Millimeter-wave frequencies offer attractive wide bandwidths in support of high-data-rate and video point-to-point communications. Already, wireless operators are making use of links at 60 GHz for backhaul communications between cellular towers and base stations. Generating signals at this and higher frequencies is a matter of developing compact, tunable oscillators, and the varactor-diode-tuned Gunn oscillator is a strong candidate for supplying low-noise millimeter-wave signals with wide tuning bandwidths.

What follows is a report on the development and performance validation of varactor-tuned Gunn oscillators for V- and W-band applications. These voltage-controlled oscillators (VCOs) typically provide tuning bandwidths of 600 MHz to 5 GHz, are simple in construction and compact in size, and are cost effective for a wide range of applications, including radar systems and communications links.

Congestion in lower-frequency bands has spurred interest in the use of higher-frequency bands, especially at millimeter-wave frequencies. low-cost applications, such as 77-GH automotive collision-avoidance systems, have even motivated the development of millimeter-wave oscillators based on silicon CMOS integrated-circuit (IC) technologies. Often overlooked, however, is a proven technology in the Gunn diode and oscillators based on that semiconductor.

Such oscillators can be designed in fairly straightforward configurations with simple DC power supplies. although monolithic-microwave-integrated-circuit (MMIC) VCOs have extended to millimeter-wave frequencies in recent years, the phase noise of Gunn oscillators is still superior at higher frequencies and the Gunn oscillator is still a practical choice for many millimeter-wave applications.

The frequency of an electronically tuned Gunn oscillator is changed by adjusting an externally applied voltage. Compared to mechanical tuning, electronic tuning offers a significant advantage in frequency tuning speed, and the use of electronically tuning also offers the option of stabilizing the oscillator by means of a phase-lock loop (PLL).

Electronic tuning may be achieved by a number of techniques, including by means of ferrite or YIG-tuned resonators, by bias tuning of active devices, or by capacitive tuning using a varactor diode. This article will focus on the latter.

Wide-range electronic tuning can be achieved by incorporating a varactor diode into an oscillator circuit. This essentially adds a voltage-variable capacitance to the oscillator's resonant circuit; controlling the amount of coupling between the two is a key factor in establishing an efficient tunable oscillator. To explore the performance capabilities of several Gunn oscillator circuits, waveguide circuit implementations will be used since they offer high quality factor (Q) for low phase noise. Also, they are relatively easier to manufacture for low-cost oscillators.

The wide variety of oscillator configurations generally fall into two categories: parallel circuits and series circuits. Several decades earlier, Cawsey^{1} analyzed equivalent circuits for both parallel and series oscillator configurations. His results can be summarized as follows: For a series circuit oscillator,

ΔF/F_{0} = K(C_{o} C_{v})/C_{v}

For a parallel circuit oscillator, ΔF/F_{0} = K(C_{o} C_{v})/C_{o} where ΔF = bandwidth, F_{0} = center frequency K = a constant C_{o} = the varactor capacitance at 0 V, and C_{v} = the varactor capacitance at voltage V.

For large capacitance swings, i.e., when C_{o}/C_{v} ->, the series circuit oscillator tends towards ΔF/F_{0}-> while the parallel circuit oscillator tends to ΔF/F_{0}->K.

These results indicate that large bandwidths are possible with a series type oscillator. It should be noted that the analysis performed by Cawsey involved the use of ideal components; when real components are used, the performance of the circuit will be degraded considerably.

It is typically more efficient and cost effective to generate millimeter-wave frequencies using a second-harmonic oscillator as opposed to a fundamental-frequency oscillator. For example, it is less expensive to generate 40 mW output power at 80 GHz with a harmonic-mode 250-mW gallium-arsenide (GaAs) device than to generate that amount of power with a 40-mW fundamental-mode indium-phosphide (InP) device. Some of the other advantages of harmonic signal generation over fundamental-mode signal generation for millimeter-wave frequencies include:

1) Wider tuning ranges: Wideband tuning at fundamental frequencies results in even broader ranges at harmonics.

2) More tolerance of load pulling: For a harmonic oscillator, the fundamental frequency is significantly decoupled from the harmonic load. As a result, a harmonic oscillator will have a much higher external quality factor (Q_{e}) than that of a fundamental oscillator. This high Qe means the harmonic oscillator is less sensitive to load pullinghence, an isolator is generally not required.

3) Improved phase noise: The close-in phase noise of a harmonic oscillator is much better than that of a fundamental-frequency oscillator. However, the noise floor is typically 6 dB worse. This makes harmonic oscillators ideal candidates for millimeter-wave frequency-modulated-continuous-wave (FMCW) radar systems, particularly where space is at a premium.

