# A Space-Borne Ka-Band Tile-Type Phased Array Transmitter Module Based on HTCC Technology
## 3 Tile-Type Transmitter Module Based on HTCC Technology
### Key Electrical Structure  
#### 3.2.1 Coaxial-Like Transition  
The RF signal transmits via coaxial-like transitions between different layers. The schematic of the coaxial-like transition is illustrated in Figure 6. The signal vias act as the inner conductor of the coaxial cable, and the GND vias surrounding the signal vias act as the outer conductor of the coaxial cable [26; 27]. As the metalized blind vias can be flexibly fabricated in multi-layer HTCC substrate, the distance between the signal vias and the ground vias can be adjusted in different layers to achieve a broadband impedance matching in vertical transmission.  
Figure 5: Schematic of the proposed SiP package.  
Figure 6: The quasi-coaxial vertical interconnection structure: (**a**) cross-sectional drawing (The yellow layers are metal. The orange layers are substrate. The green vias are shielding vias. The red dashed circles demonstrate that the electromagnetic field is confined by the shielding holes); (**b**) equivalent circuit.  
When the ohmic resistance and the conductance can be considered negligible compared to reactance and susceptance, respectively, the impedance of the transmission line can be written as  
\[Z_{0}=\sqrt{\frac{L_{0}}{C_{0}}} \tag{1}\]  
where \(L_{0}\) and \(C_{0}\) are the series inductance and shunt capacitance per unit length, respectively [28].  
When the transmission line is interconnected with the vertical signal through the hole, an extra parasitic inductance \(\Delta L\) is introduced [29]. The characteristic impedance of the transition structure can be re-written as  
\[Z_{0}=\sqrt{\frac{L_{0}+\Delta L}{C_{0}}} \tag{2}\]  
To compensate for the impedance mismatch caused by the parasitic inductance, an additional capacity \(\Delta C\) is needed [29]. A general way to introduce compensation capacity is to increase the dimension of the bonding pads at both sides of the vertical transition through the hole. The compensated impedance is written as  
\[Z_{0}=\sqrt{\frac{L_{0}+\Delta L}{C_{0}+\Delta C}} \tag{3}\]  
The value of the compensation capacity can be derived from the Equations (1) and (3) as  
\[\Delta C=\frac{C_{0}}{L_{0}}\Delta L \tag{4}\]  
The above formula indicates that by optimizing the matching circuit of the vertical transition, parasitic inductance and capacitance can be effectively compensated for, achieving broad-band impedance matching and improving overall RF performance.  
In the proposed design, a stepped impedance transformation structure is employed to achieve broadband matching, enabling the coaxial-like transition to operate effectively from 20 GHz to 30 GHz. To be more specific, the diameter of the GND vias and the width of the transmission line are optimized for broadband matching. The simulation model and results are shown in Figure 7. The return loss is better than 20 dB, and the transmission loss is less than 0.3 dB from 20 GHz to 30 GHz.  
Figure 7: (**a**) The simulation model of the coaxial-like transition. (**b**) Simulation results of inter-layer vertical interconnection.