================================================================================ INTEGRATED ANTENNA ANALYZER - MULTIBAND DIRECTIONAL COUPLER Broadband Coupled-Line Architecture: HF/VHF/UHF (1.8 MHz - 1.3 GHz) ================================================================================ OVERVIEW ──────── A three-section switchable directional coupler covering all ham bands: HF section: 1.8-70 MHz (160M through 10M) using ferrite toroid VHF section: 60-500 MHz (6M through 1.25M) using PCB stripline UHF section: 400-1400 MHz (70cm through 20cm) using microstrip Each section provides: - Forward port: Reference signal from RF source - Reflected port: Signal reflected from DUT - Both ports at -20 dB coupling factor (±1 dB across band) - Directivity > 20 dB (measures true reflection, not source leakage) - >2 GHz port isolation (filters don't interfere with neighboring bands) Coupling coefficient: -20 dB ≡ 0.1 V/V amplitude ratio SYSTEM BLOCK DIAGRAM ──────────────────── ┌─────────────────────┐ │ RF Source (AD9851) │ (0.1-100 MHz) │ or ADF4351 PLL │ (100-1400 MHz) │ Output: -4 to +5dBm│ └──────────┬──────────┘ │ v ┌──────────────────────┐ │ Band-Select RF │ GPIO control: │ Switch (PE4259) │ - HF_EN: routes to HF section │ 50Ω T-network │ - VHF_EN: routes to VHF section │ Insertion loss <0.5│ - UHF_EN: routes to UHF section └──────┬───────────────┘ │ ┌───────────────┼───────────────┐ │ │ │ v v v ┌────────────┐ ┌──────────┐ ┌──────────┐ │ HF Section │ │VHF Sec │ │UHF Sec │ │ (toroid) │ │(stripline) │(microstrip) │ 1.8-70 MHz │ │60-500MHz │ │400-1.3GHz│ └────┬───────┘ └────┬─────┘ └────┬─────┘ │ │ │ └──────────────┼─────────────┘ v ┌──────────────────────┐ │ AD8302 Vector │ │ Detector │ │ INPA: Fwd (-20dB) │ │ INPB: Reflected │ └──────────────────────┘ SECTION 1: HF DIRECTIONAL COUPLER (1.8 - 70 MHz) ───────────────────────────────────────────────── Topology: Ferrite Toroid Coupled Transformer Primary: 10T wound on FT50-43 (Fair-Rite 2643625001) Secondary: 10T wound on same core, bifilar with primary Coupling factor: 10 dB (0.316 V/V) → converts to 20 dB by attenuation Coupling directivity: >20 dB (primary-to-secondary flux dominates) Schematic (DUT on primary, couplers on secondary): RF_SOURCE (50Ω) ──[T50-43 10T PRIMARY]──→ DUT PORT (50Ω) │ (10T) │ │ bifilar wound │ [SECONDARY 10T] │ [10nF AC couple]─┬→ FORWARD ARM (to AD8302 INPA) │ │ -20 dB coupling [100Ω series R]──┤ │ │ [GND] │ │ [10nF AC couple]─┬→ REFLECTED ARM (to AD8302 INPB) │ │ -20 dB coupling [100Ω series R]──┤ │ │ [GND] │ Winding procedure (Fair-Rite FT50-43): 1. Use #26 AWG enameled copper wire 2. Wind 10 turns bifilar (two wires together): - Start with ~12 inches of lead wire - Wrap both wires around toroid simultaneously - 10 complete turns (in and out counts as 1 turn) - Leave ~12 inches of lead on each end 3. Secure with teflon tape at start/finish 4. Primary (source): Start1→10T→End1 5. Secondary (sense): Start2→10T→End2 6. Twist secondary pair for common-mode rejection Core specifications: Fair-Rite 2643625001 (FT50-43 in ferrite) - OD: 0.5", ID: 0.3", HT: 0.188" - µ_i = 850 (initial permeability) - Frequency range: 1-200 MHz - Temperature stable: 40ppm/°C Coupling stability: - Temperature: ±0.5 dB across 0-70°C - Frequency: ±1 dB across 1.8-70 MHz (main variation at LF) - Aging: < 0.1 dB per year (ferrite stable) Impedance matching (50Ω to secondary): - Secondary characteristic impedance: ~500Ω (loose coupling) - Pad with 100Ω series resistor on each output arm - 10nF coupling cap blocks DC, passes RF above ~10 kHz - Output impedance to AD8302: ~100Ω (matched via balun in next stage) Output filtering (HF coupler outputs): Each arm (forward and reflected): COUPLER_OUT ──[10nF]──┬──→ To AD8302 balun input │ [100Ω + 10nF] ← Low-pass, fC ≈ 160 kHz │ GND SECTION 2: VHF DIRECTIONAL COUPLER (60 - 500 MHz) ────────────────────────────────────────────────── Topology: PCB Stripline Coupled-Line Coupler Structure: Two parallel 50Ω striplines on internal PCB layer Spacing: 1.0 mm (gap between lines, drives coupling factor) Length: λ/4 @ 200 MHz nominal = 375 mm (electrical), ~15 mm PCB trace at er=4.6 Coupling factor: -20 dB (0.1 V/V) Directivity: >22 dB (via phase velocity matching) PCB Stack-up for VHF section (stripline): Layer 1: Signal (primary through-line) | 0.4 mm copper Layer 2: GND plane | 0.5 mm copper Layer 3: Signal (coupled secondary line) | 0.4 mm copper Layer 4: GND plane | 0.5 mm copper Impedance calculation (56 Ω nominal): ZC = sqrt(L/C) ≈ 56Ω for 1 mm gap on 0.4 mm trace Adjust trace width for 50Ω target (typically 0.8-1.0 mm) Schematic (top view): ┌─────────────────────────────────────────────────┐ │ INPUT THROUGH-LINE │ │ (50Ω) ═══════════════════════════════════════→ OUTPUT (DUT) │ │← 1 mm gap →│ │ ├─ COUPLED LINE ────────────────→ COUPLED OUT (-20dB forward) │ │ │ │ └──────────────────────────────────────→ ISOLATED (-70dB or better) │ GND PLANE ABOVE AND BELOW ALL LINES │ └─────────────────────────────────────────────────┘ Coupled output port (forward arm): - Tapped at input end of secondary line (provides forward coupling) - Isolation resistor: 100Ω series - Coupling capacitor: 10nF (blocks DC to AD8302) - Absorptive termination on isolated port: 51Ω to GND (dissipates reflection) Reflected output port (secondary arm): - Tapped at output end of secondary line - Coupling capacitor: 10nF - Isolation resistor: 100Ω series - Directional coupler directionality achieved by phase relationship Physical layout on FR-4: - Primary line: 50Ω, length 15 mm @ 200 MHz (electrical λ/4) - Secondary line: parallel, 1.0 mm gap, same length - Both lines over continuous GND plane - Ground vias every 2 mm around primary trace perimeter - Secondary trace vias at start/end only (to preserve coupling) Port connections: INPUT → From RF_SELECT switch (50Ω) THROUGH → To DUT SMA connector (50Ω) FORWARD → To AD8302 INPA (via AD8302 input buffer/balun) ISOLATED → 51Ω series terminator to GND (absorptive load) REFLECTED→ Jumper wire to next stage (UHF coupler during VHF sweep) Frequency response: -20 dB ± 1 dB @ 100-500 MHz Directivity > 20 dB across band Group delay variation < 0.5 ns (phase linearity good for magnitude measurement) SECTION 3: UHF DIRECTIONAL COUPLER (400 - 1400 MHz) ──────────────────────────────────────────────────── Topology: Microstrip Coupled-Line Coupler Structure: Two parallel 50Ω microstrip lines on top PCB layer Line width: 0.8 mm (50Ω nominal impedance on 1.6 mm FR-4) Spacing gap: 0.635 mm (drive coupling to -20 dB @ 1 GHz) Length: λ/4 @ 1 GHz = 75 mm (electrical), ~40 mm on FR-4 (er=4.6) Directivity: >24 dB (via Lange coupler hybrid, or stepped-impedance) PCB Stack-up for UHF section: Layer 1: Signal (microstrip, top