================================================================================ SCHEMATIC: Notch Filter — Manual Twin-T and Auto-Notch PLL TM-AF-001 Rev A Heterodyne / Beat Note Rejection, Tunable 300–3500 Hz ================================================================================ SECTION 1 — TWIN-T PASSIVE NOTCH ────────────────────────────────── The twin-T null network produces a notch (band-reject) response at: f_notch = 1 / (2π × R × C) Standard twin-T topology: Upper T: R–R–C/2 (two series resistors, one shunt capacitor) Lower T: C–C–R/2 (two series capacitors, one shunt resistor) For f_notch = 800 Hz (common CW heterodyne): Choose C = 100 nF, polypropylene film: R = 1 / (2π × 800 × 100e-9) = 1989 Ω → 2.0 kΩ R/2 = 1.0 kΩ C×2 = 200 nF (2× 100 nF in parallel) Notch depth (theoretical): −60 dB with matched components Notch depth (practical): −30 to −45 dB with 1% components −3 dB bandwidth: ~50 Hz each side at 800 Hz TWIN-T NOTCH — CIRCUIT DIAGRAM: R1 (2.0kΩ) R2 (2.0kΩ) IN ─────────────/\/\/────────┬────/\/\/──────────────── OUT │ C3 (200nF = 2×100nF) │ GND C1 (100nF) C2 (100nF) IN ────────┤├────────┬──────┤├──────────────────── (connect to junction above) │ R3 (1.0kΩ) │ GND Full diagram with both T networks: R1 2kΩ R2 2kΩ IN ──────/\/\/──────┬──────/\/\/──────┬──── OUT │ │ C3 (same node = OUT) 200nF │ GND C1 100nF C2 100nF IN ───────┤├─────────────┤├────────── OUT (same input, same output nodes) │ R3 1kΩ │ GND Combined layout (both paths connect same IN and OUT nodes): R1(2k) R2(2k) ┌───/\/\/──┬──/\/\/───┐ │ │ │ │ C3 200nF │ IN ───────────────┤ │ ├──────────── OUT │ GND │ │ │ └───┤C1├──┬──┤C2├────┘ 100nF │ 100nF R3 1kΩ │ GND STANDARD NOTCH FREQUENCIES (using C=100nF, vary R): f_notch (Hz) | R (Ω) | R/2 (Ω) | Use -------------|----------|----------|---------------------------- 300 | 5.305kΩ | 2.65kΩ | Low heterodyne / low beat 400 | 3.979kΩ | 1.99kΩ | 400 Hz CW sidetone alt 500 | 3.183kΩ | 1.59kΩ | 500 Hz beat note 600 | 2.653kΩ | 1.33kΩ | 600 Hz beat note 700 | 2.274kΩ | 1.14kΩ | Common CW pitch 800 | 1.989kΩ | 0.99kΩ | Standard, common AM carrier 1000 | 1.592kΩ | 0.80kΩ | 1 kHz reference / beat 1500 | 1.061kΩ | 0.53kΩ | Higher heterodyne 2000 | 0.796kΩ | 0.40kΩ | Carriers near upper SSB edge 3000 | 0.531kΩ | 0.27kΩ | Whistle / high-pitched tone ================================================================================ SECTION 2 — VARIABLE TWIN-T WITH GANGED POT ================================================================================ For continuously variable notch frequency 300–3500 Hz, replace fixed resistors with a ganged dual-pot (three-gang preferred for R, R, and R/2 tracking): Gang 1: R1 (series upper T) Gang 2: R2 (series upper T) Gang 3: R3 = R/2 (shunt lower T) — use pot in half-value configuration Option A — Three-gang pot (custom or dual pot + separate R/2 pot): Three 5 kΩ linear pots Gang 1 & 2 track together; Gang 3 tracks at half value Achievable: use 10 kΩ triple-gang + series 1kΩ fixed for R3 null Option B — Two-gang + switched C (practical): Switch between C=47nF and C=100nF to set frequency range Range 1 (C=100nF): 300–1700 Hz with 1kΩ–5kΩ dual pot Range 2 (C=47nF): 640–3600 Hz with 1kΩ–5kΩ dual pot Variable Twin-T (Option B) — modified circuit: R1 (RV1A) R2 (RV1B) IN ──────/\/\/──────┬──────/\/\/──────┬──── OUT 1kΩ–5kΩ │ 1kΩ–5kΩ │ (gang