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Design Considerations in State-Of-The-Art Signal Processing and Phase Noise Measurenentsystems

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Design Considerations in State-Of-The-Art Signal Processing and Phase Noise Measurenentsystems DESIGN CONSIDERATIONS IN STATE-OF-THE-ART SIGNAL PROCESSING AND PHASE NOISE MEASURENENTSYSTEMS bY F. L.Walls, S. R. Stein, James E. Gray and David J. Glaze Frequency and Time StandardsSection National Bureau of Standards Boulder,Colorado 80302 where S isthe spectraldensity of the noise powerand Introduction P, is tRe power availableto a matched load.For Johnson noise,the most common situation, The recentrapid improvement of oscillator phase noise has resultedin significantly more stringent re- quirements forsignal hand1 ing equipment . However, in- So = kT = 4 x J formationconcerninq the phase noiseperformance of the twomost importantiypes of circuits L amplifiers and and theachie able phase noiseperformance is -184 dB mixers - isoften difficult to find. Some generalprinci- below a rad h for a 1 V,,,s,gnal from a 500 source. plesare presented which allow one to estimatethe phase noiseperformance of anamp1 ifier. Also,techniques.are In contrast to the white phase noisewhich is added describedwhich permit one to obtainthe best possible to the carrier by the amplifier, the flicker phase noise results from thetraditional double balanced mixer. A is produced by direct phase modulation in the active measurement set-upwhich has15 to 25dB improvement in its element. It has beenfound empiricallythat a transistor noise floor is shown in detail to illustrate proper mixer stagewhich does not use emitterdegeneration typically drive and termination.Although traditional circuits can hasphase noisegiven by S+(f) = rad2/f. However with extremecare achicie % = -175 dB or slightly better, the use of local RF negativefeedback can reduce this thisis not sufficient for all present requirements. One noise power by as much as a factorof lo4. [l1 Passive techniqueto obtain an additional improvement of 10 to elementscan also contribute to the flicker phase noise. 40 dB in measurementsystem noise is to reducethe mixer [Electrolytic,ceramic, and silver micacapacitors and carbon2 and amplifier contributions to the noise floor by the use compositionresistors can give excessive flicker noise of correlationtechniques. A circuitto accomplish this and shouldonly be used innon-critical locations. is discussedalong with some preliminary results. Threedesign requirements for low phase noiseampli- One of the most frequently needed systems in the fierdesign follow from the above discussion.Firstly, study and use of oscillators is thephase-lock loop. each stage of an amplifier must incorporate emitter de- However, sincethe performance of this system is often generationto minimize the flicker phase modulation. incidentalto the ultimate goal of the experimenter, e.g. Secondly, criticalpassive components should beexamined the measurement of phase noise,the design of such a forexcessive phase noise.Finally, the signal level system is sometimes giventoo littleconsideration, re- mustbe always maintained at a high enough level to sulting in unanticipated difficulties andwasted time. achievethe desired white phase noiselevel. The design of an extremelysimple phase-lock loop which is suitable for almost all high stability oscillator ap- In order to illustrate the influence of this philos- plications is discussedwith particular attention to the ophyon the design of an amplifier, a new isolation ampli- advantaqesover more traditional circuits. fier is described.This amplifier was developed to pro- I. PHILOSOPHYOF LOW NOISE AMPLIFIER DESIGN vide a high degree of isolation between verylow phase noise RF signalswhich are used to compare atomic ana If an amplifier were drivenfrom a noiseless oscil- otherfrequency standards. lator,then the output phasespectrum would typically have a flicker noise region at low frequencies and a 11. WIDE-BAND LOW-NOISEISOLATION AMPLIFIER whitenoise region at higher frequencies. The break be- tween the two is usually between one andone hundred Hz The amplifier shown in Figure 1 is designed to op- and the white noise extends out to the bandwidth of the erate from one toseveral hundred megahertz. Inorder amplifier. The source ofthe white noise modulation can to minimize current drain a method of achieving high be identified and themagnitude estimated, but similar isolationwhich used a small number of stages was generalizations can not be made for the flicker noise. needed. Thisrequirement was satisfied by a cascaded Nevertheless,empirical guide1 ines can be established pair of comnon base transistor stages, Q1 and Q2 A whichshould ensure against unnecessarily poor flicker signalapplied to theoutput port propagates towards noiseperformance. theinput through the collector-base capacitance of Q]. The2N3904 was selected