Pros and Cons of Different High-Frequency Transmission Line Types

You already know that dating is something I struggle with. After my failed attempts at blind dates and speed dating, I decided it was time to try online dating. I found a nice woman who said she was my age and set up a date. Upon arrival, I found out that I’d been catfished--that wasn’t actually her in the picture! Needless to say, it didn’t go well. It made me realize that I should have done a bit more research about my date and online matchmaking sites before blindly jumping in. While you may be tempted to over study the intricacies of online dating sites and their patrons, you should save some time for your first love, PCBs. Something that may be of particular interest to you in the near future is high-frequency transmission lines (TLs). In the past, we could leave worrying about them to the people designing cutting edge circuits. Now, though, the everyday engineer is making boards for higher frequency applications. Since high-frequency applications are becoming more popular, this trend will continue. I may not know much about dating, but I can definitely remind you of the basics and applications for the three most commons TLs: microstrip, stripline, and coplanar waveguide (CPW).

Microstrip

Microstrip is the default TL, like unoriginal users who “like long walks on the beach at sunset.” Not bad, but not good, just normal. This TL type generally consists of one or more strips of copper on substrate, with a grounding plane underneath. It’s quite easy to fabricate, just like an online dating profile. It’s also simple to design and simple to model. If only dating relationships were as easy to predict as high-frequency electromagnetic wave transmission.

The electromagnetic (EM) radiation pattern for microstrip shows that most of the electric field is contained in the substrate. A small portion is radiated out above the strip as well. This portion that radiates from the open top of microstrip contributes to radiation losses in transmission.

Online dating is popular because it connects us to other people wirelessly. Microstrip is popular for similar reasons, which is why it’s used in patch antennas. They make good antennas when they are combined with a relatively thick substrate that has a low dielectric constant (Dk). However, this TL’s usefulness declines as you go from the RF spectrum into microwave and beyond. As frequencies rise, microstrips start to emulate the Dk of their substrates and become quite lossy.

A microstrip is the perfect TL for you if you’re working at relatively low frequencies, and want a no hassle kind of conductor.

Don’t try to use these transmission lines in your designs.

Stripline

On many matchmaking sites, you’ll find people who’ve been burned before and now hide behind a protective shell. Stripline is similar, in that it’s a microstrip that is simply hidden between two grounding planes. Technically I can’t call any material around it substrate since it surrounds the conductor, but I’m going to anyway, deal with it. This TL is a bit more difficult to manufacture than microstrip, but isn’t excessively troublesome. Things become more problematic if you decide you’d rather suspend the stripline signal in the air between its ground planes rather than substrate.

The electric fields in stripline are contained entirely in the substrate by the top and bottom ground planes. It’s basically a coax cable in disguise, especially if you include grounding vias on both sides of your line. The advantage of grounding vias is that there can be no difference in potential of your two grounding planes. They’ll also fully contain your signal.  If your conductor is surrounded on all sides by substrate, the effective Dk is simply the Dk of the material. If you’ve got it partially or fully suspended in several materials you’ll have to do the calculations yourself.

I don’t particularly enjoy other people prying into my love life. If you’re also a private person, stripline can help keep your signal free from interference. The top and bottom grounding planes will fully contain your signal, and keep out any unwanted characteristic impedance signals as well. The main loss concerns in striplines are the loss characteristics of the dielectric, magnetic resonance, and conductivity of the line itself. In addition, you’ll need to be careful about how you get your signal out of the substrate. You’ll have to use vias, and traditional vias can cause reflection and interference. I would recommend using microvias to connect your stripline to other layers.

Stripline has decent high frequency transmission line performance but falls short of being the best.

Feel free to blow kisses at the screen in thanks for this information.

Coplanar Waveguide

Do you ever find that the most perfect person for you also happens to be extremely complicated? Unfortunately for you, the best TL for high frequencies is also the most complicated. Coplanar waveguides have excellent high-frequency characteristics, even past 100 GHz. The problem is that they’re difficult to fabricate and a pain to model.

The radiation pattern in coplanar waveguides is quite interesting. The fields are primarily contained in the two side ground planes. This leads to lower radiation losses than in the open topped microstrips. CPWs can achieve lower Dks than other line types, especially if grounded. If you add a ground plane beneath your CPW it forces more electric field lines into the air above the conductor, more even than in microstrip. This lowers the effective Dk for the line and can decrease losses. The primary losses in CPW are losses in the conductor and substrate. These can be limited by using better materials.

CPWs are great for dense circuits because their geometry limits crosstalk. They have the added advantage of resisting discontinuities in their ground planes. These make CPW a great choice if you’re working with dense, high-frequency circuits.

So there you have it, your three main options with it come to transmission lines. Hopefully, you have a few more options in your love life. If not, take solace in your work and design boards using the TLs best suited for their purposes. Microstrip is fine for the everyday PCB, but if you’re looking for that something special, you’ll need to use something else. Stripline has good high-frequency characteristics and resists other meddling signals. However, it’s difficult to manufacture and needs vias to connect it to the rest of your circuit. CPW is the best but will require you to put a lot more energy into the design of your PCB.

I like to have a wingman with me if possible when I’m out trying to woo women. They remind me to tell my endearing embarrassing stories instead of the ones that are just embarrassing. As a PCB you need a good wingman as well, and by wingman I mean wing-computer software. CircuitStudio^® is just that kind of software. It has a wide variety of features that will help you present the best version of your work.

Have more questions about transmission lines? Call an expert at Altium.