Sending serial data over analog TV coaxial cable (RF-band)
TV coax is terminated in 75 ohms. If we contribute 1milliAmp to that, we get 75 milliVolts signal. With double_ended termination on TV cables (often the case), you would get 37.5 millivolts. So let us use 2 milliAmps, and have at least 75 milliVolts of data signal.
As transmitter, you can use a TTL gate running on +5volts, or a CMOS gate running of +3.3 or +5 volts.
From the TTL output, you need a series/shunt/series RCR. The capacitor is to greatly slow the TTL edge, so the RF TV signals get no interference. The sum of the 2 resistors has the purpose of providing at least 2 milliAmps into the TV coax cable. With 3 volts and 2 milliAmps, the sum of the 2 resistors needs to be 1,500 ohms.
If each is 750 ohms, the equivalent at the middle node is 375 ohms, and 1,000 pF
from middle node to Ground produces 375 nanosecond time constant or about 500,000 Hertz bandwidth, This should easily support 100,000 bits per second, yet cause little interference with the TV signals.
For signaling, use a self-clocking PWM signal, with Rising edge the &quot;start&quot;, and the pulse width being either &quot;1/3&quot; bit time, or &quot;2/3&quot; bit time.
For your receiver, use a medium speed (100 nanosecond Tpd is fine) analog comparator, with threshold at Vin/2 or 35 milliVolts. Have 5 milliVolts of hysteresis. This uses the Vin+ pin and the Vout.
On the Vin- pin, have a series R and a Shunt C; 3,300 ohm resistor and then 100pF cap right at the Vin- pin. You should add more capacitance there, if you have Bit Error problems. [had been 330 ohm and 1,000pf, but that low R would have caused reflections.]
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As Jason suggests in the comment below, the low_frequency behavior of the cable has strong influence on the signaling method chosen.
I assumed there WAS DC_TERMINATION into 75 ohms.