Frequency-switching speed and post-tuningdrift measurement of fast-switchingmicrowave-frequency synthesisers

S.D. Marougi

Abstract: Frequency-switching speed is a critical parameter in many communication systems,target detection as well as in automated test systems. Measuring frequency-switching speed isnecessary to determine the suitability of the frequency synthesiser for a particular application. Inthis paper, a method for measuring frequency-switching speed of fast-switching microwave-frequency synthesisers is described and demonstrated. The measurement set-up utilises ageneral-purpose analogue spectrum analyser to perform the measurement. Frequency-switchingmeasurement is demonstrated up to 70 GHz. This method can also be utilised in monitoringslow- and fast-frequency drifts in microwave oscillators.

1 Introduction

Frequency-switching speed is a critical performanceparameter in many modern communication systems suchas Bluetooth technology, WLAN, W-CDMA and 3G. It isalso critical in frequency-hopping radars. An example ofsuch frequency-switching is in cellular phone systems.When the phone user crosses cell boundaries, the cellularphone will have to switch to a new frequency. Duringfrequency-switching and the handover process, the cellularphone will go through a wait period during which thecommunication link is interrupted.

There is no standard definition for the criteria of theswitching speed. Typically, switching time is measuredfrom the instant the switching command is issued to afinal instant in time that varies from application to appli-cation. In analogue systems, in most cases, the final pointis defined as the instant of time beyond which the instan-taneous frequency error in the system is within a givenabsolute frequency, or within a given percentage of thefinal steady-state frequency.

Switching speed is determined by the closed-loop band-width of the phase-locked loop (PLL) of the frequencysynthesiser, the loop configuration and the frequencychange. Generally speaking, in second order PLLs,frequency-switching speed is approximately proportionalto the logarithm of the frequency change relative to thefinal acceptable settling frequency error. It is also inverselyproportional to the natural frequency of the PLL [1].Thus, to reduce frequency-switching time, the naturalfrequency of the loop will have to be increased, whichimplies increasing the closed loop bandwidth of thesynthesiser. This will lead to the well known trade-off inthe noise and spur performance of the synthesiser againstspeeding up frequency-switching [1, 2].

# The Institution of Engineering and Technology 2007

doi:10.1049/iet-smt:20060011

Paper first received 27th January and in revised form 7th July 2006

The author is with the Electronic Products and Solutions Group, AgilentTechnologies, 1400 Fountain Grove Parkway, Santa Rosa, CA 95403, USA

E-mail: salam_marougi@agilent.com

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Different methods have been used to control thesynthesizer hardware to speed up frequency-switching[2, 3]. However, in some of the fast frequency synthesisers,fast-switching speed is achieved through combinationof hardware configurations controlled by a complexfirmware algorithm. In this case, frequency-switchingspeed becomes a parameter that is partially dependent onthe hardware configuration and mostly on the firmwarealgorithm. Frequency-switching speed is a critical figureof merit for many state-of-the-art commercial frequencysynthesisers.

Various methods have been developed to measurefrequency-switching speed of frequency synthesisers.These methods vary in complexity and accuracy, dependingon the range of frequency of operation as well as the desiredmeasurement accuracy. Beat frequency measurementmethod has been implemented for measuring frequencyerror in the frequency synthesiser [3, 4]. In this method,a reference source is needed to downconvert thefrequency-switched signal and generate a signal at thebeat frequency, which will be displayed and measuredwith an oscilloscope.

A delay line frequency discriminator is also used to detectthe frequency parameter of the frequency-switched signal.The detected signal can then be monitored to determineits convergence to the final frequency [5, 6]. Other formsof frequency demodulators have been used and reportedin the literature [7, 8].

Commercial instruments are also available that are fullycapable of measuring frequency-switching and transientsin time domain, by measuring intervals between zero cross-ings of the frequency-switched signal [9, 10].

In this paper, a method for measuring frequency-switching speed of fast switching frequency synthesisersusing a general-purpose spectrum analyser is described.When a stable frequency reference is available, thismethod can also be used in monitoring post-tuning fre-quency drift in oscillators and frequency synthesisers. Themeasurement can be done over a large frequency-switchingstep and over a wide band of frequencies up to 50 GHz, orhigher when an external frequency mixer is used to extendthe frequency range of the spectrum analyser.

