154 IEEE MICROWAVE AND WIRELESS COMPONENTS LETTERS, VOL. 21, NO. 3, MARCH 2011

New Tuning Method for 94 GHz WaveguideVoltage Controlled Oscillator

Dong-Sik Ko, Sang-Jin Lee, Tae-Jong Baek, Seok-Gyu Choi, Min Han, Hyun-Chang Park, Member, IEEE,Jin-Koo Rhee, Fellow, IEEE, Joo-Ho Jung, and Young-Wook Park

Abstract—We propose a simple, yet highly effective tuningmethod to increase the bandwidth of a waveguide voltage con-trolled oscillator (VCO) by applying controlled mechanicalpressures to the packaged Gunn diode mounted in the wave-guide cavity resonator. When we applied this method to avaractor-tuned, second-harmonic 94 GHz VCO, the bandwidthwas doubled to 1 GHz with an output power over 14.8 dBm, whichis suitable for frequency-modulated continuous-wave radar sensorapplications.

Index Terms—Bandwidth, cavity resonator, Gunn diode, tuning,voltage controlled oscillator (VCO), waveguide, 94 GHz.

I. INTRODUCTION

A waveguide voltage controlled oscillator (VCO) is suit-able for W-band frequency-modulated continuous-wave

(FMCW) radar sensors due to its high output power and lownoise performance [1]. Linearity with large bandwidth (tuningrange), high output power, proper center frequency, and lowphase noise are most desired characteristics of a VCO.

To improve the bandwidth of a waveguide VCO, a carefuldesign of the cavity resonator is required, covering many de-tails including the low pass filters, the resonator disc, the dis-tance between the Gunn and the varactor diodes, and the po-sition of the sliding back short [2]–[4]. Even with this propercavity design, however, tuning of the individual VCO is usuallyrequired to achieve desired characteristics, because commer-cial Gunn diodes in the millimeter-wave frequencies, in spiteof the same part number and manufacturer, always come withnon-negligible variations in their electrical characteristics. Thistuning procedure is highly inefficient, expensive, and time-con-suming.

In this letter, we propose a simple, yet highly effective tuningmethod to increase the bandwidth of a waveguide VCO by ap-plying controlled mechanical pressures to the packaged Gunndiode mounted in the waveguide cavity.

Manuscript received November 03, 2010; accepted December 21, 2010. Dateof publication February 04, 2011; date of current version March 11, 2011.

D.-S. Ko, S.-J. Lee, T.-J. Baek, S.-G. Choi, M. Han, H.-C. Park, and J.-K.Rhee are with the Millimeter-wave Innovation Technology Research Center,Dongguk University, Seoul 100-715, Korea (e-mail: jkrhee@dongguk.edu).

J.-H. Jung is with the Defense Acquisition Program Administration, Seoul140–833, Korea.

Y.-W. Park is with the Institute of Defense Acquisition, Kwangwoon Univer-sity, Seoul 139–701, Korea.

Color versions of one or more of the figures in this letter are available onlineat http://ieeexplore.ieee.org.

Digital Object Identifier 10.1109/LMWC.2010.2103933

Fig. 1. Schematics of the waveguide VCO. (a) cross section, (b) equivalentrectangular oscillator, and (c) top view.

II. DESIGN OF THE WAVEGUIDE VCO

Fig. 1(a) shows a cross-sectional view of the 94 GHz wave-guide VCO. A packaged Gunn diode, either of GaAs or InP,and a packaged GaAs varactor diode are mounted in a wave-guide cavity. A replaceable resonator disk, with a diameter (D)ranging from 2.10 mm to 2.14 mm for tuning purposes, is in-stalled on the bias post for the Gunn diode. The distance be-tween the disk and the top of the Gunn diode is fixed at 0.1 mm.Thickness of the disk is 0.14 mm. The widest portion of theGunn diode package (the stud) is buried slightly into the lowerjig. The buried depth (M) is 0.07, 0.10, 0.15, or 0.33 mm, as an-other means for frequency tuning.

