This paper presents the characterization results of several new passive millimeter wave circuits integrated on very thin ceramic substrate. The work is focused on the design and characterization of a novel rounded Wilkinson power divider, a 90° hybrid coupler, a rat-race coupler, and a novel six-port (multiport) circuit. Measurements show the wideband characteristics, allowing therefore their use for multi-Gb/s V-band wireless communication systems.
The use of the 60-GHz band has attracted a great deal of interest over the last few decades, especially for its use in future compact transceivers dedicated to high-speed wireless applications in indoor environments (57–64 GHz) [
Nowadays, there are few promising high-quality fabrication technologies, yielding potentially low-cost millimeter wave components, such as the monolithic microwave integrated Circuit (MMIC) on GaAs or SiGe for large-scale production, and the miniature hybrid microwave integrated circuit (MHMIC) technology on very thin ceramic substrates, for small-scale production and prototyping [
Moreover, several technologies have been intensively used for the millimeter wave circuit design and in-house prototype fabrication. We particularly note the coplanar, the substrate integrated waveguide (SIW), and the microstrip technology. The coplanar technology assures high-quality component design but is not well suited for low-cost production due to the difficulties in automating wire-bonding implementation, necessary for obtaining repeatable performances. On the other hand, the SIW technology assures high-quality component design on thin ceramics [
As known, the microstrip line width is related to the characteristic impedance, substrate relative permittivity, and its thickness. It is to be noted that, due to reduced guided wavelength in high permittivity ceramic substrates, in order to keep the required circuit aspect ratio (guided wavelength versus the line width), the substrate must be as thin as possible. The optimal choice for frequencies greater than 60 GHz is the 127
Initial designs and circuit characterization results of several MHMIC passive circuits on very thin ceramic substrate, designed for advanced millimeter wave systems operating in 60–90 GHz band, have been published few years ago [
This paper presents novel circuit designs, together with major improvements obtained in fabrication and characterization process in recent years.
Measurement performance mainly depends on the accuracy of the calibration technique and its standards used for correcting the imperfections of the measurement system. These imperfections depend on several factors such as nonideal nature of cables and probes and the internal characteristics of the vector network analyzer (VNA) itself. In order to simplify calibration procedures and to obtain more accurate and reliable measurement by introducing much smaller systematic errors, the on-wafer calibration and measurement with picoprobes were adopted.
Typically, on-wafer calibration standards are fabricated either on the wafer including the device under test (DUT) or on a separate impedance standard substrate (ISS). The reference plane is usually taken at the probe tips. Nevertheless, for the DUT measurement in microstrip technology, on-wafer standards fabricated on the same wafer as the DUT are required since the probe-to-standard transition can be designed to be very similar to the transition to the DUT. It sometimes happens that the transition between the probe tips and the coplanar line end is not well matched and parasitic and some wave modes occur at the contact of the probe tips. By taking the probe tips as measurement reference plane, the errors due to this transition are not corrected and may affect the measurement results.
Different calibration procedures or standards have been used for measuring microstrip-based circuits; among the most commonly used are line-reflect-match (LRM), line-line-reflect-match (LLRM), and thru-reflect-line (TRL) [
One of the most robust and popular technique is the TRL calibration, which is well suited to the on-wafer measurements at millimeter wave frequencies. According to previous comments, the reference plane is considered at the middle of the thru line.
The TRL calibration was done using on-wafer microstrip structures and the TRL algorithm supported by our vector network analyzer E8362B of Agilent Technologies.
A nonzero length thru is used to extend the reference plane a physical distance of 2286
One microstrip delay line of 477
For the reflect standard, the designer can choose between the open and the short. In our opinion, the open standard is a better option at millimeter wave frequencies, due to the complex nature of the short-circuit design having repeatable performances in microstrip technology, especially when via-holes are used.
In our designs, millimeter wave RF short circuits are implemented with quarter wavelength sectors, avoiding via-holes.
Figure
Microphotograph of the fabricated circuits on thin ceramic substrate of 2.54 cm
Due to the vulnerability of the very thin gold layer metallization (1
Typical measurement results over 60–90 GHz band of a microstrip line, after calibration, show matching results better than −50 dB at both ports and a quasi-perfect transmission of 0 dB (with no more than 0.5 dB ripple, the intrinsic error of the VNA) when the picoprobes are properly aligned and positioned.
In order to integrate complete millimeter wave front-ends on ceramic substrates, the first step is to design basic circuits, such as couplers and power dividers/combiners. These components will be further utilized in antenna array and six-port downconverter or direct modulator designs.
As mentioned, the MHMICs have been designed and fabricated on a very thin ceramic substrate having a relative permittivity of 9.9 and a thickness of 127
In order to perform on-wafer measurement of the
The symmetry of circuits is used to reduce the number of fabricated circuits required for complete characterization. For example, the full characterization of four port couplers requires minimum three different circuits due to the two-dimensional symmetry. Only two circuits are needed for the Wilkinson power divider, because of its one-dimensional symmetry. Finally, five circuits are requested for the full characterization of our six-port design. All these circuits can be easily identified in Figure
Even if the allowed frequency band starts from 57 GHz, all circuits are measured from 60 GHz because of measurement set-up capabilities (WR-12 rectangular waveguides modules for the 60–90 GHz millimeter wave extension of the VNA). However, the results can be extrapolated in the 57–60 GHz band by symmetry and comparison with electromagnetic simulations.
Figure
Microphotograph of the branch-line coupler.
