Output Power Improvement of a Push-push FET DRO by Using an Additional DR

Technical Note

Output Power Improvement of a Push-push FET DRO by Using an Additional DR

In this article, the output power level and phase noise properties of nine conventional push-push dielectric resonator oscillators (DRO) have been experimentally investigated while adding an identical dielectric resonator (DR) at the drain ports. The nine oscillators were designed to generate 20 GHz from a 10 GHz fundamental oscillation frequency. They have been tested with three different power combiners at the output port. It was observed that the output power level of a push-push FET DRO can be improved by placing a second DR, while the phase noise characteristics remain approximately equal to the ones obtained before adding the DR.

Ihn S. Kim, Chisung Jo and Yongin Han
Kyung Hee University
Suwon, South Korea

As the information age progresses, higher frequency signals are required for the transmission of larger amounts of information. The push-push oscillation scheme^1-5 is one of the many techniques used for increasing the oscillation frequency. The push-push oscillator circuit configuration enhances the second harmonic and suppresses the fundamental frequency output, doubling the fundamental frequencies so that higher operating frequencies can be obtained, beyond the limitation caused by the cut-off frequency of available three-terminal active devices.¹ Not only higher frequencies are necessary, but also a higher output power is required for all types of solid-state signal sources including the push-push FET DRO.

All of the push-push FET DRO circuits described in the literature^2-5 have used only one DR between the two microstrip lines connected to the gate ports to obtain the second harmonic output condition, as shown in Figure 1 . The function of the DR is to provide two fundamental frequency signals, 180° out of phase, to the gates of the FETs, and to enhance the two second harmonic frequency signals in phase at the gate ports. Since more output power at the second harmonic can be anticipated if the same function can be realized with a second DR at the drain port, a series of experiments have been carried out to measure the effect of adding one more identical DR between the drain port and the power combiner of the oscillators. Higher output power levels for the oscillators can be obtained by using the second DR (TE01d mode), while their phase noise characteristics are maintained approximately the same as before adding the DR.

Fig. 1 Push-push oscillator's (a) schematic diagram and (b) circuit model with dielectric resonators.

Experimental Circuit Topology

To investigate the variation in output power level and phase noise property of the conventional push-push FET DRO by adding an identical DR at the drain port, the circuit topology described in references 2 through 5 has been chosen as a breadboard model. The push-push FET DRO circuit model has been designed to enhance the second harmonic (20 GHz) of the 10 GHz fundamental frequency of the oscillator. A total of nine oscillators have been realized with two FETs (ATF-13780) and DRs (DRD051U E022) in a microstrip circuit using a substrate with a dielectric constant of 2.5 and a thickness of 0.54 mm. A photograph of one of the nine oscillators is shown in Figure 2 .

Fig. 2 A push-push DRO using two dielectric resonators.

The purpose of the experiment has been to measure the output power level and phase noise property of the oscillators when placing a DR, identical to the one used at the gate ports between the drain ports and the power combining circuit. The optimum location for the second DR has been found to be approximately a half wavelength at the fundamental frequency from the FET drains.

However, since the oscillators reported in the literature^2-5 have used three different types of power combiners, such as a T-junction,^2,4 a Wilkinson combiner³ and a rat race⁵ at the output port, three of the nine oscillators have been tried with a T-junction, three with a Wilkinson combiner and the rest with tapered6 combiners. In addition, the three sets of two FETs, mounted in the oscillators using T-junction, have been used again in the oscillators using either a Wilkinson or a tapered combiner.

Fig. 3 Output spectrum of a conventional push-push DRO.

Fig. 4 Output spectrum when the second DR is added.

