Stable generator for VHF transmitter. 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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For successful work with distant stations on the 144 MHz band, it often becomes necessary to work on the same frequency as the correspondent. This becomes especially obvious when working in competitions, when dozens and even hundreds of stations are listening on the band, creating strong mutual interference, or during a QSO at a round table.

Master oscillators assembled according to mixing circuits, as well as tunable quartz oscillators, have been repeatedly described earlier, but they are all rather complicated.

The proposed master oscillator is simple, almost as stable as a quartz oscillator and does not require significant alterations if used in an already finished transmitter. Possible frequency overlap is 400-500 kHz.

The principle of operation of the master oscillator is based on the following phenomenon. If in a quartz oscillator assembled according to the capacitive "three-point" scheme, a coil is connected in series with the resonator, the generation frequency will decrease relative to the frequency of the quartz. If we turn on (also in series) a capacitor, the frequency will increase. In both cases, the degree of frequency change will depend on the values ​​of inductive (XL) and capacitive (Xc) resistances. In this generator (see figure), a serial circuit L1C1 is included in the quartz circuit. At voltage resonance (ХL=Хс and Z=0), the generator operates near the frequency of the series resonance of quartz. Changing the capacitance of the capacitor in one direction or another from the resonance position will lead to the predominance of the influence of either capacitance or inductance.

In order for the generator frequency at the maximum capacity after multiplication to be equal to 144 MHz, it is necessary to use quartz with a frequency corresponding to the harmonic of 144,25-144,33 MHz (4010, 6015, 8020, 12030 kHz, etc.). Since many radio amateurs do not have such quartz, any quartz at 4, 6, 8, 12 MHz can be used, reducing the silver layer with an elastic band to obtain a resulting frequency of 144,25 MHz. It should be taken into account that with such an increase in frequency, the activity of quartz and the overlap decrease somewhat.

The variable capacitor C1 should have a maximum capacitance of 100-150 pF and the smallest possible value of the minimum capacitance. Coil L1 has a diameter of 8-10 mm. and wound in bulk with PELSHO 0,15 wire. The number of turns is 30-60 (depending on the quartz used). The data of the coil L2 and the capacitance of the capacitor C2 depend on the frequency of the applied quartz and therefore are not indicated.

When setting up the generator, set the capacitor C1 to the position of maximum capacity. If there is no generation, it is necessary to unwind part of the turns of the coil L1. If the generation breaks down in the minimum position of the capacitor C1, then in parallel with it it is necessary to connect a constant capacitor with a capacity of 5-7 pF. To obtain maximum overlap, it is also useful to choose the capacitance of capacitors C3 and C4.

The anode circuit L2C2 is tuned to the corresponding harmonic. If you use two or three switchable quartz, then you can block the 144-145 MHz section, in which the vast majority of radio amateurs work. When reworking an already finished transmitter, it is enough to turn on the L1C1 circuit in the open circuit of the quartz and select the parameters of the elements.

This device has been successfully used by RA6LAF, RA6LDC, UW6MA and UK6LAA for several years.

Author: V. Glushinsky (UW6MA) Rostov-on-Don; Publication: N. Bolshakov, rf.atnn.ru

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