copper) | 0.035 mm Dielectric (FR-4) | 1.6 mm Layer 2: GND plane (bottom copper) | 0.035 mm Microstrip impedance (50Ω): w/h ≈ 2.0 (w = line width = 0.8mm, h = substrate height = 1.6mm) Z = 60/sqrt(εr) × ln(4h/w) ≈ 50Ω Coupling vs frequency (empirical for 0.635mm gap): 600 MHz: -19 dB 1000 MHz: -20 dB (design point) 1400 MHz: -21 dB Coupling factor relatively flat across band (good!) Schematic (top view, microstrip): ┌──────────────────────────────────────────────────┐ │INPUT PRIMARY LINE (50Ω) │ │50Ω ──/\/\//──═══════════════════════════════────→ DUT (50Ω) │ │←0.635mm→│ │ │ SECONDARY LINE (coupled) │ │ ───────────────────────→ FORWARD │ │ (tapped @ input) port │ │ (-20dB) │ │ │ │ ───────────────────────→ REFLECTED │ │ (tapped @ output) port │ │ (-20dB) │ │ │ │ ════════════════════════════════════════════════ │ │ GND PLANE (bottom layer) │ └──────────────────────────────────────────────────┘ Coupling implementation (two tap methods): METHOD A: Distributed Taps (Recommended) - Forward: Capacitor (10nF) to coupler input point - Reflected: Capacitor (10nF) to coupler output point - Each tap: 0.5 mm trace perpendicular to secondary line - Isolation resistor: 100Ω in series on each tap METHOD B: Lumped Coupler (Simpler PCB) - Hybrid rat-race coupler using microstrip - Four ports: input, through, forward, isolated - 2.5 λ/4 total length (~ 100 mm @ 1 GHz) - Phase balance via λ/8 stub junctions - (More complex layout, better directivity) Absorptive termination: - Isolated port: NOT used (left unconnected or 51Ω to GND) - Prevents reflection back into forward path - Resistor must be film type (0603, 1% tolerance) mounted flush on trace Connection topology: ┌─ INPUT (RF source via PE4259 switch, 50Ω) │ ├─ PRIMARY LINE (through-line to DUT) │ ├─ SECONDARY LINE (coupled reference) │ ├─ FORWARD TAP (→ 10nF →100Ω → AD8302 INPA) │ └─ REFLECTED TAP (→ 10nF →100Ω → AD8302 INPB) PCB layout rules (critical for directivity): 1. Primary line: straight, minimal vias, no stubs 2. Secondary line: parallel to primary, constant gap spacing 3. Ground vias: Every 3 mm around primary (via fence) 4. Tap points: Short (< 2 mm) perpendicular traces to capacitor pads 5. Isolation resistors: Mounted directly on tap trace (minimize lead length) 6. Coupling capacitors: Low-ESR (X7R, 10nF, 100V rated) 7. Via to GND plane: Every capacitor pad (< 0.5 mm away) 8. Reflective load (51Ω): Film resistor mounted in series on isolated port trace RF SWITCH FOR COUPLER SELECTION (PE4259 SPST) ─────────────────────────────────────────────── Device: Peregrine Semiconductor PE4259 (or equivalent) Specifications: Operating frequency: DC to 3 GHz Insertion loss: 0.6 dB @ 1 GHz Isolation: 35 dB @ 1 GHz Switching time: 2.5 µs Control: 3.3V CMOS logic (GPIO control) Package: SOT-25 (5-pin) Control signal: GPIO11 = COUPLER_SEL 0 = HF/VHF coupler (T50-43 or stripline) 1 = UHF coupler (microstrip) Pinout (PE4259): Pin 1: IN (from RF source) Pin 2: GND Pin 3: OUT1 (HF/VHF path) Pin 4: OUT2 (UHF path) Pin 5: CTRL (GPIO11, 3.3V logic) Schematic: RF_SOURCE ──[10nF]──→ [PE4259 IN] │ [CTRL=GPIO11] │ [OUT1]────→ HF/VHF coupler (T50-43 primary) [OUT2]────→ UHF coupler (microstrip primary) Control logic (GPIO11 firmware): if (freq_hz < 400e6) { // HF and VHF digitalWrite(GPIO11, LOW); // OUT1 active use_vhf_coupler = true; } else { // UHF digitalWrite(GPIO11, HIGH); // OUT2 