Experimental Results

The output power levels and phase noise properties of the nine oscillators have been measured with a spectrum analyzer and are summarized in Table 1 . DR1 is the case when only one DR is used, and DR2 is the case when one more (identical) DR is added. The differences in the output power level and phase noise property between DR1 and DR2 for the nine oscillators can be compared from the data. The output power level has always increased when one more identical DR has been placed between the drain ports and the power combiner. Among the test data, a maximum 3.2 dB increase in output power has been obtained for the case of the oscillator using a Wilkinson combiner. Figure 3 shows the output spectrum for the case of the conventional push-push FET DRO and Figure 4 represents the oscillator with the second DR. The phase noise characteristics have been maintained approximately equal to those before adding the DR.

Conclusion

In this article, the output power level and phase noise property of nine conventional push-push FET DROs producing a 20 GHz signal from a 10 GHz fundamental frequency of oscillation have been measured by placing a DR identical to the one used at the gate ports between the drain ports and the power combiner. Three identical pairs of FETs have been used for each of the three different power combiners. In each case the output power level has increased when adding the second DR, while maintaining approximately the same phase noise as for the conventional push-push FET DRO. In this experiment, a maximum 3.2 dB increase in output power has been obtained. The reason why the output power level has been improved may be that the oscillation power at the fundamental frequency is reflected from the second DR to the drain ports and the power at the second harmonic frequency is enhanced.

References
1. J.R. Bender and C. Wong, "Push-push Design Extends Bipolar Frequency Range," Microwaves & RF , Vol. 22, No. 10, October 1983, pp. 91-98.
2. A.M. Pavio and M.A. Smith, "A 20-40 GHz Push-push Dielectric Resonator Oscillator," IEEE Transactions on Microwave Theory and Techniques , Vol. MTT-33, No. 12, December 1985, pp. 1346-1349.
3. C.M. Liu and C.Y. Ho, "On the Design of a Voltage-tuned Push-push Dielectric Resonator Oscillator," Microwave Journal , Vol. 33, No. 6, June 1990, pp. 165-174.
4. A.S. Hyun, H.S. Kim, et al., "K-band Hair-pin Resonator Oscillator," 1999 IEEE MTT Symposium Digest, 1999, pp. 725-728.
5. F.X. Sinnesbichler, B. Hautz and G.R. Olbrich, "A Si/SiGe HBT Dielectric Resonator Push-push Oscillator at 58 GHz," IEEE Microwave and Guided Letters , Vol. 10, No. 4, April 2000, pp. 145-147.
6. H. Kobeissi and K. Wu, "Design Technique and Performance Assessment of New Multiport Multihole Power Divider Suitable for M(H)MIC's," IEEE Transactions on Microwave Theory and Techniques , Vol. 47, No. 4, April 1999, pp. 499-505.

Ihn S. Kim received his BE degree in electrical engineering from Kyung Hee University, Suwon, South Korea, in 1974, and his MASc and PhD degrees, both in electrical engineering, from the University of Ottawa in 1983 and 1991, respectively. He has worked for the Canadian Space Agency, Com Dev Ltd., General Instrument of Canada and the Korean Broadcasting System. From February 1999 to February 2000, he was on sabbatical at ETRI (South Korea), ETH (Switzerland) and Motorola Florida Research Lab (US). He is currently a professor teaching microwave engineering in the school of electronics and information at Kyung Hee University. His research involves commercial application of radar technology, modeling of microwave structures by numerical methods (FEM, FDTD and TLM) and their application to filters and power divider/combiners, and nonlinear microwave circuit development such as oscillators and mixers. He can be reached via e-mail at ihnkim@khu.ac.kr or by phone at +82-31-201-2587.

Chisung Jo received his BE degree in electrical engineering from Kyung Hee University, Suwon, South Korea, in 1999. He is currently working on his MAS degree, also in electrical engineering. His research interests are in the areas of microwave couplers, oscillators and self-oscillating mixers.

Yongin Han received his BE and MAS degrees in electrical engineering from Kyung Hee University, Suwon, South Korea, in 2000 and 2002, respectively. He is currently working for Sewon Telecom, Seoul, as a research engineer. His research interests are in the areas of power dividers/combiners, microwave oscillators and RF part design in CDMA PCS phone.