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Design of Directional Coupler in Bulk-CMOS Switch Technology

Bogdan Tanc1,2, Valentyn Solomko1, Daniel Kehrer1, Ivan Bogdanov2

1Infineon Technologies, RF and Protection Devices, Neubiberg, 85579, Germany2Politehnica University of Timisoara, 300006, Timisoara, Romania

Abstract The design of a directional coupler is presented. The goal is to achieve performance required for RF front-ends of mobile handheld applications using monolithic integration approach. The technology used for designing such a coupler is C11NP, which focuses on RF switch applications. The coupler is initially approximated based on an ideal model of a short directional coupler and later on finely tuned using electro-magnetic (EM) simulation. Designed directional coupler achieves worst-case insertion loss of 0.26 dB at frequencies between 0.5 GHz and 3.8 GHz, directivity of 25 dB within and mention frequency range and coupling factor of 25 dB at the middle of the frequency range.

Keywords: Directional coupler, coupling factor, directivity, transmission lines, transformer, winding.

I.INTRODUCTION

The largest market in terms of volumes in consumer electronics is the smartphones market. Due to ever-increasing demand on performance of smartphones while constantly pushing down the cost of a mobile handset there is a need for highly reliable and low-cost components with small form-factor. This also applies for radio-frequency (RF) front-ends.

Directional coupler is a part of RF front-end of a mobile phone. Its role is to couple a transmitted signal back to the system for power amplifiers control. Additionally, the signal reflected from antenna due to mismatches is sensed by a coupler and can be used to implement antenna tuning functionality.

Fig. 1. Block diagram of an RF Front End The block diagram in Figure 1 represents a RF front-end which consists of a Power Amplifier (PA), an Antenna Tuner and a Directional Coupler.

The goal of this paper is to describe the design of directional coupler, by achieving reasonable performances through C11NP technology.II.SPECIFICATION FOR DIRECTIONAL COUPLER

Directional couplers are RF passive device which couple part of the transmitted power by a specific proportion out through another port, using two transmission lines which are located close enough to each other, such that the energy passing through one is coupled to the other[2]. A directional coupler has typically four ports: input port, transmitted port, coupled port and isolated port, as shown in Figure 2.

Fig. 2. Directional coupler

A directional coupler can be characterized by the following set of parameters:

Coupling Factor. The amount of incident power that is delivered to the coupled port. It is defined as the ratio between the power at the coupled port to the power at the input port:

Isolation. Represents the amount of incident power parasitically coupled into the isolated port. Ideally this value should be zero, but due of imperfect isolation a small amount of power leaks to the isolated port.

Directivity. It represents the directional couplers ability to isolate forward and reverse waves. This indicates the accuracy of the measured power levels in a specific direction.

Insertion loss. Indicates how much power is lost between input and transmitted ports due to power and/or mismatch losses. The insertion loss of an ideal directional coupler will consist entirely of the coupling loss.

Due to Long-Term Evolution (LTE) and Wireless Communications Standards, a directional coupler is required to operate at frequencies between 0.5 GHz and 3.8 GHz. Within this frequencies range, the suitable performance for a coupler are insertion-loss lower than 0.4 dB, coupling factor in the range of 25 dB and directivity greater than 25 dB.

III. DESIGN OF IDEAL DIRECTIONAL COUPLER

The coupler shown in Figure 2 can be designed with two coupled transmission lines at the center frequency of the band through the following equations [1]:

where subscripts e and o denote even and odd mode, C represents the coupling coefficient expressed in decibels with positive sign and Z0 is the terminating impedance.

Also, the same coupler can be modeled as a lumped-element equivalent circuit (EC) as shown in Figure 3. The values for L, M, Cg and Cc in terms of Z0e, Z0o and are obtained as follows [1]:

where f0 is the center frequency and =90 at f0. Therefore if the coupling is known, using (7) and (8), the Z0e and Z0o can be determined and then the lumped-element values are calculated using (9) and (10).

Fig. 3. Lumped-element model of a coupler

To design a compact directional coupler with tight coupling the coupler is implemented as a spiral magnetic transformer in a planar fabrication process. For achieving a small size of the coupler, the spiral transformer is printed on a substrate material with small pitch between the conductors of the transformer. High coupling factor is achieved by letting windings of the transformer run parallel to each other over the whole length of the conductor. To obtain accurate design of such structure an electromagnetic (EM) simulation is required [1].

