Tag Archives: Couplers
The most common question we receive about our stripline directional couplers, low loss airline directional couplers, and high directivity directional bridges is ‘How much power can it handle?’. The reason is that directional couplers are frequently used for load-pull testing of amplifiers or monitoring a signal after a power amplifier. In either case, the directional coupler will be placed as close to the output of the power amplifier as possible, which means it must perform within spec at high operating powers.
Despite the importance of directional coupler performance under high power inputs, most directional coupler vendors offer a single number without any background or context. As we will show in this post, the static value commonly provided for the power handling of a directional coupler is an oversimplification of the matter.
We’ve talked a lot about IQ mixers in the last few posts, about their theoretical underpinnings and applications as a phase detector and phase modulator. In upcoming posts we will discuss applications of image reject and single sideband mixers as well. The key to all of these circuits is the quadrature phase shift, both at the LO side for an IQ mixer, and at the LO and IF side for an image reject or single sideband mixer.
Remember: a phase shift is not the same as a time shift. This is one of the most difficult concepts to grasp in RF, microwaves, and optics. We will begin with the trivial example of a time delay, just to show that this doesn’t work for anything but the most simple circumstances. We go on to show how different techniques can be used to create more flexible and useful quadrature phase shifts to ultimately realize our goal of an ideal, broadband IQ/image reject/single sideband mixer.
I was thinking about the difference between power dividers, baluns, and couplers, and realized that they could all be thought of as power splitters. The characteristics that make them different are the relationship between the outputs in terms of amplitude, phase, and attenuation between outputs. Here is a brief chart that explains them all:
We get this question a lot: how much power can part XYZ handle?
Power handling is a difficult topic, because the ways in which a device can fail depend so much on the operating conditions that it is subjected to.
We specify the max power on (for example) the PD-0165 as 1 watt only to be extremely conservative. Here are some use scenarios for the PD-0165 and the power handling I would estimate:
– Ideal use case: 50 ohm matched at all ports, using the device as a power divider. In this case the device is only dissipating the excess insertion loss. Depending on the heat sinking it has attached, it should be able to handle 10s of watts of CW power or more at 43 GHz. At a high enough power the connectors will fail.
– Worst case CW performance: Out of phase reflections at both output ports, or use as a power combiner with two signals that are 180° out of phase. In this case all the power will be dissipated in the isolation resistors, which means that the power is limited to what the resistors can dissipate. This is where the power handling will be limited to about 1 W before the resistors pop.
– Pulsed case: In this situation the power is limited by the voltage breakdown in the device. If the peak power is high enough the voltage will break down the dielectric either in the connectors or the substrate, this isn’t clear. The amount of power it can take depends on the pulse width and hence the peak power.
So the amount of power that you can put through the device depends on how you are using it and how much heat sinking you provide to it.
This will be in an upcoming app note, but I couldn’t find a good explanation on the internet of the difference between a directional coupler and a dual directional coupler, so here it is:
A dual directional coupler is exactly the same circuit as two directional couplers placed back to back.
The prime reason most companies make, and most people use, dual directional couplers is that most commercially available directional couplers only have one coupled port accessible. The other port is generally terminated in a near 50 ohm load that is tuned to improve the directivity. Because Marki directional couplers are capable of superior directivity without tuning on the reflected port our customers are able to measure both the forward and reflected power using a single directional coupler instead of a dual directional coupler.
Marki makes dual directional couplers with all four coupled ports accessible. In this case the two inner ports can be terminated with 50 ohm loads, and the outer ports can be connected to potentially mismatched loads without affecting the directivity or output at the other coupled port.