because of its smalloutput Providedthat the integrated noise of the capacitance, 4 pF. Sincethe base of Q1 is grounded amplifier over its entire bandwidth is small compared to through a capacitor and theemitter looks into the high thesignal power, halfthe thermal noise power contrib- output impedance ofthe preceding stage, the signal is utesto the phase modulationof the signal. Thus the low pass filtered. A second stage of filteringis per- spectraldensity of phase fluctuations due to formedby the transistor Q2 inthe same way. It noise of the amplifier is is also possible for a signalto propagate from the outputto input through the bias chain. Transmission throughthis path is reduced to the same level as trans- SJf) = S0/2PS (1 1 missionthrough the transistor chain by the cascadedlow 269 pass filters.Typical isolation which is achieved is The bestperformance has been obtained with units which greaterthan 120 dB at 5 MHz degradingto 100 dB at 50 usehot carrier diodes in the ring. Some differences may MHz. alsoresult from the type of transformers in the coupling circuits. The noiseobserved at the output of the mixer, The common emitterinput stage determines the col- consistingof mixer and amplifiercontributions, is nearly lectorcurrent of the transistors. The27n dc emitter constantover a range ofinput power level. However, the resistanceproduces an average collector current of outputsignal, proportional to the phase fluctuations, 40 mA. Noiseperformance isgenerally best when the increaseswith the drive power. The bestsignal-to-noise amplifieroperates well within the class A region.With ratiofor Fourier frequencies in the white noise region a 50n loadthis amplifier can produce an output of nearly isobtained at very high drive levels. For one type of 1 V (13 dBm) with minimum distortion. The gainof the mixer,using a singlediode in each arm, thebest noise amprlfieris determined by theload resistance and the floor was obtainedwith approximately 30 mA rms current unbypassedportion, 27n, ofthe emitter resistance. With at each input.This drive level exceeded themanufac- thevalues shown, thefull. output swing occurs for an turer's maximum drivecurrent specification. The optimum inputof approximately 1.5 Vptp. driveis not necessarily the same forall Fourier fre- quencies. The same mixerperformed best. below 40 Hz at The white noise floor which onewould estimate for lowerdrive level. Since such a doublebalanced thisamplifier is S+ = -184 dB. Themeasured noise mixer is a dynamicimpedance theaverage drive current floor is shownas curve A ofFigure 2. The noisefloor does notsufficiently describe the operating conditions. appears to beonly -174 dB, butsince this level cor- Theoptimum method of coupling to the mixer also depends responds to the measurementsystem noise it canonly be upon theoutput impedanceof the signal source. Although saidthat the amplifier is not worse than this. The theuse of 50n pads toattenuate the drive level is tradi- measured flickgr phase noise of the amplifier is tional, a seriesresistor whose valueis chosen to set S = rad /f. Thisperformanre level reachedis thedesired current often gives superior performance. btcauseeach transistor has a reasonable amount of local The improvementswhich are observed may bedue to reduced RF negativefeedback. The emittersof Q1 and Q2 both ringing of the drive currents. lookinto the high dynamic impedance ofthe preceding stage while the emitter of Q3 hasthe unbypassed 270 The signal-to-noise ratio at the mixer output is also resistor.For a givenapplication, this unbypassed re- affectedby the type of termination used. Since the mixer sistorshould be made as largeas possible, limited has a lowoutput impedance, near 500, thedc termination onlyby the necessity of having full output voltage mustbe high impedance compared to 50R. Failureto ob- swing. servethis may result in 6 dB or more lossin signal level. However, it hasbeen determined empirically that the mixer In additionto achieving low noise levels it is mustbe terminated differently at F!F. In thecircuit shown .!n necessaryto minimize microphonics and pickup of power- Figure 4 theimpedance to ground at the output of the mixer linefrequencies and othersignals. For this reason, no is 1 kR at dcand approximately 50R at 10 MHz. The net usehas been made of filter inductors or coupling trans- result of the high drive level and theoutput termination formers. It isalso possible that temperature changes is illustrated in Figure 5, which shows thebeat frequency could cause sufficient collector current variation to betweenthe two oscillators in Figure 4 atlow drive degradethe flicker performance. As a result,the level(sinewave) and high drive level (clipped waveform). diodehas been included inthe bias chain to further The slopeof the clipped waveform at the zero crossings is stabilizethe collector current. It shouldbe placed in morethan twice the slope
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