Quasi-optical network amplifiers and oscillators have been power-limited to less than one watt at Ka-band until recently. Additionally, measurements of grid structures are made with far-field techniques, making reflection measurements difficult.
Introduction
Approaches
Various monolithic designs have been successfully demonstrated J showing lower available powers than hybrid designs but have the added benefit of simpler fabrication that may be suitable for moderate to high volume production. Among the monolithic designs, grid amplifiers have been shown to operate up to Ka- and V-band frequencies.
Modeling
De Lisio extended Weikle's method and included the method of moments to generate a better estimate of the current distribution on the grid metal patterns [28]. The vertical dotted lines show De Lisio's division of the strip into smaller strips, and a current source will be associated with each.
Goal and Thesis Organization
Compton, "'n Monopool-sonde-gebaseerde kwasi-optiese versterkerskikking," IEEE Transactions on lvIicwave Theory and Tech. Rebeiz, "A Planar Quasi-Optical Mixer Using A Folded-Slot Antenna," IEEE Transactions on Al1:crowave Theory And Techniques, 43: (4) pp.
Design of The First Grid and Performance
Heat Removal
- Lumped-Element Models
- Full Electromagnetic Model
The first simulation assumed a uniform heat flux (equal to the power dissipated in the amplifier) over the GaAs area of the network amplifier. The measured performance of the amplifiers designed with 2.8(b) matched the modeled performance quite well, but the gains were lower than expected [8].
Unit Cell Design
The result is that the devices in the center of the grid (most of the transistors in the array) would have a bias of almost 0 V drain-to-source, and the few edge devices would have almost 3 V drain-to-source. The electrical plane of symmetry in the center of the cell provides a virtual ground, so a direct metal connection will short the input signal.
Stability
The symmetry of the currents excited by the common-mode oscillation imposes electrical wall boundary conditions on the unit cell. Since the fields are driven by the substrate, the input and output traces connected to the gates and drains of the transistors appear as transmission lines in the circuit model.
First Grid Performance
For design, then, the procedure is to sweep the values of the line impedance and the electrical length over a sufficiently wide range to record what may be physical. In this case, the impedance values of the lines were swept from almost 0 to 1000 n to watch for possible oscillations. As the figures show, the bias profile of the grid prevented most of the devices from turning on at all.
Gate voltage measurements show significant variations from the center of the network to the edges. But the slope of the gate voltage curve varies in each cell; the shape is parabolic, concave down. A pHEMT, under normal DC bias conditions, should show very little leakage (less than IJ.LA).
Each row of the grid can tolerate one resistance fault since the bias is connected on both sides of the grid.
High-Power Design and Performance
Successful Grid Amplifier
Since a large part of the transmitted power affects the network amplifier, the vector network analyzer can be used directly as a power source. The setup in Figure 4.2 was used to measure the small-signal performance of a network amplifier. Some additions to the system in Figure 4.2 are made to measure the power of the network amplifier.
Analysis of the model shows that the transfer function of the circuit is given by. The difference may be due to the presence of the substrate and the polarizer in the measurements. Therefore, the 17% efficiency of the network amplifier in Chapter 3 would change to 16%, a change of about 1%.
Although this structure provides excellent feedback, tuning requires movement of the structure relative to the grid amplifier surface.
Thermal Measurements
Measurements
Description of Far-Field Measurement Techniques
Gaussian Beam Optics
Placing the grid amplifier at the focal point of the system ensures that most of the current in the beam is delivered to. As a result, the power source for small-signal measurements can be provided by a network analyzer, enabling direct vector measurements of all four of the network amplifier's dispersion parameters. Treatment of Gaussian beam optics can be found in several sources [6-7], and a derivation of some of the important properties of Gaussian beams is given in the appendix.
The fundamental Gaussian beam mode is given by [6]:. where w is the 1/ e radius of the field; k is the propagation constant; and R is the beam's radius of curvature. For a system matrix of the form operating on an input beam parameter qin, it can be shown that the output beam parameter is given by [6]. However, the size may not be uniform due to the focused property of the beam.
