================================================================================ SCHEMATIC: RESISTIVE BRIDGE DIRECTIONAL COUPLER — WIDEBAND TM-COUP-001 Rev A Wideband resistive bridge: DC to 500 MHz Coupling: −20 dB nominal Directivity: >20 dB (1–200 MHz), >15 dB (200–500 MHz) Insertion loss: <0.5 dB (frequency dependent) ================================================================================ REFERENCE DOCUMENTS: sch_toroidal_bruene_coupler.txt — Better directivity for HF-only use sch_detector_swr_output.txt — Detector circuits ================================================================================ SECTION 1 — RESISTIVE BRIDGE PRINCIPLE ================================================================================ A resistive (Wheatstone) bridge directional coupler works on the principle that a balanced bridge produces a null output when loaded with the reference impedance. Unlike toroidal couplers, the resistive bridge works from DC upward, making it suitable for wideband spectrum analyzer connections, pulse power measurement, and envelope detection. TRADE-OFF: The resistive bridge inherently has higher insertion loss than a toroidal coupler (typ. 0.3–1.0 dB vs. <0.05 dB for toroidal). This is the fundamental cost of wideband, transformer-free operation. BASIC RESISTIVE BRIDGE TOPOLOGY: R1 (Z0/2 = 25Ω) IN ─────────────┬──────────────────┬──────────────── OUT │ │ │ R3 (Z0 = 50Ω) │ │ R2 D1 → FWD output (Z0/2=25Ω) │ │ R4 (Z0 = 50Ω) │ │ └──────────────────┘── D2 → REF output │ GND OPERATION: With Z_L = Z0 (matched load): Forward wave: Bridge balanced for forward traveling wave → D1 responds Reflected wave: Bridge balanced for reflected traveling wave → D2 responds Directivity: Limited by resistor precision and stray inductance/capacitance PRACTICAL 4-RESISTOR BRIDGE (Quadrature Bridge): The standard practical implementation (used in HP/Agilent wattmeters): R1 = 16.67Ω R2 = 16.67Ω IN ───┬────[R1]────────────────────[R2]────┬──── OUT │ │ │ │ │ R3 R4 │ │ (50Ω) (50Ω) │ │ │ │ │ │ D_fwd D_ref │ │ │ │ │ └────────────────┴────────┘────────────────┘ GND R1 = R2 = Z0/3 = 16.67Ω (use 16Ω + 0.68Ω series or 18Ω trimmer) R3 = R4 = Z0 = 50Ω INSERTION LOSS CALCULATION: IL = 20 log₁₀(R2/(R1+R2)) = 20 log₁₀(2/3) = −3.5 dB ← HIGH This is the fundamental minimum insertion loss of a resistive bridge. A hybrid coupler (hybrid T or 3 dB splitter) cannot do better without reactive elements. IMPROVED CONFIGURATION — DIRECTIONAL POWER SAMPLER: For low insertion loss AND reasonable directivity, use an asymmetric topology: 16.67Ω 33.33Ω IN ────────[R_s]──────[R_s2]──────────────── OUT (< 0.5 dB IL) │ │ [C1] [R_term = 50Ω] │ │ [R1=100Ω] [R2=100Ω] │ │ D1(FWD) D2(REF) │ │ [C2=100pF] [C2=100pF] │ │ GND GND R_s total = 50Ω; divides between R_s and R_s2 in series with Z_load. Actual IL depends on R_s / Z0 ratio. ================================================================================ SECTION 2 — RECOMMENDED CIRCUIT: HYBRID RESISTIVE + CAPACITIVE SAMPLE ================================================================================ For best wideband performance, combine resistive sensing with small capacitive coupling to compensate for transformer-free HF roll-off: COMPLETE CIRCUIT: ┌──────────────────────────────────────────────────────────────────┐ │ │ │ IN MAIN LINE (50Ω) OUT │ │ J1 ──────┬──────────────────────────────────┬────── J2 │ │ SMA/BNC │ │ SMA/BNC │ │ │ │ │ │ R_s1=5.6Ω R_s2=5.6Ω │ │ (0.5W) (0.5W) │ │ │ │ │ │ ─┴─ C1=100pF ─┴─ C2=100pF │ │ │ (shunt to main line) │ (shunt) │ │ │ │ │ │ ┌─┴──────────────────────────────────┴─┐ │ │ │ BRIDGE DETECT NETWORK │ │ │ │ │ │ │ │ V_A ──[47Ω]──D1(BAT41)──[2.2kΩ]──FWD │ │ │ │ │ │ │ V_B ──[47Ω]──D2(BAT41)──[2.2kΩ]──REF │ │ │ │ │ │ │ R_null=50Ω (between V_A and V_B) │ │ │ └──────────────────────────────────────┘ │ │ │ └─────────────────────────────────────────────────────────────────┘ COMPONENT VALUES: R_s1 = R_s2 = 5.6Ω (0.5W metal film) — sampling resistors (low value → low IL) C1 = C2 = 100 pF (NP0/C0G) — high-frequency coupling supplement R_null = 50Ω (trimmable for null/directivity adjustment) D1, D2 = BAT41 matched pair (Schottky, Vf ≈ 0.3V at 1mA) R_det = 47Ω summing resistors R_L = 2.2kΩ (detector load) C_filt = 1 nF (detector filter) INSERTION LOSS (resistive bridge with 5.6Ω sampling resistors): IL = 20 log₁₀(Z0 / (Z0 + R_s1)) = 20 log₁₀(50/(50+5.6)) = −0.43 dB This is acceptable for spectrum analyzer tapping, power monitoring. For transmitter final stage monitoring: <0.1 dB required → use toroidal coupler. ================================================================================ SECTION 3 — FREQUENCY RESPONSE AND