Ultrawideband (UWB) RF Signal Source

Ulrich L. Rohde, Fellow, IEEE, and Ajay K. Poddar, Senior Member, IEEE

Abstract-Ultrawideband (UWB) has drawn the interestamong the research and wireless communication communitiesdue to its configurability and adaptability, which enables it tocoexist with many concurrent services. There are currentlyvarious mobile communication standards in use worldwide. Themobile communication industry is under transition phase andexperiencing a shift from single system connectivity to multi-system connectivity. The objective is to communicate overvarying distances using varying bit rates with a high-speed dataand coverage. One of the challenging blocks to design in suchtransreceiver is the multi-standard low noise ultrawideband(UWB) RF signal source (VCOs) that can provide seamlessconnectivity among the various standards considered. The VCOsdesign approach demonstrated in this work can satisfy thepresent demand of tuning range and noise performance, andreconfigurable for integration in chip form. The measured phasenoise is better than -112 dBc/Hz at 100 kHz offset over the tuningrange (0.5-2 GHz/2-8 GHz), and to our knowledge this is the bestphase noise and highest tuning band of operation using N-Pushtopology with discrete components so far reported.

Index Terms-UWB, N-Push, Phase noise, VCO.

I. INTRODUCTION

D ECENT advances in the wireless communication marketlhave led to the coexistence of several networks such ascellular network, personal area network (PAN), wireless localarea network (WLAN), etc. along with several different airinterfaces (802.11 a, 802.11g, Bluetooth, wireless codedivision multiple access (WCDMA), etc. Thus, all the wirelessdevices need to be compatible with the differentcommunication standards in order to enable "global roaming".The technology of the wireless devices also needs to allow fora smooth migration to future generations of communicationstandards with higher data rates. Hence, a compact and power-efficient implementation of such multi-standard terminals callsfor the need of an intelligent RF front-end that can achievemaximum hardware sharing for various standard. Such acompact RF front-end would require reconfigurable widebandVCO blocks rather than switched radio architectures.

There are currently various mobile communicationstandards in use worldwide. Software-defined radio (SDR)enables the creation of multi-standard terminals, which can be

This work was supported in part by the U.S. govemment (DARPA and U.SArmy).

Ulrich. L. Rohde is with the University of Cottbus, Germany and Chairmanof Synergy Microwave Corporation, NJ 07504 USA (phone: 973-881-8800;fax: 973-881-8361; e-mail: ulr@ synergymwave.com).

Ajay K. Poddar is with the Synergy Microwave Corporation, NJ 07504USA. (phone: 973-881-8800; fax: 973-881-8361; e-mail:akpoddar@synergymwave.com).

flexibly used in various mobile communication systems bysimply rewriting their software. SDR is a conceptual radiofrequency design where functions may be defined in software,and can be viewed in the context of wireless convergence. Assuch, an ideal SDR is a multi-band, multi-carrier, and multi-standard radio with dynamic capability defined throughsoftware in all layers of the protocol stack, including thephysical layer. However, to be compatible with differentstandards, the transreceiver must be able to deal with variousfrequency bands and brought a demand for ultrawideband(UWB) RF signal source [1]-[2].The coexistence of second and third generation wireless

system require multi-mode, multi-band, and multi-standardmobile communication systems, therefore, requiring awideband source that may replace several narrow band voltagecontrolled oscillator (VCO) module by a single low noisewideband VCO. Transreceiver components such as VCOs,power dividers, amplifiers, and phase shifters must also becapable of wideband performance to cover the frequency bandof various systems. The different standards operating in thefrequency range of up to 8 GHz, with even higher frequencieswith the introduction of ultrawideband (UWB) techniques,arises the need and gives a key role to multi-standard RFtransreceiver which combine several cellular and cordlessphone standards as well as wireless LAN functionalities in oneunit. This place more demand on the topologies andtechnologies used to implement reconfigurable widebandVCO operation with low-power and low phase noisecharacteristics [9]-[1 1].As the frequency band for the wireless communication

shifts higher, generation of the power efficient ultra low noisewideband and thermal stable compact signal sources with lowcost become more and more challenging due to the frequencylimitations of the active devices. A high frequency signal canbe generated either based on an oscillator operating at afundamental frequency or a harmonic oscillator.

Various techniques, such as switching between VCOs forseparate bands, utilizing inter-modal multiple frequency,using switched resonators for band selection have beenproposed. But these result in large size of the circuit, narrowband and generation of only discrete frequencies. A typical LCoscillator consists of an inductor and a capacitor. The inductorand capacitor can be made switchable. A perfect example is abinary coded set of inductors, which are switched by PINdiodes or other types of switches and one or more tuningdiodes. Fig.1 shows an example of an oscillator, where bothinductors and capacitors are being selected with diodeswitches, thereby, reasonably good phase noise and widetuning range is achieved simultaneously.