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2 Principle of the measurement

This frequency-switching speed measurement is based on thefrequency discriminator method. However, the frequencydetection is done using the frequency-response slope of abandpass filter. A number of such filters can be found in theIF stage of the spectrum analyser. These filters determine theresolution bandwidth of the spectrum analyser, where, inmost spectrum analysers, the resolution bandwidth may varyin discrete steps from a few Hz up to a few MHz. A typical fre-quency response of such a filter is shown in Figs. 1 and 2. Thefrequency response of these IF filters indicates that, whenthe filter is slightly off-tuned from the input signal frequency,the filter output will be linearly proportional to the input signalfrequency as well as the input power level. The IF filter outputis envelope detected and displayed on either linear or logarith-mic scale by the spectrum analyser. Thus, when keeping theinput power level constant, the off-tuned filter can operate asa frequency discriminator over the linear slope portion of theresolution filter skirt. The sensitivity of this discriminator isproportional to the slope of the linear skirt region of the off-tuned filter. This, in turn, is dependent on the bandwidth ofthe filter. In Fig. 1, the frequency response of a 3 kHzresolution-bandwidth filter is displayed in which the slope is4.75 dB/kHz as measured by the two markers indicated inFig. 1. Filters with wider resolution bandwidth will provideless steep response, as shown in Fig. 2 in which the sensitivityis 1.4887 dB/kHz as indicated by the two markers.

Hence, an off-tuned spectrum analyser can be utilised as anarrow-band frequency discriminator. This discriminator isformed by the linear slope of the frequency response of theresolution-bandwidth filter, the linear or logarithmic ampli-fiers that follow the filter and the envelope detector. Thesensitivity and dynamic range of this frequency detector isdependent on the gain of the amplifiers and the bandwidthof the resolution filter. The frequency demodulation of theinput signal can then be performed by placing the centre fre-quency of the input signal at the centre of the linear portionof the filter skirt.

This implies that the spectrum analyser would have to beoff-tuned from the centre frequency of the input signal by anamount Df shown in Fig. 1. On the other hand, as we like todisplay the envelope of the detected waveform, the spec-trum analyser will have to be set on zero span and the band-width of the video filter should be set wide enough toaccommodate all harmonics representing the frequencytransition in the detected signal.

Fig. 1 Frequency response of the 3 kHz resolution-bandwidthfilter

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3 Frequency-switching speed measurement

When trying to measure the frequency-switching speed of amicrowave signal using the aforementioned frequencydetector, it becomes obvious that the range offrequency-switching could be much wider than the linearregion of the frequency detector. This is true for all resol-ution filters that can be used in the spectrum analyser.This in fact is not a limitation on this method because theobjective of the measurement is not to display the patternof the frequency change during transition, the objective isto measure the time required to go through the desired fre-quency transition. To measure the frequency-switchingtime, the spectrum analyser is off-tuned by an amount Df(shown in Fig. 1) from the final frequency of the switchedsignal, and set to zero span. This will allow monitoringthe frequency transient in the switched signal as itapproaches its steady state. The spectrum analyser sweepis externally triggered and set to continuous. The triggersignal utilised is the same signal used to trigger thefrequency-switching in the frequency synthesizer, asshown in Fig. 3. Additionally, in many frequency

Fig. 2 Frequency response of the 10 kHz resolution-bandwidthfilter

Fig. 3 Simple block diagram of the measurement set up

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synthesisers, switching time is expected to be in the order ofa few milliseconds, in which case, the spectrum analysershould be set to delay the trigger by a few milliseconds,to allow for the final portion of the frequency transientto be clearly displayed to measure the total frequency-switching time.

