================================================================================ active_noise_canceller_ascii.txt — TM-RFI-SCH-003 Rev A Active RFI Noise Canceller: Phased Auxiliary Antenna Nulling 2-Channel Analog Phase/Amplitude Combiner with Op-Amp Subtractor Document: TM-RFI-SCH-003 Rev A ================================================================================ OVERVIEW -------- An active noise canceller uses a reference (auxiliary) antenna aimed at the noise source to sample the noise waveform. After phase-shifting and amplitude- matching the reference to the noise component in the main antenna signal, the reference is subtracted from the main signal, cancelling the noise. The desired signal (from a distant station) arrives from a different direction than the local noise source. If the reference antenna has very little desired signal and mostly noise, cancellation is selective: noise is removed while the desired signal is preserved. PRINCIPLE: Main antenna: V_main = V_signal + V_noise_main Ref antenna: V_ref = V_noise_ref (negligible V_signal) After scaling: V_ref' = A × e^(jφ) × V_noise_ref Cancel output: V_out = V_main − V_ref' ≈ V_signal (if A and φ set correctly) SYSTEM BLOCK DIAGRAM ---------------------- [MAIN ANTENNA] ─────────────────────────────────────────────┐ ├──> [COMBINER] ──> [SDR/TRX] [REF ANTENNA] ── [RF PREAMP] ── [PHASE SHIFT] ── [GAIN] ──┘ (0–360°) (0–+20dB) KEY SPECIFICATIONS ------------------ Frequency range: 0.5 – 30 MHz (HF), with modification to 50 MHz Noise cancellation: 20–40 dB typical (depends on noise source geometry) Phase adjustment: 0° to 360° continuous Gain adjustment: −20 dB to +20 dB (40 dB range) Reference antenna: short (1–3m) vertical whip or small loop Input impedance: 50Ω (main and reference ports) Output impedance: 50Ω Supply: ±12V or +12V single-supply Current: 50 mA typical SCHEMATIC — REFERENCE ANTENNA PREAMPLIFIER ------------------------------------------- Short whip antenna has very low radiation resistance (~1Ω for 1m whip at 7 MHz) and high capacitive reactance. A high-impedance JFET buffer is essential. REF ANTENNA INPUT (SMA J1): J1 center ─── R1 (100kΩ) ─── gate of Q1 (J310 JFET) J1 center ─── C1 (10 pF) ─── GND (ESD/static bleed) J310 JFET source follower: Gate (G): from antenna (hi-Z, ~100 MΩ + C_gate) Source (S): R_S (2.2 kΩ to GND) → output via C_couple (1 µF NP0) Drain (D): +12V via R_D (470Ω) JFET Q1 (J310 or BF245A): +12V │ R_D (470Ω) │ ant ── R1 ──── G ─────────────────────────────┐ (100kΩ) │ J310 │ D │ │ C2 S ─── R_S (2.2kΩ) ─── GND (1µF) │ │ └──────────────────────────────┘ ── to phase shifter Gain: ~0.9 (source follower, near unity) Output impedance: R_S / (µ + 1) ≈ 2200/30 ≈ 73Ω → close to 50Ω Optional JFET buffer + NE5534 op-amp buffer for lower output impedance: JFET source ── NE5534 buffer ── 50Ω output SCHEMATIC — ALL-PASS PHASE SHIFTER (0°–360°, VARIABLE) --------------------------------------------------------- Two-stage RC all-pass network: each stage shifts 0°–180°, cascaded gives 0°–360°. Op-amp based for flat amplitude response across phase shift. STAGE 1 (0°–180° shift): ┌────────────────────┐ │ NE5532 A │ IN ── R2 ─────────┤+ (non-inv) out ├──── to Stage 2 / gain control (5.1kΩ) │ │ │ │ - (inv) │ └── RV1 ──── ┤─ (inv input) ───── + (10kΩ) └────────────────────┘ │ C3 (10 nF NP0) │ GND The all-pass phase shift at frequency f: φ = 2 × arctan(2π × f × C3 × RV1) At RV1 = 0: φ = 0°; at RV1 = ∞: φ = 180° For 7 MHz full 180° sweep: C3 = 10 nF, RV1 max needed = 1/(2π × 7e6 × 10e-9) = 2.27 kΩ Use RV1 = 10 kΩ pot (covers full 180° range + margin at 7 MHz) For broadband: C3 = 10 nF covers 1–30 MHz (phase center shifts with frequency) STAGE 2 (SECOND 0°–180°): Identical circuit with RV2 (10 kΩ pot) and C4 (10 nF). Total phase: 0° to 360° by adjusting RV1 and RV2 independently. PRACTICAL NOTE: Because noise source frequency may vary, a simpler single-stage 90° shift + gain inversion covers 0°–270° adequately for many installations: - Stage 1 (RV1 = variable, 0°–180°) - INVERT (via NE5532 inverting amp): adds 180° - Total: 180° to 360° Combined with non-inverted path: covers 0° to 360° COMMERCIAL ALTERNATIVE: MFJ-1026 Noise Cancelling Antenna Phaser (~$100 used). Uses similar principle. Study its circuit for reference. SCHEMATIC — VARIABLE GAIN STAGE --------------------------------- After phase shifting, amplitude must match noise contribution in main antenna. Use a variable-gain inverting amplifier (NE5532): Phase shift out ── R3 ─────┐ (10kΩ) │ NE5532 B │ -input ──────── output (gain-controlled) │ +input ──── GND (virtual) │ RV3 (log pot, 100kΩ) ── GND Gain = −(RV3 setting) / R3 = 0 to −10 (0 to +20 dB inverting) For non-inverting gain: use non-inverting topology or split gain range COMBINED GAIN/PHASE CONTROL: In practice: use one continuous pot for gain (0 to +20 dB) and two pots for phase (0–360°). Adjust in order: 1. Set phase while observing noise reduction 2. Fine-adjust gain for deepest null 3. Iterate (phase and amplitude interact) SCHEMATIC — RF COMBINER (SUBTRACTOR) -------------------------------------- Wideband RF transformer combiner-subtractor: OPTION 1: Transformer Combiner (passive, insertion loss ~6 dB): Use a 1:1:1 trifilar transformer (3 equal windings on FT-37-43): Main antenna (50Ω) ─── winding 1 Reference signal (50Ω) ─── winding 2 (reversed polarity = subtraction) Output (50Ω) ─── winding 3 10 turns trifilar on FT-37-43: Winding 1 start → antenna Winding 2 start → reference IN, END reversed (subtract, not add) Winding 3 start → output OPTION 2: Resistive Hybrid Combiner (6 dB insertion loss, flat amplitude): Standard 2-to-1 combining: Main ──(R=50Ω)──┬── 50Ω output Ref ──(R=50Ω)──┘ Reversed connection on Ref port = subtraction. Loss: 6 dB (half power lost in isolation resistors). Acceptable for RX. OPTION 3: Active Op-Amp Subtractor (zero insertion loss, low IP3): For receive-only use with op-amp: Main (via voltage divider to op-amp level) ── NE5532 + input Ref (via gain stage, inverted) ── NE5532 − input NE5532 output ── 50Ω series R ── to SDR Caution: op-amp IP3 limits dynamic range. NE5532 IP3 ≈ +20 dBm. Adequate for receive-only. Do not use on transmit path. COMPLETE CIRCUIT BLOCK ------------------------ [50Ω SMA] [50Ω SMA] MAIN ANT ──┬──────────────────────────────────────────┐ │ (through) │ │ COMBINER ──> OUT (to SDR) │ REF ANT ──[JFET preamp]──[phase RV1+RV2]──[gain RV3]──┘ │ +12V supply ADJUSTMENT PROCEDURE (FIELD): 1. Connect noise source antenna (short whip, loop, or directional) to REF port. 2. Connect main antenna to MAIN port. 3. Tune radio to noise QRM frequency. 4. Slowly adjust RV1 (phase 1) while monitoring noise level on receiver. 5. Adjust RV2 (phase 2) for further null. 6. Fine-adjust RV3 (amplitude) for deepest null. 7. Noise null should be 20–40 dB below original level. 8. Check that desired signal is preserved (may be slightly attenuated in null direction). POWER SUPPLY -------------- Single +12V input ─── 7812 → +12V for reference supply └─── 7912 → −12V (if dual-supply op-amp design) Or: use +5V single-supply op-amps (TLV2372 or MCP6022) on single +5V. TLV2372: rail-to-rail, ±5V, GBP 2.4 MHz — adequate for HF audio/IF use. Note: op-amp GBP must exceed 10 × max operating frequency. For 30 MHz: use LMH6702 or similar wideband op-amp. BILL OF MATERIALS — ACTIVE NOISE CANCELLER -------------------------------------------- Qty Part Value/Type Notes ────────────────────────────────────────────────────────────────── 1 Q1 J310 JFET N-channel or BF245A 2 U1,U2 NE5532 Dual low-noise op-amp DIP-8 1 T1 FT-37-43 trifilar transformer 10 turns each winding 3 Pot 10 kΩ RV1, RV2, RV3 linear taper (audio log for RV3) 6 Cap 10 nF NP0 C3–C8 timing caps for phase shift various Resistors 1% metal film 5.1kΩ, 10kΩ, 470Ω, etc. 1 Reg 7812+7912 ±12V regulators if dual-supply 3 SMA connectors 50Ω MAIN, REF, OUT 1 Enclosure 3D printed see noise_canceller_box.scad 2 Knobs for front panel pots RV1, RV2 (phase), RV3 (gain) ================================================================================