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How do I route a high speed LVDS signal near an RF circuit without causing interference?

How do I route a high speed LVDS signal near an RF circuit without causing interference? LVDS (Low-Voltage Differential Signaling) is one of the better-behaved digital interfaces for mixed-signal environments, but careful routing is still needed to prevent coupling to RF circuits: (1) Why LVDS is better than single-ended: LVDS uses differential signaling with 350 mV swing (much lower than CMOS 3.3V or LVCMOS). The differential mode fields cancel in the far field, reducing radiated EMI by 20-30 dB compared to single-ended signals. Common-mode emissions are the primary concern (caused by skew, asymmetric routing, and coupling imbalance). (2) Routing guidelines near RF: keep LVDS traces ≥ 10 mm from RF traces (25 mm preferred for GPS/cellular receivers). Route LVDS on inner layers (stripline) to benefit from ground plane shielding. Route LVDS perpendicular to RF traces when crossing is unavoidable (minimizes coupled length). Maintain tight differential coupling (spacing ≤ 2× trace width) to maximize common-mode rejection. Match P and N trace lengths within ±2 mil to minimize mode conversion and common-mode radiation. Use ground via fencing between LVDS and RF routing regions (vias every 2-3 mm). (3) Return current management: the LVDS return current flows in the ground plane directly beneath the traces. If the LVDS traces cross a ground plane gap or change reference layers: the return current must find an alternative path, creating a large current loop (antenna). Ensure continuous ground reference under the entire LVDS route. When transitioning layers: place ground vias adjacent to the signal vias (within 15-20 mil) to provide a return current path. (4) Clock vs data: LVDS clock signals have a narrowband spectrum (strong harmonics at f_clk and multiples). If a clock harmonic falls in an RF band: the harmonic is a strong, coherent interferer. LVDS data signals have a spread spectrum (data transitions are random, energy is distributed across the Nyquist bandwidth). Data coupling is less likely to cause narrowband interference but contributes to the broadband noise floor. Prioritize isolation of LVDS clocks over LVDS data.

Category: Signal Integrity and High Speed Digital

Updated: April 2026

Product Tie-In: PCB Materials, Shielding, Capacitors

LVDS Routing near RF

LVDS is widely used for camera interfaces (MIPI CSI-2), display interfaces (FPD-Link), and FPGA I/O in mixed-signal products, making proper routing near RF circuits a common design challenge.

LVDS EMI Reduction Techniques

(1) Common-mode choke: add a common-mode choke on the LVDS pair at the point closest to the RF section. The choke attenuates common-mode noise (the EMI source) by 15-30 dB while passing the differential signal unaffected. Use when the LVDS route must pass within 5-10 mm of the RF section. (2) EMI filter IC: integrated LVDS-compatible EMI filters (e.g., STMicroelectronics ECMF02/ECMF04) provide common-mode filtering in a tiny package (1 × 0.5 mm). These are designed for LVDS data rates up to 1 Gbps. (3) Shielded flex cable: if the LVDS interface connects to a separate board (e.g., camera module): use a shielded flex cable with continuous ground on both sides. The shield provides 20-30 dB of isolation from the RF section.

LVDS near RF Guidelines

LVDS swing: 350 mV (lower EMI than CMOS 3.3V)
Differential: 20-30 dB less radiation vs single-ended
Separation: ≥ 10 mm (25 mm for GPS/cellular)
Length match: ±2 mil P/N for low mode conversion
Ground via fence: every 2-3 mm between LVDS and RF

Common Questions

Frequently Asked Questions

Is LVDS always safe near RF?

LVDS is safer than CMOS/TTL (lower swing, differential cancel), but not inherently safe. A 200 MHz LVDS clock has a 10th harmonic at 2.0 GHz (near cellular Band 1). The harmonic level from a well-routed LVDS pair: approximately -60 dBm (before isolation). With 60 dB required isolation to reach -120 dBm at the receiver: 10-25 mm separation + ground plane may be borderline. Always verify with frequency planning and post-layout EMI simulation or measurement.

What about MIPI CSI-2 near RF?

MIPI CSI-2 uses a similar low-voltage differential signaling. Data rates: 1-4.5 Gbps per lane (harmonics extend to 10+ GHz). The lower swing (200 mV) reduces EMI, but the higher data rate creates broadband noise at frequencies that overlap with Wi-Fi 5/6 GHz and 5G FR1. Mitigation: use the MIPI-recommended shield layer in the flex cable, add common-mode filtering at the connector, and route the CSI-2 lanes as far from the RF antenna as possible.

Can I route LVDS and RF on the same layer?

Strongly discouraged. On the same layer: there is no ground plane between the LVDS and RF traces, and the coupling is maximized. Best practice: route LVDS and RF on different layers, separated by a ground plane. If they must be on the same layer: maintain ≥ 25 mm spacing and add a ground trace between them (connected to the ground plane with vias every 2-3 mm).

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