VHF SAME Receiver Design with Complete Technical Documentation

# TECHNICAL REQUIREMENTS ## VHF SAME Receiver and AFSK/FSK Demodulator **Document:** RT-RF-SAME-001 **Revision:** 1.1 **Application:** EAS-SAME/SASMEX Signal Receiver **Target Frequency:** 162.425 MHz **Operating Band:** 162.400–162.550 MHz --- ## 1. OBJECTIVE Design and develop an electronic module capable of receiving the VHF signal at **162.425 MHz**, demodulating the NFM transmission, recovering the baseband signal, and demodulating the **AFSK/FSK data used by SAME (Specific Area Message Encoding)**. The module shall cover the complete signal chain: **Antenna/RF Input → RF Filtering → VHF Receiver → NFM Demodulation → Baseband Conditioning → AFSK/FSK Demodulation → Digital DATA_OUT** The final output shall be ready for connection to an external microcontroller that will implement the SAME protocol decoder. Implementation of the complete SAME message parser and earthquake alert logic is outside the scope of this development. --- ## 2. RF REQUIREMENTS ### 2.1 Frequency Primary operating frequency: **162.425 MHz** The design should preferably support software/configuration-based tuning across: **162.400–162.550 MHz** without PCB or component changes. ### 2.2 RF Input * Nominal impedance: **50 Ω** * External antenna connection: preferably **SMA** * Antenna suitable for approximately 162 MHz * RF layout shall follow appropriate controlled-impedance and grounding practices. ### 2.3 Modulation The receiver shall support the **Narrowband FM (NFM)** transmission carrying the SAME AFSK data. The NFM demodulator shall provide a baseband signal with sufficient fidelity for subsequent digital data recovery. ### 2.4 Sensitivity Required design target: **≤ –110 dBm** Preferred target: **≤ –115 dBm** The final sensitivity shall be experimentally characterized based on successful digital data recovery, not audible reception only. ### 2.5 Selectivity and Stability The receiver shall provide sufficient selectivity to reject adjacent/out-of-channel VHF signals. A crystal, TCXO, PLL, or equivalent stable frequency reference shall be used to maintain reliable tuning during continuous operation. The designer shall specify the achieved frequency accuracy and stability. --- ## 3. RECEIVER ARCHITECTURE The designer may select an appropriate architecture, including: * integrated VHF receiver; * superheterodyne; * low-IF; * direct conversion; * RF transceiver configured for receive operation; * equivalent solution. The selected solution shall prioritize: * sensitivity; * selectivity; * frequency stability; * component availability; * manufacturing repeatability; * long-term availability. The receiver should preferably provide **RSSI (Received Signal Strength Indicator)** through an analog or digital interface. --- ## 4. BASEBAND REQUIREMENTS The NFM demodulated output shall preserve the AFSK frequencies used by SAME: **MARK: 2083.3 Hz SPACE: 1562.5 Hz Nominal data rate: 520.83 bit/s** The baseband signal shall not be excessively modified by audio processing, filtering, deemphasis, AGC, or other functions that could affect reliable data recovery. A test point identified as **TP_BASEBAND** shall be provided. --- ## 5. AFSK SIGNAL CONDITIONING The baseband signal shall be filtered and conditioned for reliable discrimination of the MARK and SPACE frequencies. as a design reference, the signal-processing chain should preserve approximately: **1.2 kHz to 2.5 kHz with minimal distortion at 1562.5 Hz and 2083.3 Hz Filtering may be implemented using: analog active/passive filters; digital filters; DSP; or a combination of these methods."}"]}