Radio receiver AM signals on a special chip
The sensitivity of the receiver when the signal-to-noise of 12 dB is about 1 µv/m Selectivity adjacent channel - 32 dB (depends on the settings used piezoceramic filter). Selectivity mirror channel - 26 dB. Power amplifier audio frequency of 100 mW at a load impedance of 8 Ohms. The receiver operates at supply voltages from 4 to 9 V.
Figure 21. Receiver amplitude-modulated signals.
A schematic diagram is given in figure 21. The signal from the antenna is supplied to the base of transistor VT1, which acts as a resonant high-frequency amplifier. The circuit L1, C3 determines the selectivity of the receiver mirror channel. The amplified signal is input to the frequency Converter, made on the VT2 transistor according to the scheme combined with a local oscillator whose frequency is stabilized by a quartz ZQ1. With the load of the Converter, the inductor DR1, the intermediate frequency signal is supplied to piezoceramic filter ZQ2, which set of frequencies allocates an intermediate frequency of 465 kHz. Filtered inverter is input to the HRC chip DA1. The output stage is enabled by the HRC non-standard scheme, the role of the load of the HRC performs the resistor R8. This slightly degrades the quality of detection, but allows you to refuse the use of the inverter circuits and their settings. From the output of the voltage detector audio frequency is fed to the volume control R10 and to the input of the power amplifier of the chip. With the release OSC signal through the capacitor C13 is supplied to the load is a loudspeaker or head phones. Transistors VT1, VT2 can be replaced by CT, KT368. All resistance in the circuit - type MLT-0,125, resistor R10 - type SL. Coil L1 is wound on a ferrite rod with a diameter of 2.8 mm and a length of 14 mm and has 16 turns of wire sew 0.23 mm with a branch from the 1st round, counting from the bottom on the diagram. Choke DR1 wound on the same core and contains 150 turns of wire sew 0.1 mm. Setting begin with installation of the collector currents of transistors VT1 and VT2 is equal to 0.7 mA and 2 mA, respectively, by selection of resistors R1 and R3. The resistor R8 is selected by minimum distortion of the audio signal with minimum noise level at the output USC. The circuit L1, NW tuned to the frequency of a high frequency signal.
Figure 22. The high-frequency part of the circuit of the receiver.
To upgrade this receiver by applying another specialized chip - DA2 CPS. It acts as a mixer and local oscillator and has a higher transmission coefficient. The high-frequency part of the circuit (figure 21) is replaced by the diagram presented in figure 22. Designation of parts diagrams continued figure 21. Coil L1 is made without challenge. VT3 transistor performs the role of the balun device.