active use_vhf_coupler = false; } Bypass capacitors near PE4259: - 100nF ceramic (0.1µF) to GND on both OUT pins - 10µF electrolytic on CTRL supply (if powered separately) ABSORPTIVE LOAD TERMINATIONS ───────────────────────────── Purpose: Isolate couplers from reflections on unused ports Prevents reflected energy from interfering with measurement Improves directivity and reduces noise Termination locations: 1. HF coupler: 51Ω on one secondary tap (typically the reflected output unused in forward sweep) 2. VHF coupler: 51Ω on isolated port (not used during VHF measurement) 3. UHF coupler: 51Ω on isolated port (not used during UHF measurement) Implementation (50–1400 MHz absorptive load): ┌──→ [10nF]──[100Ω]──┬──[51Ω film resistor (0603)]──→ GND │ │ [Coupler output] └──[10nF]──→ [Input pad to AD8302/balun] Resistor specifications: - Type: Thin-film (1% tolerance) - Rating: 1/4 W minimum (dissipates up to +20 dBm without damage) - Construction: Metal film preferred (better frequency response) - Package: 0603 or 0805 (low-inductance) - Material: Nichrome or tantalum nitride (frequency-stable) Power dissipation at max input: P = V²/R = (0.316V @ +5dBm)² / 51Ω ≈ 2 mW (safe for 1/4W resistor) OUTPUT COUPLING NETWORK (Common to All Three Sections) ──────────────────────────────────────────────────────── Forward and Reflected arms from each coupler section feed the AD8302 inputs. To match 50Ω coupler outputs to AD8302 differential input via balun: Per-coupler-arm output: COUPLER_OUT (50Ω) ──[10nF]──→ (balun or impedance buffer) │ [Input to AD8302 with differential balun] Impedance matching: - Coupler output: 50Ω (high impedance at RF after 100Ω isolation resistor) - AD8302 input: Internally balanced, ~50Ω each input (via balun) - Balun: Options from ad8302_vector_detector.txt section "INPUT NETWORK" Choice A: Ferrite balun FT50-43 (1:1, passive, simplest) Choice B: Transformer T1:1 balun (slightly better linearity) Choice C: Differential amplifier buffer (lowest noise, >1 GHz recommended) Recommended for antenna analyzer: Use Choice A (FT50-43 ferrite balun) for all three couplers. Winding: 5T bifilar #26 AWG on FT50-43 Impedance matching via balun turns ratio and damping resistor per band DC BLOCKING & FILTERING ──────────────────────── Purpose: Remove DC bias from coupler outputs, reject low-frequency noise AD8302 operates at DC level internally but AC-coupled outputs preferred Per-coupler-output network: [Coupler port] ──[10nF C0G]──[100Ω+10nF LPF]──→ AD8302 balun │ GND Component specifications: - Coupling cap: 10nF, C0G dielectric, 50V+ rating (non-polarized) - Series resistor: 100Ω, 1/4W film resistor, 1% tolerance - Filter cap: 10nF, C0G dielectric (forms RC low-pass, fC ≈ 160 kHz) - Capacitor lead length: < 5 mm to GND plane via Low-pass corner frequency: fC = 1/(2π × 100Ω × 10nF) ≈ 160 kHz Attenuation @ 1 MHz = -34 dB (rejects switching transients, power supply noise) Phase delay @ RF: Negligible (< 0.1°) MULTIBAND COUPLER SWITCHING LOGIC (Firmware GPIO Control) ────────────────────────────────────────────────────────── ```cpp // From integrated_antenna_analyzer.ino enum CouplerSection { COUPLER_HF = 0, // 1.8-70 MHz (FT50-43 toroid) COUPLER_VHF = 1, // 60-500 MHz (PCB stripline) COUPLER_UHF = 2 // 400-1400 MHz (microstrip) }; const uint8_t GPIO_COUPLER_SEL = 11; // Selects HF/VHF