The dimension of the waveguide cavity resonator was de-termined based on an equivalent internal rectangular oscillatormodel shown in Fig. 1(b). The distance between the Gunn postand the varactor post (L) should be one half of the guided wave-length at the operating frequency of the Gunn diode,

1531-1309/$26.00 © 2010 IEEE

KO et al.: NEW TUNING METHOD FOR 94 GHz WAVEGUIDE VCO 155

Fig. 2. Proposed method of applying pressures to the packaged Gunn diode.

which is 47 GHz. H was fixed at 1.27 mm for WR-10 wave-guide. For a mode, W and L are related to the oscillationfrequency through the following equation:

where c is , and both and are 1 [4]. Throughthe eigenmode and the de-embedding simulations using HFSS,we obtained W of 4.08 mm and L of 4.92 mm [5]. Put into theabove equation, these values produced the required oscillationfrequency of 47 GHz.

As seen in Fig. 1(c), only the 94 GHz second harmonic is out-putted through the Iris filter of the cavity [5]. A m-adjustableback short is included to tune for maximum output power.

III. PROPOSED TUNING METHOD

A cavity resonator such as the one shown in Fig. 1, whichperforms satisfactorily with a particular Gunn diode, may notdo so with another diode of the same part number due to the in-evitable variations in their electrical characteristics. One mayresort to tuning by changing D or M. Even with this tuning,however, it is often difficult to achieve desired bandwidth, op-erating frequency, and output power simultaneously. For ex-ample, the bandwidth is observed to improve by increasing D,but this may reduce the operating frequency or the output powerbelow the target values [6], [7]. In this case, it is generally re-quired to change the cavity design itself, which is expensiveand time-consuming. We propose a simple, yet highly effectivetuning method that greatly increases the bandwidth of a VCO.

The operating frequency of a VCO is affected by the imped-ances of the Gunn and the varactor diodes. We find that by ap-plying controlled mechanical pressures to the packaged Gunndiode, we can change the impedance of the Gunn diode andachieve much larger tuning range from the VCO. This methodis very easy to use, inexpensive, and quite reproducible.

Fig. 2 illustrates this method. We mount a packaged Gunndiode upside-down to a jig. Using a flip-chip bonder such asM9 from Laurier, we apply vertical pressures to the center of thescrew tap of the packaged Gunn diode three or five times, witha force of 500 g. We mount this tuned Gunn diode into the VCOcavity and measure the characteristics. If sufficient bandwidth isnot achieved, the diode is removed from the VCO and additional

Fig. 3. External appearances of the Gunn diode (a) before tuning, and (b) aftertuning.

Fig. 4. X-ray images of the inside of the Gunn diode package (a) before tuning,and (b) after tuning.

Fig. 5. Cross section of the packaged Gunn diode (a) before tuning, and (b)after tuning.

Fig. 6. Equivalent circuit of the packaged Gunn diode.

pressures are applied one or two more times. We also found thatan experienced person could produce quite reproducible and sat-isfactory results as well by applying gentle manual pressuresusing a hard object such as a long nose plier.

The pressure apparently produces physical changes to theGunn diode. Fig. 3 shows the external appearances of the orig-inal and the tuned Gunn diodes. The center of the stud under theceramic ring protrudes up by 30 m after tuning.

Fig. 4 shows the x-ray images of the inside of the twopackaged diodes. The vertical position of the diode chip in thepackage has been changed. Fig. 5 shows the changes usingcross-sectional schematics.

Fig. 6 shows an equivalent circuit of the packaged Gunndiode, where and refer to the capacitance and thenegative resistance of the diode chip, while R, , and referto the parasitic resistance, the packaging inductance, and thepackaging capacitance, respectively. We believe that our tuning

156 IEEE MICROWAVE AND WIRELESS COMPONENTS LETTERS, VOL. 21, NO. 3, MARCH 2011

Fig. 7. Changes in the impedance of the Gunn diode after tuning.

Fig. 8. Output characteristics of the VCO using Gunn diode A, original andtuned.

mostly affects , and , changing the overall impedance ofthe packaged Gunn diode [4], [8], [9].

Fig. 7 shows the changes in the impedance of the packagedGunn diode of Fig. 5 using HFSS de-embedding simulation. Weobserve that tuning produced negligible changes in the resis-tance, but significant changes in the reactance, both in its mag-nitude and the slope with respect to the frequency. We con-clude that this is why the overall operating frequency and thebandwidth of the VCO change after tuning. The change in themagnitude of the reactance produces the change in the oscilla-tion frequency, while the increased slope in the reactance re-sults in larger frequency changes with respect to the varactorbias voltage, producing increased bandwidth.

