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Advances in microwave and mm-wave oscillator and VCO technology challenge system designers' creativity.

Advances in Microwave and mm-Wave Oscillator and VCO Technology Challenge System Designers' Creativity

Introduction

Advances have been made in microwave and mm-wave Gunn oscillator and VCO technology. They present forward looking opportunities to system designers in addressing current and future needs for new and retrofitted EW, radar and communication systems, and for instrumentation needs. The work reported in this paper is presented in three parts, including modeling, design and performance of fundamental mm-wave, lumped-element, Gunn sources; ultra-wideband, second-harmonic, mm-wave, lumped-element, Gunn VCOs; and multioctave microwave Gunn VCO sources, and frequency and temperature compensation of lumped-element, Gunn VCOs.

The basis for state-of-the-art microwave wave oscillator and VCO performance is the use of a lumped-element circuit form, which is preferred over a distributed-type circuit, for example, a waveguide or microstrip. The advantages of a lumped-element circuit include its inherently broadband nature, circuit loss that is comparable to a waveguide circuit, miniature circuit size and simple circuit form. Demonstrated VCO performance advantages include broadband, continuous and spurious signal-free varactor tuning and high output power levels (to 0.25 W)

The lumped-element circuit form also has provided a new means to model accurately and quantify the equivalent circuit parameters of fundamental and second harmonic InP and GaAs Gunn VCOs and oscillators under large signal conditions. The modeling has provided previously unavailable resolution into the values of RF voltage and power dissipation at the individual elements comprising a VCO or oscillator circuit, including the Gunn and varactor diodes. It has thereby enhanced the understanding of Gunn diode and VCO operation and performance, and provided new opportunities in VCO and oscillator design to meet advanced system needs.

Technical Discussion

The lumped-element concept is based on the use of circuit elements that are sufficiently small (electrically) and therefore characterized as lumped components.(1,2) The basic layout of a lumped-element circuit used for fundamental VCOs in the 26 to 60 GHz range is shown in Figure 1. The VCO circuit is a simple and low cost ensemble that has been reduced to an elementary form as evident by the minimum parts count and small circuit size (0.089 X 0.089"). The discrete circuit elements consist of a packaged Gunn diode, a GaAs chip varactor and three chip capacitors. In fixed frequency oscillators, the varactor and its bias choke and bypass capacitor are replaced with a single fixed capacitor, and capacitor [C.sub.TF] is eliminated. The inductive elements [L.sub.T] and [L.sub.L] are short lengths of transmission line above ground that provide the electrical connection between the discrete circuit elements. The transmission lines are sufficiently short so they constitute lumped elements. The exception is the bias chokes that are nominally a quarter wavelength long. The simplicity of this VCO circuit, the small number of circuit elements and the functional use of all interconnections between the element's discrete components as the required circuit inductances have minimized parasitic elements and enabled wideband, continuous and spurious signal-free performance to be achieved at mm-wavelengths.

The measured tuning and output power characteristics of a lumped element Gunn VCO centered at 55 GHz is shown in Figure 2. The VCO was built in the circuit form shown in Figure 1 and used an InP Gunn diode and a GaAs hyperabrupt junction tuning varactor. The VCO circuit was assembled in an 0.089 X 0.089" slot under the screw attached cover in the waveguide housing shown in Figure 3a. A short length of small diameter coaxial (0.034") transmission line extends from the VCO circuit through the top wall of the output waveguide. The center conductor of the coaxial transmission line extends into the waveguide and is sized to adapt from the nominal 50 [OMEGA] output of the VCO circuit to the nominal 400 [OMEGA] impedance level of the output waveguide. The probe in the output waveguide also provided for dual output waveguide ports. A fixed short was used at one of the two output ports.

The dual output port arrangement eliminated the need for an external power divider or a directional coupler in such applications as provided coherent local oscillator drives to dual mixers and phase locking, which resulted in a significant saving in size and cost. A desired power split at the output ports was obtained by use of an appropriately sized apertured waveguide shim at one of the output ports.

The VCO (Figure 2) is tuned from 54.25 to 55.38 GHz with a corresponding tuning voltage range of -4 to -14 V. Output power increased monotonically from a minimum of +21.2 dBm (132 mW) at the low frequency end of the band to +23.8 dBm (240 mW) at the high end. The average power over the tuning range was +22.8 dBm (190 mW). Varactor current was zero over the tuning range and the DC to RF efficiency of the VCO was 7 percent. This VCO was intended for use as a transmitter source.(2)

In Ka-band, 26 to 40 GHz, VCO performance over a tuning range of 35.3 to 40.7 GHz with an average power of +19 dBm (80 mW) has been achieved. At lower power levels, tuning ranges up to 10 GHz have been obtained. In all cases, tuning varactor current was zero over the tuning range of the VCO. These data illustrate the state-of-the-art performance capabilities of VCOs in lumped-element circuit form.

