1991

Real-Time Microwave Signal Processing Using Microstrip TechnologyJoshua D. Schwartz', Jos' Azaia2, and David V. Plant'

'Photonics Systems Group - McGill University, Montreal, QC, H3A 2A7, Canada. 2Institut National de laRecherche Scientifique - Energie, Materiaux et Telecommunications, Montreal, QC, H5A 1K6, Canada

Abstract - We experimentally demonstrate two applicationsof microstrip technology in the domain of real-time microwavesignal processing. Both applications employ a modulated stripwidth featuring a linearly distributed modulation period (i.e.linear chirp). The high-dispersion, low-loss nature of such amicrostrip makes it suitable for real-time Fouriertransformation, whereby an incident signal's frequency contentbecomes temporally ordered and can be detected or processedusing time-domain methods. In addition, such a microstrip iscapable of providing adjustable true-time delay by means of up-and down-conversion through conventional mixers, withpotential applications for phased-array antennas.Index Terms - Microstrip circuits, signal processing, time

domain measurement.

I. INTRODUCTION

It has been recently shown that microstrips with low,uniform insertion losses, linear group delay (quadratic-phase)and high time-bandwidth products can be readily designed andmanufactured [1]. By varying the strip-width of aconventional microstrip on a material with a moderate-to-highdielectric constant, an incident electromagnetic signal can besubjected to a periodic variation of impedance. When thisperturbation satisfies the well-known Bragg reflectioncondition for an incident wavelength, that wavelength willcouple to the same but counter-propagating mode at thelocation of the local perturbation. Such structures have cometo be known as electromagnetic bandgap (EBG) devices. If, inaddition, the period of the strip-width modulation is varied ina linear fashion along a length of microstrip (commonlyrefeffed to as linear chirp), different frequency components ofthe incident signal will experience Bragg reflection at differentlengths along the device, thus the device exhibits a lineargroup delay.

In this work we investigate the potential of these low-loss,high dispersion devices for various microwave signalprocessing applications. In particular, we experimentallydemonstrate (i) real-time spectral analysis and (ii) tunabletrue-time delay (TTD) systems based on microstriptechnology. In the former application, the spectrum of a giveninput signal is mapped into the time-domain in real-time bypropagation through a high-dispersion microstrip line; thisoperation would enable filtering, convolution and correlationfunctions of broadband microwave signals to be performedusing time-domain processing [2]. In the latter application, thesame microstrip is used to demonstrate a TTD system capableof adjustable delays on the order of nanoseconds, limited in

bandwidth principally by the state of voltage-controlledoscillator (VCO) technology. This TTD system may proveuseful in application to phased-array antennas (PAAs) andradars, which require parallel TTD elements with largeenough delays to achieve full-scale angular deflection in thearray.

Fig. 1. Photograph of test microstrips. The center microstrip featureslinearly chirped width modulation.

II. TEST MICROSTRIPFor purposes of demonstrating the aforementioned

applications, a test microstrip was fabricated according to theequations laid out in [1]. The strip width modulation waschosen to vary the microstrip impedance around 50 Q from z-L/2 to z = L/2, according to the form:

Zj(z)=50.exp{A.W(z).sin(2.z+C.(z2 _ L )) (1)

where 'C' (m-2) is the chirp parameter, 'A' is dimensionlessand fixes the maximum depth of modulation, 'ao' is the centralperiod of the device, and W(z) is a tapering function (in thiscase, a Gaussian) which helps to suppress long-path Fabry-Perot type resonances due to abrupt impedance transitions atthe ends of the device. The general form of the impedancemodulation is exp(sin(cwooz)) because this offers the benefits ofsmooth and continuous impedance variation (which reducesparasitic effects), while at the same time suppressing theharmonic resonances that would occur at multiples of coo witha simple sinusoid. An integration constant of -C (L2/4) isincluded to ensure that at each extreme of the structure (z±L/2) the impedance is 50 Q-matched.

