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Oscillator Phase-Noise Reduction Using Low-Noise HigResonators

M t a Ni k d Ami M ta aiD pa tm t f l ti al gi i g d C mp t Si

Uiv si t f Mi higa A A b MI 48109, USAbs a This paper describes a method for the design of a

low phase-noise planar oscillator based on a compact low-noiseactive elliptic lter for its frequency stabilization. The phasenoise of the oscillator is signi cantly reduced by takingadvantage of the high frequency-selectivity and low-noisecharacteristics of the active lter. The lter occupies a relativelysmall area due to its two-pole dual-mode structure, making itsuitable for the fabrication of very compact low phase-noiseoscillators. As a proof of concept, a X-band oscillator using apackaged SiGe HBT transistor is designed and tested. Theoscillator, operating at GHz, achieves a measured phase-noiseof 50dBc Hz at MHz frequency o set with 0dBm outputpower. To the best of our knowledge, the oscillator demonstratedin this paper presents the lowest phase-noise among publishedplanar oscillators, to date.

d T msActive resonator, dual-mode lter, oscillator,phase noise, quality factor.

. N R D C NOscillator's phase-noise plays an important role in the

performance of communications systems. ow phase-noise levels can be achieved by utilizing high-quality resonators for

equency stabilization. Dielectric and cavity resonators o er the highest unloaded-quality-factors among available

microwave resonators, but are not amenable to low-cost integrated-circuit fabrication. Transmission-line-based resonators such as hair-pin [1] and ring [2] resonators are widely used to design planar oscillators. Unfortunately, these resonators do not provide high unloaded-quality-factors and hence don't allow for the low phase-noise operation of the oscillators.

n this paper, a very low phase-noise pla ar oscillator at X band is demonstrated by utilizing a two-pole dual-mode active elliptic lter in its feedback loop for equency stabilization. The lter provides a very high equency-selectivity due to the high unloaded-quality-factors of its constituent resonators, and the presence of a close-to-passband transmission-zero its

transfer nction. Fur ermore, the active resonators the lter are designed so as to minimize their noise impact on the phase-noise of the oscillator. As a result, the oscillator demonstrates a measured phase-noise of -150 dBc/Hz at 1 MHz equency o set. To e best of our knowledge, the oscillator presented in this paper achieves the lowest phase noise perfo ance among published X-band planar oscillators to date. Moreover, the small size of the lter makes it very

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use l for e design of low phase-noise oscillators with compact structures.

.PH N R D C N N An oscillator can be designed by provid g feedback to a

stable ampli er. According to the eeson's formula [3], oscillator's phase-noise is proportional to the inverse-square of the feedback network's group-delay de ned as

T = W1w Wo (1) where

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Fig. (a): A dual-mode active elliptic lter designed for lowphase-noise oscillator applications. (b): Compact version of the lterusing a meandered-loop resonator.

. C L W N D L D C LL C F L RD GN

A compact pseudo-elliptic bandpass lter can be implemented by designing a two-pole dual-mode lter asshown in Fig. 1 (a). The dual-mode lter design pr nciples have been addressed the literature [9], [10]. The structure inFig. 1(a) posses a pair of degenerate resonant modes, along its horizontal and vertical axis. The resonant modes are mutually coupled by the microstrip perturbations at the two inner co ers in order to form a bandpass lter response. A single transmission-zero is realized by source-load cross-coupl g provided through the asymmetric design of the feeding lines. The size of the lter can be fur her decreased by using a meandered-loop resonator as shown Fig. I(b). Thisstructure occupies a relatively small area of about /8 x 8, making it suitable for applications with size constraints.

To obtain high unloaded-quality factors and compensate for their losses, the resonators are coupled to negative-resistance networks. The negative-resistance circuits are implemented by using series feedback at the source terminals of low-noise pHEMT transistors in a common-gate con guration. t is very important to minimize the effect of the added-noise introduced by the negative-resistance circuits because the active resonators are intended for low phase-noise oscillator applications. This can be accomplished through designing for the minimum noise-measure of the negative resistance circuits by providing the optimum reactive terminations to the source and gate terminals of the transistors, as described in [11]. Matching-networks are then used to transform the drain impedances to the proper negative-resistance values required to fully compensate for the resonator losses.

