An important characteristic of phase shifters

Updated January 2, 2010

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New for January 2010! On this page you will learn how to judge different phase shifter concepts based on a single characteristic that is of huge importance, especially if you are considering using the phase shifter in a phased array. That characteristic is the ability to provide a flat phase shift over frequency.

In a phased array, when you steer the beam, you adjust the phase of the elements so that individual signals line up at the desired beam-pointing angle (theta). Having phase settings that don't change with frequency help keep the beam pointed where you want it when you shift frequencies.

Comparison of delay line and high-pass/low-pass phase shifters

Note to readers... the switched delay line is just one way to create a phase shifter that has phase shift that varies with proportionally with frequency.... see our discussion of quadrature phase shifters for another example.

Suppose you were to make a four bit phase shifter, two different ways. In the first phase shifter, you use nothing but delay line bits. Shown below is a simple ADS schematic of a two-state delay bit which provides 180 degree phase shift at 10 GHz, the other bits would look similar. R1 and R2 allow the parameter "ON" to set the phase state, if ON=1, the delay line is connected, if ON=0, the delay line is bypassed.

Delay line phase shifter

Shown below is a schematic of a high-pass/low-pass phase bit that is parameterized so that its phase is set by "PS", its center frequency is set by "FC", and its characteristic impedance is set by "Z0". Again, R1 and R2 allow the "ON" parameter to control the state.

Using the multi-state ADS phase shifter circuit, let's compare the phase states across frequency, and the RMS phase errors. In the delay line phase shifter,

High-pass/low-pass phase shifter

Characteristics of delay line phase shifter

Characteristics of high-pass/low-pass phase shifter

So, at the center frequency, dither approach will provide (in theory) zero degrees phase error. If you need bandwidth, the delay line phase shifter quickly falls apart, while the filter-based phase shifter does not. If you have ever designed any microwave circuits, you know that they don't always function like the simulation, and center frequency of a design often shifts up or down. Therefore, you should avoid the delay line phase shifter when possible.

Implications of phase shifter choice on phased array performance

Now let's look at the bandwidth implications of having a constant phase shift over frequency, versus a delay line, when you are designing or operating a phased array system. We used the phase array spreadsheet that is available in our download area to create these charts. We set up a four-bit phase shifter, centered at 10 GHz, with element spacing lambda/2 (1.5 cm), and we scanned off to 45 degrees.

Then we shifted the center frequency to 12 GHz while keeping the 10 GHz calibration. By "calibration", we mean the phase settings. Below is the result, using a constant-phase phase shifter. The pattern is slightly shifted from 45 degrees, but the gain is almost unaffected (the radial axis is magnitude of field intensity). Sidelobes are slightly higher.

Now look what the result is if you used delay-line phase shifter bits. The angle is off slightly more, but the magnitude has dropped significantly and more sidelobes are popping up. You have just killed the gain of your antenna! Note that there would be no effect if your phase shifter provided true time delay, a topic we'll deal with in the future. True time delay for every element across an entire phased array is almost impossible to pull off.

In real life, few phased arrays have the bandwidth be able to jump 20% in frequency without at least using a different calibration file. But the point we are trying to make is that a constant phase shift is a better characteristic than a constant delay line, when you are building a phased array, you'll get more bandwidth, and fewer calibration files are needed over frequency.