In typical frequency-switching speed measurement, thetime interval is measured from the instant thefrequency-switching trigger is applied to an instant whenthe transient frequency is within a certain percentage ofthe final steady-state frequency. In some commercial fre-quency synthesisers, the percentage is 0.1/millionth of thefinal frequency. In the measurement shown in Fig. 4, thefinal frequency is 40 GHz. Thus, the final switching timeis measured when the test signal is within 4 kHz of thefinal frequency. When projecting 4 kHz on the skirt of the3 kHz resolution filter used in the measurement, the 4 kHzwill correspond to 19 dB difference in the detected signallevel from the final steady-state frequency. Thus, d (dmarker) shown in Fig. 4 is set to 19 dB, where the markerindicates the instant of time at which the time measurementis done. The time interval from the beginning of the trace tothe marker location indicates 1.6667 ms. When the triggerdelay (not indicated on the spectrum analyser display) isadded to this number, the final result will provide the timeinterval taken to switch the signal from 4 GHz to within4 kHz of the final frequency, which, in the case of Fig. 4,is 40 GHz as measured on Agilent’s 8565E spectrumanalyser. A similar measurement is performed on thesame frequency synthesiser, in which the synthesiser fre-quency is switched down from 60 GHz to 5 GHz. Thespectrum analyser trace is shown in Fig. 5. In this case dis 2.38 dB, which corresponds to 0.1/million of the 5 GHzfinal frequency reflected on the slope of the 1 kHzresolution-bandwidth filter.

The frequency range of the spectrum analyser can beextended by bypassing the front-end mixer of the spectrumanalyser and replacing it with an external mixer. The8565EC (or E) spectrum analyser facilitates this frequencyextension with the LO signal accessible from the frontpanel of the spectrum analyser and an external IF inputport to route the downconverted signal from the externalmixer back to the spectrum analyser [11]. An 11970Vharmonic mixer is used with the spectrum analyser toextend its frequency range up to 75 GHz [12]. Note that,when an external mixer is used, the spectrum analyser isno longer calibrated to perform absolute power measure-ment of the detected signal. However, in performing

Fig. 4 Frequency-switching speed measurement for step switch-ing from 4 GHz to 40 GHz

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frequency-switching speed measurement, only relativepower (d in Fig. 4) needs to be measured and the spectrumanalyser with its external mixer is capable of performing itaccurately, as long as the detected signal is operating on thelinear segment of the resolution-bandwidth filter asexplained in the preceding sections. Fig. 6 illustrates aplot obtained for a V-band frequency synthesiser whosefrequency is switched from 4 GHz to 70 GHz in one step.In this case, d, which corresponds to 0.1/million of thefinal frequency, is 33.25 dB as can be evaluated from theslope of the filter skirt shown in Fig. 1. The trigger delay,which is not indicated on the spectrum analyser screen,should be added to the time offset of the marker from thestart of the screen shown in Fig. 6.

In the measurements shown in Figs. 4–6, the triggersignal is taken at the end of a SCPI command sent to thefrequency synthesiser to switch to the new frequency. Thetrigger signal is extracted from the IEEE-488 data businterface connecting the controlling computer to thefrequency synthesiser.

4 Frequency drift measurement

It is known that the operation of both integer andfractional-N frequency synthesisers is based on convertinga reference frequency to the desired synthesised frequency.In most cases, the reference oscillator is an ovenized crystaloscillator operating in the MHz frequency range, typically

Fig. 5 Frequency-switching speed measurement for step switch-ing from 60 GHz down to 5 GHz

Fig. 6 Frequency-switching speed measurement for step switch-ing from 4 GHz to 70 GHz

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at 10 MHz. In practical frequency synthesisers, to upconvertthe relatively low frequency of the reference oscillator to afrequency in the higher microwave frequency bands, acombination of phase-locked frequency synthesis and anupconversion frequency mixing scheme are utilised toachieve the final frequency. In most cases, the signal atthe final frequency is phase-locked to the reference crystaloscillator. As the frequency ratio between the referenceand final frequencies is high, any small frequency drift inthe reference oscillator will be significantly magnified inthe final synthesised frequency. It is well known that theaging process of the crystal reference oscillator causesfrequency drift, which will be at a relatively faster paceduring the ‘infancy’ period of the crystal oscillator. Thespectrum analyser method described in this paper can alsobe used to measure frequency drift in the frequencysynthesiser, simply by focusing on the steady-statefrequency of the synthesiser, rather than the frequencytransient that happens during frequency-switching. Thus,no trigger signal is needed to initiate the spectrum analysersweep or to trigger frequency-switching in the frequencysynthesiser. This method also utilises the capability ofthe spectrum analyser to display two signal tracessimultaneously and storing each one of them independentlyof the other.

