================================================================================ SCHEMATIC: DETECTOR, LOG AMP, AND SWR CALCULATION CIRCUITS TM-COUP-001 Rev A Forward/reflected power detection: peak, RMS, and logarithmic SWR calculation: analog and digital methods Calibrated output for power meter or spectrum analyzer ================================================================================ REFERENCE DOCUMENTS: sch_toroidal_bruene_coupler.txt — Upstream coupler (HF) sch_transmission_line_coupler.txt — Upstream coupler (VHF/UHF) rf_power_monitor_esp32.ino — ESP32 firmware (ADC, display, BLE) ================================================================================ SECTION 1 — DETECTOR TYPES AND SELECTION ================================================================================ THREE DETECTOR METHODS FOR DIFFERENT APPLICATIONS: METHOD A — SCHOTTKY PEAK DETECTOR (simplest, field use): Best for: CW power measurement, SWR bridge, field wattmeter Frequency range: DC – 500 MHz (BAT41, 1N5711); DC – 2 GHz (HSMS-2850) Dynamic range: 20–30 dB (limited by diode forward voltage) Accuracy: ±0.5–1 dB with calibration table Output: DC voltage proportional to RF peak amplitude METHOD B — LOG AMP DETECTOR (AD8307, calibrated): Best for: Precision power measurement, spectrum analyzer input, wideband logging Frequency range: 1 MHz – 500 MHz (AD8307) Dynamic range: 92 dB (−75 dBm to +17 dBm) Accuracy: ±1 dB over temperature (-40 to +85°C) Output: 25 mV/dB, 600 mV at 0 dBm reference Cost: ~$4 each (AD8307ARNZ, SOIC-8) METHOD C — RMS DETECTOR (AD8361): Best for: modulated signals (SSB, FM, AM), accurate power regardless of waveform Frequency range: DC – 2.5 GHz Dynamic range: 36 dB (−30 dBm to +6 dBm input range) Output: 7.5 mV/mW_rms into 50Ω (scaled output) Notes: Required for SSB power measurement (CW-calibrated meter reads wrong on SSB) SELECTION GUIDE: CW-only field wattmeter: Method A (simplest, cheapest) All-mode HF monitor (SSB/CW/FM): Method B (AD8307 logarithmic) SSB power accuracy: Method C (AD8361 true RMS) Spectrum analyzer reference: Method B (log scale output natural for SA) ================================================================================ SECTION 2 — SCHOTTKY PEAK DETECTOR CIRCUIT ================================================================================ DUAL PEAK DETECTOR (forward + reflected): FWD RF in (from coupler) │ D1 (BAT41) ─┤►├─────────────────────── FWD_DC anode ───┘ └─── cathode │ C1 = 1 nF (ceramic, 500V) │ R_L = 2.2 kΩ │ GND REF RF in (from coupler) │ D2 (BAT41) — matched to D1 ─┤►├─────────────────────── REF_DC │ C2 = 1 nF │ R_L = 2.2 kΩ │ GND FWD_DC and REF_DC → ESP32 ADC inputs (via voltage divider if >3.3V) DETECTOR DC VOLTAGE vs. POWER: V_dc = V_peak_RF − V_f_diode V_peak_RF = √(2 × P_coupled × Z_det) With −20 dB coupler, P_coupled = P_input × 0.01: At P_in = 100W: P_coupled = 1W; V_peak = √(2×1×50) = 10V After BAT41 (Vf ≈ 0.23V at peak): V_dc ≈ 9.77V (exceeds ESP32 ADC input!) VOLTAGE DIVIDER REQUIRED: Divide V_dc by 4 for 3.3V ADC range: R_div1 = 10kΩ, R_div2 = 3.3kΩ → V_adc = V_dc × 3.3/13.3 = 0.248 × V_dc At 100W input: V_adc = 9.77 × 0.248 = 2.42V (within 3.3V ADC range ✓) At 1W input: V_adc = 0.097 × 0.248 × ... but Vf dominates below ~0.5V. DIODE THRESHOLD CORRECTION: Below about 100 mW input to coupler (P_coupled < 1 mW → V_RF < 0.32V): The diode Vf (0.23–0.35V for BAT41) dominates; detector response is non-linear. Below this threshold: use AD8307 log amp for accurate low-power measurement. Two-range design: High power (>1W): Schottky peak detector (linear in V²) Low power (<1W): AD8307 log amp (linear in dBm) Switchover at ~1W: ADC multiplexer or two detector channels ================================================================================ SECTION 3 — LOG AMP CIRCUIT (AD8307) ================================================================================ AD8307 APPLICATION CIRCUIT (FWD channel — duplicate for REF): FWD RF from coupler (BNC/SMA, 50Ω) ───────────────────────────────────── │ AC coupling capacitor C_ac C_ac = 0.1 µF (electrolytic