Robert M. Weikle

A 100-MESFET planar grid oscillator

A 100-MESFET oscillator which gives 21 W of CW effective radiated power (ERP) with a 16-dB directivity and a 20% DC-to-RF conversion efficiency at 5 GHz is presented. The oscillator is a planar grid structure periodically loaded with transistors. The grid radiates and the devices combine quasi-optically and lock to each other. The oscillator can also be quasi-optically injection-locked to an external signal. The planar grid structure is very simple. All of the devices share the same bias, and they can be power and frequency tuned with a mirror behind the grid or dielectric slabs in front of it. An equivalent circuit for an infinite grid predicts the mirror frequency tuning. The planar property of the oscillator offers the possibility of a wafer-scale monolithically integrated source. Thousands of active solid-state devices can potentially be integrated in a high-power source for microwave or millimeter-wave applications. >

A grid amplifier

A 50-MESFET grid amplifier is reported that has a gain of 11 dB at 3.3 GHz. The grid isolates the input from the output by using vertical polarization for the input beam and horizontal polarization for the transmitted output beam. The grid unit cell is a two-MESFET differential amplifier. A simple calibration procedure allows the gain to be calculated from a relative power measurement. This grid is a hybrid circuit, but the structure is suitable for fabrication as a monolithic wafer-scale integrated circuit, particularly at millimeter wavelengths. >

Bar-grid oscillators

Grid oscillators are an attractive way of obtaining high power levels from the solid-state devices, since potentially the output powers of thousands of individual devices can be combined. The active devices do not require an external locking signal, and the power combining is done in free space. Thirty-six transistors were mounted on parallel brass bars, which provide a stable bias and have a low thermal resistance. The output power degraded gradually when the devices failed. The grid gave an effective radiated power of 3 W at 3 GHz. The directivity was 11.3 dB, and the DC-to-RF efficiency was 22%. Modulation capabilities of the grid were demonstrated. An equivalent circuit model for the grid is derived, and comparison with experimental results is shown. >

Quasi-optical power-combining arrays

Semiconductor devices have limited power handling capabilities at high frequencies, particularly at millimeter-wave frequencies. A method is presented for overcoming this problem by combining the outputs of several devices quasi-optically in a resonator cavity. This method has been applied to a number of solid-state devices, including Gunn diodes and MESFETs. The devices do not require an external locking signal because they lock to a mode of the resonator cavity. Effective radiated powers of 22 W for a 4*4 array of Gunn diodes and 25 W for a 10*10 array of MESFETs have been achieved.<<ETX>>

A 100-element planar Schottky diode grid mixer

The authors present a Schottky diode grid mixer suitable for mixing or detecting quasi-optical signals. The mixer is a planar bow-tie grid structure periodically loaded with diodes. A simple transmission line model is used to predict the reflection coefficient of the grid to a normally incident plane wave. The grid mixer power handling and dynamic range scales as the number of devices in the grid. A 10-GHz 100-element grid mixer has shown an improvement in dynamic range of 16.3 to 19.8 dB over an equivalent single-diode mixer. The conversion loss and noise figure of the grid are equal to those of a conventional mixer. The quasi-optical coupling of the input signals makes the grid mixer suitable for millimeter-wave and submillimeter-wave applications by eliminating waveguide sidewall losses and machining difficulties. The planar property of the grid potentially allows thousands of devices to be integrated monolithically. >

Transistor oscillator and amplifier grids

Although quasi-optical techniques are applicable to a large variety of solid-state devices, special attention is given to transistors, which are attractive because they can be used as either amplifiers or oscillators. Experimental results for MESFET bar-grid and planar grid oscillators are presented. A MESFET grid amplifier that receives only vertically polarized waves at the input and radiates horizontally polarized waves at the output is discussed. These planar grids can be scaled for operation at millimeter- and submillimeter-wave frequencies. By using modern IC fabrication technology, planar grid oscillators and amplifiers containing thousands of devices can be built, thereby realizing an efficient means for large-scale power combining. >

Failures in power-combining arrays

We derive a simple formula for the change in output when a device fails in a power-combining structure with identical matched devices. The loss is written in terms of the scattering coefficient of the failed device and reflection coefficient of an input port in the combining network. We apply this formula to several power combiners, including arrays in free space and enclosed waveguide structures. Our simulations indicate the output power degrades gracefully as devices fail, which is in agreement with previously published results.

Oscillator and amplifier grids

Presents the largest recorded output power for a quasi-optical power-combining array and a new planar heterojunction bipolar transistor (HBT) grid amplifier design. A 16-element MESFET grid oscillator has been fabricated that generated an effective radiated power of 28 W at 9.21 GHz. The total radiated power was estimated to be 2.0 W, giving a DC to RF efficiency of 28%. A new planar grid amplifier is also presented that is suitable for monolithic fabrication. The planar amplifier grid is a hybrid design using HBT transistors monolithically fabricated in a differential pair configuration and wire bonded to a Duroid substrate. The grid amplifier had a measured gain of 11 dB at 9.9 GHz.<<ETX>>

Element efficiency and noise in grid arrays

The element efficiency of a phased array is the ratio of the radiated-to-available power of a single element, when only that element is excited. We relate this element efficiency to the output noise power generated by a quasi-optical grid amplifier array. Both electromagnetic and thermodynamic derivations are presented. These ideas are used to predict the total noise power and noise radiation pattern of grid arrays. The results are also extended to show that the output noise temperature of the entire array will be the same as the output noise temperature of a single element.

Beam diffraction by a planar grid structure at 93 GHz

Using the transmission-line model approach, the authors attempt to demonstrate that the beam can be steered by building a grid structure without diodes to give a fixed beam shift. A simple transmission-line model is used to convert the zero-order diffracted wave to the first order. This model is useful because it can easily include semiconductor devices with biasing circuits. In the grids considered, diodes were replaced by gaps with different sizes to obtain different capacitances needed to steer a beam at 93 GHz. The results show a successful beam shift of 30 degrees with a loss of 2.5 dB.<<ETX>>