================================================================================ SCHEMATIC: ACTIVE FIELD STRENGTH METER TM-FSM-ACTIVE Rev A MMIC Preamplifier + AD8307 Logarithmic Detector + Analog Meter Frequency Range: 1 MHz – 500 MHz Dynamic Range: −75 dBm to +10 dBm (85 dB) ================================================================================ OVERVIEW -------- Active field strength meter providing calibrated logarithmic output. ERA-3SM MMIC preamplifier (20 dB gain, 0.1–3000 MHz) feeds an AD8307 logarithmic detector (25 mV/dB slope, −75 to +17 dBm). Output: 0–2V DC proportional to input power in dBm. Drives analog meter M1 (1 mA FSD) and audio tone oscillator. Power: 9V battery (single supply). Includes input attenuator (0/−10/−20/−30 dB) for high-field environments. COMPLETE SCHEMATIC — ACTIVE FSM --------------------------------- +9V BATTERY │ PROBE INPUT [SW_PWR] SMA J1 (50Ω) │ │ [F1 0.5A] ─────┴───── │ PROTECTION [C_B1 100μF] Bulk bypass │ │ [D_LIM pair] [C_B2 0.1μF] HF bypass BAT54S clamp │ to 3.3V / −0.3V ┤ +9V rail │ ─────┴───── ATTENUATOR SW_ATT (4 pos): 0 / −10 / −20 / −30 dB │ Pi attenuators (all 50Ω in/out): −10 dB: R=96.2Ω shunt, R=71.6Ω series, R=96.2Ω shunt −20 dB: R=61.1Ω shunt, R=247.5Ω series, R=61.1Ω shunt −30 dB: R=53.3Ω shunt, R=789.4Ω series, R=53.3Ω shunt (Use nearest E96 values) │ ┌──────────┴──────────┐ │ │ [C_IN 100pF] BYPASS PATH DC block (0 dB pos) │ +────┴────────────────────────────+ │ ERA-3SM U1 │ │ │ IN ──┤1 (IN+) (OUT) 4 ├──── RF_AMP_OUT │ │ │ Pin 1 = RF IN │ R_BIAS = (Vsupply − 4V) / I_bias │ Pin 2 = GND │ I_bias ≈ 65 mA (ERA-3SM nominal) │ Pin 3 = GND │ At 9V: R_BIAS = (9−4)/0.065 = 77Ω │ Pin 4 = Vcc / RF OUT │ Use 75Ω ±5% 1/4W +────────────────────────────────+ │ Pin 4 [R_BIAS 75Ω 1/4W] ←── Bias through RF choke is preferred [RFC1 100 nH] but R_BIAS alone works for broadband │ +9V │ │ C_OUT 100 pF RF_AMP_OUT ─────┴───[C_OUT]───────────────────────────────────── │ +───────┴───────+ │ AD8307 U2 │ 50Ω IN ──┤INHI V+ ├── +5V* │INLO VOUT ├──── VOUT (25mV/dB) R_SER 51Ω │COM TADJ│ (float TADJ) │INHI FILT ISET│ +───────────────+ │ GND * AD8307 requires 2.7V–5.5V supply. Use 78L05 (TO-92) from 9V. AD8307 draws 8–10 mA. 78L05 handles up to 100 mA; fine. VOUT: 0 to ~2.5V over −75 to +17 dBm input range Slope: 25 mV/dB Intercept: −84 dBm VOUT = 0.025 × (Pin_dBm − (−84)) = 0.025 × (Pin_dBm + 84) Input match: Place 51Ω between INHI and the RF input node. Bypass INLO to GND with 100 pF. Bypass VPS (pin 7) with 0.1 μF directly to GND. Place 1 nF on FILT pin (pin 6) to GND for smoothing. METER DRIVER CIRCUIT ---------------------- AD8307 VOUT ─────────────────────────────────────┐ │ [R4 1kΩ] (current limit) │ VOUT_FILT │ [C3 10μF + C4 100nF] to GND (low-pass; ~16 Hz cutoff with 1kΩ) │ +────┤+ LM358 U3A ├────┐ │ │ │ │ (−) [R5 10kΩ] [R6 10kΩ] feedback │ │ │ └────┘ OUTPUT │ VR1 (10kΩ) │ METER OUTPUT calibration │ offset adj [R7 15kΩ] series resist │ ┌───┴───┐ │ M1 │ 1 mA FSD │ meter │ analog └───┬───┘ │ GND M1 scale: Mark dBm values at key points. At −75 dBm: VOUT = 0.025 × (−75 + 84) = 0.225V → meter ~10% FSD At −30 dBm: VOUT = 0.025 × (−30 + 84) = 