Millimeter-wave lumped-element VCOs have been built for direct interface with integrated circuits (Figure 3b) and with dual coaxial output ports (Figure 3c) in addition to the waveguide port arrangement shown in Figure 3a.

The miniature size of the lumped-element oscillator circuit (0.125 X 0.125") is such that the oscillator circuit could be assembled in a notch in the front-end housing itself rather than as a separate attachment as shown. In the arrangement shown in Figure 3d, the entire circuit of a VCO, whose output frequency is centered at 46.6 GHz, was assembled on the 0.115" diameter flange of a standard studded Gunn diode package. An antenna, nominally a quarter wavelength long, was used to accomplish the impedance transform from the VCO circuit to waveguide impedance.

The realization of optimum performance characteristics from a Gunn VCO or oscillator mandates the use of an accurately characterized model, including the Gunn and varactor diodes under large signal conditions. The use of a distributed circuit for modeling and quantifying the equivalent circuit parameters of a Gunn VCO or oscillator at mm-wavelengths is unsatisfactory due to uncertainties incurred in de-embedding the parameter values from measured data in such a circuit, and due to their limited frequency range or validity. These deficiencies have been overcome with a new large signal modeling means that is based on the use of a lumped-element circuit from for mm-wave Gunn VCOs and oscillators. The accuracy of the modeling is evident by the excellent agreement that has been obtained between measured and calculated performance over oscillation frequency ranges as wide as 44 to 60 GHz.

The basis for the design of lumped element VCOs and oscillators was the general design model shown in Figure 4. The elements in the model are the Gunn diode and its package parasitic elements, the varactor and various circuit elements The capacitive circuit elements [C.sub.L1] and [C.sub.L], associated with output coupling, were not present in the VCO circuit shown in Figure 1. The capacitive element [C.sub.T] designates either a varactor or a resonator capacitor, corresponding respectively to VCO and fixed frequency oscillator configurations.

The equivalent circuit elements [C.sub.D], [L.sub.p], [C.sub.p], [L.sub.L] and [L.sub.T] in the VCO model were quantified from measurement of oscillation frequency vs. tuning capacitance [C.sub.T]. The circuit elements [C.sub.T], [R.sub.T] and [R.sub.O] were quantified from standard measurements at low frequency. A calculated value was used for miniature parallel plate capacitor [C.sub.TF] and 50 [OMEGA] was used for [R.sub.L] (based on calculation and indirect measurement).

The measured tuning characteristics of a VCO (Figure 2) or the measured oscillation frequency vs. resonator capacitance characteristic of an oscillator, was the basis for the process of de-embedding the unknown circuit elements, [C.sub.D], [L.sub.p], [C.sub.p], [L.sub.L] and [L.sub.T] in the model. A computer program, in conjuction with a SuperCompact(*) optimization, was used to obtain a best fit to the measured tuning characteristics and thereby establish the value of these circuit elements. In the case of a VCO, the varactor capacitance was corrected for large signal effects. The large signal correction of varactor capacitance was, for example, an increase in capacitance of 13.5 percent at the low voltage end and 3.8 percent at the high voltage end of the C-V characteristic of a GaAs varactor with gamma of 0.7. The measured output power characteristic of a VCO subsequently was used as input data in a computer program to delineate the RF voltage and power values at the various circuit elements noted in the model (Figure 4).

Typical values of the circuit elements of 55 GHz GaAs and InP Gunn VCOs are shown in Table 1. The accuracy of the modeling is exemplified by the excellent agreement between the measured and calculated tuning characteristics for the 55 GHz VCO, shown in Figure 5. The modeling and design analysis of this VCO accurately predicted the RF voltage swing across the varactor and the onset of varactor current flow at a tuning voltage of -3.8 V.

[Tabular Data Ommitted]

The unavailable level of resolution into the operating conditions internal to the VCO gives insight into the understanding and control of VCO performance, for example, post-tuning drift with a step in tuning voltage. Knowledge of the peak RF voltage swing at the varactor ([V.sub.j]) provides knowledge of the voltage margin to breakdown, a realiability factor, and the onset of varactor current that would normally set a lower limit on VCO tuning range. Knowledge of the voltage swing at the Gunn diode ([V.sub.D]) gives insight into the operating mode, that is, quenched domain and delayed domain. Another example of the resolution capability of the modeling is shown in Figure 6, which shows the change in Gunn diode domain capacitance with Gunn bias voltage, a determinant of the pushing of the VCO.