0-7803-9206-X/05/$20.00 ©2005 IEEE 255

The coupled N-Push oscillators consists of an array of Noscillator modules that share a common resonator and produceN duplicates of the oscillation signals with a 3600/N (900 for4-Push, as shown in Fig. 2.) phase difference between theadjacent oscillator modules. The advantage of the N-Pushtopology is the extended frequency generation capabilities andthe reduction of the phase noise in comparison with the singleindividual uncoupled oscillator by the factor N, where N is thenumber of uncoupled oscillator modules.

II. NOISE ANALYSIS OF N-COUPLED OSCILLATORSThe phase noise of the ith individual oscillator, and the total

phase noise in an N-coupled oscillator system as shown in Fig.3 are given by [3]-[7] as

(1)

Fig. 1. (a) UWB VCO using PIN diodes for L and C switching

Frequency High Kvco-----f Ow rapf > ihKC

Switching -wr1ap LowKvcoDiscreteCapacitors/Inductors ---------- Overlp

VtuneFig. 1. (b) Tuning sensitivity ofUWB VCO using switching mechanism

The drawback of band-switching approach is powerconsumption and extra noise due to the switching spikegenerated from PIN diodes. This limitation has made it moreattractive to pursue altemative approaches, such as coupledoscillator N-Push topology. The coupled oscillator designapproach using N-Push topology as shown in Fig. 2, improvesthe phase noise and extends the operating frequency of theavailable active device but at the cost of circuit complexity.

2(0)= I;,1

Osc#3 i

(2)N ~~~~

|A fi0)|2 = | ,(W)12y | 1

j=l

AV,.,., (w) =2 | Avfl ; i = l, e2,3,4...N

where Vf(o)) is the phase fluctuations for an oscillator in a

coupled oscillator system as Jyv..(a)j2= yI (w))|= J§v (v)12(assuming identical noise power spectral density of the Nidentical oscillator noise sources in the system of N-coupled

oscillators); and E | fl is the coupling parameter thatj=l i=l,

can be determined based on the coupling network [7].From (1) and (2), the total phase noise of the N-coupled

oscillator in terms of single uncoupled oscillator is given by

IA ,01a1 (w)V =I

1 go (ao})l ; i = 1,2,3,4...NN(3)

From (3), system of the N-coupled oscillator extends thefrequency generation capabilities, and improves the phasenoise in comparison with the single individual uncoupledoscillator by the factor of N, where N is number of theuncoupled oscillators in the coupled N-Push topology.

Osc#1

N-Push Oscillator 4-Push Oscillator (Quadruple-Push)Fig. 2. N-Push coupled oscillator (Ex: 4-Push oscillator)

OSC# lh

Fig. 3. N-Coupled oscillator coupled through arbitrary coupling network [7]

256

III. QUADRUPLE-PUSH/4-PUSH VCOFig. 4 shows the schematic of the 4-Push (quadruple-push)

oscillator, where Oscl#l, Osc#2, Osc#3, and Osc#4 oscillateat fundamental frequency fo (2 GHz), and 4-Push output isgiven by 4fo (8 GHz).As shown in Fig. 4, 4-Push (N=4) oscillator consists of four

identical VCOs having a phase shift of 900 among the fourfundamental signals oscillating at fundamental frequency fo,simultaneously, in a mutual injection mode [7]. Theoscillating signal from a neighboring oscillator module isinjected into another oscillator module and is again injected toother modules, and so on so that all the oscillator modules canoscillate in the same fundamental frequency fo. The fouridentical modules oscillate in the same fundamental frequencyfo, and the fundamental oscillating signal of each sub-oscillatorcircuits has a phase difference of 900, 1800 and 2700, thereby,the undesired fundamental signals fo, the second harmonicsignals 2fo, the third harmonic signals 3fo, and the fifthharmonic signals Sfo, etc. can be suppressed in principle andthe desired fourth harmonic signal 4fo is obtained when thesessignals are all combined in phase.The time varying oscillating signals of each oscillator

module for a 4-push oscillator can be given as

VI (t) =IA,,eina = A,e-j +A2ej2 + A3ej'' + A4e i 2+.. A,,ei' ') (4)n

V~,(1) = EA"e n't_2=Ae 2+jA2e 2+ Ae 2+ A4e 2 +. .Ae 2

V3(t)=A,,e"x'w')' =4e'q"'-f +Ae2e'-4r +Ae/3'4i-,T> +Ae,,w,,,f>+ --A4ea,'~) (6)n

nt,t-)(to-- j21.t,1t-- 13(Ot,1 -_ ) 14(e),i--) _n()-)