The authors' microstrip design targeted an operatingbandwidth of 4 GHz with a center frequency of 6 GHz. Themicrostrip was 10 cm in length and was fabricated on a 1.27

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mm alumina substrate (F. = 9.41, tan 6 = 0.0007) with gold-alloy metallization. The microstrip was mounted on analuminum baseplate and connectorized to SMA cables (Fig 1).The design used a chirp parameter of C = -2600 m2, whichcorresponds to a delay slope of a = -0.38 ns/GHz. Shown inFig 2 are the simulated and measured SI, and group delayresults for the microstrip, obtained respectively by Agilent'sMomentumTM software and a vector network analyzer. Thegroup delay features oscillations as a result of end-to-endresonances which were not completely eliminated by theapplied tapering technique; however the effect of theseoscillations on the overall performance is small.

20 4 51Frequency (GHz)

Fig. 2. Microstrip S11 & group delay, simulated (dash) & measured (solid)

III. REAL-TIME FOURIER TRANSFORMATION

Real-time spectral analysis is a powerful signal processingtool with a myriad of interesting applications. The domain ofReal-Time Fourier Transforms (RTFTs) has been exploredover the years in many different kinds of media. Particularinterest has been generated in mapping the spectrum of agiven input signal into the time-domain in real-time; thisoperation would enable filtering, convolution and correlationfunctions to be performed using time-domain processing [2].Such frequency-to-time mapping has already beendemonstrated in optical media using dispersive fiber gratings[3]. We provide here an experimental demonstration of anRTFT based on microstrip technology.

It has been shown that a signal si(t) constrained in time to A\tand subjected to a linear frequency-chirp with delay slope a(ns/GIHz) will decompose temporally into its FT componentsif the following condition is satisfied (corrected from thereference) [3]:

At<<1 (2)040

In the authors' design, the condition of (2) was satisfied forsignals within a time window of t «< 1.2 ns, althoughevidence suggests that this condition can be somewhat relaxed

in practice and good results can be obtained for time windowsapproaching 1 ns. In order to test the performance of themicrostrip, a broadband 6-dB directional coupler was used toisolate the reflected response. The authors suspect that infuture designs such a coupler could readily be integrated withthe microstrip itself. The other end of the microstrip was 50-ohm terminated. Time-windowed test signals were generatedwith a high-speed pulse-pattern generator and patterns werechosen to contain frequency peaks and nulls within thefrequency band of interest. It was deemed that a temporalwindow of 0.6 ns would be sufficient to satisfy (2). Various 6-bit patterns at frequencies of 10 and 12 Gbps (yielding spectralpeaks and nulls around 5 and 6 GHz) were applied,surrounded by strings of zeros to emulate a time window. Itshould be noted that although the microstrip was tested herewith digital bit-signaling, it is in no way limited to suchsignals, they were employed strictly for the experimental easewith which time-windowing could be applied.

For each input, the microstrip yielded a dispersed outputsignal approximately 1.5ns in length (corresponding to a =

0.38ns/GHz over a designed bandwidth of 4 GHz). The signalcontained the Fourier transform information modulated on achirped-sinusoidal carrier frequency which varied from 4 to 8GHz, the designed band of the microstrip. Experimentalmeasurements were recorded using a digital samplingoscilloscope and the Fourier transform information was thenextracted from the chirped sinusoid by peak-detection of theoutput signal (although any real-time averaging mechanismcould be used instead).

Time (ns)0.19 0.38 0.57 0.76 0.95 1.14 1.33 1.52

4 4.5 5 5.5 6 6.5 7 7.5 8Frequency (GHz)

Fig. 3. The normalized spectra of 6-bit signal '100111 ' as measured usinga spectrum analyzer (dashed, frequency domain) and a dispersive microstrip(solid, time-domain). Microstrip measurements are presented using simplepeak-detected interpolation.

One example of an RTFT is presented in Fig. 3 and isdirectly compared to measurements taken using aconventional spectrum analyzer by scaling the time-domaindetected response to the corresponding frequency range usingthe known dispersion slope a of the device and the simpletransformation f = t/ . Due to the negative delay slope of the

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microstrip, higher frequencies were reflected first and so formthe leading edge of the temporal signal, thus the signal hasbeen reversed in time for the benefit of comparison. Somedeviations, particularly near the frequency extremes, are theresult of end-connector effects which would not exist in anintegrated microstrip circuit. In practice, device resolution of amicrostrip RTFT is limited due to the limitations of peak-detection using relatively few peaks, an effect which would bemitigated in higher-frequency bands of operation where thesinusoidal modulation yields more peaks in the temporalwindow of interest.

