The Image Transmission Module PCB captures raw video from one or more cameras, compresses it using H.264/H.265 hardware encoders, and formats the compressed stream for transmission over the UAV's RF datalink — all with end-to-end glass-to-glass latency under 100 ms. This board must handle 4K/60fps video streams with 10-bit color depth while fitting within the tight size and power budgets of a tactical UAV payload bay. Let's examine the high-speed serial interfaces and video processing hardware that make this possible.
MIPI CSI-2 Camera Interface
The primary video input to the image transmission module is the MIPI CSI-2 (Camera Serial Interface) — a differential, source-synchronous serial bus that carries pixel data from the image sensor at rates up to 2.5 Gbps per lane. A typical 4K/60fps camera uses 4 lanes operating at 1.5 Gbps each, for an aggregate bandwidth of 6 Gbps. PCB layout requirements for MIPI CSI-2 are stringent:
Differential impedance: 100 Ω ±10% for each lane. Traces are routed as edge-coupled differential pairs on the top layer with a continuous reference plane on layer 2.
Intra-pair skew: Less than 5 ps (approximately 0.7 mm in FR-4) within each differential pair.
Inter-pair skew: Less than 200 ps (approximately 30 mm) across all lanes within a CSI-2 bundle. This is achieved through serpentine length matching on the shorter pairs.
Via minimization: Ideally zero vias in the CSI-2 path from the FFC/FPC connector to the SoC. When vias are unavoidable, paired ground-return vias flank each differential via pair.
Connector selection: 0.5 mm-pitch FPC connectors (Hirose FH35C) with ground pins between every signal pair for return-path continuity.
H.265 Hardware Encoder
Raw 4K/60 video at 10-bit depth generates approximately 12 Gbps of uncompressed data — far exceeding available UAV datalink bandwidth. Hardware video encoding compresses this to 10–40 Mbps with minimal latency. The encoder SoC — typically an Ambarella CV5, Movidius Myriad X, or HiSilicon Hi3559A — includes dedicated H.265/HEVC encoding pipelines that achieve sub-30 ms encode latency.
On the PCB, the encoder SoC is typically a large BGA (0.65 mm pitch, 400+ balls) requiring a minimum 6-layer HDI stackup for breakout. The DDR4 memory interface runs at 2133 MT/s, demanding fly-by topology with address/command length matching to ±25 mils and data byte-lane matching to ±10 mils. Power delivery for the encoder core (1.0V at 5–8A) uses a multi-phase buck converter with remote sense to compensate for IR drop across the PCB.
Video Output Interface
The compressed video stream is output to the data link modem via either:
Ethernet (RGMII/SGMII): For IP-based datalinks. RGMII operates at 125 MHz with 4-bit wide data paths and requires 50 Ω controlled impedance with length matching to ±50 mils within each clock domain.
SDI (HD-SDI/3G-SDI): For broadcast-grade links. 75 Ω controlled impedance on the PCB, with a cable driver IC (LMH0302) placed within 10 mm of the BNC connector.
Latency Optimization
Every millisecond of latency in the video pipeline degrades the operator's ability to control the UAV. The image transmission PCB minimizes latency through line-based processing in the encoder (encoding begins before the full frame is captured) and direct framebuffer access between the camera and encoder via shared DDR memory, bypassing the host CPU entirely.
Conclusion
The Image Transmission Module PCB embodies the convergence of broadcast-quality video processing and SWaP-constrained avionics packaging. With 6 Gbps MIPI interfaces, multi-gigabit DDR4 memory buses, and sub-100-ms encode pipelines, these boards demand precision fabrication. Superb Tech delivers image transmission PCBs with 100 Ω differential MIPI routing, HDI BGA breakout, and IPC Class 3 quality.