As usual in our millimeter wave designs, the shape of the circuit is rounded, ensuring better
As explained earlier, all via-holes are replaced with wideband RF short circuits. Details, such as 50 Ω integrated resistor or the trace of the picoprobes on gold layer metallization after measurement, can be seen in the picture.
Figures
Measured input return loss for the 90° hybrid coupler.
As seen in Figure
The measured transmitted power is well splitted between the two outputs, especially around the central frequency allowed for V-band communications (60.5 GHz). The magnitude unbalancing is practically zero at 60 GHz and less than 1 dB at 64 GHz, as can be seen in Figure
Measured transmission
Measured transmission
Figure
Because in a six-port circuit the signal path crosses over two such couplers, these low values of magnitudes and phases unbalances are considered appropriate for modulation/demodulation schemes having up to 16 symbols.
A rate-race coupler has been also designed and fabricated on a separate die, similar to that illustrated in Figure
Figure
Microphotograph of the rat-race coupler.
In order to have a better idea of circuit size, let us see the same dimensions: the line widths are 126
Figures
Measured input return loss for the rat-race coupler.
Figure
As regards the transmitted power plotted in Figure
Measured transmission
Measured transmission
Figure
The rat-race coupler performances are ideal to design a V-band six-port-based downconverter using the quasi-conventional architecture (with two pairs of antiparallel diodes connected to rat-race quadrature outputs), similar to those presented in [
It is known that a conventional mixer uses such a rat-race coupler and a pair of antiparallel diodes. The six-port downconverter design is completed by adding a 90° hybrid coupler to LO port and a Wilkinson at RF port and two diode mixers [
Figure
Microphotograph of the Wilkinson power divider/combiner.
In order to avoid via holes, as in previous measurements, the 50 Ω loads use a quarter wavelength open stub as millimeter wave RF short circuit (see port 3).
Figures
Measured return loss for the Wilkinson power divider/combiner.
As seen in Figure
The power is almost equally split over the band, as illustrated in Figure
Measured transmission
Measured transmission
The phase difference between the two outputs is less than 2° over the considered band, as shown in Figure
The glitches at 60.5 GHz are due to an internal error of our VNA millimeter wave heads, which cannot be totally cancelled by calibration. It remains in the tolerance measurements of VNA, for both magnitude and phase measurements.
When the power divider circuit is part of a six-port, same conclusion as for previous circuits, the low values of magnitudes and phases unbalances are considered appropriate for the use of the six-port in modulation/demodulation schemes having up to 16 symbols.
Six-port (multiport) quadrature downconversion and direct modulation is an innovative approach in millimeter wave technology. A complete theory, validated by various simulations and measurements of V-band direct conversion receivers, has been published in recent years [
Figure
Six-port circuit block diagram.
In downconversion techniques, it has been demonstrated that the six-port technology allows improved results in terms of conversion loss and requires reduced LO power, as compared to the conventional methods (as low as −20 to −25 dBm to perform an efficient frequency conversion) [
A novel six-port circuit, having an improved symmetry and rounded shapes, has been designed using the novel Wilkinson power divider/combiner and the 90° hybrid couplers presented in previous Sections
Measurements are performed, as explained for other circuits from 60 GHz, due to our measurement equipment capabilities. Once again, extrapolation of measurements and comparison with simulations help us to estimate the circuit behavior from 57 to 60 GHz.
The microphotograph in Figure
Microphotograph of the novel millimeter wave six-port circuit in a typical
In order to measure the most important six-port
Figures
Measured RF inputs return loss and isolation for the proposed six-port circuit.
Figure
Figure
Typical measured transmission magnitudes (
The power splitting between the RF port (port 6) and two adjacent output ports,
Typical measured transmission magnitudes (
The measured return losses at output ports are illustrated in Figure
Typical measured outputs matches for the proposed six-port circuit.
Figure
Typical measured transmission phase difference of (
Typical measured transmission phase difference of (
As regards the phase difference between the two typical transmission
Novel V-band MHMICs, including a rounded shape six-port circuit, have been presented in this paper. In order to improve circuits’ performances, these MHMICs are fabricated in microstrip technology on very thin ceramic substrate.
Measurement results show that the proposed circuits are wideband components. The measured supplementary insertion losses, amplitude, and phase unbalancements are considered more than acceptable to build modulators/demodulators for modulation schemes having up to 16 symbols (BPSK to 16 QAM, PSK, or dual star). Keeping in the account the 7 GHz bandwidth allowed for V-band communication systems, the data-rates can reach quasi-optical values.
Six-port computer models have been implemented from the previous full-port measurements of Wilkinson and couplers. The two-port measurements of each circuit on die have been imported into ADS using data access components (DAC). Each model uses multiple DAC, interconnected according to the corresponding schematic of the six-port. The
In conclusion, this new fabrication run has allowed us to improve the performances of the six-port circuit in order to be integrated in our future design of an entire millimeter wave front-end on a 2.54 cm × 2.54 cm thin ceramic substrate. The die will integrate a 2 × 8 elements patch antenna array, a MMIC low noise amplifier, and a six-port quadrature downconverter.
The authors declare that there is no conflict of interests regarding the publication of this paper.
The authors would like to acknowledge the support of the “Centre de Recherche en Électronique Radiofréquence” (CREER) of Montréal, funded by the “Fonds du recherché du Quebec–Natures and Technologies” (FRQNT), for the MHMICs fabrication. Manuscript received July 25, 2013. This work was supported in part by the National Science Engineering Research Council of Canada.