Converter for SI-BI radio station. Scheme, description

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ENCYCLOPEDIA OF RADIO ELECTRONICS AND ELECTRICAL ENGINEERING
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Converter for SI-BI radio station. Encyclopedia of radio electronics and electrical engineering

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Encyclopedia of radio electronics and electrical engineering / Civil radio communications

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A simple converter, which is described in this article, will allow the owner of a CB radio station to join the world of amateur communication, and at his leisure to listen to broadcast radio stations.

If you have an AM / FM radio station, then the converter will allow you to receive broadcasts from HF and even MW broadcasting stations, listen to music, and find out the news. And if the radio station is with SSB, then it will be possible to watch the work of radio amateurs in the ranges of 160, 80, 40, 20 m, listen to the "round table", learn radio amateur news. Especially such a converter will be convenient in a car, where the possibility of installing additional radio equipment is limited.

The converter circuit is shown in fig. 1. Its main components: mixer on the DA1 chip; local oscillator with quartz frequency stabilization on the transistor VT1; matching stage on the transistor VT2. Since the converter was planned to be used in conjunction with the "Dragon SS-485" transceiver, which has an SSB signal reception path, for stable reception of such signals, the converter's local oscillator must have high frequency stability, i.e., be quartz. The operating frequency range of such a transceiver is from 25,16 to 29 MHz (bandwidth - 66 MHz), so a local oscillator with four ratings of generated frequencies selected by the SA4,5 switch was used. In order to simplify the design and tuning, quartz resonators were used that operate at the first harmonic (i.e., with frequencies not exceeding 1.2 MHz) and with "round" ratings in order to simplify the comparison of the transceiver readings with the frequencies of the received signals. Therefore, 30, 10, 15 and 20 MHz crystals were chosen, although the best option would be to install quartz oscillators at a frequency above 30 MHz, but this would have to complicate the local oscillator.

(click to enlarge)

The use of these resonators made it possible to obtain four sub-ranges of received frequencies: with a local oscillator of 10 MHz - 15,16 ... 19,66 MHz; with a 15 MHz local oscillator - 10,16 ... 14,66 MHz; with a local oscillator 20 MHz - 5,16 ... 9,66 MHz; with a 30 MHz local oscillator - 0, 34 ... 4,84 MHz. Knowing the frequency of the local oscillator, you can determine the frequency to which you need to tune the transceiver in order to receive the desired signal in the HF band. Thus, the converter, together with the indicated transceiver, makes it possible to cover almost the entire range of MW and HF.

Other multi-grid transceivers have a slightly different frequency range and therefore the resulting range will differ from what is shown. In addition, if the transceiver does not have an SSB path, then it will be possible to receive only stations with AM, that is, broadcasting, and therefore, the local oscillator can be made without quartz stabilization, on LC circuits.

In the off state, the converter input (XS1) is connected directly to the output (XS2) through the relay contacts K1.1 and K2.1. After the converter is turned on, voltage is supplied to the windings of these relays. They work, and the signal from the antenna goes to the L1 coil. The input circuit consists of coil L2, capacitor C1 and one of the capacitors C2-C7, depending on the selected range. The signals selected by the input circuit are fed to the input of the mixer DA1. The other input from the resistor R2 receives a local oscillator signal, the frequency of which also depends on the selected range.

The converted signal from pin 2 of the DA1 microcircuit is fed through the L3C16 notch filter to a matching amplifier assembled according to the emitter follower circuit (VT2). The notch filter is set to 30 MHz and rejects the 30 MHz LO signal as well as its harmonics if it operates at 10 and 15 MHz. These signals lie close to the operating frequency range of the transceiver and can have a negative impact on the quality of reception, so it is desirable to suppress them. At the output of the amplifier, a high-pass filter with a cutoff frequency of 25 MHz (C18L5C19L6C20) is installed, which suppresses HF signals and local oscillator signals with frequencies of 10, 15 and 20 MHz. Diode limiters (VD1VD2 and VD3VD4) are installed at the input and output, which are primarily designed to protect the converter from the powerful transmitter signal of the transceiver itself.

