequencies below 20 ... 25 MHz. So, for example, quartz with an inscription on the 6 MHz case at C4 "100 pF normally generated this frequency (6 MHz), but when C4 was reduced to 10 pF, it also began to generate at a frequency of 18 MHz! As it turned out, at least a third of such non-harmonic quartz crystals can be made to generate at a frequency 3 times higher than indicated on their case. It is also worth noting that even those quartzes (both at the 1st and at the 3rd harmonic) are normally excited in the proposed scheme, which usually do not generate in other schemes (low active). Setting up the circuit with serviceable elements consists only in selecting C4 to obtain the required generation frequency. To do this, through a 50-ohm attenuator, we connect a frequency meter to the output of the circuit, and select the capacitance C4. At a 50-ohm load, the circuit at Up \u12d 200 V produces an RF voltage of about 3 mV. There are, unfortunately, quartzes that "do not want" to work on the XNUMXrd harmonic (with the frequency that is written on the case). These are mainly imported miniature quartz, where, probably, not quartz itself is used as a working material, but special ceramics. Literature 1. Polyakov V. Stable quartz oscillator. - Radio, 1999, No. 6, S. 62. Author: V. Artemenko (UT5UDJ), Kiev; Publication: N. Bolshakov, rf.atnn.ru
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