or ceramic) │ ┌────────┴───────┐ │ AD8307 │ ─────[R_i]─┤ INP (pin 8) │ │ │ 50Ω term─[R_t=52.3Ω]─┤ INM (pin 1) │ │ │ │ VOUT (pin 4) ├── LOG_OUT → ADC │ │ │ VPS (pin 7) ├── +5V │ │ │ COM (pin 3) ├── GND │ │ │ ENB (pin 6) ├── +5V (enable) └─────────────────┘ R_i = 52.3Ω (with R_t = 52.3Ω forms 50Ω input impedance: R_i || R_t = 26.15 ≈ 25Ω... Actually: Use R_t = 50Ω directly to GND; R_i = 0Ω (direct connect for 50Ω source). AD8307 input impedance = 1.1kΩ || 1.4 pF; shunted by R_t=50Ω → ≈ 50Ω system. CORRECT WIRING: INP (pin 8) ──── RF input (directly, no R_i needed for 50Ω source) INM (pin 1) ──[50Ω to GND] ← single-ended 50Ω termination VOUT (pin 4) → ESP32 ADC VPS (pin 7) → +5V, decoupled with 0.1µF + 100pF ceramic COM (pin 3) → GND ENB (pin 6) → +5V (always enabled); or GPIO for power save TRANSFER FUNCTION: V_out = slope × (P_in_dBm − intercept) slope ≈ 25 mV/dB (0.025 V/dB) intercept ≈ −84 dBm (output = 0V when input = −84 dBm) V_out = 0.025 × (P_dBm + 84) At P_in = +10 dBm (10 mW): V_out = 0.025 × (10+84) = 2.35V At P_in = 0 dBm (1 mW): V_out = 0.025 × (0+84) = 2.10V At P_in = −20 dBm: V_out = 0.025 × (−20+84) = 1.60V At P_in = −75 dBm: V_out = 0.025 × (−75+84) = 0.225V INPUT POWER FROM ADC READING: P_dBm = (V_adc / 0.025) − 84 = V_adc × 40 − 84 POWER FROM COUPLED PORT (−20 dB coupler): P_main_dBm = P_coupled_dBm + 20 (add back coupling factor) P_main_watts = 10^((P_main_dBm − 30) / 10) AD8307 FREQUENCY RANGE: Usable: 1 MHz – 500 MHz (specified to 500 MHz) With care: extends to 900 MHz with >1 dB uncertainty Below 1 MHz: output valid but ripple increases; add larger C_ac (10 µF) ================================================================================ SECTION 4 — SWR CALCULATION CIRCUIT (ANALOG AND DIGITAL) ================================================================================ ANALOG SWR METER (simplified): V_fwd_dc ──────────────────────────────────── S1(SWR meter, normalized) (signal derived from ratio) SWR calculated from: ρ = |Γ| = V_ref / V_fwd (reflection coefficient, voltage) SWR = (1 + ρ) / (1 − ρ) ANALOG ρ COMPUTER: Requires V_fwd − V_ref and V_fwd + V_ref. An op-amp difference and sum circuit: V_fwd_dc ──[R]──┬────────────── (+) input of op-amp OA1 V_ref_dc ──[R]──┤────────────── (−) input → V_diff = V_fwd − V_ref └────────────── (and sum separately) Analog SWR computation: SWR_voltage = (V_fwd + V_ref) / (V_fwd − V_ref) Implement with dual op-amp (TL072 or LM358) + resistor divider. Output drives analog meter movement (200µA full scale). DIGITAL SWR (ESP32 firmware — see rf_power_monitor_esp32.ino): Preferred; higher accuracy; display on CYD; data logging via BLE. SWR METER DRIVE (for panel meter, analog output): Some field applications require a traditional SWR meter movement. Output: 0–1 mA for SWR 1.0 to ∞. CIRCUIT: V_swr (from op-amp) ──[R_meter = 4.7kΩ]── meter movement (1mA FSD, 50Ω) Scale: V_swr at SWR=2 → calibrated on front panel ================================================================================ SECTION 5 — CALIBRATED POWER METER OUTPUT ================================================================================ TWO OUTPUT CONFIGURATIONS: CONFIGURATION A — ANALOG DC VOLTAGE (0–3.3V): Suitable for ADC input (ESP32 or standalone panel meter). Range: 0–3.3V = 0–P_max watts (scaled by voltage divider). Calibration: V_out = k × √P_in where k = calibration constant. CONFIGURATION B — BUFFERED LOG AMP OUTPUT (25 mV/dB): Suitable for spectrum analyzer REF input (calibration reference). Output impedance: 50Ω (buffer op-amp with 50Ω series resistor). Direct connect to SA AUX input or calibration port. BUFFER AMPLIFIER: +5V │ AD8307_VOUT ──[0.1µF]──┬── (+) LMH6702 ──┤── 50Ω ──── VOUT_buffered │ │ 100kΩ 100kΩ │ │ GND (feedback, unity gain) LMH6702: 1.7 GHz unity-gain bandwidth; suitable for buffering up to 300 MHz signals. ALT: OPA690 or AD8057 for lower cost. POWER METER CALIBRATION TABLE: P_out (W) | P_coupled (mW) | V_peak (V) | V_DC (after diode) | ADC counts ----------|----------------|------------|---------------------|------------ 0.001 (1mW)| 0.01 µW | 0.001V |