================================================================================ SCHEMATIC: TUNED RADIO FREQUENCY (TRF) CRYSTAL RADIO TM-CR-001 Rev A Three tuned circuits, improved selectivity, no battery AM broadcast 530–1700 kHz; dual-gang and triple-gang tuning ================================================================================ OVERVIEW -------- A TRF receiver uses multiple tuned circuits all tuned to the same frequency, providing dramatically improved selectivity compared to a single-tuned circuit. This design uses three coupled resonators (triple-tuned) in a purely passive configuration. Advantages of triple-tuned TRF: • 3× improvement in selectivity (narrower passband) • Better adjacent-channel rejection (>40 dB typical) • Higher out-of-band attenuation • Reduced image response at other stations entering detector Disadvantages: • Requires triple-gang variable capacitor (or three separate caps) • All three circuits must track in frequency (tracking error = distortion) • More components; larger physical size SELECTIVITY COMPARISON (at 1 MHz): Single-tuned (Q=80): −3 dB BW = 12.5 kHz; −40 dB BW = 397 kHz Double-tuned (Q=80): −3 dB BW = 7.3 kHz; −40 dB BW = 84 kHz Triple-tuned (Q=80): −3 dB BW = 5.2 kHz; −40 dB BW = 40 kHz Formula: BW_n = BW_1 × √(2^(1/n) − 1) for n cascaded identical stages. SCHEMATIC — TRIPLE-TUNED TRF CRYSTAL RADIO -------------------------------------------- ANT ──┬───────────────────────────────────────────────────────────────────── │ │ STAGE 1 STAGE 2 STAGE 3 │ ┌─────────────────┐ ┌──────────────────┐ ┌───────────────┐ │ │ L1: 220µH │ │ L2: 220µH │ │ L3: 220µH │ │ │ C1: 365pF var │ k │ C2: 365pF var │ k │ C3: 365pF │ │ │ (gang 1) │ ←→ │ (gang 2) │ ←→ │ (gang 3) │ └───┤ ANT tap │ │ │ │ ├──→ DETECTOR └────────────────┘ └──────────────────┘ └───────────────┘ │ │ │ GND GND GND Coupling between stages: Stage 1 → Stage 2: loose inductive coupling (shared ferrite rod or air gap) Stage 2 → Stage 3: loose inductive coupling Coupling coefficient: k ≈ 0.008–0.012 (critical coupling at Q=100) FULL DETAIL: ANT ─── [L1 antenna tap (25t)] ───────────────────────────────────────────── │ ┌─── Top of L1 ────────┴─────────────────────────────────────── A1 │ │ │ [C1: 365 pF variable, gang 1] │ │ └────── GND ──────────────────────────────────────────────────────────────── Coupling from A1 to Stage 2: A1 ── [C_k12: 10–22 pF coupling cap] ─── B2 ┌─── B2 (bottom of L2) ────────────────────────────────────── B2 │ │ │ [C2: 365 pF, gang 2] │ │ └────── Top of L2 ─────────────────────────────────────── A2 Coupling from A2 to Stage 3: A2 ── [C_k23: 10–22 pF coupling cap] ─── B3 ┌─── B3 (bottom of L3) ────────────────────────────────────── B3 │ │ │ [C3: 365 pF, gang 3] │ │ └──── Top of L3 ──────── DETECTOR ──────────────────────────── to earphone DETECTOR: Top of L3 ─── [D1: 1N34A] ─── AF output │ [C4: 0.01µF] [earphone] │ │ GND ─────────── GND CAPACITIVE COUPLING BETWEEN STAGES ------------------------------------ Coupling capacitors C_k12 and C_k23 control inter-stage coupling. Smaller value = weaker coupling = narrower passband = more selective. Larger value = stronger coupling = wider passband = more signal. C_k (pF) Coupling type Passband (3 tank circuits) -------- ------------- -------------------------- 5 pF Undercoupled Very narrow, some loss 10 pF Near critical Best balance (recommended) 15 pF Critical Maximum flat passband 22 pF Overcoupled Double hump appears 47 pF Overcoupled Two distinct peaks For crystal radio: start with 10 pF; adjust to maximize volume while maintaining single-peaked response. NOTE: Coupling caps ALSO add capacitance to each tank circuit. Correct: reduce variable cap maximum by C_k at high end of band. In practice: 10 pF coupling adds 10 pF to each circuit → shift of ~5 kHz at 1 MHz (negligible; within tuning range). INDUCTIVE COUPLING ALTERNATIVE -------------------------------- For smoother inter-stage coupling, use inductive coupling: All three coils L1, L2, L3 wound on SAME ferrite rod, spaced apart: L1 at position 0–50 mm (with ANT tap) L2 at position 80–130 mm L3 at position 