FM transmitter for video and audio low-frequency signals. Scheme, description

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ENCYCLOPEDIA OF RADIO ELECTRONICS AND ELECTRICAL ENGINEERING
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FM transmitter of video and audio LF signals. Encyclopedia of radio electronics and electrical engineering

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Encyclopedia of radio electronics and electrical engineering / Телевидение

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This transmitter circuit is designed to transmit AUDIO and VIDEO signals over the radio channel. The microwave generator itself is taken from a ready-made VCO manufactured by MURATA (or similar) at a frequency of 1GHz. You will have to change something in it, raise the frequency (by soldering a piece of silver-plated (required) wire parallel to the inductance of the frequency-setting circuit so that the frequency rises ~ 1.3GHz), while the control slope should be ~ 25-40MHz / V, otherwise you will have to install a video amplifier: (

VIDEO is fed to the control input through a tuning resistor, which sets the FM depth. The signal from the sound subcarrier generator is also "injected" there. The subcarrier generator is tuned (elements L1, C1) to a frequency of 6.5MHz. The AUDIO level should be such that the frequency deviation does not exceed 75kHz. With a trimmer, we set the subcarrier level so that it does not affect the image quality. by selecting resistors in the PA bases, consumption currents are achieved: for the first stage ~ 8mA, for the second ~ 15mA and for the third ~ 45mA.

(click to enlarge)

The whole circuit is assembled by double-sided mounting on a 48x13mm board.

If you do not have the opportunity to order a three-layer board, do it the way I did: make two boards on textolite d = 0.5mm with double-sided metallization 48x13mm in size. One side remains intact (it will act as a dividing screen, and the circuit is placed on the second. The first board has a DC / DC converter (I used MAX1674), a microphone amplifier (two stages) and a 6.5MHz generator. On the second, the entire microwave part (do not forget follow the rules for microwave mounting, maximum ground, short conductors, etc.) After that, the boards are folded with the screen sides and sewn together with copper clips.

The entire structure (i.e. the board, the CMOS video camera and the container for the AA element) has dimensions of 125mm in length and 18.5mm in diameter.

A little subtlety: the board is powered through L=80-120nH chokes, and the microwave signal from the output stage of the TX is fed through a separating capacitance to the metal case of the power container serving as an antenna.

Publication: cxem.net

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