A) C3 (2×C_SW) (gang B) │ GND C1 (C_SW) C2 (C_SW) IN ───────┤├─────────────┤├──────── OUT │ R3 = wiper of RV1A/2 tracked at half value (see note: use op-amp follower to halve pot output) │ GND R/2 TRACKING CIRCUIT (op-amp divider): Take RV1A wiper voltage, feed to voltage divider 10k/10k, buffer with op-amp Op-amp output drives gate of JFET (2N5457) in drain-source ohmic region JFET R_DS used as the R/2 element Limitation: JFET R_DS non-linear; distortion at large signal levels Better solution: use a 3-gang 5kΩ pot (Alps type, custom order) ================================================================================ SECTION 3 — ACTIVE NOTCH WITH ENHANCED DEPTH ================================================================================ Bootstrapping the twin-T shunt resistor to an op-amp follower GREATLY increases notch depth (limited only by op-amp gain-bandwidth product): R1 2kΩ R2 2kΩ IN ──────/\/\/──────┬──/\/\/──────────── OUT │ C3 200nF │ GND C1 100nF C2 100nF IN ────┤├──────┬──┤├────────────── OUT │ R3 bootstrapped: ┌───────── output of U_boot │ R3_fixed (1kΩ) goes to V_mid point V_mid driven by op-amp follower: U_boot (+) input: follows V at top of R3_fixed U_boot (−) connected to output (unity follower) U_boot output feeds BOTTOM of R3_fixed, not GND Bootstrapped Active Twin-T — deeper notch, >60 dB practical: C3 (200nF) ┌────┤├────┐ R1(2kΩ) │ │ R2(2kΩ) IN ───────/\/\/─────┼──────────┼──/\/\/──── OUT │ │ C1(100nF) C2(100nF) in series upper path └──────────┘ (same IN–OUT path, see twin-T standard) Shunt R3 path replaced with: R3a(1kΩ) → node A → R3b(1kΩ) → GND Node A ──► U3A (+) input U3A (−) = output = Node_B Node_B drives: junction of C1/C2 shunt path Result: notch depth improves from ~35 dB to ~55–65 dB ================================================================================ SECTION 4 — AUTO-NOTCH (NE567 TONE DECODER PLL) ================================================================================ The NE567 is a phase-locked loop tone decoder that can track and notch a single frequency tone automatically within ±14% of center frequency. Principle of operation: 1. NE567 PLL locks to interfering tone 2. PLL VCO tracks the tone frequency 3. In-phase VCO output drives active notch circuit 4. Notch follows tone if it drifts (carrier offset in AM, heterodyne drift) NE567 AUTO-NOTCH CIRCUIT: +9V ──────────────────────────────────────────── Vcc (pin 8) │ ──┴── AF IN ──────────────────────────────────────── │ 3 │ Input (pin 3) │ │ Lock detect LED ◄──────── 470Ω ──────────── │ 8 │ Output (pin 8, open collector) │NE │ C_filter (2.2µF) ──────────────────────────── │ 2 │ Loop filter (pin 2) │ │567│ GND │ │ │ 5 │──── Timing R (VCO freq set) VCO freq: f_0 = 1/(1.1×R_T×C_T) │ 6 │──── Timing C For f_0 = 800 Hz: │ │ R_T = 10kΩ, C_T = 0.1µF → f = 909 Hz │ 7 │──── VCO out (reference signal) R_T = 12kΩ, C_T = 0.1µF → f = 758 Hz ✓ └───┘ R_T = 10kΩ, C_T = 0.12µF → f = 758 Hz ✓ │ GND NE567 output pins: Pin 5 (timing R): connect to VCO frequency-setting resistor R_T Pin 6 (timing C): connect to VCO frequency-setting capacitor C_T Pin 7 (VCO out): 50% duty cycle square wave at tracked frequency Pin 2 (LPF out): loop filter node; sets tracking bandwidth Pin 8 (output): logic LOW when locked (active LOW) AUTO-NOTCH PRINCIPLE USING NE567 + ANALOG SWITCH: Phase-shifted version