160–178 mm (end of rod) Coupling coefficient between adjacent coils: k ≈ 0.005–0.015 (depends on rod material permeability and coil separation distance) Advantage: No coupling capacitors needed; physically simpler. Disadvantage: Ferrite rod must be 7" minimum (178 mm) to space 3 coils; tight fit. Use 10" rod for comfortable spacing. Coil separation for k ≈ 0.01 (critical coupling at Q=100): On mix-77 rod: ≈ 25 mm between coil ends On mix-61 rod: ≈ 15 mm between coil ends (lower µ, less coupling at distance) TRIPLE-GANG VARIABLE CAPACITOR -------------------------------- Option A: True triple-gang 365 pF capacitor. Available from vintage radio suppliers; Surplus sales. All three gangs on same shaft; tracking typically ±2 pF. Option B: Three separate 365 pF capacitors with ganged shaft. Connect shafts with flexible coupling (3D printed; see openscad). Requires alignment and tracking adjustment on each band. Option C: Two separate capacitors (compromise 2-stage version): Use dual-gang for stages 1 and 2; fix stage 3 as a parallel resonator. Set fixed stage 3 cap to resonate at band center (1115 kHz nominal). Re-tune only stages 1 and 2; stage 3 provides fixed-frequency selectivity boost at band center only. TWO-STAGE VERSION (SIMPLER, STILL EFFECTIVE) --------------------------------------------- For home construction without triple-gang, a dual-gang double-tuned TRF: ANT tap → [L1/C1] → [C_k: 10pF] → [L2/C2] → DETECTOR This still provides: −3 dB BW ≈ 7–8 kHz at 1 MHz −40 dB BW ≈ 80–90 kHz (vs. 40 kHz for triple-tuned) Adjacent station rejection: ≈20 dB at 10 kHz offset (much better than single) TRF WITH SHIELDED COILS (REDUCED INTER-COIL COUPLING) ------------------------------------------------------- For maximum control over inter-stage coupling, shield each coil in a separate aluminum or tin enclosure. Coupling is then ONLY through C_k caps, and k can be set precisely. Shield dimensions: coil diameter + 30 mm clearance on all sides. For 35 mm dia coil: use 75 × 75 × 75 mm aluminum enclosure. Drill holes for coil lead wires; ground enclosure to circuit ground. Crosstalk without shields (ferrite rod): −20 to −30 dB adjacent coils. Crosstalk with shields: <−60 dB (coupling is entirely through C_k). TRACKING ALIGNMENT PROCEDURE ------------------------------ After construction, the three gang sections may not track perfectly. Alignment ensures all three circuits are at the same frequency: EQUIPMENT: Signal generator (or known local station), earphone. 1. Set C1/C2/C3 to mid-range (all gangs mid-position). 2. Inject signal at antenna frequency = 1000 kHz. 3. Peak L1 position (slide coil on rod or adjust trimmer cap on C1 gang). 4. Peak L2 position similarly. 5. Peak L3 position. 6. Repeat steps 3–5 until no further improvement. Then check at 600 kHz (low end) and 1600 kHz (high end). Adjust small trimmer capacitors (10 pF) across each gang if available. Triple-gang air variables often have individual gang trimmer screws — use these for low-end and high-end tracking adjustments. PERFORMANCE SPECIFICATIONS — TRIPLE-TUNED TRF ----------------------------------------------- Selectivity (−3 dB): ≈5 kHz at 1 MHz Selectivity (−40 dB): ≈40 kHz at 1 MHz Adjacent channel: −20 dB at +10 kHz; −40 dB at +20 kHz Sensitivity: ≈3–8 mV/m (better than single-tuned due to higher effective Q from multiple resonators) Image rejection: Not applicable (no IF conversion; only in-band) Spurious response: Harmonics of local stations may appear weakly Audio bandwidth: Limited by C4 (RF bypass) and earphone response PARTS LIST — TRIPLE-TUNED TRF ------------------------------- Ref Qty Value/Part Description --- --- ---------- ----------- L1–L3 3 220 µH Ferrite rod coils (3 identical windings) C1–C3 3 365 pF Variable caps (triple-gang preferred) C_k12 1 10 pF Stage 1–2 coupling (NP0/C0G) C_k23 1 10 pF Stage 2–3 coupling (NP0/C0G) D1 1 1N34A Germanium detector C4 1 0.01 µF RF bypass R1 1 470 kΩ Earphone shunt (optional) J1 2 binding posts ANT + GND J2 1 3.5 mm jack Earphone ================================================================================