COMPENSATION ================================================================================ RESISTIVE BRIDGE FREQUENCY RESPONSE (uncompensated): Frequency | Coupling | Directivity | Notes -----------|-----------|-------------|--------------------------- DC | −20.0 dB | N/A | Works at DC (unique to resistive bridge) 1 MHz | −20.0 dB | >25 dB | Excellent; no reactive effects 10 MHz | −20.1 dB | >24 dB | Still excellent 50 MHz | −20.5 dB | >22 dB | Stray reactance beginning 100 MHz | −21.2 dB | >20 dB | Noticeable; compensate with C1/C2 tuning 200 MHz | −22.5 dB | >17 dB | Needs compensation; directivity marginal 500 MHz | −25 dB | >12 dB | Use transmission-line coupler above this COMPENSATION WITH C1/C2 CAPACITORS: C1/C2 provide a capacitive bypass at high frequency, partially compensating for the roll-off from stray inductance in R_s1/R_s2. Optimal value: C_opt = R_s / (2 × π × f_corner × Z0²) At f_corner = 200 MHz, R_s = 5.6Ω, Z0 = 50Ω: C_opt = 5.6 / (2 × π × 200e6 × 2500) = 1.8 pF Use 2 pF NP0 (adjust empirically on VNA for flattest coupling). PCB LAYOUT CRITICAL POINTS: - Keep R_s resistors as short as possible (2mm body, 0402 or 0603 preferred) - Stray inductance of 1 nH causes 0.5 dB additional IL at 200 MHz - Ground plane directly under coupling region - D1/D2 Schottky diodes: place within 5mm of bridge node - Shield coupler from RF environment (all unused surfaces connected to GND) ================================================================================ SECTION 4 — SPECTRUM ANALYZER DIRECT-CONNECT COUPLER ================================================================================ For connecting a 50Ω spectrum analyzer to a transmitter output for monitoring: REQUIRED: −30 to −40 dB coupling (analyzer input range: −30 to +30 dBm) A 100W transmitter: P_out = 50 dBm Required SA input: 50 dBm − 40 dB = 10 dBm → safe for most SA inputs (max +30 dBm) SPECTRUM ANALYZER COUPLER DESIGN (−30 dB, resistive, DC–500 MHz): CIRCUIT: IN (50Ω) ──[R_s=1.56Ω]──────────────────── OUT (50Ω, load) │ [100Ω] │ [100Ω] │ ─┬─ Node A │ [50Ω] ← termination (R_load of SA input = 50Ω) │ GND Coupling = −20 log₁₀(Z0 / (Z0 + R_s)) − ... Simplified: use voltage divider R_s + R_sense with R_sense = 1Ω: V_coupled/V_in = R_sense/(Z0+R_s) = 1/(50+1) = 0.0196 = −34 dB POWER HANDLING: With 100W into 50Ω: V_in_peak = √(2×100×50) = 100V Current through R_s: I = 100V/50Ω = 2A; P_Rs = I²×R_s = 2²×1 = 4W Use 5W wirewound for R_s at this power level. PARTS LIST (SA COUPLER, −30 dB): R_s 1 1.56Ω, 5W wirewound (bulk resistance, main shunt) R1 1 620Ω 0.25W (divider top leg) R2 1 10Ω 0.25W (divider bottom; adjusts division ratio) C1 1 100 pF NP0 (bypass, extends HF) J1 1 SO-239 (TX output, 100W rated) J2 1 SO-239 (to antenna) J3 1 SMA or BNC female (SA or power meter port) ================================================================================ SECTION 5 — INSERTION LOSS BUDGET ================================================================================ INSERTION LOSS BREAKDOWN (100 MHz, resistive bridge): Loss mechanism | Contribution | Notes -----------------------------|-------------|------------------ R_s resistor (5.6Ω / 50Ω) | 0.43 dB | Fundamental; reduce R_s for less IL Connector return loss | 0.02 dB | Quality SMA: RL>30 dB PCB trace resistance | 0.01 dB | 2oz copper, short traces Core material (none in R-bridge)| 0 dB | Advantage over toroidal Stray radiation at 100 MHz | 0.05 dB | Shielded enclosure mitigates TOTAL | 0.51 dB | Typ. spec: <0.5 dB COMPARISON: Resistive bridge: 0.43–0.51 dB (acceptable for monitoring; not for inline PA use) Toroidal coupler: 0.02–0.05 dB (preferred when low IL required) Transmission line: 0.05–0.15 dB (VHF/UHF; acceptable) ================================================================================ SECTION 6 — DIRECTIVITY OPTIMIZATION ================================================================================ RESISTIVE BRIDGE NULL PROCEDURE: A trimmer resistor R_null (50Ω single-turn cermet) between V_A and V_B nodes allows fine adjustment for null (maximum directivity). PROCEDURE: 1. Apply 1W CW at operating frequency into J1. 2. Connect matched 50Ω load to J2 (termination). 3. Monitor REF port (J4) with DC voltmeter. 4. Adjust R_null trimmer until V_ref is minimum. 5. Lock trimmer with thread-lock compound. 6. Note: optimal null varies slightly with frequency; set at most-used frequency. EXPECTED NULL DEPTH AFTER ADJUSTMENT: HF (< 30 MHz): >30 dB directivity (V_ref < 3% of V_fwd) VHF (30–200 MHz): >22 dB (resistive bridge practical limit without reactive comp.) ================================================================================