================================================================================ SCHEMATIC: DIRECTIONAL COUPLER — 20 dB, 1 MHz - 500 MHz TM-DL-COUPLER Rev A ================================================================================ PURPOSE: Samples forward and reflected power from dummy load (or any 50-ohm line) to drive power and SWR display. Used with CYD monitor. TWO DESIGNS: COUPLER-HF: Bruene bridge type, 1-30 MHz, toroid transformer COUPLER-VHF: Resistive dual-directional, 1 MHz-500 MHz, broadband ================================================================================ COUPLER-HF: BRUENE DIRECTIONAL COUPLER (1-30 MHz) ================================================================================ TX IN (J1) LOAD OUT (J2) | | +-----+------[LINE]--------+-----+ | | | | T1 primary (1T thru-line) | | | | | T1 secondary (10T, bifilar) | | | | | FWD tap REF tap | | | | | | D1(BAT54) D2(BAT54) | | | | | C1(100pF) C2(100pF) | | | | | R2(1k) R3(1k) | | | | | | J3 FWD J4 REF | | SMA SMA | | | | | +-----+--------------------+----+ GND bus (continuous, low inductance) FULL SCHEMATIC: J1 T1 (FT-37-61 toroid) J2 IN ----+---(1-turn thru line, #14 AWG)---+---- OUT (to dummy load) | | | T1 secondary winding: | | 10 turns, #26 AWG, bifilar | | on FT-37-61 (or FT-50-61) | | | Tap A (+) Tap B (-) | | R4 (50R) R5 (50R) <-- termination resistors | | +------+ +--------+ | | | | D1 D2 (BAT54 Schottky pair) (FWD) (REF) | | C1 C2 (100pF, NP0 capacitor each) | | R6(1k) R7(1k) <-- detector load | | J3 FWD J4 REF (SMA female, panel mount) | | GND GND COMPONENT VALUES: T1: FT-37-61 or FT-50-61 ferrite toroid Primary: 1 turn #14 AWG through toroid (thru-line) Secondary: 10 turns #26 AWG, bifilar wound Results in 20 dB coupling (20:1 voltage ratio = 10:1 turns) D1,D2: BAT54 or 1N5711 Schottky diodes C1,C2: 100 pF, NP0/C0G, 50V R4,R5: 51 ohm, 1/4W, 1% (termination, must be accurate 50 ohm) R6,R7: 1 kohm, 1/4W (detector load resistor) J1,J2: SO-239 (HF) or N-type (HF/VHF) panel connectors J3,J4: SMA female, panel mount (detector outputs) CALIBRATION: Apply known power (e.g., from calibrated QRP rig: 5W = 37 dBm) Measure V_dc at J3 with load terminated into dummy load. Calibration factor K = P_in / (V_dc * V_dc) Example: 5W into load, measure 0.50V at J3 K = 5 / 0.25 = 20 SWR calculation in CYD code uses this K factor. PERFORMANCE: Frequency range: 1 MHz - 30 MHz (-1 dB) 0.5 MHz - 60 MHz (-3 dB) Directivity: > 25 dB (1-20 MHz) Coupling: 20 dB ± 1 dB Insertion loss: < 0.15 dB Power handling: 1.5 kW (limited by thru-line current) ================================================================================ COUPLER-VHF: RESISTIVE DUAL DIRECTIONAL COUPLER (1 MHz - 500 MHz) ================================================================================ Wheatstone bridge topology, also called a "Hybrid ring" or "rat-race" in its transmission-line form. This is the lumped-element version. J1 (TX IN) | +--------+--------+ | | R1(25R) R2(25R) <- series arm | | +---+ +---+ | | | | R3(25R) R4(25R) <- shunt arm | | J3 (FWD) J4 (REF) detector detector | | GND GND | | +---------------+ | J2 (LOAD OUT) | 50-ohm dummy load RESISTOR VALUES for 20 dB coupling (voltage ratio 10:1): R1 = R2 = R3 = R4 = 25 ohm This creates a 12-dB coupler when loaded. For exact 20 dB: see Pozar "Microwave Engineering" Ch.7 for design. PRACTICAL 20 dB COUPLER: This topology: 3-resistor Wheatstone bridge J1 IN ----R1(16.7R)----+----R2(16.7R)---- J2 OUT | R3(100R) | +---+---+ | | D1(FWD) D2(REF) | | C(100p) C(100p) | | R4(1k) R5(1k) | | FWD out REF out (to ADC) Actually, properly: J1 IN ---+-----(16.67R series)-----+---- J2 LOAD OUT | | FWD REF detector detector (diode+RC) (diode+RC) | | GND GND Full design (Pozar-based resistive divider coupler): Insertion resistors: R1=R2=8.33 ohm (two 16.7R in parallel) Coupling resistors: 100 ohm each to detector taps Detector: BAT54 + 100pF + 1kohm each port NOTE: Resistive couplers have insertion loss proportional to coupling. For 20dB coupler: insertion loss ~0.8 dB. Acceptable for test purposes. For VERY LOW insertion loss, use transformer coupler (COUPLER-HF above). PERFORMANCE (resistive VHF coupler): Frequency range: 1 MHz - 500 MHz (flat to ±1 dB) Directivity: > 20 dB (50 MHz - 500 MHz) Insertion loss: 0.8 dB Coupling: 20 dB ± 2 dB Power handling: 200W continuous (R1, R2 are in power path) Use R1,R2 as 2W metal film resistors ================================================================================ DETECTOR CIRCUIT (for both couplers) ================================================================================ Used at J3 (forward) and J4 (reflected) ports: SMA IN ----+---- D1 (BAT54A) ----+---- V_dc OUT (to ADC) | (anode to SMA) | (50R) C1 (100pF NP0) to | GND R1 (10k) | GND D1: BAT54A or HSMS-2852 Schottky (low-forward voltage, fast) For frequencies > 200 MHz: HSMS-2852 or SMS7621 preferred C1: 100 pF, NP0/C0G, detector bypass R1: 10 kohm, detector DC load (slow decay for averaging) Change to 1k for faster response (peak power reading) Change to 100k for slow averaging (average power reading) Voltage-to-power calibration: V_dc = 0.707 * V_peak * coupling_factor * detector_efficiency P_fwd = V_dc^2 / (50 * coupling_factor^2 * detector_efficiency^2) Calibration constants (empirical, measured with known source): K_fwd, K_ref (stored in CYD EEPROM, see cyd/power_monitor_cyd.ino) ================================================================================ PARTS LIST — COMPLETE COUPLER BOARD ================================================================================ Qty Reference Value Description --- --------- ------------------ ---------------------------------- 1 T1 FT-50-61 toroid HF coupler core (or FT-37-61) 2 D1,D2 BAT54A SOD-123 Schottky detector diodes 4 C1-C4 100pF NP0 0402 Detector bypass caps 2 R1,R2 51R 1% 0805 2W Coupler termination 2 R3,R4 1k 1% 0402 Detector load 2 R5,R6 51R 1% 0402 SMA termination at detector ports 2 J3,J4 SMA female flange Forward/reflected output ports 2 J1,J2 Per antenna design Main line connectors (N or SO-239) 1 -- FR4 PCB 50x30mm Coupler board ================================================================================