================================================================================ SCHEMATIC: NULL DETECTOR WITH AUDIO AMPLIFIER TM-NB-NULLDET Rev A Sensitivity: < -60 dBm (usable null indication) Audio output: 8-32 ohm headphones or speaker ================================================================================ TWO DETECTOR MODES: MODE A: External receiver/transceiver (recommended for best sensitivity) MODE B: Built-in audio detector (standalone, no receiver needed) MODE C: AD8307 logarithmic detector (for digital readout, CYD display) ================================================================================ MODE A: EXTERNAL RECEIVER CONNECTION ================================================================================ The simplest null detector is a communications receiver or SDR tuned to a frequency within the noise band. Connect J_DET (BNC) on bridge to antenna input of receiver. Set receiver to AM mode (SSB also works). PROCEDURE: 1. Tune receiver to desired frequency (e.g., 14.200 MHz for 20M check). 2. Enable noise source. S-meter should show S3-S7 noise. 3. Adjust R_cal and X_cal until S-meter reads minimum (null). 4. Read R and X values. ADVANTAGES: Best sensitivity (receiver NF = 15-20 dB typical). Frequency-selective (ignores other noise sources). DISADVANTAGES: Requires separate receiver. Cannot use transceiver in TX mode. ================================================================================ MODE B: BUILT-IN AUDIO DETECTOR ================================================================================ FULL SCHEMATIC: BRIDGE DETECTOR PORT (J_DET, BNC) | R1 (51 ohm, 1%) <-- input matching | C1 (100pF NP0) <-- DC block | +------+------+ | U1 | | [ERA-3SM] | <-- MMIC preamplifier (20dB gain, NF=7dB) | or MAR-6SM | Bias: same as noise generator section +------+------+ | C2 (100pF NP0) <-- DC block | D1 (1N60 or BAT54A) <-- envelope detector diode | (anode to signal, cathode to filter) C3 (10nF) <-- detector filter (RC with R3) R3 (10k, 1/4W) <-- detector load | C4 (10uF/16V, ELCO) <-- audio coupling | VR2 (10k log taper audio pot) <-- volume control | +------+------+ | U2 | | LM386-N4 | <-- audio amplifier (1W, DIP8) | 20x gain | +------+------+ | R5 (10 ohm, 1W) <-- speaker current limiter (for 8 ohm speaker) C5 (220uF/25V) <-- output coupling cap | J5 (3.5mm stereo TRS) <-- headphone output LM386 FULL CIRCUIT: VCC (+9V) | +----- pin 6 (V+) | C6 (100uF/16V from pin 6 to GND) <-- bypass C7 (10uF from pin 7 BYPASS to GND) <-- extra ripple rejection Input from VR2 wiper: pin 3 (+input) pin 2 (-input): to GND pins 1,8: open (20x gain default) or 10uF cap between pins 1 and 8 (200x gain) pin 5 (output): via C5 to headphone jack NOTE: LM386 is susceptible to RF oscillation. Install: - 10 ohm + 0.047uF series RC from pin 5 to GND (Zobel network) - Do not route pin 5 or output traces near input traces - Shield LM386 section from RF stages if PCB area permits GAIN SELECTION (pins 1 and 8 of LM386): Pins open: 20x gain = 26 dB 10uF cap across: 200x gain = 46 dB Use 20x gain for initial setup. If headphones are quiet, add cap for 200x. Too much gain causes instability -- verify layout first. NULL DETECTION PROCEDURE (MODE B): 1. Enable noise source. Hiss audible in headphones. 2. Adjust R_cal (R pot): hiss changes character as R changes. 