Bandpass Filter for Bell103A Originator Modem

Bell103A modem standard specify that the originator (the caller) uses 1070Hz and 1270Hz frequency shift keying for transmission, and listen for 2025/2225Hz FSK transmitted by the answering modem. This bandpass circuit filter out any frequency beyond 2025-2250 Hz to improve originator’s decoding of the received FSK signal. Here is the schematic diagram of the circuit:

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LTC1045 TTL RS-232 Converter

A simple way to build a battery-powered RS-232 receiver  circuit is shown on the following schematic diagram. This circuit convert RS232 level to TTL level. Because the 100 KΩ resistor prevents device damage, the input voltage can be driven as much as +- 30V without adverse effects. The LTC1045 draws only 100 μA of quiescent current, and has a propagation delay […]

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LM359 Fast Photodiode Amplifier

Photo-diode can be used for high speed digital transmission, but you have to provide high speed signal conditioner for that purpose. The amplifier circuit shown in the schematic diagram below satisfy the high-speed requirement. The circuit is based on current mode (Norton) operational amplifier, that’s why the response is faster than regular op-amp.  The frequency response of this circuit is […]

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