CHARACTERIZATION OF PHASE SHIFTERS ON A KU-BAND PHASED ARRAY ANTENNA

ESA/ESTEC, NOORDWIJK, THE NETHERLANDS 3-5 OCTOBER 2012

J. Arendt (1), R. Wansch (1), H. Frühauf (1)

(1) Fraunhofer IIS, Am Wolfsmantel 33, 19058 Erlangen, Germany, Email: {johannes.arendt, rainer.wansch, holm.fruehauf}@iis.fraunhofer.de

ABSTRACT

In this article an approach is given for a quick, exact and cost effective characterization of phase shifters on a phased array antenna. The proposed algorithm offers the ability to measure phase shifter after the mounting on a printed circuit board (PCB). The presented measurement setup is easy to rebuild in a laboratory environment or in a production line. It uses standard vector network analyzer (VNA) and a horn antenna as measurement equipment. This simple setup is verified by measurements in an anechoic chamber. Additionally typical results from the measurement campaign are presented. 1. INTRODUCTION

The NATALIA (New Automotive Tracking Antenna for Low cost Innovative Applications) is a phased array antenna for satellite reception in Ku-Band. The antenna was realized within the ESA-project NATALIA (funded under contract number 18612/04/NL/US). It consists of 156 single radiating patches. By the use of two monolithic microwave integrated circuit (MMIC) based phase-shifters per patch the polarization of the antenna is steerable. Each MMIC contains of an LNA and a 4-bit phase-shifter. The MMIC are mounted on the bottom side of the antenna and connected via bond wires. After assembly it is not possible to measure the RF path of a single MMIC phase-shifter directly. A measurement method is needed to characterize each MMIC even if no direct connection to the chip is possible. To solve these problem two methods have been developed. The first approach injects an excitation signal via a waveguide onto every MMIC phase-shifter. By analyzing the signal attenuation and phase shift of every path is determinable [2]. The second approach uses the MMIC phase-shifter itself to generate orthogonal excitation signals for the characterization. By analyzing the received signals the contribution of each phase-shifter is determinable. Both measurement methods are tested with a NATALIA antenna in the laboratory. The second approach that uses  the  MMIC’s  for  coding is additionally tested in an anechoic chamber.

The waveguide setup showed severe problems in positioning and alignment of the setup. Therefore, we focus on the results of the latter one. In this paper the NATALIA antenna is presented more detailed. This is followed by a description of the measurement algorithm. Thereafter the measurement setup is presented following by the results of the measurement campaign. 2. NATALIA

Goal of the NATALIA project was the development of a satellite receive antenna for automotive purposes. The key parameters of the antenna are:

Operation frequency 10.7 GHz – 12.75 GHz Linear polarization G/T: -6 dB/K (Figure of Merit) Cross polarization discrimination > 15 dB Antenna size: 20 cm diameter Scan range 20° - 60° in elevation from horizon

0° - 360° in azimuth A more detailed description of the antenna can be found in [1]. The complete build up which is shown in Figure 1 consists of the NATALIA antenna and a second PCB mounted below. This second hosts the power supply and a microcontroller.

Figure 1: NATALIA antenna with attached waveguide

The topology of the NATALIA antenna is shown in Figure 2. The planar array consists of 156 dual polarized microstrip stacked patches arranged in a hexagonal grid. Both received orthogonal (linear) signals (V & H) are for each patch first converted into two circularly polarised components via a 90° hybrid coupler and fed into a combined phase shifting unit [1]. This unit is realised by a GaAs MMIC, called core chip, which contains Low Noise Amplifier, phase shifter and digital steering logic. The phase shifters are controlled via a serial interface. A microcontroller (µC) generates the necessary steering sequences.

Figure 2: Schematic overview of the NATALIA topology [1]

A combiner network combines the different signal paths and a down converter converts from Ku-band to L-band. 3. CONTROL CIRCUIT ENCODING (CCE)

The idea behind control circuit encoding is to use the phase shifter on board the antenna itself to modulate orthogonal excitation signals. This algorithm was first published by Silverstein [3] and practically implemented by Lier [4] for a 16-element phased array antenna. The schematic shown in Figure 3 describes the NATALIA antenna from the characterization point of view. A VNA inserts an excitation signal into every path of the phased array antenna.