IV. TUNABLE TRUE-TIME DELAY

Tunable delay lines have received a great deal of attentionfor their use in a variety of applications, most notably for theirapplication to phased-array antennas (PAAs) and radars.Interest in systems capable of true time-delay, as opposed tophase-shifters, was motivated by the desire for widebandPAAs wherein the angle of radiation was not sensitive to thefrequency of the input. Since then, several schemes have beenproposed for implementing tunable true time-delay (TTD)lines, including (but not limited to) signal conversion to theoptical domain [4], and semiconductor and ferroelectricvaractors [5], [6]. Conversion to the optical domain enableswide bandwidths of operation with fast switching times, butcan involve expensive electro-optic modulation. Nonlineardelay line (NDL) approaches (e.g. varactors) can also providevery wide bandwidths and are more easily integrated, andtypically provide delay variations on the order of tens ofpicoseconds. NDL approaches are inherently limited tooperations well below the design Bragg frequency, and can besensitive to temperature conditions.

Linearly chirped microstrips present another opportunity fora tunable TTD system implemented entirely in the electricaldomain and capable in principle of adjustable delays on theorder of nanoseconds with signal bandwidths limited by thestate of VCO technology. The setup is shown in Fig 4, inwhich a pair of identical mixers is used to up-convert anddown-convert an input signal to and from a frequency withinthe operating bandwidth of the chirped microstrip. Supposingan input centered at frequency tin, having a narrow bandwidthcompared to that of the chirped microstrip, one can drive themixers with a local oscillator (LO) such that the resultingintermediate frequency (IF), taken as the difference or sumfrequency WLO ± Win =: WF, falls within the bandwidth of thechirped microstrip. The signal is then reflected at a particularlength along the microstrip where the Bragg condition issatisfied. The reflected signal passes to the second mixer,restoring the signal to its original frequency (neglecting, forthe moment, the issue of image-frequency filtering). By tuningWLO (for example, with a high-power VCO) one can cause thesignal to travel different path lengths into the microstripbefore experiencing reflection; thus the chirped microstrip

enables a tunable time-of-flight delay.

Coupler

50 OhM

Fig. 4. TTD block diagram.

The maximum possible variation in time delay

introduced by this mechanism is controlled by the length of

the microstrip and the effective permittivity of the medium

according to the following:

2L 8eff

c(3)

Microstrip length, bandwidth of operation, and delay slopeare intimately related. For a given microstrip length, higherbandwidths of operation require lower delay slopes. Ingeneral, the microstrip bandwidth should be designed inaccordance with the bandwidths of the available VCO in thesystem, as this will typically be the limiting factor. The speedat which the full-scale delay may be scanned is limited by thestate-of-the-art in VCO technology. An example VCO(Herley 's V6040) capable of supplying + 10 dBm ofLO powerover a bandwidth of 4 GHz has a switching speed on the orderof 50 ns. Some restriction on the bandwidth of the inputsignals is necessary to avoid dispersion-induced distortion ofthe signal. Overall system attenuation of the signal will bedominated by the conversion loss ofthe mixers. Active mixingor amplification may be employed to compensate for systemlosses. Insertion losses incurred by the microstrip itself areminimal (the authors achieved an S1 of about -1 dB within theoperating band with ease). In addition, proper filtering must beadded to ensure image-frequency rejection.

For this scheme to be employed in a parallel manner (e.g.for a PAA with many elements), system size would bedominated by the length of the microstrip, for which a typicalstructure can be on the order of several centimeters. This canbe mitigated by raising the frequency of operation of themicrostrip, resulting in shorter structures. Such structures willexhibit higher losses (in proportion to [ ), however ajudicious use of tapering techniques and asymmetricwindowing applied to the microstrip modulation can raise thereflectivity of higher frequencies and remove undesiredharmonics [1]. Tapering techniques also improve the linearity

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of the microstrip group delay by reducing long-pathresonances that cause group delay ripple.