1.35V → meter ~54% FSD At 0 dBm: VOUT = 0.025 × ( 0 + 84) = 2.10V → meter ~84% FSD At +10 dBm: VOUT = 0.025 × ( 10 + 84) = 2.35V → meter ~94% FSD METER SCALE MARKINGS (for 1mA FSD, R7 = 15kΩ, range 0–2.5V): dBm: −75 −60 −40 −30 −20 −10 0 +10 %FSD: 9% 15% 24% 36% 50% 68% 84% 94% NOTE: The AD8307 is inherently logarithmic; the meter deflection IS approximately linear in dBm. The scale is nearly linear in dBm. AUDIO TONE OSCILLATOR ----------------------- AD8307 VOUT ────┤+ LM358 U3B (comparator) ├─────┐ │ │ │ [R8 1kΩ] [R9 100kΩ] feedback (oscillation) │ +────────────┘ GND │ [C5 0.1μF] │ GND 555 ASTABLE OSCILLATOR (alternative, simpler): +9V ─────[R10 10kΩ]─────+─────[R11 VR2 100kΩ]────+ │ │ [C6 0.01μF] Pin 7 (discharge) │ │ GND NE555 U4 │ +─────────────────+│ Pin2,6 ───┤THR/TRG DIS 8─┘ │ OUT 3─────[R12 100Ω]──── SPEAKER+ +5V ──────┤VCC RST/CV 4├─── +5V (RST) │ │ GND GND +────────────────+ VR2 (control voltage input): Wire VR2 wiper to AD8307 VOUT through R_ctrl (10kΩ). As VOUT increases (stronger field), 555 frequency increases. Tone range: approximately 200 Hz (weak, VOUT=0.2V) to 3000 Hz (strong, VOUT=2.0V). Speaker: 8Ω, 0.1W piezo buzzer or small dynamic speaker. Add 100μF bypass on +9V near 555. POWER SUPPLY -------------- 9V battery BT1 ─── SW_PWR ─── F1 (0.5A polyfuse) ─── +9V rail │ [78L05 U5] │ +5V rail (for AD8307) Current budget: ERA-3SM: 65 mA AD8307: 10 mA LM358: 1 mA 555 + LED: 5 mA Total: ~81 mA Battery life: 550 mAh / 81 mA ≈ 6.8 hours (PP3 alkaline) GPIO / CONNECTOR SUMMARY -------------------------- J1 SMA female Probe RF input (50Ω) J2 BNC female Secondary probe input (parallel path) J3 3.5mm mono jack Audio output (speaker/headphone) M1 Panel meter 1 mA FSD, 60×40 mm face SW_ATT 4-pos rotary 0/−10/−20/−30 dB input attenuator SW_PWR SPST rocker Power on/off VR1 10kΩ trimmer Meter calibration offset VR2 100kΩ pot Audio tone sensitivity BT1 9V PP3 Battery ATTENUATOR PI VALUES (50Ω system) ------------------------------------ dB Rshunt (each) Rseries --- ------------- ------- 0 ∞ (open) 0 (wire) 10 96.2 Ω 71.6 Ω 20 61.1 Ω 247.5 Ω 30 53.3 Ω 789.4 Ω Nearest E96 values: 10 dB: 95.3Ω || 71.5Ω || 95.3Ω 20 dB: 61.9Ω || 249Ω || 61.9Ω 30 dB: 53.6Ω || 787Ω || 53.6Ω Use 0402 or 0603 SMD thin-film, ±1%, 1/10W minimum. CALIBRATION REFERENCE ----------------------- At 14.175 MHz, 10 mW (+10 dBm) into J1 with 20 dB attenuator in: Effective input to ERA-3SM = 10 dBm − 20 dB = −10 dBm ERA-3SM output = −10 + 20 = +10 dBm (near P1dB = +12 dBm: OK) AD8307 input = +10 dBm VOUT = 0.025 × (10 + 84) = 2.35V → near FSD At typical QRP field near antenna (0 dBm received at probe): With 0 dB attenuator, ERA-3SM output = +20 dBm (MMIC will clip!) Use −20 dB attenuator for high-field measurements near transmitters. CAUTION: Maximum safe input at J1 = +10 dBm (10 mW) with 0 dB attenuator. ERA-3SM P1dB = +12 dBm; exceeding this clips output and gives false low readings. Input protection diodes (BAT54S) clamp at ±3.3V (±17 dBm), providing safety margin. However, do not rely on protection diodes as a routine operating mode. ================================================================================