144 MHz transmitter on a 74HC00 chip. Scheme, description

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144 MHz transmitter on a 74HC00 chip. 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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Digital microcircuits of the 74HC series can operate at frequencies up to 150 MHz and higher. This allows you to build on their basis low-power transmitter circuits operating at frequencies of the 2-meter range (144 MHz).

144 MHz transmitter on a 74HC00 chip

Figure 1 shows a schematic diagram of a micro power narrowband chirp radio transmitter operating at 144,855 MHz.

The master oscillator is made on the D1.1 element according to the quartz multivibrator circuit. The generation frequency is set by the quartz resonator Q1. In order for the transmitter to fall into the range of 144-146 MHz, the resonator must be at a frequency within 16..16,22 MHz. In this case, the resonator is at 16095 kHz.

As you know, the frequency of such a multivibrator to some extent depends on the capacitances connected between the outputs of the resonator and the common minus. One of these capacitances is formed by a circuit consisting of a capacitor 1 (it performs the functions of a separating one, excluding the penetration of the modulating voltage to the input D1 1), and a varicap, with the help of which frequency modulation is carried out. Capacitor C2 can adjust the generation frequency within a small range.

Element D1.2 - buffer cascade. The circuit on the elements D1.3 and D1.4 is used to generate short pulses. At the output of D1.4, a signal is formed consisting of many harmonics. The ninth harmonic corresponds to the frequency of the two-meter range.

The antenna is connected directly to the output of the element D1.4. The emitted signal has many harmonics and the frequency at which the maximum will be depends on the antenna tuning. For operation at 144 MHz, the antenna is a 50 cm long rod. You can use a shorter rod, but this reduces the reception range.

The transmitter power is very small (5-10 mW).

The main advantage of the transmitter, the circuit of which is shown in Figure 1, is the absence of coils in the circuit.

It is possible to improve the purity of the output signal and increase its power by supplementing the circuit with a power amplifier based on a transistor with a resonant output, which is tuned to the used antenna. A diagram of such a transmitter is shown in Figure 2. The only difference is in the power amplifier on VT1. In its collector circuit, a circuit is included, tuned, together with the antenna, to 144.855 MHz.

Coil L1 on a frame made of plastic with a diameter of 4 mm, with a tuning brass core. Contains 6 turns of PEV 0.43 wire.

The output stage is adjusted with a working antenna, adjusting the circuit and the ratio of the capacitive transformer, using the trimmer L1 and condensates C9 and C8. The setting is controlled by a resonant wavemeter.

144 MHz transmitter on a 74HC00 chip

The output power (Fig. 2) is about 200mW.

Author: Andreev S.; Publication: radioradar.net

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Vyacheslav Mikhailovich Svateev
Varicap VV909V - what kind of animal is this? There is no data on it. It is also not on sale. Where to buy it? If there is an equivalent, please let me know.

Ivan
KV121=VV909

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