================================================================================ SCHEMATIC: PRECISION 50-OHM LOAD — VNA CALIBRATION (DC TO 6 GHz) TM-DL-VNA-CAL Rev A ================================================================================ OVERVIEW -------- Precision 50 ohm termination for VNA SOLT calibration, nanoVNA use, and RF reference measurements. Two versions: CAL-SMA: SMA connector, DC-6 GHz, 1W maximum CAL-N: N connector, DC-3 GHz, 2W maximum CRITICAL DESIGN PRINCIPLES: 1. Minimum parasitic inductance and capacitance 2. All resistors within 2mm of connector reference plane 3. Controlled 50-ohm ground plane under resistor pads 4. No through-hole components (use 0402 or 0603 SMD only) 5. Resistor tolerance: 0.1% minimum (0.05% preferred) -------------------------------------------------------------------------------- VERSION A: SMA PRECISION LOAD (DC-6 GHz, 1W) -------------------------------------------------------------------------------- TOP VIEW (PCB, actual size approximately 15mm x 15mm): +-------+-------+-------+ | | | | | R1 | R2 | GND | | 100R | 100R | PLANE | | 0402 | 0402 | | +---+---+---+---+-------+ | | +---+---+ | [SMA CENTER PIN] (solder direct to R pads) | [SMA SHELL] = GND PLANE (all of bottom PCB layer) Cross-section view: ___ R1 (+)---[100R]---| |---GND (bottom copper) R2 (+)---[100R]---| |---GND (bottom copper) Signal IN --------|-| (SMA center pin, 1.27mm diameter) PARALLEL COMBINATION: R1 || R2 = 100 || 100 = 50 ohm Power: 2 x 500mW = 1W maximum PREFERRED RESISTORS: Vishay FC0402E1000BTT1 (100R, 0402, 0.1%, 63mW, +/-25ppm/C) Vishay CRCW04021000FRT2 (100R, 0402, 1%, 63mW) Susumu RG1005P-101-B-T5 (100R, 0402, 0.1%, 63mW) Bourns CRP0402B1000TDF (100R, 0402, 0.1%, thin film) For DC-6 GHz accuracy: THIN FILM resistors only. Thick film (CRCW) acceptable to 3 GHz. Wirewound: NEVER use, even "non-inductive" types. PCB STACKUP: Layer 1: Signal + resistors (35um copper) Layer 2: Ground plane (full copper pour, 35um) Material: Rogers RO4003C or Isola I-Tera MT40 (Dk=3.5 at 10GHz) Thickness: 0.508mm (20 mil) for SMA launch compatibility Copper pour: Solid, no voids, minimum 10mm from resistors to board edge SMA ATTACHMENT: Use end-launch SMA: Southwest Microwave 1092-01A-6 or equiv Solder directly to pads using silver solder (melting point consideration) Keep solder fillet below PCB plane -- do not build up on signal pads Torque to manufacturer spec (1 in-lb typical end-launch SMA) HOUSING (see openscad/enclosure_vna_cal.scad): Small brass or aluminum body, precision-machined or 3D printed PCB mounts inside body, SMA connector protrudes through front Body provides mechanical support, ESD protection, phase stability EXPECTED PERFORMANCE: Frequency Return Loss SWR DC-1 GHz > 40 dB < 1.02:1 1-3 GHz > 30 dB < 1.07:1 3-6 GHz > 22 dB < 1.17:1 (PCB quality dependent) These values require Rogers/Isola substrate. FR4 degrades above 2 GHz. With FR4: expect > 20 dB return loss to 2 GHz, 15 dB to 3 GHz. -------------------------------------------------------------------------------- VERSION B: N-TYPE PRECISION LOAD (DC-3 GHz, 2W) -------------------------------------------------------------------------------- RESISTOR ARRANGEMENT: 4x 200 ohm, 0603 thin film, in parallel = 50 ohm, ~1W total Arranged in quadrant pattern around N-type center pin All leads < 1.5mm +---------+---------+ | | | [200R] [200R] (N center pin connects | | to all 4 resistor hot ends) +---------+ [200R] [200R] | | +---------+---------+ | GND PLANE Board OD: 22mm (fits inside N connector body) Material: Rogers RO4350B, 0.508mm EXPECTED PERFORMANCE: Frequency Return Loss SWR DC-1 GHz > 36 dB < 1.03:1 1-2 GHz > 28 dB < 1.08:1 2-3 GHz > 22 dB < 1.17:1 -------------------------------------------------------------------------------- DC RESISTANCE VERIFICATION (both versions) -------------------------------------------------------------------------------- After assembly, measure DC resistance with precision LCR meter: Acceptable: 49.5 to 50.5 ohm (±1% for standard version) 49.9 to 50.1 ohm (±0.2% for precision version) Temperature coefficient (Vishay thin film): ±25 ppm/C typical At 50C rise: resistance change = 50 x 25 = 1250 ppm = 0.125% This produces SWR change of < 0.001:1 -- negligible -------------------------------------------------------------------------------- PARTS LIST — SMA VERSION (CAL-SMA) -------------------------------------------------------------------------------- Qty Reference Value/Part Number Description --- --------- --------------------------- --------------------------- 2 R1,R2 100R 0.1% 0402 thin film Vishay FC0402E1000BTT1 1 J1 SMA end-launch SW Microwave 1092-01A-6 1 -- Rogers RO4003C PCB 15mm x 15mm, 0.508mm 1 -- Brass housing CNC or cast brass body PARTS LIST — N-TYPE VERSION (CAL-N) -------------------------------------------------------------------------------- Qty Reference Value/Part Number Description --- --------- --------------------------- --------------------------- 4 R1-R4 200R 0.1% 0603 thin film Vishay FC0603E2000BT24 1 J1 N-type male, precision Amphenol 172138 or equiv 1 -- Rogers RO4350B PCB 22mm diameter disc, 0.508mm 1 -- N-connector body Machine brass ================================================================================