Microwave Couplers on PCB

3.2.1 Motivation

Directional couplers are important elements in microwave integrated circuits (MIC’s) [3.2.1]. It can be used in design of baluns, power dividers/combiners, filters, attenuators and instrumentation systems. A hybrid coupler, i.e. a coupler with 3-dB coupling, is found useful in building a power distributed amplifier [3.2.2], a balanced mixer [3.2.3], etc. A fully planar conventional microstrip coupler has a coupling less than 10 dB due to lower realizable limit of slot width in microstrip technology. This leads to the development of many methods. In the vertically installed planar (VIP) circuit [3.2.4], coupled lines are constructed perpendicularly to the main microstrip circuit board. In [3.2.5]- [3.2.6], a semi reentrant microstrip section is used to realize the tight coupling. In [3.2.7], couplers are designed in a multilayer structure with two- and three-strip couplings. These approaches rely on the broadside coupling through a dielectric layer between the microstrip lines.

It is also possible to use an original or unfolded Lange coupler for 3 dB coupling. The design concept is based on multifold constructive edge couplings to enhance the coupling capacitance between the two main lines in multiple coupled microstrip. It is found that bond wires are inevitably required in circuit realization, since nonadjacent lines are assumed to have identical potentials.

In this paper, we try to make a conventional single-stage microstrip coupler with a coupling level as high as possible. The high-level coupling is achieved by fabricating a narrow gap between the coupled microstripe using low cost IC process.

The design is based on the structure of a 3-dB coupler. Measurement shows that the fabricated coupler has a 4-dB coupling. The measured good results are due to the small coupling gap used in our previous RF passive devices. Note that the entire circuit is fully planar, requires no bond wire, and retrieves the simplicity of the original circuit design.

3.2.2 Filter Design and Fabrication

For a coupler with coupling C, the even and odd mode impedances are required to be [1] additional λ/4 converter impedance (Zt) of 46.0 Ω is used for Device 2 to match the 50 Ω. The smaller Z0 and Zt in Device 2 are smaller for the conductor loss in

εr = 10.2 and thickness = 1.27 mm is chosen to realize the circuit. The EM simulator IE3D [10] is used to validate the circuit design before the circuit is fabricated. The required line width are 309 µm and 4 µm; the interline spacing are 560 µm and 4 µm for Device 1 and 2, respectively. The circuit is fabricated by conventional low cost IC process with a >1-µm resolution and standard FeCl etching. An infrared red aligner is used for pattern exposure, and HFD5 solution for circuit development. The solution is heated up to 50 – 60 ^oC to have a better etching in the valley of the central gap.

Circuit measurements are performed by an Agilent/HP 8720C vector network analyzer.

3.2.3 Results and Discussion

Figure 3.2.1(a) shows the photo of the fabricated coupler where an enlarged pattern of a part of the coupled lines is displayed in Figure 3.2.1 (b). Transmission line sections with 50Ω characteristic impedance are used to extend the circuit ports for saving an enough spaces of the SMA connectors for measurement. Since the chemical solution etching is isotropic, the fabricated circuit inevitably has a V-shape coupling gap. From the enlarged circuit picture in Figure 3.2.1 (b), the upper opening of the V-shape gap can be estimated to be 24 µm, since total distance between the outer edges of the strips is 620 µm. The performance of the coupler is simulated by

µm width at valley. Figure 3.2.2 compares the simulated and measured responses of Device 1. In Figure 3.2.2 (a), the measured |S21| (through) and |S31| (coupling) responses have very good agreement over a frequency range from 3 to 9 GHz. In Figure 2 (b), the isolation and return loss responses of the fabricated coupler are better than 20 dB.

Figure 3.2.3 (a) and 3.2.3 (b) show the measured coupling response, return loss and isolation loss of Device 2, respectively. Good coupler characteristics of high coupling of 3.8 dB, small insertion loss of -3.9 dB and very wide bandwidth (1dB) of 52% are achieved that are comparable to even better than the reported very wide bandwidth data. The slightly inferior return loss of -24.3 dB and directivity of -21.0 dB may be due to small power reflection in non-ideal impedance transformer. The reason of good agreement between measured and modeled coupler characteristics is due to the sharp and accurate etching profile close to EM simulation shown in the insert picture of Figure 3.2.1. However, since the used IC technology is ~2 decades old with only line definition of >1µm, this process may provide simple and low cost solution for fabricating high performance coupler and other RF devices.

Table 3.2.1 summarizes the measured and simulated data of the couplers at 5GHz. The Device 1 has a coupling level (|S31|) of 3.8 dB in simulation and 4.0 dB in measurement. The measured isolation and return loss data of the fabricated coupler

are better than 20 dB, and the 1 dB bandwidth of the circuit is more than 50%. The Device 2 also has a coupling level (|S31|) of 3.8 dB in simulation and 3.9 dB in measurement. These simultaneously measured high coupling, high directivity, and broad bandwidth are simply due to the smaller coupling gap from the fundamental theory analysis that was justified by the close match between measurements to EM simulations.

3.2.4 Conclusions

A microstrip coupler with a 4-dB coupling level is fabricated on PCB. It is achieved by etching a 14-µm gap. Entire circuit is fully planar and needs no bond wire. This simple technology will have applications to high performance RF passive devices on PCB.

3.3 Conclusions

A fully planar microstrip coupler with a high coupling level up to 4 dB is fabricated on PCB. Based on the conventional coupled-line structure, a gap size of coupled lines about 14 µm is realized by low cost IC process. It has a single stage and requires no bond wire. In measurement, a bandwidth more than 50%

and isolation and return loss better than -20 dB are achieved. The measured responses have good agreement with EM simulation data. A high-performance ring resonator filter is fabricated on PCB with aid of the IC lithography technique. The filter, designed at 1.9 GHz, has a very large fractional bandwidth (up to 30%), an insertion loss of 0.50 dB, a flat |S21| response in pass-band, a return loss better than ~30 dB at two transmission poles and a rejection level of –40 dB at attenuation poles. In addition, the measured characteristics are very close to ideal design by EM simulation. The achieved excellent broadband device characteristics are due to a small coupling gap in the filter. In contrast, the conventional method can only have a large coupling gap with relatively small bandwidths and relatively large insertion losses. The excellent device performance reflects that this technique makes the ring resonator filter applicable from narrow band to broadband designs.

Fig. 3.1.1 Structure of a dual-mode ring resonator filter

θp, Zp

Fig. 3.1.2 Normalized frequencies foe/fo and foe/fo as a function of electrical length 2θp

Odd mode

Even mode

Kz=0.4

Kz=0.8 Kz=0.4 K =0 8

1.0 1.5 2.0 2.5 3.0

Fig. 3.1.3 Simulated responses of the dual-mode ring resonator filter with center frequency fc = 1.9 GHz. (a) Coupled gap = 240 µm, ∆ = 10%. (b) Coupling gap = 40 µm, ∆ = 30%.