Encyclopedia of radio electronics and electrical engineering / Civil radio communications

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Electromagnetic relays are widely used for switching transceiver circuits. But how to competently organize the order of their switching in the device? How to avoid burning contacts, especially when switching RF circuits of a power amplifier? The device offered to your attention, which complements the transceiver control scheme, will help to solve this problem.

When using the same antenna for both reception and transmission, switching of the high-frequency circuits of a separate power amplifier is carried out, as a rule, according to the scheme shown in Fig. 1.

When the contacts of the "Receive / Transmit" switch (pedals) are closed, the transceiver is switched on for transmission and the relays K1 and K2 are activated. Electromagnetic relays have inertia - it takes some time to switch them, so the RF voltage at the input of the power amplifier appears before the relays have time to switch. In other words, the switching of both relays occurs in the presence of high-frequency voltages on their contacts. Switching high frequency currents causes much more burning of the contacts compared to switching DC or power frequency current. For this reason, RF relays (especially relay K2 at the output of the power amplifier) ​​often fail.

It is possible to eliminate the burning of the relay contacts if, when the radio station switches from reception to transmission, HF voltage is applied to their contacts with some delay relative to the moment the voltage is applied to their windings. And vice versa, during the transition from transmission to reception, the relays should be de-energized only after the RF voltage on their contacts is already absent.

In most transceivers, switching of RF circuits is carried out by electronic switches and electromagnetic relays. As a rule, electromagnetic relays switch the powerful output signal of the transceiver and power amplifier, and electronic switches switch the voltages in the signal conditioning paths. Therefore, high RF voltages on the relay contacts can only be when the electronic switches are already switched to transmission, and when working with the telegraph, the telegraph key circuit is also closed.

Based on this, I propose to divide the control circuits of the transceiver and the power amplifier into two parts. The first is the windings of electromagnetic relays. The second is the control circuits of electronic switches and the telegraph key circuit of the transceiver. In many transceivers, this division is already incorporated in the circuit - the first circuits are controlled by an external "Receive / Transmit" switch (pedal), the second - by a telegraph key; and in some transceivers there is no relay at all. Therefore, most often, the transceiver itself does not need to be modified. When switching from reception to transmission, it is necessary to first switch the first circuits (relays), and then (with a delay) the second ones (electronic switches and the telegraph key circuit). When switching from transmission to reception, on the contrary, it is first necessary to switch the second circuits, and then, with a delay, the first ones (Fig. 2). The duration of the delays must exceed, respectively, the response time twork and the release time tp of the inertial relay of the RF path itself (as a rule, this is a relay at the output of the power amplifier).

The device shown in fig. 3, allows you to control the switching of the radio in compliance with the above conditions. Its use completely eliminates the presence of voltages on the relay contacts at the moment of their switching, including in case of erroneous actions of the operator. It provides the operation of the radio station by telegraph and telephone both using the "Receive / Transmit" switch (pedals) and with automatic switching (half duplex, VOX). At the same time, the device minimizes the number of relay switching of the high-frequency path of the radio station - when working in a half-duplex telegraph, the radio station does not switch from transmission to reception in short pauses between telegraph messages, characters and words.

The inputs of the device receive signals from the telegraph key, the "Receive / Transmit" switch (pedal) and from the voice control system (VOX) of the transceiver. All electromagnetic relays of both the power amplifier and the transceiver itself are connected to output 1 of the device ("Relay"). From output 2 ("Electronic switches"), voltage is supplied to the input of the transceiver "Telegraph key", as well as to all electronic switches of the transceiver, which switch the circuits common for receiving and transmitting (they are most often already connected to the input "Telegraph key" in the transceiver). "). Output 3 is used when a signal is required to switch the electronic switches of the transceiver, inverse to the signal at output 2. Active levels for both inputs and outputs of the circuit are low (short to common wire).

Elements DD1.1, DD1.2 and DD1.4 control electronic switches and telegraph key circuits of the transceiver (perform manipulation). When the telegraph key is closed, a low logic level appears at the input 2 of the DD1.1 element. Element DD1.3 controls the operation of the relay. When you press the pedal at the input 9 element DD1.3 will be a low logic level. It can be seen from the diagram that the radio station relays are triggered when the output 10 of the DD1 chip has a high logic level (logical "1"). In turn, the electronic switches are switched to the "Transmission" mode when the output 11 of the DD1.2 element has a low logic level (logical "0"). A necessary condition for the presence of a low logic level at this pin is the presence of a high logic level voltage at its input 13 . It occurs at this output only after the appearance of a high logic level at the output 10 of the DD1.3 element with a delay determined by the time constant of the R7C4C5 circuit.

Thus, with the necessary delay, the above condition for switching on the manipulation and switching the electronic switches to transmission is ensured only after switching to transmission of the electromagnetic relays. In turn, when the transceiver telegraph key circuit is closed, as well as electronic switches are switched to transmission (which is a condition for the presence of RF voltage at the transmitter output both in telephone and telegraph modes), the low logic level voltage from output 11 of element DD1.2 through the diode VD4 is fed to the input 8 of the element DD1.3. As a result, at the output 10 of this element, even when the pedal is released, a high logic level will remain, which means that switching the relay to receive will be impossible until a high logic level is reached at pin 11 of the microcircuit. When the manipulation is stopped and the pedal is released, the relay will not switch to receive immediately, but after a period of time necessary to charge the capacitor C7 through the resistor R8.

The R8C7 circuit time constant is selected to be greater than the R7C4C5 circuit time constant. Its value is chosen in such a way that if the operator accidentally (or, perhaps, deliberately to increase efficiency in work) releases the pedal before the end of the transmission with the key, he will still complete the transmission of not only the current telegraph message, but also the sign, letter, phrase . And when working in half-duplex, relay switching does not occur in pauses between telegraph messages, signs and words, which reduces wear on the contacts of electromagnetic relays and eliminates unpleasant pops in the beat of manipulation.

When working in telephone mode, switch SA1 is closed. The resistance of the resistor R2 is much greater than the resistance of the resistor R6. Therefore, thanks to the diode VD2, the logic level at input 1 of the element DD1.1 repeats the logic level at input 9 of the element DD1.3. As a result, when the pedal is pressed, the output 3 of this element will be a high logic level, as when closing (pressing) a telegraph key. When working using the VOX voice control system, the signal from this system with an active low level should be applied to input 9 of element DD1.3.

When telegraphing in half-duplex mode (switch contacts SA2.1 are closed), pressing the key, among other things, also causes the same action that occurs when you press the pedal. In order to avoid a noticeable shortening of the duration of the first telegraph message during half-duplex operation, the delay between the moment the voltage is applied to the relay windings and the moment the manipulation is turned on is reduced. Switch SA2.2 disables capacitor C4, which in other modes is connected in parallel with capacitor C5. The use of powerful inertial relays at the output of the amplifier is not an obstacle to the radio station being able to operate in half duplex. In this case, the contacts of the switch SA2.2 should be replaced with a jumper and the capacitor C4 will be permanently connected to the circuit. But then, to switch from reception to transmission, you need to start transmission with a key from an extra point (letter "E"), which will not be transmitted on the air.

Elements R3, C1, R4, C6 protect the circuit from high-frequency pickups on the wires of the key and pedal, and also reduce the effect of contact bounce.

The capacitance of capacitors C4, C5 and C7 (Fig. 3) is selected depending on the speed of the relay installed at the outputs of the transceiver and power amplifier. As a transistor VT3, you can use any silicon npn transistor with a maximum allowable collector current that is not less than the total current of all relays connected to output 1.

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