Figure 3: Schematic from characterization point of view

Every core chip psk contain of 4 delay lines 𝑑 (𝑘), 𝑑 (𝑘),  𝑑 (𝑘)  and  𝑑 (𝑘), which perform the phase shift. Goal of the characterization is the measurement of these delay elements. The straight through path 𝑠(𝑘) contains all influences between the transmitting antenna and the receiving point. These are without requirement on completeness:

Influence of the channel Influence  of  the  LNA’s Influence of the beam forming network Bond wires

The description of the algorithm is divided into three parts. First the encoding step is described. The encoding is done in every measurement step that is described in the following section. The last section describes the decoding which 3.1 ENCODING

The coding is done by a Walsh sequence. Walsh sequences are simple generated out of an Hadamard matrix. A single matrix has the following form

𝐇𝟐 = 1 11 −1 (1)

To generate larger matrixes the following scheme is applied

𝐇𝟒 =𝐇𝟐 𝐇𝟐𝐇𝟐 −𝐇𝟐

=1 1 1 11 −1 1 −11 1 −1 −11 −1 −1 1

(2)

The general formula to generate matrizes of size 2𝑀 is

𝐇𝟐𝑴 = +𝐇𝑴 +𝐇𝑴+𝐇𝑴 −𝐇𝑴

(3)

M stands for the size of the Hadamard matrix. A modulation step works in the following way: The row of the matrix 𝐇 gives the rule how to switch the stages dedicated to perform the modulation. +1 stands for stage on −1 stands for stage switched off. It is important to note that the modulation switches all core chip phase shifters. The rows are exchanged for every measurement step. So the first row gives the switch law for the first modulation step, the second row for the second modulation step and so one. The column of the matrix indicates to the core chip. The measured attenuation and phase for every step will be stored in a vector�⃗�.

core chip psk

core chip ps1

s(k) d1(k) d2(k) d3(k) d4(k)

s(1) d1(1) d2(1) d3(1) d4(1)

s(2) d1(2) d2(2) d3(2) d4(2)

VNA

The following example describes the encoding algorithm with four core chips. We use stage 4 (180°) for modulation. The first row contains only 1. So all stages 4 of all phase shifters must be switched on. The measured value will be saved in �⃗�(1). The second row of a 4x4 Hadamard matrix is [1|−1|1|−1]. So bit 4 of phase shifter 1 and 3 must be switched on. The measured value will be saved in �⃗�(2). The third row is [1|1|−1|−1]. So bit 4 of phase shifter 1 and 2 must be switched on. The measured value will be saved in �⃗�(3). The last step uses the forth row [1|−1|−1|1] so bit 4 of phase shifter 1 and 4 must be switched on. The measured value will be saved in �⃗�(4). These steps described the so-called forward measurement. Forward measurement means a measurement with a modulation using a control matrix 𝐇. To calculate the phase shift of every delay element we have to repeat the modulation using a inverted control matrix –𝐇. This is called reverse measurement. At the end we get a vector �⃗� for the forward measurement and a vector �⃗� for the reverse measurement. The variable µ indicates the stage used for modulation; the variable n indicates the stage under characterization, which is needed in the measurement steps. 3.2 MEASUREMENT

By using the previously described modulation and coding steps we measure by switching all stages in the following way:

Step 1: Switch stages 1, 2, 3 of all phase shifters off, modulate with stage 4 (µ=4). The results are the vectors �⃗� and �⃗� .

Step 2: Switch stage 1 of all phase shifters on,

stages 2, 3 off, modulate with stage 4. The results are the vectors �⃗� and �⃗� .

Step 3: Switch stage 2 of all phase shifters on,

stages 1, 3 off, modulate with stage 4. The results are the vectors �⃗� and �⃗� .

Step 4: Switch stage 3 of all phase shifters on,

stages 1, 2 off, modulate with stage 4. The results are the vectors �⃗� and �⃗� .

Now we have to characterize stage 4.

Step 5: Switch stages 1, 2, 4 of all phase shifters off, modulate with stage 3 (µ=3). The results are the vectors �⃗� and �⃗� .

Step 6: Switch stage 4 of all phase shifters on,

stage 1, 2, 4 off, modulate with stage 3 It is also possible to use other stages like stage 1 and stage 2 for modulation. Other combinations are also possible. These are the minimum number of steps for a characterization of the phase shifters. 3.3 DECODING

The next step is to decode the measured values. This is done by the following formula.

𝑧 = 𝐇  �⃗� −   �⃗� (4) Attenuation and phase shift for the 𝑛 stage of phase shifter 𝑘 is given by the following formula

𝑑(𝑘) = ( )( ) (5)

Attenuation and phase shift for the straight through path of phase shifter 𝑘 is given by the following formula

𝑠(𝑘) = ( )( ) (6)

4. MEASUREMENT SETUP

The measurement arrangement consists only of a VNA, a horn antenna and a control PC running Labview. A block diagram of the setup is shown in Figure 4. The NATALIA antenna is controlled by a serial interface connected to the PC. A microcontroller controls the phase shifter core chips.