30

20

S 10

0)(O

2 -10

-20

-30

Test Signal at Delay Extrema

-2 -1 0 1 2Time (ns)

Fig. 5. Example of signal output at the full-scale delay extrema for an inputsignal centered at 4 GHz. 1.5 ns delay (dash) is demonstrated as compared toa reference (solid).

agreement with the prediction. Some variation in signalattenuation was observed over the range of delay values,attributable to frequency-dependent mixer conversion loss anddirectional coupling, neither of which were optimized. Plots ofmeasured signal delay recorded for two identical testmicrostrips are shown in Fig. 6 and demonstrate that a delayslope of approximately -0.39 ns/GHz was achieved in thedevice, with some anticipated ripple based on the group delayin Fig. 2.

V. CONCLUSION

Low-loss, high-dispersion filters based on microstriptechnology have been fabricated and demonstrated forimplementing two different operations of practicalsignificance, namely real-time spectral analysis, andadjustable true time-delay. These results demonstrate thepotential of chirped microstrips for a myriad of analog signalprocessing applications, such as real-time filtering,convolution and correlation functions of broadbandmicrowave signals, as well as potential applications in phased-array antennas and radars.

Relative Delay vs. Intermediate Frequency

4nC.)

-)

1.5

0-I5 4 4.5 5 5.5 6 6.5 7 7.5 8

Intermediate Frequency (GHz)Fig. 6. Measured relative delay introduced into a bit sequence by tuning thelocal oscillator. Two data sets (o) and (+) are shown for two identical testmicrostrips, along with a linearization of (o) indicating delay slope.

A simple proof-of-concept demonstration was carried outusing the previously introduced microstrip. The microstripfeatures an effective permittivity of eff = 6.34 for a

corresponding delay range, according to (3), of ST = 1.68 ns.

Employing commercially available mixers and a synthesizedlocal oscillator, a 16-bit alternating sequence was used toapproximate a short, narrow-band signal centered at 4 GHz.The LO was tuned between 7.5 GHz and 12 GHz, resulting inIF values in range of the microstrip. Experimentally measuredoutput signals from the TTD system are shown in Fig. 5,demonstrating the full-scale delay of approximately 1.5 ns, in

ACKNOWLEDGEMENTS

The authors gratefully acknowledge the support of Mr.Robert Morawski and Professor Tho le-Ngoc for theirexperimental assistance.

REFERENCES

[1] M. A. G. Laso, T. Lopetegi, M. J. Erro, D. Benito, M. J.Garde, M. A. Muriel, M. Sorolla, and M. Guglielmi, "Chirpeddelay lines in microstrip technology", IEEE Microwave.Wireless Comp. Lett. (USA), vol. 11, pp.486-488, Dec. 2001.

[2] M. A. G. Laso, et al. "Real-time spectrum analysis inmicrostrip technology", IEEE Trans. Microwave Theory Tech.,vol. 51, pp.705-717, Mar. 2003.

[3] J. Azana, L. R. Chen, M. A. Muriel and P. W. E. Smith,"Experimental demonstration of real-time Fouriertransformation using linearly chirped fiber bragg gratings",Electron. Lett. , vol 35, pp.2223-2224, Dec. 1999.

[4] J. L. Corral, J. Marti, J. M. Fuster, and R. I. Laming, "Truetime-delay scheme for feeding optically controlled phased-arrayantennas using chirped-fiber gratings," IEEE Photon. Tech.Lett., vol. 9, pp.1529-1531, Nov. 1997.

[5] C. Chang, C. Liang, R. Hsia, C. W. Domier, and N. C.Luhmann Jr., "True time phased antenna array systems based onnonlinear delay line technology," in Proc. Asia-PacificMicrowave Conf:, Taipei, Taiwan, R.O.C., Nov. 2001, pp. 795-799.

[6] D. Kuylenstierna, A. Vorobiev, P. Linnr ,and S. Gevorgian,"Ultrawide-band tunable true-time delay lines using ferroelectricvaractors," IEEE Transactions on Microwave Theory and Tech.,vol. 53, pp.2164-2170, Jun. 2005.

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