A simple frequency synthesizer. Scheme, description

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ENCYCLOPEDIA OF RADIO ELECTRONICS AND ELECTRICAL ENGINEERING
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A simple frequency synthesizer. Encyclopedia of radio electronics and electrical engineering

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Encyclopedia of radio electronics and electrical engineering / Radio amateur designer

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The device offered to the attention of readers makes it possible to obtain a grid of highly stable frequencies with a step of 1 kHz at frequencies up to 7 MHz. The advantage of the circuit design of the device is the visibility of setting the generated frequency, without any additional calculations, the absence of programmable microcircuits and expensive components.

The diagram of the device is shown in the figure.

(click to enlarge)

The basis of the device is a generator assembled on a transistor VT1 according to an inductive three-point circuit. Through the buffer follower, made on the transistor VT2, and the chain R8, C3, the output sinusoidal signal of 0,4 V (effective value) is fed to the output of the device. At the same time, from the VT1 drain, a pulsed (trapezoidal) signal is fed to the input of the rectangular pulse shaper, made on the DD1 chip. The chain of logic elements DD1 1 - DDI 3, covered by a negative DC feedback, is brought to a linear section and works as a limiter amplifier. Such a shaper is recommended for use by V. L. Shilo in the book "Popular digital circuits" (publishing house "Radio and Communication", 1989), since it provides high stability of the counters up to 7 MHz.

From the output of the shaper, rectangular pulses are fed to the input of decimal counters DD5-DD8 with built-in decoders, on which a divider with a variable division ratio (CVD) is made. Elements of DD9.1. DD9.2 form a coincidence circuit that generates a pulse to reset the counters when they reach the set value. The DPKD division coefficient is set using jumpers (switches), units of kilohertz correspond to jumpers associated with the DD5 microcircuit, thousands correspond to the DD8 microcircuit.

From the output of the DPKD (pin 3 of the DD8 microcircuit), the signal of the tunable oscillator, divided to the value of the reference frequency, is fed to the input of the comparison device, the role of which is played by the frequency-phase detector (FPD), made on triggers DD10.1, DD10.2, transistors VT3, VT4 and diodes VD3, VD4 The master stage of the reference frequency generator is quartz and is made on the elements DD2 and counters DD3, DD4 - a divider by 100.

The generated lock-in voltage is integrated by the capacitor C5 and fed through an additional low-pass filter R16C6 to the varicaps VD1, VD2, thereby closing the PLL loop. This is done to reduce the settling time in the resulting automatic control system and to exclude possible self-excitation. But the use of the PIF entails a number of significant drawbacks related to the fact that it suppresses the variable component of the reference frequency signal that comes with the constant voltage to the varicaps of the tunable oscillator and thereby causes parasitic frequency modulation of the oscillator. To combat this, a precision notch filter tuned to the reference frequency is usually installed between the PIF and the varicaps, and this filter, which was supposed to obtain high performance, turns out to be rather complicated and expensive.

In this design, due to the relatively high reference frequency, it was possible to get by with a simple integrator providing a settling time of less than 0,5 s and an additional noise filter with a cutoff frequency of about 33 Hz. This solution made it possible to obtain a minimum of frequency-phase noise at an acceptable frequency settling rate without the use of accurate RC components. With the type of coil L1 used and the trimmer in the middle position, the device can produce a frequency grid in 1 kHz steps ranging from 1,5 to 1,7 MHz when tuned using jumpers. Using other coils, frequencies up to 7 MHz can be obtained.

The device uses MLT-0,125 resistors, capacitors of types KT1 M47 (C1, C4), K53-4-20 V (C5, C14), K73-17 (C6), K50-35 (C7), K10-7 (C3 and all blockers). Transistors KT315B are interchangeable with KT3102B, KT361B - with KT3107B, KP3O3G - with KP307G. As a DD9 chip, you can try to use the domestic K561LA8. Coils L1, L2 - FSS coils of the radio receiver "ABAVA RP 8330" passport ISH4.777.240-04.

Setting up the device. Set the DPKD jumpers in accordance with the average frequency of the range that you need. A high-resistance voltmeter (Rin > 5 MΩ) should be connected to the capacitor C1, by phasing the L2 coil on and rotating the L1 coil trimmer (necessarily with a screwdriver made of dielectric material) to achieve a voltage across the capacitor close to half the supply voltage.

The proposed design was developed by the author back in 1992 and was used as a master oscillator of one Moscow medium-wave broadcasting station, for which it was actually created.

In conclusion, I would like to say that by replacing the jumpers with hardwired switches with the appropriate inscriptions, you can introduce operational manual frequency tuning. It is only important that the connecting conductors coming from the switches have a minimum length, especially if it is supposed to work at frequencies close to the maximum.

Author: Ya.Tokarev, Moscow

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Comments on the article:

Victor
Everything is fine, just what you need, as a generator for Clark's treatment methods. Here's the PCB layout. Thank you.

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