V'(1)-=A2e 2 2'+A2e 2 +A3e 2 +A4e 2 + A,e 2

(7)

The output of the 4-Push oscillator is given by

[Vo,, (t)]4 P-,,h =E V,, (t) = K4e j4a%0t + K8e j8ajt + K,2 +*j*w *

(8)n=l

The undesired fundamental signal fo, the second harmonicsignals 2fo, the third harmonic signals 3fo, and the fifthharmonic signals 5fo are suppressed due to the phase relationsabove, while the desired fourth harmonic signals 4fo arecombined because of their in-phase relations. The higherorder harmonics (8ob, 124o ) are filtered out.A system ofN nonlinear oscillators can in principle operate

in any one of N frequency modes [7], however, at a steady-state oscillation conditions, typically, only one of these modesmeets the phased criteria of the N-coupled oscillator system.The crucial point of the 4-push (N=4) oscillator is to realize

the phase difference of the fundamental oscillating signals of90 degree, 180 degree and 270 degree between one sub-circuitand the other three ones. In order to achieve these phaserelationship, microstrip/stripline ring as a common resonator isused, which is shown in Fig. 4.

Ring resonators have unique resonance characteristics suchas orthogonal resonance modes due to the symmetricalconfiguration so that each sub-circuit (Oscl, Osc2, Osc3 andOsc4) resonates with the other ones through the common ringresonators. Every two contiguous sub-circuits can oscillate ina mutual interaction with a phase difference of 900 , and everytwo opposite sub-circuits are mutually interacted out of phaseby 1800 at fundamental frequency of oscillation fo.

Fig. 5 and 6 illustrate the phase relationship of RFfundamental base current (fo=2 GHz) of the sub-circuitsOsc#1, Osc#2, Osc#3 and Osc#4.

I

Fig.5. RF fundamental base current for Osc# I and Osc# 3 of Fig. 4

I

Fig. 4. 4-Push coupled oscillator (fo=2 GHz; 4fo=8 GHz)

2.00_

4.oo_X .........~~~~~~~~~~~~~~.........

-200-

.o o.io o.Jo o.io o.o40040 0/0 O

Fig.6. RF fundamental base current for Osc# 2 and Osc# 4 of Fig. 4.

257

IV. VALIDATIONTheoretical and experimental results demonstrate the new

design approach for multi-octave-band tunability. Fig. 7 showsthe 4-Push UWB VCOs, which consists of sub-circuit Osc#1,Osc#2, Osc#3 and Osc#4 that oscillate for the same operatingfundamental frequency (fo), and having identical tuning range(0.5-2 GHz) as a function of tuning voltage. The powercombining of the four fundamental frequency sub-circuits(Osc# 1, Osc#2, Osc#3 and Osc#4) in proper phase relationsdescribed in section III, results in the suppression of theundesired harmonic signals such as fo, 2fo, 3fo, Sfo and so on,while the desired fourth harmonic signal 4f0o can be obtainedfrom the centre point of the common ring resonator.

Experimental results have shown that a poor mismatch andresults in discontinuous tuning due to the non-uniform phaseshift between the sub-circuits (Osc# 1, Osc#2, Osc#3 andOsc#4) over the tuning range. This mismatch in phase shiftbetween the sub-circuits is due to possible componenttolerances, package parameters, and the phase associated withthe path difference over the tuning range. Therefore, thesystem of 4-Push (N=4) may go out of the locking range dueto the mismatch in phase shift. The problem of discontinuoustuning can be overcome by incorporating a phase detector;thereby, the phase error is minimized and dynamicallycorrected for 4-Push operation over the tuning range; this is afurther extension of this research work for the purpose ofcovering the millimeter frequency range employing N-Pushtopology in MMIC/RF-MEMS domain [12].

Fig. 8 shows the measured phase noise plot to verify thenew approach. The measured phase noise is better than -112dBc/Hz ( 100 kHz offset over the band (2-8 GHz), and agreewith the predicted value within 2dB as given in (3). Thecircuit operates at 5V and 40 mA.

Fig. 7. 4-Push coupled UWB (0.5-2 GHz/2-8 GHz) VCOs

11- 170041 4.08 P1

noyw 4.*.o mOuSbd*tumTUos Liftl dbci"t

0,0

-100 _

-200-

-400 -

-6.00

0- 82GHz (Sub -C io:uil1a1 I) (_ W

-$001

'12tG-3 | | =h~~~~~GH

-1200 r

-1401

-1 0 001k 00k

Fig. 8. Measured phase noise of 4-Push coupled UWB (0.5-2 GHz/2-8GHz) VCOs

V. CONCLUSIONThe This work demonstrates the state-of-the-art of

designing reconfigurable low noise UWB RF signal sourcethat can be extended to millimeter wave, and is amenable forintegration in chip form.

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