Clich here to go to our original March hybrid coupler page
Click here to go to our main page on rat race couplers
Click here to go to our main page on Gysel couplers
Click here to go to our main page on combiners
QUCS is freeware, the acronym stands for quite universal circuit simulator. It can perform SPICE simulations as well as RF circuit analysis. We used it for the latter. We are not sure how to pronounce it, should we say "cwuks" or "cuks" or what? Saying cuks at a meeting will result in some snickering for sure. Why use QUCS? You never know when you are gonna get laid off, and all that cool software you take for granted disappears. You can download QUCS here:
https://qucsstudio.de/download/
QUCS has some annoying issues, especially trying to figure out how to solve syntax errors. And you can forget about cutting and pasting images, you need to resort to screen grabs. Is it as good as Microwave Office or ADS? No. But it is an amazing contribution to the industry created by collaboration among volunteers. Cheers!
Steven March's coupler was published in 1968 as an alternative to the rat-race hybrid coupler, but it can also work similarly to the Gysel in-phase coupler. March's idea was that the 180 degree line is replaced with a shorted coupled-line coupler that gives perfect 180 phase across frequency, extending the useful bandwidth.
Below is our QUCS schematic of the March coupler. Our improvement is that we added quarter-wave transformers to all of the ports. Z1 is the ring impedance (including the impedance of the coupled line coupler!) and Z2 is the transformer impedances at the four ports. We added a phase shifter to port 3, which is used to compute phase errors when the coupler acts as a 180 degree hybrid. More on this later.
The network's line impedance are controlled by these equations, which can be enabled one at a time to compare results. When you think about it, In a classic rat-race the 70.7 ohm ring acts as a single-section transformer to make 50 ohm ports behave internally as 100 ohms. When you look into two 100 ohm ports in parallel, you see 50 ohms.
The Equation block on the left represents the original March coupler, where the port transformers are set to 50 ohms and become transparent and teh ring impedance is 70.7 ohms. Port reflection coefficients and isolations are shown below. Port 1 (S11)and port 3 (S33) show identical responses. The return loss is a single-point match, and therefore kind of narrow. Isolations S32 and S41 are also identical, and are remarkably better than 24 dB across all frequencies. You can predict the identical responses by noting the symmetry of the network.
The transmission coefficients for the original March coupler are shown below. On the left is S43 and S42, which are the 180-degree phase paths, and on the right are S21 and 21, the in-phase paths. S21 on th3 left is the same as S43 on the right, again, due to symmetry. You will notice that the overall bandwidth is slightly better when the couple is used for in-phase combining. In both cases the usable bandwdith might be 8 to 12 GHz depending on how you define it.
Now let's add th transformers We found the best result with the ring impedance at 40 ohms, and the transformers ~36 ohms. Check out the port matche, it behaves like an equal-ripple impedance transformer. Maybe someone could explain to us why that is. The isolation on the right is slightly degraded to 19 dB worst case. There is no free lunch.
Now admire the transmission coefficients. You could certainly use this from 6 to 14 GHz, which is double the bandwidth of the original March coupler.
We still need to look at phase errors for this coupler. We used an equation block to compute them, it took some time to figure out how to do ths. Thanks to Hadrien for his help!
For the in-phase coupler, the phase shifter on port 3 is set to 0 degrees. Phase error is shown below. We did not include a plot of the phase errors of the original March coupler but the impedance transformers don't cause any significant degradation. Inside the usable bandwidth of 6 to14 GHz the worst phase error is 8 degree.
For the 180 degree version the phase shifter is set to 180 degrees to center the phase errors around zero, as shown below. Here, the worst-case ohase error over 6 to 14 GHz is 7 degrees.
That wraps up our first QUCS circuit design. If anyone has any pointers or comments, please send them!
References
Steven March, "A Wideband Stripline Hybrid Ring" by Steven March, IEEE Transactions on Microwave Theory and Techniques, vol. 16, issue 6, pp. 361-361, June 1968.