Cutting a beam so that 9~)% of the power is transmitted results in a near-field ripple of 17%.
Calibration
- Calibration For Transmission Measurements
- Calibration For Reflection Measurements
This is the case when the focal point of the measurement beam is larger than the structure of interest. Alternatively, a known transmission standard can be inserted into the measurement plane which rotates the polarization of the beam. One of the complicating factors in measurement is that a matching standard may be impossible.
Tuning usually involves some movement of the polarizers after the amplifier is mounted during measurement. Referring to Figure 4.10, three stands can be constructed on a sheet of aluminum nitride, identical to the heat spreader of the grid amplifier. After calibrating the system with this technique, a reflectance measurement of the grating amplifier was performed without the polarizers.
Reflecting the beam from the heat spreader at a target allows control of the grid's angular position.
Power Measurements
An attempt to control the location of the lattice structure was made by setting up two laser beams to intersect in the focal plane. This can control the longitudinal position of the grating reasonably accurately, but the lateral positioning is limited to the eyeball alignment position of the grating so that the beam crossing occurs in the center of the standards and the grating. A third laser beam was set up to control the angular positioning of the standards and the grid.
Note that a reflected beam can be aligned with a marker on a target to track the angular position of the grid. Because the gain of the TWTA is constant4 over the power sweep range, the saturation curve of the grid amplifier can be measured by recording its gain at (~any drive power level (as measured by a power meter connected to the couplers). The gain of the TWTA remained constant to well below 0.1 dB over the entire power sweep range.
Since the gain of the TWTA is about 60 dB, very little drive produces quite a lot of power.
A Kim Oscillator
Oscillator Theory
The various effects of K's feed-forward behavior can be included in a feedback analysis by modifying the model in Figure 5.1 to include a forward path. Suitable devices can be connected to ports 2 and 4 of the rotator to display the polarizer. To check the accuracy of the model, a twist reflector was built that was significantly larger than the measurement beam.
Its function is to resolve the polarization of an incident ray into components parallel and perpendicular to the wires of the polarizer. By changing the position of the twist reflector, the frequency of the oscillator can be adjusted. Assuming that the surface of the oscillator has a uniform field distribution, Friis' formula can be used to estimate the radiated power of the electrical grid.
Simple experimentation (by typing the table) suggests that a significant source of the phase noise here is mechanical vibrations.
Future Work
A 10-W Grid
Provided that the gates of the transistors draw negligible DC current compared to the follower current, a resistive divider can be used to set the gate voltage. Control of the gate bias voltage can be achieved by varying the gate bias supply. Combining all the various design improvements together, figure n.5 shows a cartoon view of the next generation grating amplifier.
A comparison is shown for a standard TElO waveguide to verify the accuracy of the probing technique. The frequency of the grid oscillator can then be adjusted electronically without having to move the rotating reflector. As such, it replaces the polarizer of the rotated reflector to provide electronic tuning of the grating oscillation frequency.
The solution we were looking for should have a Gaussian amplitude profile, but with the j in the second term of the exponent it doesn't look promising.
Waveguide Feed
Electronically Tuned Grid Oscillator
Electronic tuning allows integration of the lattice oscillator into a phase-locked loop to reduce bandwidth and improve the noise performance of the structure. The design is also well suited for the waveguide power supply so that its output can be routed to a standard guide. Metal pattern resonates at a frequency determined by the capacitance of the varactor diodes, and behaves like a metal polarizer at that frequency.
Gaussian Beams
Looking for a solution that is symmetric about the propagation axis, {)2/84>2 = 0 so the wave equation is written as. To guess the solution to this simplified wave equation, start with knowledge of the far-field distribution of a gaussian field at a reference plane. Provided that the amplitude of the field at the reference plane is Gaussian-distributed, and its phase is fiat, the far-field amplitude is Gaussian, and the phase is spherical.
As such, we expect the solution to have a Gaussian amplitude component and a spherical phase component. But since qo is another constant of integration, we are free to choose it. Now there is a Gaussian amplitude term, but we've made a real mess with the In term.
A little fiddling with algebra finally results in the expression for the electric field we were looking for:
