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Como diseñar Arrays de Fase de Antenas, por KEYSIGHT - Contenido educativo

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Subido el 8 de abril de 2023 por Pedro Luis P.

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Descripción del concepto de ARRAYS de antenas, y software de keysight para su diseño

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Hi, my name is Murthy Upmaka. I am an Applications Engineer for Keysight Technologies. In this 00:00:00
video I will discuss the most important considerations for a face-to-face system design. I used a 00:00:14
simulation and modeling tool called SystemVue to illustrate various steps in a face-to-face 00:00:21
system design. At the end, you will be able to download the SystemVue workspaces I used 00:00:27
in the video. 00:00:32
How is the far field of a phased array computed? Traditionally, if we know the complex voltages 00:00:34
or current waveforms at the input of each of the radiators in a phased array, we can 00:00:41
compute the far field by summing them at a faraway distance. We typically build the schematic 00:00:46
with the number of RF channels equal to the number of array elements and simulate each 00:00:52
channel individually. If we have a large array, simulating these voltages can take a significant 00:00:57
amount of time. You can see how this time grows in the graph shown here with the size 00:01:04
of the array. This translates to a 20-hour simulation for a 256-element phased array, 00:01:10
scanning over plus-minus 20 degrees in azimuth and plus-minus 10 degrees in elevation with 00:01:18
a 1-degree resolution. Fortunately, the tool used here has a technique that makes the simulation 00:01:24
run several orders faster. To use this technique, we just need to construct one schematic and 00:01:31
simulate any arbitrary-sized array. For example, even a 10,000-element phased array simulates 00:01:38
in 2 seconds. With this kind of speed, the earlier 256-element array example will simulate 00:01:45
in just 14 minutes. Having this technique makes the phased array design a lot more efficient 00:01:53
and fun. To begin, let us first understand heuristically how a phased array works. If 00:01:59
we place a set of omnidirectional radiators along a line and observe the waves emanating 00:02:08
from them in all directions, we can easily notice that the waves add coherently in some 00:02:15
directions, not so coherently in some other directions, and incoherently in other directions. 00:02:21
This creates a distinct pattern at a faraway distance from the array where it becomes independent 00:02:28
of distance. This pattern will have a main lobe, several side lobes, and a back lobe. 00:02:34
The pattern is called far-field pattern. A three-dimensional far-field pattern of a 00:02:42
20-by-20 rectangular array can be seen here. The far-field pattern and its characteristics 00:02:49
are of utmost importance to baseband designers, RF designers, and antenna designers. 00:02:57
In a phased array system design, the key parameters of a phased array architecture are the number 00:03:05
of elements, distance between elements, which is usually less than lambda by two, the geometry 00:03:12
of array, such as uniform linear, uniform rectangular, circular, etc., the frequency 00:03:18
of operation, and finally, the far-field pattern of individual elements. By choosing these 00:03:24
parameters judiciously, we can arrive at a desired far-field pattern. 00:03:30
How does a far-field pattern affect overall system performance? Let us consider a simple 00:03:36
example. A base station is sending signals to four users through four distinct beams. 00:03:41
In this case, the base station is operating in a shared array mode where all four users 00:03:48
are utilizing all the elements of the array. Let us measure the quality of the signal received 00:03:54
by user 1 in terms of constellation and error vector magnitude. Since we made everything 00:04:00
in this system ideal, we expect a very good constellation and low EVM, but that does 00:04:07
not seem to be the case here. We wonder why this is so. 00:04:14
Purely by virtue of the architecture of the array, a side lobe of user 2 is pointing in 00:04:21
the same direction as the main lobe of user 1. Hence, user 1 is receiving not only the 00:04:27
desired signal, but also an interferer from user 2. At this point, the problem can be 00:04:34
fixed by increasing the signal-to-interference ratio, by increasing the power of the user 00:04:40
1 signal, or reducing the side lobe level of user 2, or increasing the number of elements 00:04:46
of the array, which will increase the gain of the array, as well as produce lower side 00:04:53
lobes. As we have seen in this example, the far-field 00:04:59
pattern has a significant influence on the performance of this system. 00:05:03
Before we go further, let us break and then learn a few interesting facts. In the early 00:05:08
days of World War I, they used acoustic waves to form a phased array, as you can see here. 00:05:16
And in the modern world today, the largest phased array is a project called Space Fence 00:05:28
for U.S. Air Force, and you can see that the transmit array has 36,000 antenna elements, 00:05:35
whereas the receive array has 86,000 antenna elements. You can also see that the canopy 00:05:45
