Coaxial cables are the mainstay for long-distance signal transmission at radio frequencies. They are not generally used for digital data or pulse transmission due to their bandlimited nature. However, they can be used to transmit signals in specialty analog applications, including modulated signals that will provide data to a receiver. The use of coaxial cables can also be extended to another case: differential analog signals.
The use of differential signals in analog applications is uncommon; this type of signaling may be used in specialty measurement applications with a custom-built differential driver, or a differential op-amp. That being said, it is still something supported by common component sets, and so it is possible to route differential analog signals into a connector, including an SMA connected to a coaxial cable. One instance where this might be used is in a noisy environment, where the goal is to take advantage of the cable shielding to prevent noise pickup.
If you are planning to transmit a coaxial differential signal over coaxial cables, pay attention to some of these design points. Eventually, you may find that the frequency or power handling becomes problematic in your system and you will require a different approach.
Differential Signals in Coaxial Cables
One of the advantages of routing differential signals is suppression of common-mode noise, as well as the reciprocal suppression of radiated noise from the opposite-polarity signals in a differential channel. This applies in both differential digital signals and analog signals. On a PCB the level of noise suppression is controlled through the placement of the ground plane and the spacing between the traces in a pair. Larger distance to ground requires smaller spacing between the pairs to hit a given impedance target.
When you route each side of a differential pair to a coaxial cable, such as an SMA connector for impedance-matched coaxial cables, there are some important points to keep in mind. The most important are frequency and impedance. We can see why this is the case from the example graphic below, where a differential pair splits into two sections that connect to the coax.
The routing strategy is easy; route each half of the differential pair into its own coaxial cable. This will ensure the impedance seen on each side of the pair matches the spec for the connector. Right at the region where the pairs split off, the impedance will start increasing. This is because the mutual inductance and capacitance between the two traces starts decreasing. This leads to reflections that should be suppressed, and this is easily done by designing the traces correctly.
Reflections at the Cable Input
An impedance mismatch leads to a reflection at the input of these connectors. For short cables and low frequencies, this is nothing to be concerned with (see the section below). It’s when the cable becomes long that you need to enforce impedance matching.
While you could design a narrowband filter circuit to match the impedance, a simpler solution is to ensure the single-ended impedance of each trace in the pair is very close to the characteristic impedance. This means routing the differential pairs with somewhat higher spacing and setting the ground plane closer in the stackup.
To ensure termination, you can use two strategies:
- Design the differential pair with ground closer to the top layer
- Use larger spacing between the traces in the pair
Both strategies will cause the trace impedance to be much closer to the characteristic impedance, and it will help reduce noise emission/reception from the pair. You then need to ensure the individual trace impedance matches the connector/cable impedance at your operating frequency.
Which Impedance to Use?
Remember, the actual impedance of a trace in a differential pair is the odd-mode impedance, which is always slightly less than the characteristic impedance, as shown below.
If you follow the guideline above, and you use larger spacing between the traces, the odd-mode impedance will be much more similar to the characteristic impedance. In this case, you can always design the trace to a specific characteristic impedance that matches the connector and cable impedance. This will ensure maximum power transfer into the connector and the cable.
Low Frequency vs. High Frequency
If your differential analog link is operating at low frequency, meaning the connector body and (trace + signal launch) structure are electrically small compared to signal wavelength, then don’t worry about the above points. In that case, the driver circuitry will only interact with the load impedance directly. At low frequencies (such as sub-1 MHz), the cable impedance will only dominate once the cable length gets to the order of 1 m or longer. Specialty measurement applications are generally compact and will have shorter cable links, so the impedance requirements are often ignored except at the receiver.
At high frequencies requiring a differential analog signal, it’s best to just design to a target impedance throughout the system (normally 50 or 75 Ohms). This is because the physical length of the link will be much larger than the wavelength of the propagating signal, and you will find that reflections can occur even at somewhat lower frequencies. Use a ground plane close to the differential traces to provide shielding and consistent impedance. The link can also be evaluated using differential-mode S-parameters, both in simulation and in measurement.
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