Micropower FM transmitter. Encyclopedia of radio electronics and electrical engineering
Encyclopedia of radio electronics and electrical engineering / Audio equipment
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This micropower transmitter can be used as a radio microphone, operating at a frequency of 87-9 MHz specially designated for such purposes. The signal is received by any VHF FM receiver having this range (VHF-2 or FM). The transmitter is powered by one galvanic cell of the AA or AAA series.
Transmitter Specifications
- Output power, mW, no more ...... 0,5
- Supply voltage, V ...... 1,5
- Current consumption mA......2
- Range, m, not more than ...... 50
The device, the scheme of which is shown in Fig. 1 is a generator assembled according to the inductive three-point circuit on a field-effect transistor with an insulated gate VT1. The oscillatory circuit is formed by the coil L1 and the varicap matrix VD1. The connection with the antenna is inductive, using the L2 coil. An antenna can be a piece of any wire from 0 2 to 0,8 m long, for compactness it can be twisted into a spiral.
The modulating signal from the electret microphone through the isolation capacitor CI and the resistor R3 enters the varicap matrix, which causes a change in its capacitance and, as a result, frequency modulation (FM) of the generator. The voltage divider R1R2 is used to set the operating point of the matrix varicaps. The passport value of its capacitance (at a bias voltage of 4 V) is about 30 pF. With a bias voltage of only 0.75 V, as in this design, the capacitance is larger and reaches 60…100 pF. But the steepness of the capacitance-voltage characteristic also increases, which makes it possible to do without additional amplification of the AF signals.
Transmitter coils are frameless. They are wound on a mandrel with a diameter of 4 mm with PEV-2 0 44 wire and contain: LI ~ 7 turns with a tap from the 3rd turn, counting from the lead connected to the common wire: L2 - 1 turn. The microphone can be any electret, for example MkE-3 or similar imported. The KVS111 varicap matrix can be taken with any letter index, the transistor also, but you need to select an instance with an initial drain current in the range of 1,5 ... 4 mA. The drain current can be measured on the assembled transmitter by closing the terminals of the L1 coil with a short conductor and connecting a milliammeter in series with the battery.
A drawing of the printed circuit board on which the transmitter is assembled is shown in Fig.2.
There are no holes in the board, and the leads of the parts are soldered directly to the pads.
The transmitter tuning is reduced to setting the frequency to 87,9 MHz by compressing and stretching the turns of the L1 coil. This is easy to do by placing a receiver near the VHF FM device and listening to the signal on it. Communication with the antenna is selected according to the maximum signal, choosing the distance between the coils L1 and L2. It may be necessary to move the monitoring receiver further away from the transmitter.
The communication coil is placed coaxially with the contour coil, closer to its output connected to the common wire.
Author: N.Tokarev, Moscow
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