================================================================================ SCHEMATIC: WIDEBAND NOISE GENERATOR TM-NB-NOISE-GEN Rev A 1 MHz - 60 MHz, -25 dBm nominal output into 50 ohm ================================================================================ TWO DESIGNS GIVEN: NGZ: Zener avalanche noise source (simpler, lower noise floor) NGT: Transistor reverse-bias noise source (higher output, more uniform) ================================================================================ DESIGN NGZ: ZENER NOISE SOURCE ================================================================================ SCHEMATIC: +9V (battery) | R1 (4.7k, 1/4W) <-- bias to zener avalanche point | +--+--+ | | D1 C1 (100pF NP0) <-- RF bypass, keeps noise off supply rail 5.1V | zener | MAR-6 or ERA-3SM MMIC AMPLIFIER | | (50 ohm, 20 dB, DC-2 GHz) C2 (100pF) <-- RF coupling | [MAR-6 pin 1 (RF IN)] | [internal 50 ohm matching] | [MAR-6 pin 3 (RF OUT)] | C3 (100pF NP0) <-- DC block | R3 (51 ohm, 1%) <-- output pad / level control | J1 (SMA or BNC output) --> to bridge T1 winding MMIC BIAS CIRCUIT: +9V | RFC1 (10uH, RF choke, SRF > 50 MHz) | R2 (130 ohm, 1/4W) <-- bias resistor for MAR-6 | I_d = (9V - 5V) / 130 = 30.7 mA (typ for MAR-6) [MAR-6 pin 3 also connects here] | C4 (10uF tantalum + 100pF NP0 in parallel) <-- supply decoupling | GND COMPONENT VALUES: D1: 1N4733A (5.1V, 1W, DO-41) or BZX84C5V1 (5.1V, 500mW, SOT-23) R1: 4.7k, 1/4W, carbon film (noise from R1 negligible vs zener) R2: 130 ohm, 1/4W, 1% R3: 51 ohm, 1/4W, 1% (output termination + level pad) RFC1:10uH, Murata LQH3NPN100M or similar, SRF > 100 MHz C1: 100pF, NP0, 0805 (supply rail RF bypass) C2: 100pF, NP0, 0805 (coupling to MMIC input) C3: 100pF, NP0, 0805 (MMIC output DC block) C4: 10uF/16V tantalum || 100pF NP0 (supply decoupling) U1: MAR-6SM (Mini-Circuits) or ERA-3SM (Mini-Circuits) Alternatives: GALI-3+ (25dB gain), BGA614 (Infineon) J1: SMA female, PCB edge mount ZENER NOISE THEORY: A reverse-biased zener near its avalanche breakdown voltage generates broadband white noise. The noise power spectral density (PSD) is: Noise PSD = 4*k*T*R_n (Johnson equivalent) where R_n (equivalent noise resistance) for a 5.1V zener at 1mA: R_n ≈ 50-200 ohm (measured empirically, varies by device) For BZX84C5V1 at 0.9mA: noise figure ≈ 15-22 dB above thermal noise. After MAR-6 (NF=7dB, G=20dB): system NF ≈ 7dB (MMIC dominates) Output noise level: approx -25 dBm into 50 ohm (integrated 1-30 MHz) FLATNESS: ±4 dB from 1-30 MHz (acceptable for bridge null detection). Above 30 MHz: MAR-6 gain rolls off, reduces flatness. For 50 MHz use: replace R1/R2 with 68 ohm + adjust MMIC for ERA-3. ALTERNATIVE MMIC: ERA-3SM If MAR-6 is unavailable: ERA-3SM is a direct substitute. Adjust R2 to 120 ohm (ERA-3 Id=50mA, Vd=3.6V: R=(9-3.6-0.7)/50mA = 94 ohm Use 100 ohm). ================================================================================ DESIGN NGT: TRANSISTOR NOISE SOURCE (HIGHER POWER, FLATTER SPECTRUM) ================================================================================ SCHEMATIC: +9V | R1 (10k, 1/4W) <-- base bias resistor | Q1 base (2N3904 or BC547) | Q1 collector -----> NC (floating) | Q1 emitter -----> GND (emitter grounded) The B-E junction is reverse biased: Vbe = -9V (through R1) Avalanche noise at B-E junction is higher than zener noise. FULL NGT SCHEMATIC: +9V ----R1(10k)---- Q1 Base | (B-E reverse biased: Vbe = -9 + 0 = -9V) +9V Q1 Emitter ---- GND | R2 (1k) Q1 Collector ---- NC | C1 (100pF) <-- couples noise from base (high impedance node) to amp | [MAR-6 pin 1] | [same MMIC bias circuit as NGZ above] | C2 (100pF) | J1 output DESIGN DETAIL: The transistor B-E junction reverse biased at 8-10V avalanches and generates approximately 3-6 dB more noise than a zener at the same current. The noise is somewhat less flat than zener but provides more output power for easier null detection. Q1: 2N3904 (NPN, TO-92) -- use any small-signal NPN with V_ebo > 8V SCREEN EACH TRANSISTOR: Not all 2N3904 have adequate V_ebo for deep reverse bias. Measure: apply -9V to base via 10k with emitter grounded. If base current flows (meter deflects), the junction has broken down. If no current: transistor is unsatisfactory. Most 2N3904 work. Alternatively: BFR93 or BFT25A RF transistors have predictable B-E breakdown and very flat noise spectrum to 1 GHz. NOISE SWITCH: Install S1 (SPST) in series with 9V supply to noise source. This allows keying the noise on/off for comparison. Some bridge users prefer to null with noise off (null depth more obvious). ================================================================================ PCB LAYOUT NOTES (BOTH DESIGNS) ================================================================================ Critical layout rules for noise generator: 1. Keep D1/Q1 noise element lead length < 5mm to MMIC input. 2. Ground plane: solid copper pour on bottom layer, no voids. 3. C1, C2, C3: place within 3mm of MMIC pins. 4. RFC1: 10mm minimum from MMIC output trace. 5. Supply decoupling C4 directly at MMIC pin 4 (Vcc pin). 6. 50 ohm microstrip trace from MMIC output to J1: On 1.6mm FR4 (Dk=4.5): trace width = 2.9mm for 50 ohm. On 0.8mm FR4: trace width = 1.4mm. 7. Shield the noise generator from the bridge circuit with a copper partition or separate shielded compartment. EXPECTED OUTPUT LEVEL: 1 MHz: -24 dBm (approx, into 50 ohm) 7 MHz: -25 dBm 14 MHz: -25 dBm 28 MHz: -24 dBm 50 MHz: -28 dBm (roll-off starting) 100 MHz: -33 dBm These levels produce a comfortable S3-S6 noise signal on a typical HF transceiver when the bridge is well off null. ================================================================================ PARTS LIST — NGZ DESIGN (RECOMMENDED) ================================================================================ Qty Ref Value/Part# Description --- ----- ------------------ -------------------------------- 1 D1 1N4733A (5.1V, 1W) Zener, DO-41 1 U1 MAR-6SM MMIC amplifier, SOT-89 or MSC-07 1 RFC1 10uH 0805 RF choke, Murata LQH3NPN100M 1 R1 4.7k 1/4W 5% Bias resistor 1 R2 130R 1/4W 1% MMIC bias 1 R3 51R 1/4W 1% Output pad 3 C1-C3 100pF NP0 0805 RF coupling / bypass 1 C4 10uF/16V tantalum Supply decoupling (bulk) 1 C5 100pF NP0 0805 Supply decoupling (HF) 1 S1 SPST mini toggle Noise on/off 1 J1 SMA PCB edge Output ================================================================================