For having an overview of the performances of ideal directional coupler the equations above are used to calculate an ideal coupler and plot its characteristics over the frequency. With the coupling coefficient of C=17dB at a center frequency of f0=1 GHz the highest value of directivity is obtained at f0, which proves the ideal nature of the coupler. In the Figure 4, the curves for calculated ideal coupler are presented.

Fig. 4. Coupling factor and directivity over frequency

Furthermore, if the series resistance of the inductance is taken into consideration (Rs=1), the directivity of the coupler becomes finite, which is demonstrated in the same Figure 4. Series resistance also degrades the insertion loss of the coupler.

Fig. 5. Insertion loss over frequency

Figure 5 illustrates the insertion loss of an ideal coupler and the effect of series resistance on it.

Fig.6. S11 for 0.5 GHz to 3 GHz As can be observed in Figure 6, the matching point is obtained at f0=1GHz - at this point the maximum power transfer is achieved. As the frequency increases, the structure becomes capacitive.

IV.C11NP TECHNOLOGY

The C11NP technology is a triple-well 130 nm fabrication process from Infineon Technologies targeting RF switch applications.

This fabrication process offers 6 levels of metallization separated by Silicon dioxide (SiO2) over a substrate. The SiO2 insulation layers between metal layers have a permittivity of .

Fig. 7. Metal stack of C11NP technology

In Figure 7 the metal stack of the C11NP technology is illustrated. In Table 1 the parameters for each metal layer are presented.

Table 1. Metal stackMetalMaterialWidth [m]Sheet Resistivity [/]

M6Aluminium2.80.01

M5Aluminium0.60.05

M4Copper0.350.06

M3Copper0.350.06

M2Copper0.350.06

M1Copper0.290.07

Through this technology, inductors can be made with different kind of metallization, from M6 down to Poly, but as the inductors are dependent on metal resistivity, substrate coupling and mutual inductance, an inductor should be made using thick M5 or M6 layers for having good performances.

V.DESIGN AND SIMULATION RESULTS

The directional coupler was designed as a transformer with two windings. Due the high transmitted power (up to 35 dBm) the RMS current may reach 300 mA requiring wide conductor of the primary winding: 35 m. The secondary winding has a width of 10m. The transformer windings were made of M5 and M6 metallization layers. The coupler layout is illustrated in the Figure 9.

Fig. 9. Layout of directional coupler

The coupler has been simulated with Sonnet. To perform a fine analysis of the structure the simulation using the Adaptive Band Synthesis (ABS) between 0.5 GHz and 6 GHz has been done. The coupling characteristic is a first order response ranging from 30dB 20dB between 1GHz and 3GHz. The coupler demonstrates a directivity exceeding 20 dB within the mentioned frequency range which shows the broadband performance of the coupler. Coupling and isolation curves are demonstrated in Figure 10.

Because the second winding has only one turn, the coupler achieves state-of-art performances regarding insertion loss. The Figure 11 illustrates the losses of the coupler.

Furthermore, little mismatch is observed at input and transmitted ports of the coupler with is another reason for low overall insertion loss. This is shown in Figure 11.

Fig. 10. Simulated coupling and isolation over frequency

Fig. 11. Insertion loss variation with frequency

Fig. 12. S11 variation in the range of 0.5 GHz 6 GHz

VI.CONCLUSIONS

The directional coupler was design in C11NP technology from Infineon Technologies. The device layout is made of M5 and M6 metallization layers, with a total layout size of 400X300m2. Because of high current densities (300 mA RMS) the width of the primary winding is chosen to be 35 um. With a directivity of 20dB-25dB between 1GHz and 5GHz, the coupler could be used as a broadband coupler, achieving very low insertion loss, 0.15dB 0.3dB, and very reasonable coupling in the same range of frequencies. REFERENCES

[1] Inder Bahl., Lumped elements for RF and microwave circuits, Artech House, 2003.[2] Reema Sidhwani, Antenna tuning for WCDMA RF front end, Maser Thesis, Aalto University, 2012