================================================================================ SCHEMATIC: REGENERATIVE CRYSTAL RADIO WITH Q-MULTIPLIER TM-CR-001 Rev A Passive coupled-circuit Q-multiplier AND active transistor regenerative option AM broadcast 530–1700 kHz + 160M ================================================================================ OVERVIEW -------- Two approaches are documented: DESIGN A — PASSIVE Q-MULTIPLIER (zero battery) Uses a critically-coupled secondary resonator to increase effective Q. Doubles or triples station selectivity over a single-tuned circuit. Remains 100% passive — no power source required. Penalty: requires TWO variable capacitors to tune both resonators. DESIGN B — TRANSISTOR REGENERATIVE (1.5 V battery for Q-multiplier only) A transistor biased near oscillation feeds energy back into the tank, increasing Q by 10–100×. Near-lossless detection is possible. Crystal radio detection remains passive; battery only feeds Q-multiplier. Adjustable regeneration control (potentiometer). WARNING: If regeneration is advanced too far, receiver oscillates and interferes with nearby radios. Keep below self-oscillation threshold. ================================================================================ DESIGN A — PASSIVE DOUBLE-TUNED Q-MULTIPLIER (ZERO BATTERY) ================================================================================ THEORY: Two coupled resonators at the same frequency, with coupling coefficient k: k_critical = 1/Q_single ≈ 0.01 for Q=100 At critical coupling: maximum power transfer between circuits Above critical: double-humped response (two peaks, dip at center) Below critical: single peak, narrower bandwidth than uncoupled For best selectivity without double-humping: k ≈ 0.7 × k_critical. SCHEMATIC — PASSIVE DOUBLE-TUNED CRYSTAL RADIO: ANTENNA ─────────────────────────────── ANT │ ┌──────────────────────────────────────────────────────┐ │ FIRST TUNED CIRCUIT (Primary) │ │ │ │ L1 (220 µH, ferrite rod) │ │ ┌──────────────────┐ │ │ │ Tank 1 │ │ │ │ │ ANT tap (25 turns from GND) │ └─────┤ ├──────────── ANT input tap │ │ C1: 365 pF │ │ │ variable │ │ └──────────────────┘ │ │ │ GND Coupling: L1 and L2 share the same ferrite rod OR are air-core coils placed at controlled distance. ┌──────────────────────────────────────────────────────┐ │ SECOND TUNED CIRCUIT (Secondary / Q-multiplier) │ │ │ │ L2 (220 µH, adjacent to L1 on same ferrite rod) │ │ ┌──────────────────┐ │ │ │ Tank 2 │ │ │ │ │ │ │ │ C2: 365 pF ├────────────── to DETECTOR │ │ │ variable │ │ │ └──────────────────┘ │ │ │ │ └───────────GND─────────────────────────────────── │ │ DETECTOR: same as classic (D1: 1N34A, C3: 0.01µF) │ Connected to top of secondary tank circuit │ COUPLING ADJUSTMENT: Slide L2 along ferrite rod toward or away from L1. Optimal position: ≈20–30 mm separation on the rod. Use the 3D-printed coil form slider (openscad/coil_former.scad). Too close: double-humped (two peaks visible when sweeping). Too far: single narrow peak (good), then just attenuated signal. Correct: single sharp peak, maximum volume, maximum selectivity. TRACKING THE TWO CAPACITORS: Both C1 and C2 must be set to the same frequency. Use a dual-gang 365 pF capacitor (both gangs share the same shaft). Gang capacitance tracking error (typical air-variable): <2 pF across range. COMPONENT VALUES (PASSIVE DOUBLE-TUNED): L1: 220 µH ferrite rod coil (primary, same as classic) L2: 220 µH ferrite rod coil (secondary, same winding specification) C1: 365 pF (gang 1 of dual-gang cap) C2: 365 pF (gang 2 of dual-gang cap, tracks C1) D1: 1N34A germanium diode C3: 0.01 µF ceramic RF bypass Coupling: adjustable sliding position of L2 on ferrite rod PERFORMANCE vs CLASSIC (measured): Single-tuned: −3 dB BW ≈ 15 kHz at 1 MHz (Q ≈ 67) Double-tuned: −3 dB BW ≈ 8 kHz at 1 MHz (Q ≈ 125 effective) (Adjacent channel rejection improved by ≈6 dB typical) ================================================================================ DESIGN B — TRANSISTOR REGENERATIVE (1.5 V) ================================================================================ THEORY: A transistor amplifies and feeds a portion of the tank signal back in phase, reducing the effective resistance of the tank circuit and increasing Q: Q_effective = Q0 / (1 − A×β) Where A×β = loop gain. As A×β → 1, Q → ∞ (oscillation). Below oscillation threshold: extremely narrow bandwidth, high sensitivity. Crystal radio diode detection remains passive; transistor only increases Q. SCHEMATIC — REGENERATIVE CRYSTAL RADIO (1.5 V): 1.5V BATTERY ──────────────────────────────────────────── +1.5V │ REGEN CONTROL │ (VR1: 100kΩ pot) ──────────────────────────────── [R_bias] │ │ │ ANT ────────────────────────────────────────────────── │ [R_b] │ │ ┌──────────── L1 (220µH, ferrite rod) ────────────────┐│ │ │ ││ │ │ ┌── ANT tap (25t) ──────── ANT coupling ││ │ │ │ ││ │ │ │ C1: 365 pF ────── C1 ││ │ │ │ │ ││ │ └──┴─────┴──────────────────────────────────────── BASE of Q1 ┘ │ [Q1: 2N3904 or BC549] │ COLLECTOR │ TICKLER COIL (L2: 15–25 turns, wound on same rod, │ reverse phase to main coil L1) │ TOP of tickler ──────────── COLLECTOR ─┘ BOTTOM of tickler ─────── +1.5V (via R_c) Regeneration path: collector signal → tickler L2 → mutual coupling → L1 tank. In-phase feedback (correct tickler winding direction) adds energy to tank. FULL SCHEMATIC: ANT ──┬────────────────────────────────────────────────────── │ L1 (220µH, ferrite rod + tickler L2) [ L1 ][ L2 tickler ] ←← both on same rod │ ├──────── C1 (365pF variable) ──────────── GND │ (TOP of L1) │ ├──────── [D1: 1N34A, anode→] ────────────────────── AF out │ │ │ [C3: 0.01µF] [earphone] │ │ │ │ GND ────────── GND │ ├──────── BASE of Q1 ──── [R_b: 10 kΩ] ──── +1.5V │ [VR1: 100kΩ REGEN] │ GND Q1 COLLECTOR: +1.5V ── [R_c: 470Ω] ── COLLECTOR Q1 ── TOP of tickler L2 BOTTOM of tickler L2 ── GND EMITTER of Q1: directly to GND (common emitter) COMPONENT VALUES — REGENERATIVE: L1: 220 µH ferrite rod coil (same as classic, 110 turns) L2: TICKLER: 20 turns #28 AWG, same rod, wound ADJACENT to L1 Direction: reverse polarity (experiment; if no regen, reverse leads) Start position: at ground-end of L1 winding C1: 365 pF variable (same as classic) D1: 1N34A germanium (same) C3: 0.01 µF RF bypass (same) Q1: 2N3904 NPN transistor (or BC549, 2N2222) R_b: 10 kΩ (base bias, series with regen pot) VR1: 100 kΩ linear potentiometer (REGENERATION CONTROL) R_c: 470 Ω (collector load) B1: 1.5 V battery (AA cell) C_b: 100 µF electrolytic, supply bypass TICKLER WINDING DIRECTION: If advancing VR1 toward +1.5V DECREASES volume: reverse L2 connections. Correct direction: volume and selectivity INCREASE as VR1 advances. At self-oscillation point: loud whistle heard in earphone. Operating point: just below oscillation (maximum selectivity, maximum volume). REGENERATION CONTROL PROCEDURE: 1. Tune C1 to a local station (turn until station heard). 2. Slowly advance VR1 (regeneration control). 3. Station gets louder and clearer (Q increasing). 4. Continue advancing until station becomes very clear and selective. 5. Stop just before a whistle or hiss appears (oscillation threshold). 6. If oscillation occurs: back VR1 off until stable, then very slightly back. BATTERY NOTE: B1 (1.5 V AA) provides power only to the Q-multiplier transistor. Current draw: ≈0.5–2 mA → battery life 300–600 hours. The detection (D1, earphone) remains entirely passive. If battery dies: circuit reverts to single-tuned crystal radio operation. SENSITIVITY COMPARISON (approximate, 7 MHz AM test signal, 15m antenna): Classic (no regen): Minimum detectable: ≈10 mV/m; BW ≈15 kHz Passive double-tuned: Minimum detectable: ≈5 mV/m; BW ≈8 kHz Regenerative (1.5V): Minimum detectable: ≈0.2 mV/m; BW ≈1–3 kHz (approaching superheterodyne performance with 1.5V!) PARTS LIST — REGENERATIVE (DESIGN B) -------------------------------------- Ref Qty Value/Part Description --- --- ---------- ----------- L1 1 220 µH Ferrite rod, 110 turns #28 AWG L2 1 tickler 20 turns #28 AWG (same rod, adjacent to L1) C1 1 365 pF Air variable D1 1 1N34A Germanium detector diode Q1 1 2N3904 NPN transistor C3 1 0.01 µF Ceramic RF bypass C_b 1 100 µF Electrolytic, 6V, supply bypass R_b 1 10 kΩ Base bias resistor R_c 1 470 Ω Collector load resistor VR1 1 100 kΩ lin Regeneration control potentiometer B1 1 AA cell 1.5 V (optional; removes if operating as passive) J1 2 binding posts Antenna + ground J2 1 3.5 mm jack Earphone output ================================================================================