The Telemetry RF PCB is the dedicated wireless link that streams real-time flight data — GPS position, altitude, airspeed, battery voltage, motor RPM, and sensor payloads — from the UAV to the ground control station. Unlike the command-and-control link that carries safety-critical uplink messages, the telemetry channel is primarily a high-throughput downlink (1 kbps to 2 Mbps) with relaxed latency requirements. This specialization enables design optimizations unique to the telemetry board, including spread-spectrum modulation hardware, antenna diversity switching, and error-correction codec acceleration.
Spread-Spectrum Modem Integration
Modern telemetry RF PCBs use frequency-hopping spread spectrum (FHSS) or direct-sequence spread spectrum (DSSS) modems to improve robustness against jamming and multipath fading. The Semtech SX127x (LoRa) and TI CC1310 families dominate the sub-GHz band (868/915 MHz ISM), while the Microchip AT86RF215 serves dual-band operation at both sub-GHz and 2.4 GHz.
On the PCB, the modem IC occupies a dedicated RF zone with its own regulated supply. The IC's RF output feeds a harmonic filter (LPF or BPF) before the antenna switch. The modem's digital interface — typically SPI — connects to the host MCU through a digital isolation moat, with series termination resistors placed at the modem (source) side.
Antenna Diversity Hardware
UAV attitude changes during flight can cause deep fades on a single antenna due to polarization mismatch and airframe shadowing. Antenna diversity uses two antennas (spatially separated by ≥ λ/2) and an RF switch to select the antenna with the stronger received signal. The diversity control loop includes:
Dual RF paths: Two identical 50 Ω microstrip traces from the antenna switch (PE42520 or SKY13351) to two edge-launch SMA connectors.
RSSI monitoring: The modem's RSSI (Received Signal Strength Indicator) output is sampled by an ADC, with the MCU comparing RSSI from each antenna during the preamble of each received packet.
Switch control: GPIO-driven switch control line, with a 33 Ω damping resistor to suppress ringing on the switch control input.
Forward Error Correction Hardware
Telemetry links over long ranges suffer from bit errors caused by low SNR. Hardware forward error correction (FEC) — typically a Reed-Solomon or Viterbi decoder implemented in an FPGA or dedicated FEC ASIC — reduces the required Eb/N0 by 3–5 dB for a given BER. On the telemetry PCB, the FEC engine sits between the modem's digital output and the host processor's UART/USB interface, operating transparently to the host software.
Power Budget Optimization
Telemetry transmitters operate at +14 to +27 dBm (25–500 mW), balancing range against DC power consumption. For battery-powered UAVs, the telemetry board includes a dynamic power control (DPC) circuit: an RF power detector (AD8318) samples the transmitted power through a directional coupler, and the MCU adjusts the PA bias or modem output level in 1 dB steps to maintain the minimum power required for a target RSSI at the ground receiver.
Conclusion
The Telemetry RF PCB is a specialized RF design that optimizes downlink reliability through spread-spectrum modulation, antenna diversity, and adaptive power control — all while operating within tight SWaP constraints. Superb Tech fabricates telemetry PCBs with dual-RF-path layout, controlled-impedance microstrip, and Rogers/hybrid stackups verified to full S-parameter compliance.