3. Adjust X_cal (C or L): hiss changes as reactance changes. 4. At null: hiss drops noticeably (10-20 dB drop is typical for a good null). 5. Vary R slowly, then X, iterating until minimum hiss found. 6. The null is not always complete (bridge asymmetry, leakage) -- find the deepest minimum achievable, then read R and X. SENSITIVITY: System noise floor (with MMIC preamp): N_in = kTB at 25C, B = 1 Hz: N_in = -174 dBm/Hz After MMIC (G=20dB, NF=7dB): N_out = -174 + 20 + 7 = -147 dBm/Hz In 10 kHz audio bandwidth: N_floor = -147 + 40 = -107 dBm Noise generator output at bridge null port: -40 to -60 dBm (good balance) Signal well above noise floor: null detection feasible without receiver. HEADPHONE IMPEDANCE: LM386-N4 rated for 8 ohm minimum load at 1W supply voltage. Compatible with: 8 ohm, 16 ohm, 32 ohm headphones. Do not connect 600 ohm headphones directly: use 47 ohm series resistor. ================================================================================ MODE C: AD8307 LOG DETECTOR (for CYD/ESP32 digital readout) ================================================================================ SCHEMATIC: BRIDGE DETECTOR PORT | R7 (52.3 ohm, 1%) <-- input matching (52.3 || 52.3 = 26 ohm... ) | Actually: AD8307 input Z = 1.1k || 1.4pF | Match with 52.3R shunt to GND at input pin C8 (47pF NP0) <-- RF coupling to AD8307 | AD8307 pin 8 (INP) AD8307 pin 1 (INM) ---- GND (for single-ended input) | AD8307 pin 4 (ENB) ---- 5V (enable, active HIGH) AD8307 pin 7 (VPOS)---- 5V (supply) AD8307 pin 2 (GND) ---- GND AD8307 pin 6 (VOUT)---- R8 (1k) ---- ESP32 GPIO34 (ADC1_CH6) | C9 (100nF) <-- ADC low-pass filter | GND AD8307 CHARACTERISTICS: Input range: -75 dBm to +17 dBm (dynamic range: 92 dB) Frequency: DC to 500 MHz (-3 dB) Output slope: 25 mV/dB Output at -75 dBm: ~ 0.25V Output at +17 dBm: ~ 2.55V (within ESP32 3.3V ADC range with 3.3V Vdd) IMPORTANT: Use 5V for VPOS if available. If 3.3V used: Output swing is reduced. Check datasheet for 3.3V operation. Preferred: AD8307 at 5V with 3.3V Zener clamp on VOUT. READING POWER FROM AD8307: V_out = 0.025 * (P_dBm + 84) [V, for 5V supply, 25C] P_dBm = (V_out / 0.025) - 84 [dBm] P_watts = 10^((P_dBm - 30) / 10) / 50 [watts into 50 ohm] Null condition: minimum V_out reading. Null depth: the difference in dBm between off-null and null. Typical null depth for good bridge: 20-35 dB minimum. Excellent null: 40+ dB. PARTS LIST — BUILT-IN NULL DETECTOR ===================================== Qty Ref Value/Part# Description --- ----- ------------------ ---------------------------------------- 1 U1 ERA-3SM or MAR-6SM MMIC preamplifier 1 U2 LM386-N4 Audio amplifier, DIP-8 1 U3 AD8307 Log detector IC, SOIC-8 (if digital out) 1 D1 BAT54A (SOD-123) Schottky envelope detector 1 VR2 10k log audio pot Volume control 1 R1 51R 1% 1/4W Input matching 1 R3 10k 1/4W 5% Detector load 2 R4,R5 10R 1W Zobel + speaker limiter 1 R7 52.3R 1% 0805 AD8307 input matching 1 R8 1k 1% 0805 AD8307 output filter 3 C1-C3 100pF NP0 0805 RF bypass/coupling 1 C4 10uF/16V Audio coupling 1 C5 220uF/16V Output coupling 2 C6,C7 100uF/16V LM386 decoupling 1 C8 47pF NP0 0402 AD8307 RF input 1 C9 100nF 0402 ADC filter 1 J5 3.5mm TRS stereo Headphone jack ================================================================================