ELECTRONIC_ANTENNA_SELECTOR

Published:2009/7/10 4:54:00 Author:May | From:SeekIC

The electronic antenna selector is intended to switch between two FM antennas via a logic signal. Gates IC1A and IC1B ensure a clean switching action and at the same time form the interface between the 5-V logic level (probably available from the receive) and the 12-V supply voltage for the selecton Depending on the type of gate used, a digital TTL or CMOS control signal is available in direct and inverted form at the outputs of IC1.When input A is logic high, the output of IC1A is low and that of IC1B is high. The current then flows from the positive supply line to IC1A via T2, R9, and D8. T2 is switched on and D9 lights.Because direct currents flow through R1/D1/R2 and R5/D3/R4, diodes D1 and D3 conduct and pass the VHF signal from input A to output D. At the same time, a direct current flows through R6/D4 so that D4 conducts. This arrangement ensures that any VHF signal at input C cannot reach the output via the parasitic capacitances of the relay contacts and the wiring.When A is logic low, and IC1B is therefore low, the current flows from the positive supply line to IC1B via T1, R7, and D17. T1 is then switched on and D10 lights. At the same time, the two series-connected relays, Re1 and Re2, are energized, their contacts close, and the VHF signal at input C is fed to output D. Moreover, a direct current flows through R3/D2 so that D2 conducts. Any signal at input B is then shorted to ground via D2.All resistors should be carbon-film types because these have a higher parasitic series inductance than metal-film resistors. Thus, the attenuation of the VHF signal caused by them is reduced to a minimum.The attenuation losses caused by the diode junctions (5-10 dB) are somewhat larger than those caused by the relays. It is thus advisable to connect the antenna that provides the weaker signal (normally the domestic one) to input C. If the domestic antenna is equipped with an antenna amplifier, it can be supplied via terminal E. Diodes D5 and D6 protect the circuit against high-voltage spikes that occur during the on and off switching. The selector draws a current of approximately 65 mA.



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