Encyclopedia of radio electronics and electrical engineering / Civil radio communications

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This crystal oscillator (KG) is intended for use as the first local oscillator in radio receivers, transceivers and transmitters made according to the block diagram of the UW3DI transceiver, where the first IF is tunable (for example, from 6 to 6,5 MHz), and the second is fixed ( 500 kHz). Such a scheme for constructing amateur equipment has a number of advantages, where the main one, perhaps, is a non-switchable smooth range generator (GFO). The absence of switched elements in the GPA and the limited tuning range at relatively low frequencies make it easier to solve, in particular, the problem of long-term temperature stability.

The described version of the KG (its schematic diagram is shown in Fig. 1) was used by the author in the Extra-Test radio receiver. Unlike UW3DI, the overlap (tuning) for the first IF is 5,5 ... 6,5 MHz (i.e., twice as much). The received frequency range of the receiver from 0,5 to 34,5 MHz is divided into 34 subbands with a width of 1 MHz. When receiving signals in the frequency band of 5,5 ... 6,5 MHz (tuning frequency of the first IF), it is provided to turn off the quartz local oscillator. The supply voltage is removed from it and the bypass mode (through the contacts of relay K1) of the received RF signal from the output of the UHF receiver to the input of the first IF tunable filter is switched on directly. Taking into account the fact that the receiver's GPA covers a frequency section slightly more than one megahertz (with some margin at the edges), normal reception is also provided in adjacent frequency sections, near the extreme IF frequencies of 5,5 and 6,5 MHz per overlap frequency.

Rice. 1. Schematic diagram of a crystal oscillator (click to enlarge)

The generator forms a frequency grid using the harmonics (from the first to the fourth) of twelve installed quartz resonators. To obtain a uniform level of the generator output signals when operating on different harmonics, the output signal level was adjusted (each harmonic has its own adjustment). A three-channel emitter follower is installed at the output of the KG, distributing the output signal to three loads.

The generator itself is assembled on a transistor VT1 according to the capacitive three-point scheme. Quartz resonators ZQ1-ZQ12 are connected between the base circuit of the transistor and the common wire. The switching of the resonators is carried out by two biscuit switches - SA1.1 (type KT 1211-1 with 26 positions) and SA2.1 (type 11P3NPM, nine positions are involved). The switching contact of the switch SA2.1 is connected to the base of the transistor VT1 only in the position of the switch SA1. 1, designated as "DD" (additional ranges).

In the collector circuit of the transistor VT1, switches SA1.2 and SA2.2 connect the circuits L1-L22, C4-C25, tuned to the output frequencies of the generator (to the harmonic frequencies of quartz resonators). Resistor R3 is connected in parallel with the load circuits, it smooths out possible current surges through the transistor when switching coils.

From the collector of transistor VT1, the signal is fed to a controlled buffer amplifier, made on a double-gate field-effect transistor VT5. The gain control of this cascade is carried out by two gates. Trimmer resistors R1.3-R2.3 are connected to the first gate using switches SA18 and SA21, operating in the same algorithm as the previous switch jacks, depending on the harmonic of the quartz resonators used. These resistors can be used to adjust the amplitude uniformity of the output signal of the crystal oscillator. Diodes VD4-VD7 serve to decouple the control resistors and exclude their influence on each other during adjustment.

A control signal is supplied to the second gate of the transistor VT5 from the AGC system of the receiver (0 ... + 12 V). At the output of this AGC circuit (in the receiver), a tuning resistor is included, which sets the initial gain of the cascade on the transistor VT5 (voltage +3 ... 5 V). When the AGC system is operating, its voltage increases with an increase in the input signal level and, having exceeded the voltage of the set level, it begins to increase the gain of the buffer amplifier. As a result, at the inputs of the receiver mixer, along with an increase in the input signal level, the local oscillator signal level also increases, while the ratio of the levels of these signals remains the same, which favorably affects the dynamic range of the mixer. A change within certain limits of the output signal level for the digital scale does not have a special effect on the readings of the latter, since the setting threshold of the gain of the buffer stage is slightly higher than the sensitivity threshold of the digital scale.

From the drain of the transistor VT5, the signal is fed to the base circuits of the transistors VT2-VT4. A three-channel emitter follower is assembled on these transistors. From its outputs, the signal is fed to the receiver mixer (RX), the transmitter mixer (TX) and the digital scale (DSH). In transmit mode, the receiver's AGC system does not work and does not affect the gain of the cascade on VT5.

If a ZQ12 resonator operating on the fifth harmonic is installed, then an additional circuit should be organized, consisting of two decoupling diodes, a tuning resistor and connected in parallel to the lower harmonic circuits.

The generator was mounted on a printed circuit board made of foil fiberglass (Fig. 2). Coils L1-L20 are wound on plastic frames with a diameter of 5 mm and enclosed in aluminum screens. On each frame (and there are only ten of them) there are two coils (one from each edge).

Rice. 2. Mounting the generator

Each coil is equipped with a 400NN ferrite trimmer with a diameter of 3,5 and a length of 14 mm. Coils L1-L14 are wound turn to turn with PEL wire 0,55, coils L15-L20 - turn to turn with PEL wire 0,41.

Coils L21 and L22 are frameless, wound with silver-plated copper wire with a diameter of 0,55 mm on a mandrel with a diameter of 5 mm. They are soldered directly on the contacts of the SA2 biscuit switch and are adjusted by expanding or compressing the turns.

Number of turns of coils: L1, L2 - 7; L3 - 8; L4, L5 - 9; L6, L7 - 10; L8, L9 - 11; L10-L13 - 12; L14-13; L15-16; L16-20; L17 - 25; L18 - 30; L19 - 40; L20-47; L21, L22 - 5.

Setting up the generator consists in adjusting the coils L1-L22 until the maximum signal level is reached and selecting the resistor R6 until an undistorted sinusoidal output signal is obtained.

The frequencies of quartz resonators and the frequencies of circuit settings are indicated in the diagram (see Fig. 1). The 23 MHz resonator can be replaced by a resonator at a frequency of 7,66 MHz with the release of the third harmonic, the 25 MHz resonator can be replaced by resonators at frequencies of 5; 6,33; 8,33; 12,5 MHz with selection of the corresponding harmonic. The generator output signal levels are set by trimmers R18-R21 within 0 ... 3 V, depending on the specific load and in the absence of a signal at the receiver input.

Author: Vladimir Rubtsov (UN7BV)

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