covering the transmit and the receive buildings is actually a radome, and you can also observe 00:05:53
the calibration tower to see how they calibrate these two huge phased arrays. If you are interested, 00:06:02
you can go to this website and then probe further. I hope you like these facts. 00:06:09
Having understood the importance of the far-field and its characteristics, we need to understand 00:06:16
various factors that influence the far-field pattern as part of the design process. 00:06:22
The subsystem characteristics and the integration of the subsystems can have profound influence 00:06:28
on the far-field pattern. I am going to talk about several critical factors that influence 00:06:33
the far-field pattern, AM to PM of the amplifiers in the RF electronics, the number of bits 00:06:39
of the digital phase shifter and the digital attenuator in the TR module, active impedance 00:06:47
of the antenna elements, which interestingly can be a function of frequency and scan angle, 00:06:53
and finally, random antenna element failures. I am going to discuss about all these four 00:07:00
factors and their influence on the far-field pattern. 00:07:06
Let us first understand the AM to PM of the amplifier on the far-field pattern. As the 00:07:10
input power to the amplifier is increased, the phase change the signal undergoes through 00:07:18
the amplifier is no longer a constant. Most phase-to-array designs use amplitude distribution 00:07:25
at the antenna elements to control the side lobe levels. This means the amplifiers in 00:07:33
various channels feeding to the antenna elements will be operating at different power levels 00:07:39
when amplitude tapering window is applied across the array. We see the far-field pattern 00:07:45
with and without AM to PM effect here. The side lobe levels and their shape and the number 00:07:53
all change with AM to PM. By plotting the 2D cut pattern, we can easily measure the 00:08:00
side lobe levels. We can see a 5 dB raise in side lobe level and 1.6 dB drop in directivity. 00:08:08
Next, let us study the influence of number of bits of the digital phase shifter and attenuator 00:08:19
on the far-field pattern. The phase distribution and the amplitude distribution of the signal 00:08:26
arriving at the antenna elements are achieved by programming these digital phase shifters 00:08:32
and digital attenuators. The digital control word length is limited usually to 3 to 6 bits. 00:08:38
It is interesting to see how the far-field pattern varies with the bit precision of these 00:08:46
components. The plot here shows a comparison between unlimited bits, 5 bits, and 3 bits 00:08:52
phase shifters and attenuators. The 3-bit precision clearly increased the number of side lobes 00:09:00
and their levels. We can see more details in a 2D cut pattern. We can see that the 3-bit 00:09:07
phase shifters and 3-bit attenuators narrowed the beam width by 5 degrees and increased 00:09:14
the side lobe levels by almost 25 dB. The active reflection coefficient of the antenna 00:09:21
is very important, especially if it mismatches with the electronics preceding each element. 00:09:29
I mentioned earlier that the active impedance can change with scan angle as well as frequency, 00:09:36
so we have to monitor the far-field performance while scanning the azimuth and elevation in 00:09:42
a range in which the phase delay is expected to function. We see here both the 3D plots 00:09:48
and the 2D cut pattern. It is clearly noticeable that the active impedance of the antenna mismatching 00:09:56
with the electronics in front of it raises the side lobe level significantly. We also 00:10:02
see that the way the pattern impacted depends on the scan angle. 00:10:08
Before we conclude, let us go through an interesting and important design step. Phased arrays are 00:10:16
attractive because even if a certain number of elements fail, the array will still function, 00:10:21
though with a graceful degradation of performance. How do we know which element failures and 00:10:28
how many failed elements are acceptable before the system is declared to be unacceptable? 00:10:34
This can be a difficult question to answer without a simulation tool, but thankfully 00:10:41
we can perform a Monte Carlo simulation by randomly failing a fixed number of elements 00:10:48
or we can attribute a probability of failure to each element. We can easily see here that 00:10:53
the side lobe levels degrade the performance with random failure of the elements. 00:11:01
Finally, we gained an understanding of what impacts the far-field pattern, which in turn 00:11:08
influences the system performance. We just learned that it is absolutely essential to 00:11:15
be able to simulate the array very quickly as we have to explore a lot of design space. 00:11:20
In addition, sometimes the array can be very large. Studying beamforming architecture and 00:11:27
techniques is very interesting and I hope you enjoyed learning along with me in this 00:11:33
video. You can download the system view workspaces I used from the link below for your own use. 00:11:39
Thank you for watching this video. 00:11:46
Idioma/s:
en
Idioma/s subtítulos:
es
Autor/es:
Keysight
Subido por:
Pedro Luis P.
Licencia:
Dominio público
Visualizaciones:
39
Fecha:
8 de abril de 2023 - 13:36
Visibilidad:
Público
Duración:
11′ 58″
Relación de aspecto:
1.78:1
Resolución:
1280x720 píxeles
Tamaño:
20.64 MBytes

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