Conclusion

The modeling, fabrication and performance of fundamental, mm-wave, lumped-element, Gunn VCOs and oscillators in Ka-band (26 to 40 GHz) and V-band (50 to 75 GHz) have been described. Both InP and GaAs Gunn diodes have been used as the active element and GaAs hyperabrupt junction varactors as the tuning element. Demonstrated advantages of a lumped-element circuit form include varactor-tuned performance that is inherently broadband (to 10 GHz in Ka-band), continuous and spurious signal-free tuning, high output power (to 0.25 W) by virtue of low circuit loss, and miniature circuit size (0.89 X 0.89" in V-band). The lumped-element form also constitutes a natural transitional step to a monolithic realization.

The lumped-element circuit form provided a unique means to quantify the circuit element accurately in the VCO model under large signal conditions. The modeling has given a previously unavailable level of resolution into the operating conditions internal to a VCO or oscillator circuit elements. This new modeling capability provides enhanced opportunities in analysis and design for the advance of the state of the art in VCO and oscillator performance to accommodate e creativity of designers in addressing present and future system needs.

Acknowledgment

A portion of the work reported was performed for the US Army Electronics Technology and Devices Laboratory (ETDL), at Ft. Monmouth, NJ, under contract no. DAAK20-84-C-0395 and was directed by A. Paolella (ETDL) and V. Higgins, formerly with ETDL.

The remainder of the work was performed under IRAD programs in the RF and Receivers Department of AIL Systems Inc. in Melville, NY, under the direction of J. Whelehan, Senior Manager, Advanced Technology Division, H. Paczkowski, Department Head and G. Irvin, Group Leader. The contributions of E. Dolphy and E. Woodson to VCO and oscillator assembly is gratefully acknowledged.

PHOTO : Fig. 1 Layout of a 55 GHz lumped-element VCO.

PHOTO : Fig. 2 Measured performance of a lumped-element InP Gunn VCO.

PHOTO : Fig. 3 Various forms of mm-wave lumped-element Gunn VCOs; (a) a waveguide port arrangement, (b) direct interface with integrated circuits, (c) with dual coax output ports, and (d) the entire circuit of a VCO assembled on the 0.115" diameter flange of a standard studded Gunn diode package.

PHOTO : Fig. 4 Equivalent circuit model of varactor-tuned or fixed-frequency, lumped-element, Gunn oscillator.

PHOTO : Fig. 5 Measured and calculated tuning performance of a lumped-element InP Gunn VCO.

PHOTO : Fig. 6 Ka-band, lumped-element, Gunn VCO measured Gunn diode domain capacitance vs. bias voltage.

References

1. L.D. Cohen and E. Sard, "Recent Advances

in the Modeling and Performance

of mm-Wave InP and GaAs VCOs

and Oscillators," IEEE Int. Microwave

Symp. Dig., 1987, pp. 429-431. 2. L.D. Cohen, "VCO Transmitter Sources

for mm-Wave Radios," AIL Final Report

to US Army Laboratory Command, Electronics

Technology and Devices Lab

(ETDL), FT. Monmouth, NJ, January 1987,

Contract No. DAAD20-84-C-0395. 3. P. Meier, J. Calviello, A. Cappello, R.

Pomian, L. Cohen, and P. Bie, "Ka-Band

Front End with Monolithic, Hybrid and

Lumped-Element ICs" IEEE Trans. MTT,

Vol. MTT-34, No. 4, April 1986, pp. 412-419. (*) SuperCompact is a trademark of Compact software.

Leonard D. Cohen received his BEE degree from the College of the City of New York and his MSEE degree from the Polytechnic Institute of New York. In 1972, he joined AIL Systems Inc. and currently is a senior staff engineer in the RF and Receiver Department of the Advanced Technology Division. His responsibilities include the design and development of solid-state sources, components and advanced receivers for the microwave and mm-wave frequency ranges. Prior to 1972, Cohen was a member of the technical staff at GTE Research Laboratories. He is a senior member of the IEEE, Eta Kappa Nu and is a registered professional engineer in New York State. He holds 10 patents and has three pending.

Eugene W. Sard received his BSEE and MSEE degrees from MIT in 1944 and 1948, respectively. From 1944 to 1946, he was a radar officer with the USNR. In 1948, Sard joined AIL Systems Inc., where he was a senior staff consultant until his retirement in 1989. Presently he is a part-time consultant at AIL. Sard has been involved with the analysis and design of low noise parametric and FET amplifiers, frequency converters and multipliers, high power limiters, filters, channelizers and oscillators in the microwave and mm-wave regions. He also has contributed to the fields of infrared, optics, radar and communication technology. He serves as a reviewer for the IEEE, is a member of Sigma Xi Honor Society and a fellow of the IEEE.
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Title Annotation:voltage controlled oscillator
Author:Cohen, Leonard D.; Sard, Eugene
Publication:Microwave Journal
Date:Sep 1, 1990
Words:2482
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