Stable local oscillator VHF converter. 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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As non-tunable local oscillators for converters in the 144-146 MHz range, tube or transistor self-oscillators are usually used, stabilized by quartz, generating in the range of 10-50 MHz (the quartz resonator is excited at the third - fifth mechanical harmonic), and two - three frequency multiplication stages. At the same time, single-stage transistor local oscillators stabilized by quartz at the highest (up to the eleventh) mechanical harmonics have a number of advantages: the absence of low-frequency harmonics with sufficiently large amplitudes in the signal spectrum; small dimensions, which allow completely screening and temperature control of the local oscillator unit by fairly simple means; low temperature instability of the output signal amplitude (due to the presence of a minimum number of narrow-band circuits); greater reliability (due to fewer elements).

A practical circuit of a local oscillator at a frequency of 115 MHz, designed for a converter in the 144-146 MHz range at an intermediate frequency of 29-31 MHz, is shown in the figure. It is assembled on a transistor T1, connected according to a common base circuit, with a quartz resonator in a positive feedback circuit between the collector circuit and the emitter circuit. Such a self-oscillator steadily generates at frequencies up to 250-300 MHz. To achieve optimal conditions for self-excitation and maximum stability of the generation frequency, the inclusion factors of the transistor and quartz in the collector circuit are chosen to be 0,65 and 0,25, respectively.

Quartz is connected through a capacitor C8 (15pF). An inductor L2 is connected in parallel to the quartz resonator, which compensates for the static capacitance of the quartz. The generator is excited on the seventh mechanical harmonic of quartz with a resonance frequency of 16,44 MHz. In this case, the amplitude of the output signal reaches 120-140 mV. In addition, it is possible to excite the oscillator at the eleventh harmonic (the quartz resonator must be with a resonance frequency of 10,45 MHz), however, the signal amplitude is somewhat smaller, in addition, the tuning of the oscillator becomes more complicated: it becomes critical even to small detunings of the collector and compensating circuits . The design of the local oscillator uses an effective decoupling of the power supply circuits using a double L-shaped protective LC filter. The output of the oscillator is designed to connect a standard 75-ohm coaxial cable and has a partial capacitive connection with the collector circuit.

The power consumed by the local oscillator in the power supply circuits is not more than 60 mW. Details and design determine the stability of the generation frequency to the same extent as the choice of circuit. Therefore, it is advisable to use fixed resistors MLT-0,5 (or MLT-0,25) in the design; resistor R3-SPO-0,15; capacitors C1 and C2 - KCO, SGM or K10-7v, C3-KSO, SGM, C4 -KPK-1 or KPK-M; loop capacitors and capacitor C7 - KT-1, and it is desirable to take C5 and C6 with positive TKE (groups P120, blue or PZZ - gray case), and C8 - with negative TKE (groups MZZ, M47 - blue, respectively, with brown or red dot). The data of coils and chokes are given in the table. Coil L1 is placed in an aluminum screen measuring 35x35x50 mm. The taps, counting from the upper end according to the diagram, are from the fifth turn - to the capacitor C8 and from the thirteenth turn - to the collector of the transistor T1.

The design is made in the form of a block on a separate (from the converter) chassis with dimensions of 80x45x75 mm. The local oscillator is tuned using a tester, a VHF GSS and an RF tube voltmeter. By selecting the resistance of the resistor R5, the emitter current is set to 2-3,5 mA (or more) until stable generation occurs. Using the resistor Lz, the voltage is selected at the base of the transistor T1. Next, the tuning core of the coil L2 adjusts the compensation circuit. Then the collector circuit is tuned to a frequency slightly higher (by 500-600 kHz) of the generation frequency - in this case, the frequency instability of the oscillator will be minimal.

The tap from the L1 coil to turn on the collector of the transistor T1 is selected within 11 and 15 turns - until stable generation appears. detuning of the collector circuit. (This detuning, when using the seventh mechanical harmonic of quartz, can reach a value of the order of 7 MHz). The maximum allowable detuning value of the compensation circuit is 5-25 kHz.

Literature

1. Radio No. 10/1971, c.27

Publication: N. Bolshakov, rf.atnn.ru

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