The design parameters of the lter, such as the bandwidth, retu -loss and the location of the transmission-zero, are determined accord g to the procedure introduced in [4] for low phase-noise oscillator applications. The active lter is designed on a Roger's RO4003C substrate and simulated using Agilent's Momentum EM solver. Two Atf-33I43

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_5 = = C -- --- !1 .- 0 - - 1 - --

'E- 5

- 2 0 ---f--- --- --- --- 25 L -_ _ _ _ l _ _ - _ 7 6 7 7 7 8 7 98 8 8 2 8 3 8 4

0 - _ 1n_ - - , --.. .. Passi flter :

Ac i l erI- ( 1 5 ___ _ _____ ___

1

___ _

r

j .. . . _ .= . ; - -

7.6 7 7 7 8 7.9 8 8.1 8 2 8 3 8 4

Fr qu ncy (GHz)ig. 2. Simulated (top) inser ion loss and (bottom) group-delay o

the active lter. The simulation results of a similar passive lter arealso included for comparison. The lter center equency is 8 GHzwith 120 MHz bandwidth, \0 dB retu loss and normalizedtransmission-zero- ocation of 1.2. The high group-delay values othe active elliptic lter make it very suitable for low phase-noiseoscillator designs

pHEMT transistors with the gains of7 dB and noise- gures of1.2 dB are utilized in the design of the negative-resistance circuits. The simulated equency-response of the active lter is shown in Fig. 2. The simulation results of a passive lter with similar design parameters are also included for comparison. t can be seen that the active lter provides a loss less transfer characteristic with very high group-delay values at the passband edge. Fur ermore, the active lter achieves low-noise characteristics, owing to the proper noise design of its active resonators. The excess noise sources of the active lter cause less than 1 dB noise- gure degradation in the passband, comparison with the passive lter.

V . C LL R D GN ND RAn X-band microwave oscillator is designed using the

active elliptic lter described in the previous section. The oscillator is designed at the equency point where the active lter yields its highest passband group-delay value, in order to minimize the phase-noise. At this equency, the active lter achieves a measured group-delay of 21 nS with 2.2 dB

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c0Ec

Fig. 3.

5

10

15

_ 40. . : . .. .,I I ::

/

--.:.

l'_- -l\ - - , - - - 3 0c I

- - - --- - - - - - .\- 2 1

. .. . . 1:. e' . . C- - - - - -- - - - -- - . 10

I I I V

07.8 7 9 8 8. 8 36 8 2 8 . 3

Freq en (GHz)

Measured frequency response of the active elliptic lter.

C , oupler

.. I

::Output-

Fig. 4. The circuit layout of the fabricated oscillator.

insertion loss, as shown in Fig. 3. This corresponds to a loaded-quality-factor value of QL =536 for the feedback loop of the oscillator. The circuit layout of the fabricated oscillator is shown in Fig. 4. The ampli er in the oscillator loop employs a packaged SiGe HBT transistor (NESG2030M04) biased at the collector voltage of 2 V and quiescent current ofIS A. A 6-dB coupler is used to deliver the output signal to a50 n load.

The output spectrum of the oscillator is shown in Fig. 5. The measured oscillation frequency is 8.15 GHz with an output power of 10 dB m. The slight shi in the oscillation equency is most likely due to the fabrication tolerances. The total consumed DC power for the oscillator is 160 mW, corresponding to 6.5% DC-RF ef ciency. The core oscillator e ciency, not includ ng the active resonators, is 25%. n this design, optimizing the DC-RF ef ciency was not a primary

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- - - - - - -- - - - 1- - - - - -

o 1 t 1 1 +- - - - - - 1

E ," 1 1

- - - - -

5 - ..L5

cy G

Fig. 5. Measured output spectrum of the oscillator.

I Iil

I I IIII II I I II i

Measu eme, .. ..S m aI

?U . .. : :1 r TI I 1 I'I " I, I1 1

1 I1 1

1 I 11 N J IQ11 ;rt - -, : i l I I I I1 .: I ,1 LL U i: J1111

1 +1+11

1 1 I I I IIII 14 - + 1111 : + I + + _ I 1 1I16 r- t 1 11I ,r . 1I1 1 1

I I 1I I !I I

1I nI I 1 I

I I 1I I I I 1I

I 1II I 1 I

1 - H+ I 1

I i i liI I I + + +1

I I I + 1 + +1 1 111 I I 11 . L u :- L l c 1 1

r cy fs t )

Fig. 6. Measured phase noise of the fabricated oscillator. hasenoise at 1 MHz offset from the carrier is about -150 d c/Hz.

goal. The oscillator's overall ef ciency can be improved by employing transistors with low quiescent-currents the negative-resistance circuits design. The oscillator's phase noise is measured by the FM discriminator technique using anAgilent's E5500A phase-noise measurement system. Asshown in Fig. 6, the oscillator demonstrates a measured phase noise of -150 dBc/Hz at 1 MHz equency offset, which is in good agreement with the simulation results om AgiJent's

ADS circuit simulator by tak g into account the thermal noisesources. Table compares the performance of the oscillator presented in this paper with other reported planar ee-running oscillators at C and X-band. To the best of our knowledge, the oscillator presented here has the lowest phase noise among microwave planar oscillators reported to date.