To use this method, the spectrum analyser and the fre-quency synthesisers should not be phase-locked to thesame external frequency reference. The spectrumanalyser should use a fairly stable frequency referencewhich is much more stable than the frequency synthesiserunder test.

To illustrate this technique, a frequency synthesiser wasset at 10 GHz. The zero-span frequency plot was thendisplayed on the spectrum analyser using an off-tuned300 Hz resolution-bandwidth filter and 2 dB/div verticalresolution, as shown in Fig. 7. A narrower resolution band-width was used to increase the spectrum analyser sensitivityto smaller frequency change, because narrower resolutionbandwidth will provide a steeper slope at the skirt of thefilter, which, in this case, was measured to be 20.876 Hz/dB. Similarly, 2 dB/div is used to further enhance the res-olution capability of the spectrum analyser to smaller fre-quency changes. The trace obtained for the 10 GHz signal(lower trace in Fig. 7) was then set to ‘MAX HOLD’ andthe spectrum analyser was switched to display trace ‘B’.To simulate frequency drift in the frequency synthesiser,the synthesiser frequency was then increased by 50 Hz to10.000 000 050 GHz and the new frequency is displayed

Fig. 7 Frequency drift measurement of a 10 GHz signal

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on the ‘B’ trace represented by the upper trace in Fig. 7.The difference between the two traces is 2.37 dB, whichcan be converted to frequency difference by multiplyingby 20.876 Hz/dB. This yields 49.477 Hz.

5 Discussion and conclusions

A versatile method has been described to measure thefrequency-switching speed of frequency synthesisers overa very wide range of frequencies. This method utilises ageneral purpose spectrum analyser to perform the measure-ment. The upper limit of the frequency-switching isrestricted by the frequency range of the spectrum analyser.Commercial frequency analysers are available up to50 GHz, such as Agilent’s 8565EC. However, thefrequency-switching measurement can be done at frequen-cies beyond 100 GHz, when an external harmonic mixeris used with the 8565EC spectrum analyser to extend itsfrequency range of operation [11, 12]. The measurementtechnique was extended to measure frequency drift in thefrequency synthesiser, although, in this case, a stable fre-quency reference is needed to monitor the small frequencydrift in the frequency synthesiser. On the same line of appli-cation, this method can be used to monitor the frequencydrift in oscillators during warm-up time. This will providean interesting insight into the short-term oscillator stability,especially when this oscillator is used as frequency refer-ence for a larger system. An important shortcoming ofthis method is that the waveform displayed by the spectrumanalyser is not frequency-dependent only, it is also sensitiveto variations in the envelope of the test signal. For thisreason, envelope variations, if they exists duringfrequency-switching, will have to be removed prior tomeasuring the frequency-switching speed, using the tech-nique reported in this paper. One exception, however, isthat if, during frequency-switching, the amplitude transientdies out before frequency transient, envelope fluctuationscan be ignored.

In frequency drift measurement, the off-tuned filterapproach may provide more accurate and highersensitivity reading for the frequency drift compared toanother approach in which the resolution filter is centre-tuned to the drifting frequency component. The reason isthat, for small frequency drifts, when the operating pointis placed on the filter skirt, the frequency displacementwill be detected by a near-linear frequency-to-voltage con-version characteristic. The sensitivity of the detector can becontrolled by the bandwidth of the resolution filter and themagnitude resolution factor of the spectrum analyser interms of dB/division. If the filter is centre-tuned on theharmonic, the frequency-to-voltage conversion character-istic is almost flat around the centre frequency, whichwill result in reduced sensitivity to small frequencydrifts.

6 References

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8 Katok, V.B., Manko, A.A., and Soloviyov, D.A.: ‘The devicefor frequency switching time measurement of UHF oscillators’.11th Int. Conf. Microw. Telecom. Techno., CriMiCo 2001,pp. 559–560

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