4) More practical fabrication: GaAs is a more mature (and more widely applied) technology than InP. GaAs is usually the preferred material for devices operating in the harmonic mode. The number of InP device suppliers is more limited than for GaAs, and InP devices have tended to be more expensive and less reliable than GaAs devices.

In comparing design approaches for series and parallel Gunn oscillators, the approach for the series VCO oscillator is similar to that described by Ondria.^{2}Figure 2 shows the cross section of the series circuit Gunn oscillator. It consists of a split block waveguide cavity, a short circuit, a three-section anodized (i.e., with insulation) coaxial filter, Gunn diode, and varactor diode. The Gunn device is screwed into the waveguide floor, which provides a good thermal heatsink. The varactor is embedded into the first low-impedance section of the coaxial filter.

A narrow slot is made along the length of the three low impedance coaxial sections; this slot accommodates an insulated wire, which is attached to the resonant disk/cone. The disk serves to transform the low impedance of the device to that of the waveguide. Neither the slot nor the insulated wire are shown in the simple series circuit oscillator diagram of Fig. 1.

The approach for the parallel VCO circuit is similar to that described in numerous articles including the work by Holzman. ^{3}Figure 2 shows the cross section of the parallel circuit Gunn oscillator. The circuit consists of a split block waveguide cavity, a short circuit, a three-section anodized coaxial filter at two locations, a Gunn device, and a varactor diode. The physical separation of the devices and associated filters determines the optimum frequency for VCO operation; typically this spatial separation is about 45 percent of the waveguide's wavelength.

Figure 2 indicates a particular arrangement of the Gunn and varactor devices (i.e., varactor diode close to the short circuit). However, this arrangement could be reversed; the latter arrangement would provide for greater mechanical tuning.

In designing a varactor-tuned Gunn oscillator with a series circuit, a good starting point would be to determine the disk/cone diameter. The minimum diameter, f, required for a given frequency, F, from Ref. 1, is f = 1.84118c/pF, where c = the velocity of light in free space. So, for F = 94 GHz, f = 1.87 mm.

In designing a varactor-tuned Gunn oscillator with a parallel circuit, the spatial distribution between the varactor and Gunn device requires the electrical path length should correspond to one-half a waveguide wavelength at the frequency of interest. Figure 3 shows a typical rectangular waveguide with waveguide width a and waveguide height b, where the dimensions are typically related by a = 2b. For c and F, the free-space wavelength, λ_{0}, can be found from λ_{0} = c/F. If the cutoff wavelength of the waveguide , λ_{c}, can be found from λ_{c} = 2a, then the waveguide wavelength, λ_{g}, can be found from λ_{g} = λ_{0}λ_{c}/(λ_{c}^{2} λ_{0}^{2})^{0.5}

If F = 94 GHz, a = 2.54 mm for WR10 rectangular waveguide, and λ_{g} is 4.10 mm, the spatial separation needs to be 2.05 mm or (N +0.5)(λ_{g}/2) where N = 0, 1, 2, 3, etc. (N should be as low as practical constraints allow otherwise multiple reflections will reduce the useful bandwidth.)

To test these design guidelines, a number of series and parallel varactor-tuned Gunn oscillators were assembled, using commercially available Gunn and varactor diodes. Typical measurement results of frequency and output power as functions of varactor tuning voltages are shown in Figure 4 and Figure 5.

Measurements indicate that a series-type circuit will provide more electronic bandwidth, as suggested by Cawsey.^{1} It has also been demonstrated that the bandwidth for each circuit can be altered by varying the amount of coupling between the Gunn and varactor diodes. Additionally, the center frequency can be altered for the series circuit, although such modifications are limited for the parallel circuit, primarily due to the spatial separation between Gunn and varactor devices.

These experiments showed that it is possible to achieve at least 600-MHz electronic tuning bandwidth at V and Wband frequencies using either a series- or parallel-type varactor-tuned Gunn oscillator circuit. For bandwidths in excess of 2 GHz, however, it is advisable to use a series-type circuit. These oscillators can be readily manufactured in volume, as shown by the examples in Fig. 6.

References

1. D. Cawsey, "Wide Range Tuning of Solid-State Microwave Oscillators," IEEE Journal of Solid State Circuits, 1970. 2. J. Ondria, "Wideband Electronically Tunable GaAs Gunn VCOs at W-Band (75 - 110 GHz)," Proceedings of the IEEE Microwave Theory and Techniques Society (MTT-S) Symposium. 1985. 3. J. Holtzman and D. Robertson, Solid State Microwave Power Oscillator Design, Artech House, Norwood, MA, 1992. 4. G. S. Hobson, The Gunn Effect, Clarendon Press, Oxford, UK, 1974.