Connecting devices through a JRE Test USB-C or USB 3 filtered interface at high data speeds requires attention to cable length, cable quality, and the specific USB revision your devices are using. This paper explains the issues, provides practical guidance for your test setup, and addresses the most common questions we receive.
A Brief History of USB Speeds — and Why Cable Matters Now
When USB was first introduced by the USB Implementers Forum (USB-IF), it operated at a modest 1.5 to 12 Mbps. Cable lengths of 5 meters or more were routine, and nobody worried about cable attenuation — at those data rates, the signal frequencies were low enough that even mediocre cable had negligible loss.
Each successive USB revision increased data speeds dramatically:
USB 1.1: 12 Mbps USB 2.0: 480 Mbps USB 3.0 (SuperSpeed): 5 Gbps USB 3.1 (SuperSpeed+): 10 Gbps USB 3.2 Gen 2x2: 20 Gbps USB4 / Thunderbolt 3: 40 Gbps Thunderbolt 4 / USB4 v2: 40–80 Gbps
But here is the critical point: while data speeds have increased by a factor of several thousand, the cables carrying those signals have not improved by anywhere near the same factor. Consider an analogy from cable television: the trunk and feeder cables you see on utility poles are typically half an inch in diameter, using foam-filled low-loss dielectric and robust aluminum conductors — and they carry frequencies far lower than what runs through a thin USB-C cable. Higher frequencies demand lower-loss cable or shorter cable runs. Physics does not offer a third option.
All USB-C, USB4, and Thunderbolt connections use the same physical USB-C connector regardless of speed. The connector looks identical whether you are running 480 Mbps or 40 Gbps. This is convenient — but it means the cable, not the connector, is what determines your actual data speed.
What's Inside a USB-C Cable
A USB-C cable is more complex than it appears. It contains two independent data paths:
A USB 2.0 connection using a single differential pair — one lane in each direction, like a two-lane road with one lane each way. This carries standard USB 2.0 data at up to 480 Mbps.
A SuperSpeed connection using multiple differential pairs — two lanes in each direction for USB-C (like a four-lane highway), or one lane in each direction for USB 3.0. This is where the high-speed data travels.
When you plug in a USB-C cable, the connected devices negotiate the fastest speed the link can support. If the cable attenuation is too high for the SuperSpeed connection to work reliably, the protocol will automatically throttle back to USB 2.0 speeds using the separate USB 2.0 pair within the same cable. This is a feature of the USB protocol, not a defect — it maintains connectivity even when the cable cannot support the highest speeds. But it means you may see your devices connect at USB 2.0 speed (480 Mbps) when you expected SuperSpeed (5+ Gbps), and the reason is almost always cable length or cable quality.
How the JRE Filters Affect the Link
The JRE USB-C filter and JRE USB 3-1 filter are completely passive — there are no active electronic components inside. The filter uses JRE's patented signal phasing topology (US 10,374,572B2 and US 10,644,670) to discriminate between the desired differential data signals and undesired common-mode RF interference. This is not low-pass filtering — there is no upper frequency limit on the data that can pass through. The filter does not care whether you are running at 5 Gbps or 40 Gbps; it discriminates based on signal mode, not frequency.
Electrically, the filter appears to the connected devices as approximately 1 to 1.5 meters of additional cable in the signal path. This is the single most important thing to understand about using these filters: the filter looks like a cable, and cable length determines speed.
If you are using a 1-meter cable from the external device to the filter, and the filter itself looks like 1 meter of cable, and the internal jumper cable to the device inside the chamber is another 0.3 meters, your devices "see" a total equivalent cable length of approximately 2.3 meters. Whether this length supports your target data speed depends on the USB revision and the quality of the cables.
What Speeds to Expect
Here is practical guidance based on our experience and the cable length limits established by the USB-IF specification (detailed at usb.org/usbc):
At 5 Gbps (USB 3.0 / USB 3.2 Gen 1): The JRE USB-C and USB 3 filters are supplied with a short jumper cable. Using this jumper inside the chamber and a similar-length cable on the outside — keeping total equivalent cable length under approximately 3 meters — will reliably support 5 Gbps. This is the most common and straightforward configuration.
At 10 Gbps (USB 3.2 Gen 2): Achievable with short, high-quality cables on both sides, keeping total equivalent length under approximately 2 meters. Results depend on the robustness of the connected devices — some USB 3.2 Gen 2 controllers are more tolerant of cable loss than others.
At 20 Gbps (USB 3.2 Gen 2x2): Requires very short cables and devices with robust signal integrity. Achievable, but test with your specific devices and cables to confirm.
At 40 Gbps (Thunderbolt 3/4, USB4): At these speeds, the USB-IF recommends passive cable lengths under 0.8 meters. Since the filter alone adds approximately 1 to 1.5 meters of equivalent cable length, a passive cable connection at 40 Gbps will not work without help. This is where active cables come in.