Figure 4: Block diagram of the control circuit encoding setup

The control program sends phase settings to the µC. The µC generates steering signals, which are sent to the core chips. The VNA is connected via a SCPI interface to the Labview control program. The VNA measures the transmission and provide them to the Labview control program.

NATALIAADS

PCLABVIEW

R&S ZVA24NVA

Horn Antenna

SCPI

uC RS232

Figure 5: CCE Measurement Box Setup

Figure 5 shows a picture of the box measurement setup. The horn antenna is mounted on top of the box and connected to the VNA. We added a temperature control to monitor the temperature inside the box. Additionally we assembled fans to keep the temperature inside the box stable. The temperature was at 23° Celsius. The antenna under test is mounted inside the box and connected to the VNA and the control PC. The position of the AUT inside of the box had no influence on the results. To verify this box setup we repeated the measurements inside an anechoic chamber. 5. RESULTS

The results are based on the following measurements: 34 measurements done in the box 25 measurements done in an anechoic chamber

Inside a measurement all stages are used for modulation. We were able to characterize 310 phase shifter core chips (2 core chips were out of order and not addressable) Additionally we measured 415 frequency points from 10.5 GHz – 13 GHz for every core chip.

Figure 6: Phase distribution take into account all 310 core chips and all valid measurements

Figure 6 shows a histogram of all measured core chips at a frequency of 11.494 GHz. There are high peaks at the expected phases It has to point out that the measurement algorithm has no information about the actual designed phases of the core chips. To compare the two measurement setups the mean over all values is calculated. The results are shown in Table 1. Stage Phase

Value Mean Box Mean

Chamber 1 22.5° -22.36 -21.82 2 45° -44.94 -44.43 3 90° -84.01 -83.75 4 180° 169.41 170.08 Table 1: Comparison of box and anechoic chamber measurement There is a slight difference between the measurements done in the box and inside the anechoic chamber but this difference is below 0.5°. It has to point out that the VNA has a measurement accuracy of 1°. The following results are based on data captured with the box setup. There are typical results for complete functional core chips, core chips with a defective stage and complete out of order core chips. In addition typical results for a functional and a defective stage are presented. The results of complete out of order core chips are completely random. Figure 7 shows the mean phase between 10.5 GHz and 13 GHz for stage 1 (22.5°) of a functional phase shifter. The vertical bars showing the standard deviation

Figure 7: Typical results for a well functional core chip

−200 −150 −100 −50 0 50 100 150 2000

20

40

60

80

100

12011.494 GHz

Phase in deg

Every stage is measured by modulation of the other stages. The curve d21 indicates the phase of stage 1 (22.5°) measured by modulation of stage 2. The curve d31 shows also the phase of stage 1 but modulated by stage 3 (90°). The high standard deviation at 12.5 GHz is caused by a calibration error inside the VNA.

Figure 8. Typical results for a defective phase shifter stage

Figure 8 shows a typical example for a phase shifter with a defective stage. In this case stage 3 (90°) does not work. If this stage operates as modulator the characterization fails. As long as stage 2 and stage 4 operates as modulator the algorithm is able to characterize stage 1. This is shown in the curves d21 and d41. If stage 3 modulates, the results are random distributed. 6. CONCLUSION

The measurements inside the laboratory and the anechoic chamber were reproducible. It was possible to characterize each stage of the 310 MMIC phase shifter within the operating frequency between 10.7 GHz and 12.5 GHz. By varying the modulation stage defective stages are allocable. The measurement system detects complete out of order phase shifters and phase shifters with defect stages. Additionally it characterizes the phase shifters in the full frequency range of the antenna. 7. ACKNOWLEDGEMENTS

The authors wish to thank ESA for funding and supporting the NATALIA project under contract number 18612/04/NL/US and especially Martin Lienau and Florian Koerfer for their great support during the measurement campaign.

8. REFERENCES

[1] R. Baggen, S. Vaccaro, D. Llorens del Rio, J. Padilla, R. Torres Sánchez, “Natalia:  A  Satcom  Phased  Array in Ku-Band”, 34th ESA Antenna Workshop, 2012. [2] H. Humpfer, R. Wansch, “Coupling  Waveguide Array for a Patch Array Calibration Scanner”, Proceedings of the Fourth European Conference on Antennas and Propagation (EuCAP), 2010. [3] S. Silverstein, “Application   of   Orthogonal  Codes to the Calibration of Active Phased Array Antennas for Communication Satellites”, IEEE Transactions on Signal Processing, Vol. 45. No. 1, January, 1997. [4] E. Lier, M. Zemlyansky, D. Purdy, D. Farina, “Phased  Array  Calibration  and  Characterization  Based  on Orthogonal Coding: Theory and Experimental Validation”, IEEE International Symposium on Phased Array Systems and Technology (ARRAY), 2010