Realization of the Broadband LTCC double-balanced mixer

To implement the broadband LTCC double-balanced mixer, the broadband transformers discussed in Chapter 2 are the key components. The Marchand balun, which has several versions, is the most commonly used component in broadband double-balanced mixer. Tight coupling can be obtained by placing the coupled line in a broadside manner and by using spiral-type coupled lines, the size of quarter-wave line based integrated passive components is decreased. As a result, the required surface area could be reduced. In addition, the spiral-type coupled lines contribute to the minimization of the thickness of the substrate. Therefore, spiral broadside coupled stripline structure achieves required characteristic impedance with thinner dielectric thickness than broadside coupled stripline structure as shown in Figure 3.3-2.

Because of the increasing magnetic flux, the spiral-type transmission line has a larger inductor value than the straight-type transmission line for the same line length.

Therefore, the spiral broadside coupled stripline has larger even mode impedance than straight broadside coupled stripline. Figure 3.3-1 shows the lumped element equivalent circuit of symmetrical coupler. Equation (3.10) and (3.11) are the formulas

of the even and odd mode impedance for symmetrical coupler.

Figure 3.3-1 Lump element equivalent circuit of the symmetrical coupler

m

Dimension BCS SBCS

S [µ ] m 74 74

B [µ ] m 1588 888

Table 3.1 Dimension comparison between BCS and SBCS

Table 3.1 shows the dimension comparison between BCS and SBCS. It indicates that the same coupling (-3.5dB) can be achieved by using spiral broadside coupled stripline with minimum thickness of the substrate. Figure 3.3-2 shows the implemented Marchand balun that consists two identical broadside coupled stripline having 2 turns and fabricated with LTCC technology using conductor linewidth of 100μm and gap of 180μm. The ceramic substrate of the LTCC has the dielectric constant of 7.8 and the coupled-line ground plane spacing of 888µ as shown in m Figure 3.3-3(b). In this spiral broadside coupled stripline, the gap and the linewidth ratio is about 2:1. From the EM simulated results in the spiral broadside coupled stripline, the even mode phase velocity is faster than odd mode phase velocity. As discussed in Chapter 2, we can add a short compensated transmission line (900µm×100µm) to slow down the even mode phase velocity as shown in Figure 3.3-2 (a). Furthermore, the compensated technique can decrease the coupling between adjacent coupled line segments. The coupling between adjacent coupled line segments will degrade the bandwidth of the Marchand balun.

(a) Spiral broadside

coupled stripline Compensated transmission line

B

407µ^m S=74µ^m 407µ^m

(b)

Figure 3.3-2 LTCC Marchand balun (a) Top view of the LTCC Marchand balun (b) Side view of the Marchand balun

Ground planes

Figure 3.3-3 3D structure of the LTCC Marchand balun

Figure 3.3-3 shows the 3D structure of the LTCC Marchand balun that is implemented by two spiral broadside coupled stripline. Two spiral broadside coupled

stripline in the same plane not only increases the even mode and odd mode ratio, but also reduces the layers of the LTCC mixer. Therefore, the lower cost can be obtained.

For straight broadside-coupled stripline with linewidth of 100µ and gap between m two couplers of 74µ , the even mode and odd mode impedance are 96 and 24 , m respectively. For spiral broadside coupled stripline shown in Figure 3.3-2, the even mode and odd mode impedance are about 135

Ω Ω

Ωand 27Ω, respectively.

4

λ/ λ/4

2

5 6

dB

Figure 3.3-4 Simulated results of the Marchand balun

The simulated results of the LTCC Marchand balun was obtained using EM simulator (Sonnet). The unbalanced input impedance is 50Ω and the balanced output impedance is 70Ω. Figure 3.3-4 shows that the S11 is less than –10dB in the range of 2.3 to 6.15GHz. The differences of the amplitude and phase between the balanced

output ports are shown in Figure 3.3-5. The amplitude imbalance at balanced output ports is within 1dB, and the phase imbalance at balanced output ports is less than over the frequency range of 2.3 to 6.15GHz where |S

10o

11|< -10dB.

Amplitude unbalance (dB) Phase unbalance (degree)

Figure 3.3-5 Simulated results of the amplitude unbalance and the phase unbalance Figure 3.3-7 shows the S₁₁ is less than –10dB in the range of 2.5 to 6.6GHz. The differences of the amplitude and phase between the balanced output ports are shown in Figure 3.3-8. The amplitude imbalance at balanced output ports is within 1dB, and the phase imbalance at balanced output ports is less than over the frequency range of 2.5 to 6.15GHz where |S

10o

11|< -10dB. The capacitors serve as the lowpass filter for IF signals and the high pass filter for RF signals. The IF signals can be obtained as shown in Figure 3.3-6. The performance of the Marchand balun was degraded by the capacitors.

4

λ/ λ/4

4 3 1

IF output

7 8

Figure 3.3-6 Schematic of the Marchand balun with IF output

dB

Figure 3.3-7 Simulated results of the Marchand balun

Amplitude unbalance (dB) Phase unbalance (degree)

Figure 3.3-8 Simulated results of the amplitude unbalance and the phase unbalance

Designing the optimum capacitors is extremely important for the IF signal. When the capacitor value is too large, it will degrade the conversion loss of the higher IF frequencies. When the capacitor value is too small, two short-circuited section aren’t

perfect grounding at the RF frequencies. Therefore, the amplitude unbalance and the phase unbalance of the Marchand balun are poor.

3 5 Diodes

2

6

Capacitor

Ground planes Ground

planes IF

2

1

Two Marchand baluns

Figure 3.3-9 3D structure of the double-balanced mixer

The 3-D structure of the double-balanced mixer is shown in Figure3.3-9. The diodes will be mounted on the top of the LTCC component. The Schottky diode-quad such as Metelics MSS-40, 455-B40 can be used as mixing elements. The chip size of the LTCC double- balanced mixer is4800µm×3400µm×962µm.