You can go here and see it work.
If you want to build a link to the past via a digital modem of your own design, you have to understand a few basics. First the Barkhausen Criterion in Oscillator Design.
The Second Part of what you need is how to use Class B Push Pull Amplifiers because you will need to take advantage of Crossover Distortion.
To use Crossover Distortion as a Schmitt Trigger:
Let's say you don't know whether the phase is going to be 0=B0 to 180=B0 and useless, 360=B0 and lost in circuitry, or 540=B0 and frequency dependent, or 720=B0 and two cycles back for every one cycle forward. If you make your own coil, you will find out that it has nothing to do with resonance or the self resonance of the coil.
Let's say you build the oscillator, and when the crossover distortion has both transistor conducting when the signal in is at 0 volts, it should self start as a result of background radio noise. If the level of crossover distortion is increased below the feedback signal's output and higher than the background radio noise, it cannot self start and will require a pulse to trigger the oscillation to start. That is where you want your circuit biased. Then you need to build 2 more just like it.
OSC 1 =3D Clock Pulse High, [when on] OSC 2 =3D Clock Pulse Low, [when on] OSC 3 =3D One or Zero, [when oscillating or not]
On is considered only the presence of AC for the digital aspects of the circuit to capture the present in the past. The moment you hit send, the moment the circuit was completely turned on, you should have your digital circuitry ready. From a human perspective, if you trigger the oscillator to start, it has always been running. Turn on from the supply without a trigger pulse and you can wait all day, then turn off the circuit without sending a trigger pulse and it never oscillates. So, you will need a large value of capacitor to lead into this and a regulated power supply with limited ripple. You do not want to trigger the oscillation without the triggering pulse being the sole means and ways.
When you start the circuit the digital circuitry will respond by turning OSC 1 on. Then it checks OSC 2 and OSC 3 for data, [1 or 0]. At any point in time that a signal is found on OSC 2, OSC 1 is shut off. In the past after the data is received, the bit counter is advanced from 0 to 1, each time a signal is detected on OSC 2. If OSC
3 is on, then the bit equals 1, if OSC 2 is on and OSC 3 is off the bit equals 0. Once the data counter has advanced 1 bit in the past, it resets OSC 2 and OSC 3, then turns on OSC 1, and continues until the end of the message, or messages.Astronauts come back a few seconds younger. So, what's 200uS back in time?
Only works from send back to on time. If the circuit is on for 24 hours, you cannot send a message back farther than 24 hours.