Most parts of the converter are placed on a printed circuit board made of double-sided foil fiberglass with a thickness of 1,5 ... 2 mm, a sketch of which is shown in fig. 2. The second side of the board is left metallized and is used as a screen; it must be connected in several places along the contour with a common wire. The input and output connectors (any coaxial) are installed on the rear panel, which can be used as a piece of foil fiberglass, soldered to the board. Relays K1 and K2 are desirable to be soldered at least at one point to the board, unless, of course, their case can be tinned, otherwise they must be glued. Switches SA1 and SA2 are installed on the front panel, an LED and a resistor R7 are also placed there when a light indication of the converter is turned on is needed.

Converter for SI-BI radio station

It is permissible to use transistors KT363A, KT363B, diodes KD503A, KD521 with any letter index in the device. Trimmer capacitor C16 - KT4-25, the rest - KSO, KM, KD, KT or similar imported. Relays K1 and K2 - REK43 with a response voltage of 5 ... 5,5 V, if a relay is used for a voltage of 12 V, then each of them must be connected in parallel with the capacitor C15. Resistor R2 - SP3-19a, the rest - MLT, S2-33. Switch SA1 - PG2, SA2 - PT2, PT57. Coils L1, L2 and L3 are wound on frames with trimmers made of carbonyl iron with a diameter of 3 mm and contain L2 and L3 - 30 turns of PEV-2 wire each, and L1 - 6 turns of the same wire over L2. Inductor L4 - DM inductance 40 ... 100 μH. Coils L5 and L6 are frameless, wound with PEV-2 0,4 wire on a mandrel with a diameter of 5 mm and contain 8 turns each.

Establishment begins with checking the performance of the local oscillator. Then connect the converter to the transceiver and check the overall performance. It is better to start by receiving AM broadcasting stations. Having tuned in to one of them, preferably low-power, resistor R2 sets the minimum level of the local oscillator signal, at which the mixer transfer coefficient (station volume) does not decrease yet. Then the input circuit is tuned, starting from the 16 m band. The transceiver is tuned to the section where stations of this band should be received. To determine this section, the frequency of the signal must be added to the local oscillator frequency. By rotating the trimmer, they achieve maximum reception volume. If the tuning will be done from the air, that is, by ear, this should be done in the daytime, since this range is "daytime".

After that, the trimmer is fixed and the input circuit is tuned to other ranges, but now by selecting capacitors C2-C7. Then, with a capacitor C16, the L3C16 circuit is tuned to a frequency of 30 MHz. It is difficult to do this by ear, but if there are no instruments, you should try to tune it to a minimum of interference whistles and provided that it does not fall into the operating frequency range of the transceiver.

If you use a full-size fixed antenna for reception, then on some HF bands the level of signals and interference may be too high. Then at the input you need to put a level control, which is conveniently placed on the front panel. It is a variable resistor with a resistance of 100 ... 220 Ohms; It's better if it comes with a switch. The coil L1 is disconnected from the relay contacts, the resistor is connected in parallel with L1, and its engine is connected to the released relay contact K1.1.

For 40-channel transceivers with AM and FM, it is possible to receive stations only with amplitude modulation, so the local oscillator frequencies must be changed, and in order to simplify it, it can be made tunable. The input circuit will also have to be tuned in frequency; for this, a two-section variable capacitor or two varicaps with a large capacitance overlap ratio are used.

In principle, you can power the converter from any source with a voltage of 9 ... 12 V, but still it is better to power it from the transceiver itself, and make their work interconnected. The point here is the following: if the transceiver works separately, then there is always a danger of "burning out" the converter with a powerful signal from the transmitter of the transceiver itself. The way out of this situation is to block the transmitter while the converter is running.

This can be done in different ways. One of the options is shown below. In the transceiver itself, a special socket is installed on the rear panel to power the converter; for example, a small-sized socket for a headphone is suitable. The converter can be powered from the receiver's power bus, then when switching to the transmission mode, the converter will be de-energized and the relays K1, K2 will be released, but their speed may not be enough and then the converter will fail.

The next option will be more reliable. You will need a small-sized relay with NC contacts, with a response voltage of not more than 3 V and a winding resistance of not more than 100 Ohms, for example, a RES-55 relay with a coil resistance of 96 Ohms is suitable. The relay winding is connected between the transceiver power bus and the converter power socket, and in the converter itself, parallel to the C15 capacitor, a zener diode with a stabilization voltage of 9 ... Then, when the converter is turned on, the relay installed in the transceiver will work and its contacts will break the circuit of the "Transmit" button.

Author: Igor Nechaev, Kursk

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