The Regenerative Receiver

Establish the baseline the trick has to beat.

1. Build a single amplifying stage — a triode valve, or a JFET if you prefer to work at low voltage — with your tuned circuit as its input load.
2. Inject a small known signal and measure the output amplitude.
3. Compute the voltage gain.
4. Measure the bandwidth of the stage as you did for the bare tuned circuit.

One stage gives perhaps twenty to fifty times, and the tuned circuit’s Q is slightly WORSE than it was standalone. That degradation is the important observation: the valve’s input is not infinitely light, and whatever it loads onto the tuned circuit costs you selectivity.

Feed a little of the output back into the input, in phase, and increase it slowly.

1. Wind a small coil — the tickler — near the main tuned coil, connected in the output circuit.
2. Arrange a way to vary the coupling: physically move the tickler, or use a potentiometer to control how much output reaches it.
3. With a weak test signal applied, increase the feedback gradually while watching the output.
4. Record output amplitude against feedback setting.

Gain climbs slowly at first, then accelerates dramatically, and just before it becomes unstable it is enormous. The circuit is now supplying part of its own losses: each cycle is reinforced by the amplified version of the previous cycle, so the ringing you saw decay in the tuned-circuit blueprint decays far more slowly.

If the tickler is connected the wrong way round you will get NEGATIVE feedback instead — gain falls below the plain amplifier. Reverse the coil. That accident is worth having deliberately, because it demonstrates that the sign of the feedback is everything and the magnitude is only the fine adjustment.

Push the feedback past the point of stability and see what the circuit becomes.

1. Remove the input signal entirely.
2. Advance the regeneration control past the threshold.
3. Look at the output on the oscilloscope and measure its frequency.
4. Compare that frequency with the tuned circuit’s resonance measured earlier.

With no input at all, the circuit produces a clean sine wave at its resonant frequency. It has become an oscillator. Feedback greater than the losses means any tiny disturbance — thermal noise, switch-on transient — is amplified, fed back, amplified again, and grows until the valve runs out of linear range and the amplitude settles.

This is the single most important boundary in electronics: an amplifier and an oscillator are the same circuit, separated only by how much of the output returns to the input. Every oscillator is built by deliberately crossing this line; every amplifier that squeals or a PA system that howls has crossed it by accident. The Barkhausen condition — loop gain of one, phase shift of zero — is just this observation written down.

The useful operating point is a hair below the threshold, and finding it is a skill.

1. Connect antenna and earth, and a high-impedance earpiece or an audio amplifier at the output.
2. Tune to a station, then advance regeneration slowly until you hear the background noise rise sharply and the signal strengthen.
3. Back off very slightly, until just before the circuit whistles.
4. Re-measure the effective bandwidth by sweeping a test signal at this setting.

Sensitivity and selectivity are both dramatically better than the crystal set — often by orders of magnitude — from one valve. The bandwidth at this operating point is a small fraction of what the same tuned circuit gave passively, because the feedback is cancelling the resistance that was broadening it.

The state you were avoiding turns out to be exactly what a different signal needs.

1. Tune to a continuous-wave Morse transmission — an unmodulated carrier that simply switches on and off.
2. With regeneration below threshold, listen. You will hear almost nothing but clicks.
3. Now advance past the threshold so the circuit oscillates, and tune slightly off the carrier frequency.
4. Listen again, and vary the offset.

The Morse becomes a clear musical tone whose pitch you can set by how far off you tune. The local oscillation and the incoming carrier are mixing, and the difference between two frequencies is an audible beat note. A carrier that carries no modulation is silent by itself; heterodyning it against a local oscillator makes it audible.

Take that idea seriously, because it is the whole of the next blueprint. Mixing two frequencies produces their sum and their difference, and the difference can be placed anywhere you choose by choosing the local oscillator. Here it is used to move a signal down to audio. In the superheterodyne it is used to move any station to one fixed frequency, so a single perfectly-optimised amplifier can handle all of them.