In Amateur practice, it is challenging to obtain a highly stable oscillation frequency. Often used for this purpose crystal oscillators. Industry produces quartz frequencies up to at least 100 MHz. If you have ham radio quartz frequency, for example, 27 MHz or 45 MHz - this does not guarantee that it will happen this generation frequency. In most cases the quartz at frequencies above 20...25 MHz - harmonique (most often it is the 3rd harmonic). This means that quartz, which has the inscription of 27 MHz, will actually generate a frequency of 9 MHz, and quartz with the inscription of 45 MHz with a frequency of 15MHz.

Therefore, in many schemes considered in the literature, uses a resonant LC circuit tuned to a frequency of 27 or 45 MHz. Usually this LC-circuit is included in the collector (or drain field) of the transistor.

In addition to the complexity of the configuration of the LC-circuit, in this case, it must be escaped, since at such frequencies it is the source of the interference. In addition, when the work of the LC-circuit at low impedance loads need more good buffer amp.

As a consequence, in [1] was proposed to dispense with the LC circuit when working with harmonique quartz. However, checking the operation of the scheme [1] has shown that none of the subjects quartz (was tested over a dozen different garmonichnyh quartz) were recorded on the 3rd harmonic. Moreover, in this scheme, even those not working quartzs (1st harmonic), that work reliably in other circuits. In this regard, the author recommends not to use the scheme [1] in Amateur practice.

However, analyzing in detail the numerous schemes of portable radios on 27 MHz, it is possible to notice that when using the chip CPS (CPS) and quartz 27 MHz, without the LC circuit can be eliminated. This important conclusion the author effectively used in the development of its scheme generator operating on the same principle, but discrete, since the use of these chips is quite inconvenient due to the impossibility of obtaining 50 Ohm output without the use of buffer stages.

In the proposed scheme, the output impedance is approximately 50 Ohms.

Acyclic crystal oscillator

Work quartz ZQ1 in the scheme as possible on main and 3rd harmonic, depending on the capacity of the capacitor between the emitters of transistors (C4).

When the capacitance of about 100 pF (capacitance should be selected) most quartz works at the fundamental harmonic, i.e., the quartz, in the case of which it is written, for example, 27 MHz, generates at a frequency of 9 MHz. However, when capacitance of about 10 pF is observed generation of directly to the 3rd harmonic, i.e., we get the frequency, which is written on the case of this quartz.

In the proposed scheme at such a small capacitance C4 to the 3rd harmonic generate even negarmonichno quartz, i.e. designed to work only on the 1st harmonic. This is especially true oscillators with frequencies below 20...25 MHz. So, for example, quartz with inscription on the body 6 MHz C4"100 pF normally generate this frequency (6 MHz), but when reduced to C4 10pF he began to generate a frequency of 18 MHz! As it turned out, at least a third of such negarmonicheskikh quartz can be made to generate a frequency 3 times higher than indicated on their body.

It is also worth noting that in the proposed scheme normally get excited even those quartzs (as on the 1st and 3rd harmonic), which in other schemes usually do not generate (inactive).

Setting the schema with the reliability of the elements is only in the selection of C4 to obtain the desired frequency generation. For this purpose, using a 50 Ohm attenuator to the output of the circuit connected to the frequency counter, and select the capacitance C4. 50-Ohm load circuit when PI=12 generates an RF voltage of about 200 mV. There are, unfortunately, quartzs, which "do not want" to work on the 3rd harmonic (with the frequency, what is written on the body). It is mainly imported miniature quartz, which is probably as a working material using quartz, and spectrumco.

Literature

1. Polyakov V. Stable crystal oscillator. Radio, 1999, No. 6, P. 62.

Author: V. Artemenko (UT5UDJ), Kyiv; Publication: N. Bolshakov, rf.atnn.ru

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