Local oscillator with PLL. Scheme, description

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ENCYCLOPEDIA OF RADIO ELECTRONICS AND ELECTRICAL ENGINEERING
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Local oscillator with PLL. Encyclopedia of radio electronics and electrical engineering

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

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In amateur practice, there is often a need for a highly stable fixed frequency generator. Such a generator is necessary, for example, for a shortwave receiver or transceiver. The traditional solution to this problem is to use an oscillator with quartz frequency stabilization, which does not always suit the radio amateur, primarily because of the difficulties in acquiring quartz resonators for the required frequencies. Now, when the number of amateur KB bands has reached nine, it is economically justified to use phase-locked local oscillators containing only one quartz resonator.

A practical diagram of one of the options for such a local oscillator is shown in the figure. It generates a frequency grid in the range of 1 ... 30 MHz, a multiple of the frequency of the reference quartz resonator. In practice, this grid can have a step from 0,2 MHz to several megahertz.

(click to enlarge)

The reference oscillator with quartz frequency stabilization is assembled on the elements DD1.1 and OD1.2. The exact value of its frequency is set by a trimmer capacitor C2 with limits of capacitance change of 5 ... 50 pF. The output signal of this generator (it is close to a meander in shape) is differentiated by the R6C1 circuit, and the resulting short pulses are fed through the inverters DD1.3 and DD1.4 to the pulsed phase detector on diodes VD1 and VD2. The signal of a high-frequency voltage-controlled generator is also supplied here (it is made on transistors VT2 and VT3). The field-effect transistor VT2 is involved in the system for automatically adjusting the output level of this generator: the RF voltage rectified by the diodes VD4 and VD5 is supplied to the gate VT2, which controls the bias current of the transistor VT3 (generator itself). This automatic adjustment system is very effective - when the generator is tuned within the entire KB range, the output amplitude changes by no more than 1 dB.

The high-frequency voltage from the oscillatory circuit of the generator through two decoupling broadband amplifiers U1 and U2 is supplied, respectively, to the phase detector and to the mixer of the receiver or transmitter. Amplifier U1 must provide very good decoupling between the phase detector and the input of amplifier U2, otherwise noticeable components with other (except for the fundamental) frequencies, multiples of the frequency of the quartz resonator, will appear in the output signal spectrum. The phase detector error signal is amplified by an integrating amplifier based on the VT1 transistor (cutoff frequency is about 2 kHz) and fed to the VD3 varicap.

The choice of the operating frequency of the voltage-controlled generator is carried out by switching the coils of the oscillatory circuit (L1) and adjusting the variable capacitor C 11.

The establishment of the GPA begins with setting the voltage on the collector of the transistor VT7 with a trimming resistor R1 to about 6 V (the collector current is approximately 0,5 mA). The contacts of the SA1 switch must be open. By connecting an oscilloscope to the resistor R10, the GPA is tuned with a variable capacitor C 11. At the same time, near frequencies that are multiples of the frequency of the reference quartz resonator, a beat signal with an amplitude of up to 5 V should be observed on the oscilloscope screen. By setting the GPA frequency so that the beats are close to zero, close the contacts of the switch SA1, thereby including the PLL.

In practical work with this local oscillator, it is advisable to apply a signal from the resistor R10 when the local oscillator is tuned (before the PLL loop closes) to the audio frequency amplifier, indicating the optimal tuning for the corresponding harmonic of the B1 quartz resonator "by ear" (by zero beats).

The SN74H00 microcircuit can be replaced with 133LAZ, the 2N5459 transistor with KP303 series transistors, the 2N5140 transistor with GT329 or any other pnp microwave transistor (silicon or germanium), MBD101 diodes with KD514, 1N914 diodes with KD521 and the like. An analog of the MPS-A12 transistor is not produced in the USSR, but it can be replaced with a composite transistor made of two transistors of the KT342 or KT3102 type with a static current transfer coefficient of at least 200. MC1R microcircuits designed to amplify signals are used as decoupling amplifiers U2 and U1350 in the IF path of television receivers. An analogue of this microcircuit is not produced in the USSR.

Literature

Hawker P. Technical Topics.- Radio Communication, 1987, August, p. 582.

Publication: N. Bolshakov, rf.atnn.ru

See other articles Section Knots of amateur radio equipment. Generators, heterodynes.

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