M.P. De Lisio

Planar MESFET grid oscillators using gate feedback

A method for quasi-optically combining the output power of MESFETs in which drain and source leads couple directly to the radiated field is introduced. The design consists of a planar grid of devices placed in a Fabry-Perot cavity. Capacitive feedback is provided to the gate, allowing oscillation at much higher frequencies than previous grids. The oscillation frequency is dependent on the device characteristics, the resonator cavity, and the symmetries of the grid. A transmission-line model for the grid is discussed and used to design two oscillator arrays. A 16-element grid has produced 335 mW of power at 11.6 GHz with a DC-to-RF conversion efficiency of 20%. This design was scaled to produce a 36-element grid oscillator with output power of 235 mW at 17 GHz. These results represent a significant improvement in the performance of planar grid oscillators. The planar configuration of the grid is very convenient for monolithic integration and is easily scalable to millimeter-wave frequencies. >

A 100-element HBT grid amplifier

A 100-element 10-GHz grid amplifier has been developed. The active devices in the grid are chips with heterojunction bipolar transistor (HBT) differential pairs that include a resistive network to provide self-bias to the base. The planar metal grid structure was empirically designed to provide effective coupling between the HBTs and free space. Two independent measurements, one with focusing lenses, the other without, were used to measure the gain of the grid. In each case the peak gain of the grid was 10 dB at 10 GHz with a 3-dB bandwidth of 1 GHz. The input and output matches are better than 15 dB at 10 GHz. The maximum output power is 450 mW, and the minimum noise figure is 7 dB. Tests show that the grid is quite tolerant of failures-the output power dropped by only 1 dB when 10% of the inputs were detuned. The device amplifies beams with incidence angles up to 30 degrees with less than a 3-dB drop in power.<<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.

A 10-watt X-band grid oscillator

A 100-transistor MESFET grid oscillator has been fabricated that generates an effective radiated power of 660 W at 9.8 GHz and has a directivity of 18.0 dB. This corresponds to a total radiated power of 10.3 W, or 103 mW per device. This is the largest recorded output power for a grid oscillator. The grid drain-source bias voltage is 7.4 V and the total drain current for the grid is 6.0 A, resulting in an overall dc-to-rf efficiency of 23%. The pattern of the SSB noise-to-carrier ratio was measured and found to be essentially independent of the radiation angle. The average SSB noise level was -87 dBc/Hz at an offset of 150 kHz from the carrier. An average improvement in the SSB noise-to-carrier ratio of 5 dB was measured for a 100-transistor grid compared to a 16-transistor grid.<<ETX>>

Monolithic 40-GHz 670-mW HBT grid amplifier

A 36-element monolithic grid amplifier has been fabricated. The active elements are pairs of heterojunction-bipolar-transistors. Measurements show a peak gain of 5 dB at 40 GHz with a 3-dB bandwidth of 1.8 GHz (4.5%). Here we also report comparisons of patterns and tuning curves between the measurements and theory. The grid includes base stabilizing capacitors which result in a highly stable grid. The maximum saturated output power is 670 mW at 40 GHz with a peak power-added efficiency of 4%. This is the first report of power measurements on the monolithic quasi-optical amplifier.

Gain and stability models for HBT grid amplifiers

A 16-element heterojunction bipolar transistor (HBT) grid amplifier has been fabricated with a peak gain of 11 dB at 9.9 GHz with a 3-dB bandwidth of 350 MHz. We report a gain analysis model for the grid and give a comparison of the measurement and theory. The measured patterns show the evidence of a common-mode oscillation. A stability model for the common-mode oscillation is developed. Based on the stability model, a lumped capacitor gives suitable phase shift of the circular function, thus stabilizing the grid. A second 18-element grid was fabricated, using this theory, with improved stability.

Stability of grid amplifiers

We present a stability model for quasi-optical grid amplifiers. This model is useful for predicting and suppressing the common-mode oscillations that often occur in amplifier grids. Three stabilization techniques will be discussed. The first technique uses a capacitor to stabilize the grid. The second approach employs resistance to suppress the oscillations. The final technique stabilizes the grid by reducing the on-chip common-mode resistance, allowing greatly increased amplifier efficiencies. Experimental evidence will be presented to confirm the validity of our stability model.

A 16-element tunnel diode grid oscillator

A 16-channel tunnel diode grid oscillator has been fabricated. The grid oscillates at 1.86 GHz with an effective radiated power (ERP) of 1.3 mW. Frequency components were observed at multiples of one-third of the main frequency, but at power levels at least 16 dB lower. The average single-sideband (SSB) noise level was measured to be -76 dBc/Hz at an offset of 100 kHz from the carrier.