Category Archives: Directional 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.
In making the datasheets for the first Microlithic frequency doubler (MLD-1640), it occurred to us that not enough has been made about the difference between isolation and suppression.
In mixers and amplifiers, some parameters are expressed relative to the input powers, while some are expressed in terms of the output power, with the conversion loss or gain calibrated out. This includes third order intercept point (IP3), which can be expressed as either input IP3 (IIP3), or output IP3 (OIP3). In general it is better to use OIP3 for mixers, since what really affects the dynamic range of a system is the amplitude difference between the output signal and output spur, expressed in dB relative to the output signal or carrier (dBc). This is illustrated in the table below, where the difference between the T3 and competing mixers is even greater when the superior conversion loss of the T3 is considered.
|T3-05||33 dBm||6.5 dB||26.5 dBm|
|Imitator 1||25 dBm||10.7 dB||14.3 dBm|
|Imitator 2||30 dBm||9 dB||21 dBm|
Note that it is better to use IIP3 in amps, for the opposite reason, namely that you want to give the amp credit for it’s gain. So in parts with gain the appropriate measure is IIP3, while in parts with a loss the appropriate spec is OIP3.
When the same logic is applied to spurious products in mixers and multipliers, the input referred value in dB is called isolation, while the output referred value in dBc is called suppression. Suppression is the preferable number to use, because it expresses the important value to the system. The isolation can always be improved by increasing the conversion loss of the mixer or multiplier, but this is obviously undesirable. There are, however, some issues using suppression.
The first comes with mixers. In all mixers we express the spurious output of the LO in terms of isolation, since it is dependent on the input LO power. Since the LO power does not change the conversion loss referenced to the input, this means that the suppression can vary by several dB with different LO drive levels.
The second complication is that the input signal, converted signal, and spurious tone are all at different frequencies. For example, when using a doubler with an 8-20 GHz input range, the output doubled frequency is 16-40 GHz, and the undesired tripled frequency is 24-60 GHz. This means that the isolation curves look like this:
While the third harmonic suppression looks like this:
The curves look slightly different. The suppression is more stretched out, and distorted by the curve of the conversion loss of the doubler. This is the result of the suppression calculation, which can basically be thought of as a 1:1 mapping of the isolation through the conversion loss.
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.