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TAB E C R N W H H R R R D CR W L N R

Device

Si BJT [1]

HEMT [2]

SiGe HBT[ ]HEMT [6]

HEMT [7]

Si BJT [12]

This workSiGe HBT)

Resonator

Hairpin 9Resonator

Ring 12resonatorPassive 8 1elliptic filterActive filter 10

Active 10resonatorMultiple 5 .7 split-ringresonatorA ive elliptic 8 1filter

V . Conclusion

( (dBc/Hz@1 MHz)

-132

-116.17

-142 5

-119

-134 4

-144 5

-150

ow phase-noise oscillators can be designed by employing active elliptic- lters for equency stabilization. These lters achieve very high equency-selectivities due to the high unloaded-quality-factors of their constituent resonators, as well as e presence of close-to-passband transmission-zeros in their transfer nctions. Proper minimum-noise design of the active resonators is necessary in order to reduce the impact of their excess noise sources on the phase-noise of the oscillators. This is achieved by the minimizing the noise measure of the negative-resistance circuits used for resonators loss-compensation.

A low-phase-noise X-band oscillator was designed by employing a low-noise active elliptic lter in its feedback network. A compact two-pole dual-mode structure was used for the lter design. The oscillator, operating at 8.1 GHz with10 dBm output power, demonstrates a measured phase-noise

of -150 dBc Hz at 1 MHz equency offset. To the best of our knowledge, this is the lowest phase-noise performance among the published plan oscillators, to date. Furthermore, the relatively small size of the lter makes it use l for the design of compact low phase-noise oscillators.

The impact of the added-noise introduced by the active resonators will be discuss d in er details in a turejou al publication; ta ing into account the nonlinearities and

icker-noise sources of the resonators.

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F R NC[1] L. Dussopt, D. Guillois, and G. M. Rebeiz, "A low phase noise

silicon 9 GHz VCO and 18 GHz push-push oscillator, in IEEEMTT -S Int. Microwave Symp. Dig., vol. 2, June 2002, pp. 695-698.

[2] L.-H. Hsieh, and K Chang, "High-e ciency piezoelectrictransducer tuned feedback microstrip ring-resonator oscillatorsoperating at high resonant frequencies, IEEE Trans. Microw.Theory Tech., vol. -51, no. 4, pp. 1141-1145, April 2003.

[3] D. B. Leeson, "A simple model of feedback oscillator noisespectrum, in Proc. IEEE, vol. 54, Feb. 1966, pp. 3 9-330

[4] 1.Choi, M. Nick, and A. Mortazawi, "Low phase-noise planaroscillators employing elliptic-response bandpass lters IEEETrans. Microw. Theory Tech., vol. -57, no. 8, pp. 1959-1965,Aug. 2009.

[5] K Hof ann, and Z Skvor, "Active resonator, Int. ConTrends. Communications, EUROCON'2001, vol. 1, pp. 164166, Jul. 2001.

[6] 1.Lee, Y.-T. Lee, and S. Nam, "A phase noise reductiontechnique in microwave oscillator using high-Q active lter,IEEE Microw. Wireless Compon. Lett., vol. 12, no. 11, pp. 426-428, Nov. 2002.

[7] Y.-T. Lee, J. Lee, and S. Nam, "High-Q active resonators usingampli ers and their applications to low-phase noise free-runningand voltage-controlled oscillators, IEEE Trans. Microw.Theory Tech., vol. -52, no. 11, pp. 621-2626, Nov. 2004.

[8] Y. Sun, J. L. Tauritz, and R. G. F. Baets, "Silicon monolithicbalanced oscillators using on-chip suspended active resonators,IEEE Radio Frequency Integrated Circuits Symp., pp. 149-152,June 1998.

[9] J. S. Hong, and M. J. Lancaster, "Microstrip bandpass lterusing degenerate modes of a novel meandered loop resonator,IEEE Microw. Wireless Compon. Lett., vol. 5, no. 11, pp. 371-372, Nov. 1995.

[10] S. Amari, "Comments on "Description of coupling betweendegenerate modes of a dual-mode microstrip loop resonatorusing a novel perturbation arrangement and its dual-modebandpass lter applications , IEEE Trans. Microw. TheoryTech., vol. -52, no. 9, pp. 2190-2192, Sept. 2004.

[11] P. Gardner, and D. K Paul, "Optimum noise measurecon gurations for transistor negative resistance ampli ers,IEEE Trans. Microw. Theory Tech., vol. -45, no. 5, pp. 580-586,May 1997.

[12] J. Choi, and C. Seo, "Microstrip square open-loop multiple splitring resonator for low-phase-noise VCO, IEEE Trans. Microw.Theory Tech., vol. -56, no. 12, part 2, pp. 3245-3252, Dec.2008.

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