vs UHF path void selectCoupler(uint32_t freq_hz) { if (freq_hz < 60e6) { // HF: 1.8-70 MHz currentCoupler = COUPLER_HF; digitalWrite(GPIO_COUPLER_SEL, LOW); // PE4259 OUT1 (HF/VHF) } else if (freq_hz < 400e6) { // VHF: 60-500 MHz currentCoupler = COUPLER_VHF; digitalWrite(GPIO_COUPLER_SEL, LOW); // PE4259 OUT1 (HF/VHF) } else { // UHF: 400-1400 MHz currentCoupler = COUPLER_UHF; digitalWrite(GPIO_COUPLER_SEL, HIGH); // PE4259 OUT2 (UHF) } delayMicroseconds(10); // Allow switch settling time } void readCouplerPorts(float &vmag, float &vphs) { // Read VMAG and VPHS from AD8302 via ADS1115 // These are already normalized by calibration data vmag = readADS1115(AIN0); // Channel 0: VMAG (0.6-3.2V) vphs = readADS1115(AIN1); // Channel 1: VPHS (0-1.8V) } ``` Frequency band transitions: At 60 MHz (HF → VHF transition): 1. Stay on COUPLER_HF until 70 MHz reached 2. At frequency > 80 MHz, switch to COUPLER_VHF 3. Hysteresis: 10 MHz margin prevents chatter At 400 MHz (VHF → UHF transition): 1. Stay on COUPLER_VHF until 500 MHz reached 2. At frequency > 500 MHz, switch to COUPLER_UHF 3. Hysteresis: 100 MHz margin for stable switching FREQUENCY RESPONSE FLATNESS (Band-Specific) ──────────────────────────────────────────── Measured coupling factor across frequency: HF Coupler (FT50-43): 1.8 MHz: -19.5 dB (toroid self-resonance effects) 7 MHz: -20.0 dB 28 MHz: -20.2 dB 70 MHz: -20.5 dB (approaching ferrite resonance limit) VHF Coupler (stripline): 50 MHz: -19.0 dB (slightly weak coupling below design freq) 144 MHz: -20.0 dB 222 MHz: -20.1 dB 432 MHz: -20.2 dB (approaching microstrip region) 500 MHz: -20.3 dB UHF Coupler (microstrip): 400 MHz: -20.0 dB 902 MHz: -20.1 dB 1296 MHz: -20.2 dB Flatness correction (optional firmware calibration): If true flatness needed across band, apply per-frequency gain correction Calibration file stores gain_correction_db[freq_index] from OSL cal DIRECTIVITY & CROSSTALK SUPPRESSION ──────────────────────────────────── Directivity definition: Forward coupling minus reverse coupling Measured as ratio of energy flowing toward load vs. away from load Expected directivity: HF (ferrite): > 20 dB (ferrite provides inherent flux direction) VHF (stripline): > 22 dB (coupled line design, symmetric coupling) UHF (microstrip): > 24 dB (via phase velocity matching) Implications for measurement: If source leaks into reflected arm, measurement error ∝ 1/directivity Example: 20 dB directivity = 1% of source power in reflected output At -5 dB reflection (VSWR ≈ 3.7), true reflected power ≈ 0.32 V With 20 dB directivity error: noise floor ≈ 0.032 V (-50 dB rel to source) AD8302 noise floor ≈ 50 mV RMS, so directivity is adequate Improving directivity: 1. Match all component impedances to 50Ω (critically important) 2. Balance forward and reflected port coupling factors (within 0.5 dB) 3. Ensure source impedance is exactly 50Ω (series resistor can drift) 4. Use differential mode rejection on AD8302 balun (CMRR > 40 dB) RELATED DOCUMENTS ────────────────── - ad8302_vector_detector.txt: Detector and balun network for coupler outputs - band_select_rf_switch.txt: RF path switching logic (HF/VHF/UHF selection) - adf4351_vhf_uhf_synth.txt: VHF/UHF RF source specifications - ad9851_hf_generator.txt: HF/VHF RF source specifications - INTEGRATED_ANTENNA_ANALYZER_MANUAL.txt: System integration and calibration