The FPV Receiver Module PCB (VRX) is the ground-side counterpart to the VTX, capturing the 5.8 GHz analog video signal from the drone, demodulating it back to composite video (CVBS), and outputting it to goggles or a display. High-end VRX modules incorporate diversity reception — two independent receiver chains connected to two antennas with different polarizations or orientations — and an RSSI-driven switching circuit that selects the stronger signal on a frame-by-frame basis. This article examines the dual-channel RF architecture that defines modern FPV receiver boards.
Dual-Channel Receiver Architecture
A diversity VRX PCB contains two complete receiver chains (RXA and RXB) that are identical in design but physically separated on the board. Each chain includes:
SMA edge-launch connector → band-pass filter (SAW or discrete) → LNA (BFP840 or SKY67151) → mixer (RFFC5072) → IF amplifier → FM demodulator (RTC6715 or discrete PLL demodulator) → video buffer.
Independent RSSI output: Each demodulator IC provides an analog RSSI voltage (typically 0.3–2.5V) proportional to received signal strength, updated every 5–10 µs.
The two receiver chains are laid out as mirror images on opposite sides of the board centerline, maximizing physical separation between the two LNAs (typically 30–40 mm center-to-center on a 50 mm VRX board) to achieve >25 dB isolation between channels.
RSSI Comparator & Antenna Switching
The diversity switching circuit compares the RSSI voltages from RXA and RXB using a high-speed analog comparator (TLV3501, 4.5 ns propagation delay) or a dual-channel ADC (sampling at >100 kHz) feeding an MCU. When the comparator detects that the alternate channel's RSSI exceeds the active channel's RSSI by a configurable hysteresis threshold (typically 3–6 dB), it toggles a high-speed video multiplexer (TS5V330) that instantly switches the output to the stronger channel.
Critical PCB layout rules for the RSSI comparator circuit include:
Guard traces between the two RSSI analog traces, driven by a low-noise reference voltage to prevent crosstalk that could cause false switching.
Star grounding at the comparator's ground pin to prevent digital switching noise from the video mux from coupling into the RSSI measurement.
RC low-pass filter (1 kHz cutoff) on each RSSI line to suppress IF-frequency ripple without introducing switching delay beyond one video line period (64 µs for NTSC).
Video Signal Integrity
The demultiplexed composite video signal (1 Vpp into 75 Ω) travels from the VRX to the goggles or DVR over a 75 Ω coaxial cable. On the PCB, the video output trace must be a 75 Ω controlled-impedance microstrip — achieved with a 0.8 mm-wide trace over a 0.2 mm Rogers 4003C dielectric on layer 2. Any impedance mismatch causes ghosting (reflections visible as shifted duplicate images) in the analog video display.
A video buffer amplifier (ADA4430 or MAX4450) provides the drive capability for 75 Ω cable termination, with a 6 MHz low-pass reconstruction filter to remove modulation artifacts above the 4.2 MHz video bandwidth.
OSD Overlay Hardware
Many FPV receivers include an On-Screen Display (OSD) overlay that superimposes RSSI bars, channel number, battery voltage, and timer onto the video signal. The OSD IC — typically a MAX7456 or AT7456 — mixes digital character pixels into the analog video signal between the video mux output and the cable driver. The OSD requires a separate SPI interface from the MCU, with the SPI clock gated off during active video lines to prevent digital noise injection.
Conclusion
The FPV Receiver Module PCB is a precision analog/RF design that demands equal attention to the RF front-end (noise figure, linearity), the baseband signal path (group delay, impedance matching), and the digital control domain (RSSI sampling, OSD overlay timing). Superb Tech fabricates diversity VRX PCBs with dual-channel mirror layout, 75 Ω video trace routing, and full RF parametric test.