We applied the proposed tuning method to five Gunn diodeslabeled A to E. These include three commercial GaAs Gunndiodes (A to C) of the same part number (DC1279F-T94 fromE2V Technologies), one GaAs Gunn diode fabricated andpackaged in our facility (D), and one commercial InP Gunndiode (E) custom-built by MDT corporation (now Microsemicorporation). In Fig. 8, output characteristics of the VCO usingGunn diode A before and after tuning are compared. A GaAsvaractor diode (MV31011 from MDT corporation) was used forthe VCO in all cases. An Agilent E4470B spectrum analyzerwith an extended harmonic mixer, and an Agilent E4419BEPM series power meter were used in the measurement. Band-width became more than doubled when the Gunn diode wastuned. Output power was slightly reduced after tuning, but atover 14.8 dBm, it was still sufficiently high for the intended

TABLE IOPERATING FREQUENCY AND OUTPUT POWER OF THE

VCO BEFORE AND AFTER TUNING THE GUNN DIODES

FMCW radar sensor applications. Results for the five diodesare summarized in Table I.

Our tuning did not affect the phase noise of the VCO. Thephase noise was below 100 dBc/Hz at 1 MHz offset with res-olution bandwidth (RBW) of 100 KHz for all the Gunn diodesused. The reliability of the tuned VCO is excellent, as we havenot observed any noticeable degradation from over 80 sets ofsuch VCO’s we have fabricated so far. Some of them have beeninstalled in the FMCW radar sensor, and have been operatingover three years now without degradation in performance.

IV. CONCLUSION

We have designed and fabricated a varactor-tuned,second-harmonic 94 GHz waveguide VCO using packagedGunn diodes. By applying the newly proposed tuning methodof applying controlled mechanical pressures to the packagedGunn diode, we were able to double the bandwidth of the VCOto about 1 GHz while maintaining output power over 14.8 dBmwith good phase noise characteristics, suitable for the intendedFMCW radar sensor applications. The proposed method issimple, yet highly effective, without any modification of thewaveguide cavity resonator itself.

REFERENCES

[1] J. D. Park and W. J. Kim, “An efficient method of eliminating therange ambiguity for a low-cost FMCW radar using VCO tuning char-acteristics,” IEEE Trans. Microw. Theory Tech., vol. 54, no. 10, pp.3623–3629, Oct. 2006.

[2] J. Ondria, “Wideband electronically tunable GaAs Gunn VCO’s atW-band (75–110 GHz),” in IEEE MTT-S Int. Dig., Jun. 1985, vol. 1,pp. 375–378.

[3] H. Barth, “A wideband, backshort-tunable second harmonic W-bandGunn-oscillator,” in IEEE MTT-S Int. Dig., Jun. 1981, vol. 1, pp.334–337.

[4] E. L. Holzman and R. S. Robertson, Solid-State Microwave Power Os-cillator Design. Norwood, MA: Artech House, 1992, ch. 2, 4 and 7.

[5] D. S. Ko, S. W. Moon, M. K. Lee, S. J. Lee, D. H. Ko, S. H. Bang, Y.H. Baek, M. Han, S. G. Choi, T. J. Baek, S. D. Kim, and J. K. Rhee,“94 GHz waveguide VCO with magic_T for FMCW radar,” in Proc.38th Eur. Microw. Conf., Oct. 2008, vol. 1, pp. 1234–1237.

[6] J. E. Carlstrom, R. L. Plambeck, and D. D. Thornton, “A continuouslytunable 65–115-GHz Gunn oscillator,” IEEE Trans. Microw. TheoryTech., vol. MTT-33, no. 7, pp. 610–619, Jul. 1985.

[7] W. H. Haydl, “Fundamental and harmonic operation of millimeter-wave Gunn diodes,” IEEE Trans. Microw. Theory Tech., vol. MTT-31,no. 11, pp. 879–889, Nov. 1983.

[8] I. W. Pence and P. J. Khan, “Broad-band equivalent-circuit determina-tion of Gunn diodes,” IEEE Trans. Microw. Theory Tech., vol. MTT-18,no. 11, pp. 784–790, Nov. 1970.

[9] W. J. Getsinger, “The packaged and mounted diode as a microwavecircuit,” IEEE Trans. Microw. Theory Tech., vol. MTT-14, no. 2, pp.58–69, Feb. 1966.