of input at the interfering frequency is summed with input to produce cancellation (notch). R_sum2 (10kΩ) ┌───/\/\/─────────────────────── OUT │ R_sum1 │ IN ──────/\/\/──────┤ 10kΩ │ └───────── AF input also goes to NE567 for tracking NE567 VCO output (pin 7) → all-pass phase shifter → summing junction When phase of VCO output = 180° relative to interfering tone, summing cancels the tone. Phase shifter: 1st order RC all-pass Phase shift = −2×arctan(ω/ω_p) where ω_p = 1/(RC) For 90° shift at 800 Hz: RC = 1/(2π×800) = 198 µs → R=10kΩ, C=20nF Limitation of NE567 approach: NE567 bandwidth ±14% means capture range ≈ 800 ± 112 Hz (688–912 Hz) Works well for single-carrier heterodyne; not effective for multiple tones ================================================================================ SECTION 5 — DSP AUTO-NOTCH (ESP32 — see firmware) ================================================================================ For full-band auto-notch (tracks multiple tones simultaneously), use ESP32 with LMS (Least Mean Squares) adaptive notch in firmware. LMS algorithm overview: y[n] = x[n] − w[n]·d[n] (output = input − adaptive notch term) w[n+1] = w[n] + 2μ·e[n]·d[n] (weight update) where d[n] is a delayed version of input (used as reference) μ = adaptation rate (0.001 to 0.01; larger = faster, less stable) Benefit: automatically notches ANY steady tone in the passband, including multiple simultaneous carriers, without manual tuning. See: firmware/audio_filter_dsp.ino, function lms_autonotch() ================================================================================ NOTCH FILTER COMPARISON ================================================================================ Type | Notch Depth | Tune Method | Multiple Tones | Battery Life ------------------|-------------|-------------|----------------|------------- Fixed Twin-T | 30–45 dB | Fixed | No | N/A (passive) Variable Twin-T | 25–40 dB | Manual pot | No | N/A Active Bootstrapped | 55–65 dB | Manual | No | ~35 hr (NE5532) NE567 Auto | 20–30 dB | Auto ±14% | No | ~35 hr ESP32 LMS Auto | 40–60 dB | Fully auto | Yes (4+) | ~8 hr ================================================================================ PARTS LIST — NOTCH FILTER ================================================================================ Fixed Twin-T (800 Hz): Ref | Qty | Value | Description | Package -------|-----|----------|-------------------------------|-------- R1,R2 | 2 | 2.0 kΩ | Metal film 1%, 1/4W | Axial R3 | 1 | 1.0 kΩ | Metal film 1%, 1/4W | Axial C1,C2 | 2 | 100 nF | PP film 1% | Axial C3 | 1 | 200 nF | 2× 100 nF PP film in parallel | Axial Variable (Option B add-ons): RV1 | 1 | 5kΩ dual| Dual gang linear pot | 16mm or 24mm SW_range| 1 | SPDT | C range switch 47/100nF | Panel toggle C_alt | 2 | 47 nF | PP film for range 2 | Axial Active bootstrapped add-ons: U3 | 1 | NE5532 | Dual op-amp | DIP-8 R_boot | 2 | 1 kΩ | Split R3 for bootstrap | Axial NE567 Auto-Notch: U4 | 1 | NE567 | Tone decoder PLL | DIP-8 R_T | 1 | 10–15kΩ | VCO timing (set per table) | Trim pot C_T | 1 | 0.1 µF | VCO timing cap, PP film | Axial C_filt | 1 | 2.2 µF | Loop filter, electrolytic NP | Axial LED1 | 1 | Yellow | Lock indicator | 3mm panel R_LED | 1 | 470 Ω | LED series resistor | Axial ================================================================================