Today, for microwave circuitry, the traditional microstrip line and coplanar
waveguide (CPW) still play dominant roles as circuit media. Recently, the balanced
transmission line such as coplanar stripline (CPS) and slot line have been commonly
used to construct differential/balanced circuits for improving the circuit performance.
This dissertation provides double-sided parallel-strip line (DSPSL) investigation for
the DSPSL circuit analysis and applications to microwave component designs such as
filter, power divider and differential active circuits. The DSPSL, as another balanced
transmission line, has an important advantage over the CPS and slot line, i.e. easy
realization of low characteristic impedance. This significant feature of the DSPSL is
very useful for the integration of different microwave circuits or testing purpose
without the need of any impedance transformer.
Firstly, the properties of the DSPSL are studied by using the image method, and
the calculation method of the characteristic impedance of the DSPSL is presented in
detail for the following DSPSL circuit designs. A novel DSPSL-to-CPW transition is
developed and comprehensive parametric studies are conducted. The relative
bandwidth of the transition reaches about 150%, and the measured insertion loss of
the transition is only 0.26dB at the center frequency.
The DSPSL provides us with one more design freedom on characteristic
impedance in addition to the strip width, i.e. the offset distance between the top and
bottom strips. High characteristic impedance can be easily increased to a great extent
by offsetting the top and bottom strips without changing the strip width, which not
only enhances the circuit performance, but also improves the power-handling
capability. Two types of design examples - lowpass filter and power dividers, are
successfully designed to verify the proposed offset DSPSL technique. In the lowpass
filter design, the high impedance 202.5Ω, which cannot be realized by the microstrip
line or CPW, improves the filter performance and reduces the circuit size greatly.
Owing to the symmetrical structure of the DSPSL, the placement of a conductor
plane in the middle of the substrate cannot change the electromagnetic (EM) of the
DSPSL, and thus has no effect on the DSPSL. Meanwhile, it can directly convert the
DSPSL into a combination of two identical back-to-back differential microstrip pairs
without any discontinuity induced, implying that no transition or impedance
transformer between the DSPSL and back-to-back microstrip pairs is required. The
inserted conductor plane can also be used as an isolator for individual design of the
microstrip circuits on the top and bottom layers while the common ground plane of
the top and bottom microstrip circuits is just provided by the inserted conductor plane.
In brief, the inserted conductor plane has two functions: (1) isolating the top and
bottom circuits, (2) providing the common ground plane for the top and bottom
microstrip citcuits. This novel structure of the DSPSL with an inserted conductor
plane has been successfully applied to the design of various high-performance and
compact passive components such as filter components, out-of-phase power dividers
and so on. In the filter components, the non-identical resonators on the top and bottom
layers, isolated by the inserted conductor plane, generate their respective passband for
the construction of the dual-band bandpass filter and diplexer. Furthermore, a novel
out-of-phase power divider with arbitrary division ratio is proposed, and the design
formula is derived and presented in detail. The relative bandwidth of the
demonstrative equal out-of-phase power divider reaches about 100%. This is because
the out-of-phase feature between the top and bottom strips of the DSPSL is
frequency-independent, as a result, only impedance bandwidth is needed to be
considered in this design.
Based on the proposed multi-layer structure using the DSPSL with an inserted
conductor plane, a novel differential oscillator using two sub-oscillators on the
opposite sides of a dielectric substrate is proposed. The two sub-oscillators, sharing a
common quarter-wavelength DSPSL resonator and a common ground plane, generate
out-of-phase fundamental signals and in-phase second harmonics. At the common
DSPSL output, the second harmonics are inherently cancelled out while the
fundamental signals are well combined. By this design, an additional power combiner
at the output, as required by the conventional differential/balanced circuits, is no
longer needed. This greatly reduces the circuit size and simplifies the design
procedure. Furthermore, a varactor is added at the end of the common resonator for
tuning the oscillating frequency of the differential oscillator, constructing a novel
differential voltage controlled oscillator (VCO). Centered at 2.69GHz, the tunable
frequency range of the differential VCO is about 7.5%, and the linear range is
approximately 5.2%. The output power fluctuation is less than 1.4dB in the tunable
frequency range.
Finally, the differential VCO can be used as a RF power source to integrate with
the quasi-Yagi antenna designed using the DSPSL on the same substrate. The output
signals of the two sub-oscillators are directly fed to the antenna without needing any
bridging matching network. Noticeably, the out-of-phase feature between the two
sub-oscillators is frequency-independent in the differential VCO, which ensures the
low cross-polarization level of the antenna in the entire operation band.
| Date of Award | 2 Oct 2008 |
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| Original language | English |
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| Awarding Institution | - City University of Hong Kong
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| Supervisor | Quan XUE (Supervisor) |
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