Active Cables: The Solution for Maximum Speed
An active cable contains built-in signal conditioning electronics (equalizers or retimers) that compensate for cable attenuation. Using an active cable on the external side of the filter — between your test equipment and the filter's external connector — allows the link to operate at speeds that would be impossible with a passive cable of the same length.
Active cables are selected based on your specific devices, the target data speed, and the total cable length. They are widely available from cable manufacturers. For Thunderbolt 3/4 operation at 40 Gbps through the JRE USB-C filter, an active cable on the external side is the standard approach.
One important clarification: a cable's power rating (for example, "100W USB-C cable") refers to its power delivery capability, not its data performance. A cable rated at 100 watts can deliver that power for charging, but may still have too much attenuation for high-speed data. Power delivery and data speed are independent specifications — do not assume that a "100W" cable is automatically suitable for 10 Gbps or 40 Gbps data.
Similarly, E-Marker chips (electronically-marked cables, required by spec for cables over 2 meters and for higher power levels) identify the cable's capabilities to the connected devices, but they do not improve the cable's signal attenuation. An E-Marked cable tells the devices "I can handle 5A at 20V," but it does not make the data path any less lossy.
Why Passive? The Security Advantage
We intentionally do not put active equalization or signal regeneration electronics inside the JRE USB-C and USB 3 filters. This is a deliberate design decision, not a limitation.
Many of our customers — particularly those in defense, government, forensics, and security-sensitive industries — require a fully passive, "copper-through" signal path with no active electronics that could be targeted, compromised, or used as an attack vector. The JRE filter is a purely passive device: signals pass through it, are filtered for RF interference, and arrive at the other side with no active processing, no firmware, no microcontrollers, and no possibility of electronic interception or manipulation within the filter itself. For customers handling sensitive or classified data, this is a critical requirement.
Thunderbolt: Same Connector, Higher Stakes
Thunderbolt uses the same USB-C connector and cable, but at speeds up to 40 Gbps (Thunderbolt 3/4) and 80 Gbps (Thunderbolt 5 / USB4 v2). Everything discussed above applies to Thunderbolt, but the cable length constraints are tighter because the data rates are higher. Thunderbolt is essentially USB-C at its most demanding — 40 Gbps over a thin cable is pushing the limits of copper interconnects.
For Thunderbolt testing through a JRE USB-C filter:
- Use the shortest possible cables on both sides
- Use an active Thunderbolt cable on the external side
- Expect to test and verify with your specific devices — Thunderbolt controllers vary in their tolerance for cable loss
- The filter itself imposes no speed limit; cable attenuation is the only constraint
HDMI: A Related Consideration
While not USB, HDMI faces the same fundamental challenge — high data rates over passive cables with limited length tolerance. The JRE HDMI-1 filter uses the same patented signal phasing technology and is compliant with HDMI 1.4 and higher. For higher HDMI versions and resolutions (4K, 8K), the same active-cable approach applies: use an active HDMI cable on the external side to compensate for the equivalent cable length the filter adds to the path.
Quick Reference: Troubleshooting USB Speed Issues. If your devices are connecting at USB 2.0 speed instead of the expected SuperSpeed:
Check your total cable length. The filter adds approximately 1 to 1.5 meters of equivalent cable. Add this to your actual cable lengths on both sides. If the total exceeds the USB-IF recommendation for your target speed, the link will throttle back.
Check your cable quality. Not all USB-C cables support SuperSpeed data — many inexpensive cables are wired for USB 2.0 and charging only, even though they use a USB-C connector. Look for cables explicitly rated for the data speed you need.
Check your devices. Both the host and device must support the target speed. A USB 3.2 Gen 2 device connected to a USB 2.0 port will only run at USB 2.0 speed regardless of cable quality.
Try removing the filter temporarily. Connect the devices directly with a short cable, bypassing the filter. If the link still does not achieve the target speed, the issue is with the devices or cables, not the filter.
Try an active cable on the external side if passive cables are not achieving the desired speed.
Power delivery should still work. Even when data throttles back to USB 2.0, power delivery (charging) typically continues at the cable's rated power level. A cable that throttles data to USB 2.0 may still deliver 60W or 100W of charging power — these are independent functions.
Further Reading
For the underlying filtering technology (how the patented signal phasing works): Understanding Data Signals Through Filtered I/O Connectors.
For general guidance on selecting an enclosure and configuring I/O: How to Select and Configure an RF Shielded Test Enclosure.
For the USB-IF specifications and cable standards: usb.org and USB-C specification.
JRE USB-C filter product page: JRE USB-C-1
JRE USB 3-1 filter product page: JRE USB 3-1
If you have questions about achieving a specific data speed through the filter with your particular devices, contact us — we work through these configurations every day.
