VHF micro transmitter for telephone. Encyclopedia of radio electronics and electrical engineering

Encyclopedia of radio electronics and electrical engineering / Telephony

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If you need wireless remote listening to telephone conversations on your telephone, then you will need a miniature frequency modulation transmitter circuit designed to operate in the VHF band at frequencies of 63 ... 80 MHz in conjunction with any household radio.

Ris.2.16

The circuit (Fig. 2.16) is powered by the telephone line only during a conversation, when the handset is lifted.

The conversation is listened to by the radio receiver in the section of the range where there are no broadcasting stations. The range of the transmitter without the use of the WA1 antenna is up to 50 m, and to increase the range, in addition to using the antenna, it is necessary to use a receiver with high sensitivity. So, increasing the sensitivity of the receiver by 2 times increases the reception range by the same amount.

When connecting the device to a telephone line, observe the polarity indicated in the diagram.

Setting up the circuit consists in rebuilding the generator with the core of the L1 coil to the desired frequency of the VHF band, and after that, with the capacitor C3, it is necessary to adjust the transmitter, controlling the reception by the quality of the transmission by ear. Frequency modulation in the transmitter is obtained by changing the internal capacitance of the transistor when the supply voltage of the circuit fluctuates due to the flow of current in the TA line during a conversation.

Before setting up the transmitter, you must connect it to the telephone line and, with the receiver off-hook, measure the voltage across the resistor R4. It should be in the range from 2 to 3,5 V, and if the voltage is higher, then the resistance of this resistor should be reduced.

The transmitter circuit is assembled on a single-sided printed circuit board 20x40 mm in size, elements are soldered to the contact pads (see Fig. 2.17). The dimensions of the board allow it to be placed in a standard telephone jack housing.

Fig. 2.17

Capacitor C3 is of the KPKM type, and the rest of the resistors and capacitors used can be of any type, small-sized. Coil L1 is wound on a frame with a diameter of 5 mm with a PEV wire of 0,23 mm and contains 5 + 5 turns. The KT315G transistor can be replaced with a KT3102A, and it is not recommended to use other transistors, since this greatly increases the level of harmonics that can interfere in other ranges. With the details indicated in the diagram, the level of the second harmonic of the transmitter is 40 ... 45 dB less relative to the fundamental frequency.

Fig. 2.18

As an antenna, you can use a piece of any stranded wire 30,..40 ​​cm long.

Tuning to the desired frequency, if there is no high-frequency ferrite core, can be done by selecting the capacitance of the circuit shown in the diagram by a dotted line. Capacitors C1 and C2 can have ratings of 0,022 ... 0,068 uF.

The transmitter can find other uses as well. So, in Figure 2.18, a diagram of a set-top box for a telephone for a wireless remote calling device is shown. It can be useful for those who often like to listen to VHF radio stations. If you have several rooms in your apartment, and there is only one TA, then the call signal may not be heard.

The scheme allows, when tuning the transmitter to your favorite station, to hear a telephone call signal against the background of the sound of the received station.

In conclusion, it can be noted that the connection of these circuits does not affect the quality of the phone.

Publication: cxem.net

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