This circuit was designed and used to transmit commands over LNB coaxial cable. An LNB (or LNC) is a low-noise block downconverter typically used for satellite TV reception. It’s fitted in the focal point of a satellite dish. [LNB Cable Data Transceiver Circuit] The circuit is based on generating a modulated signal on the bus which can be decoded by a tone decoder IC like the familiar LM567 from National Semiconductor.Data and carrier signals are ORed using D1 and D2. T1 acts as a current source whose current depends mainly on the value of R3. L1 and C5 form a (damped) resonance circuit for the centre frequency of the carrier. C6 acts as a very low impedance bypass, so the impedance seen by T1 at the carrier frequency equals roughly R4.
As the current passes through R4, the voltage generated across it can be detected by IC1 which has its input coupled to the bus via C4. The low DC resistance of inductor L1 allows current to flow to the circuitry connected to the bus. Components R1 and C1 control the centre frequency of the decoder, and C2 the bandwidth.
Relevant formulas may be found in the LM567 datasheets. C3 is output filter and its value depends on the ‘data’ frequency. In accordance to what’s found in the LM567 datasheet, the carrier frequency must be at least 20 times higher than the frequency of the ‘data’ signal. The maximum detectable carrier frequency is about 500 kHz. R5 is just a load for IC1, whose output is signal in phase with ‘data’. The Carrier frequency can be generated using any simple square wave generator. In the author’s application, the carrier frequency was 100 kHz with 1200 bps data, both generated by a microcontroller. The transmitter and receiver were installed at each side of the LNB cable to create a half-duplex transceiver. Author: Sajjad Moosavi.