RF Microelectronics. Hanyang University. Oscillator. Changsik Yoo. Div. Electrical and Computer Eng. Hanyang University.

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1 RF Microelectronics Oscillator Changsik Yoo Div. Electrical an Computer Eng. anyang University.

2 Barkausen s Criterion RF oscillators can be viewe as a feeback circuit with frequency selective network. X(s) (s) Y(s) Y ( s) X ( s) ( s) ( s) β ( s) β(s) Resonator For oscillation to occur at, Barkausen s criterion ( j ) β ( j ) In RF oscillators, a frequency selective network (resonator) LC tank is inclue to stabilize the oscillation frequency.

3 Oscillation frequency Determine by LC resonator. Oscillation amplitue Oscillation Waveform Self-sustaining effect allows the circuit s noise to grow initially, but as the amplitue increases, the loop gain rops an thus the oscillation amplitue is limite. Desire output waveform of an RF oscillator Noise contribute by switching transistors in mixer is minimize if LO waveform exhibits abrupt transitions. Direct feethrough from RF input to IF output is suppresse when LO has 5% uty cycle. Thus, the ieal waveform is square wave. Practical LO waveform is sinusoi, but if the amplitue is large, it approximates a square wave in the vicinity of the zero crossings. (see Fig. 7.4) For 5% uty cycle, ifferential topologies are esirable. 3

4 Basic LC Oscillator Topologies Feeback to emitter or base? Degrae Q of resonator 4

5 Colpitts an artley Oscillator Passive impeance transformation Colpitts an artley oscillator r L eq C eq R eff ( + C C ) g m R eff ( + L L ) g m Colpitts artley 5

6 Selection of Element Values for Resonator Q of C >> Q of L an therefore Q of resonator is etermine by L. Equivalent parallel resistance of LC tank is given as ; ( ) S R where R S moels the loss of inuctor. P Leq r R For large voltage swing, large inuctor value is esirable since L eq an R S scale proportionally in typical inuctors. When L eq increases by m, so oes R P. Larger value of inuctor, however, Decrease self-resonant frequency of inuctor Tank capacitance shoul be ecrease, approaching the limit set by evice parasitics, which makes it ifficult to vary the oscillation frequency by aing a variable capacitor to the oscillator. 6

7 Voltage Controlle Varactor ioes ae to a LC tank Control voltage shoul be limite so the varactor ioe be reverse biase. If forwar biase, Q of LC tank is severely egrae. t y ( t) A cos RF t + K VCO vcont ( t) t K VCO If vcont ( t) Vm cos mt y( t) A cos t + V t RF m sin m m igh-frequency component appearing at its control input is rejecte. 7 If K V VCO m << ra, narrow ban FM approximation applies, an the output spectrum consists of the main component at RF an two siebans at ±. RF m

8 Phase Noise Noise in oscillators Amplitue noise is negligible if the amplitue is sufficiently large. Phase noise is critical ; ranom variation of zero crossing points. ( t + φ ( )) x( t) A cos t If φ ( t) n C << ra, x(t) n ranom phase noise A cos C t Aφ ( t) sin The spectrum of φ ( t ) is translate to ± n n C. C t Ieal oscillator Actual oscillator C Phase noise is quantifie at an offset from the carrier C. C If carrier power is -Bm an the phase noise power is -7Bm when measure in kz banwith at an offset of Mz, the phase noise is {-7Bm-log(kz/z)}-(-Bm)-98Bc/z. 8

9 Effect of Phase Noise in RF Communication Receiver with ieal oscillator Reciprocal mixing Phase noise of Tx oscillator can corrupt the Rx sensitivity of ajacent channel users 9

10 Calculation of Allowe Phase Noise 3kz 6B P f n, tot f L S n ( f ) f ; total noise power in the wante channel Assuming Sn( f ) is constant an equal to S, Pn, tot S( f Psig SNR, for SNR 5B; S f f ) ( L ( S Psig ) 5B log( f f ). ( S P ) 5B log( f f ) 6B log L log L L f ). ( ) Since log P sig 5B log(3kz) 6B -B/z P 6B, Phase noise shoul be less than -Bc/z at 6kz offset.

11 Q of an Oscillator Rea Section 7.4. an unerstan the various efinition of Q an the meaning of loae Q.

12 anyang University Phase Noise in Signal Path () ) ( ) ( ) ( ) ( s s s X s Y j j + + ) ( ) ( oscillation, In the vicinity of an typically ) ( << j ( ) ( ) ( ) j j j j X j j Y ) ( ) ( ) ( ) ( ( ) { }. multiplie by is Noise component at +

13 3 anyang University Phase Noise in Signal Path () Noise spectrum is shape as shown below ; ( ) { } ( ) j X Y + φ φ φ j j e j e + ) ( φ +

14 4 anyang University Phase Noise in Signal Path (3) oscillation. vicinity of in the & In LC oscillator, << φ φ φ + ( ) ( ) 4 ) ( φ φ j X Y 4 Q φ Q igher Q of LC resonator gives ; () Better noise shaping function () Lower power issipation

15 Phase Noise in Control Path 5 Noise on the control voltage ; V cosnt Narrowban FM approximation AV nkvco vout ( t) A cos t + { cos( + n ) t cos( n ) t} n ( KVCO m ) Vm Noise power at two siebans ; 4 K VCO is proportional to the carrier frequency, meaning more noise for higher frequency. This type of noise becomes more prominent as making /f noise in the control path harmful. m n ecreases,

16 Noise-Power Trae-Off 6 If the output voltages of N ientical oscillators are ae in phase, total carrier power is multiplie by N whereas the noise power increases by N (assuming noise is uncorrelate among oscillators). Thus, the phase noise ecreases by a factor N at the cost of increase of power issipation. For fair comparison of phase noise, power issipation shoul be consiere.

17 Frequency Division an Phase Noise x( t) A cos { t + φ ( )} n t Divie by N gives ; x cos N / N ( t) A φn ( t) t + N From narrowban FM approximation, phase noise power /N 7

18 Oscillator Pulling 8 Large interferer close to LO frequency Power amplifier on the same substrate as oscillator Power supply noise ; especially when power amplifier is turne on an off.

Bipolar an CMOS LC As technology scales own, lower

VCO much more ifficult because Lower supply voltage

19 Bipolar an CMOS LC As technology scales own, lower supply voltage shoul be use which makes the esign of VCO much more ifficult because Lower supply voltage means lower voltage swing, thereby yieling higher relative phase noise. Reverse bias voltage range of varactor ioe becomes narrower, thus limiting the frequency tuning range. 9

20 Negative-Gm Oscillator Rin g m Negative-Gm compensates for the loss in the resonator. If the voltage swing excees about 4mV, the bipolar transistors enter saturation region. Capacitive voltage ivier

21 Variations of MOS

22 Noise in CMOS LC Oscillator

23 Tuning Issues Large swing at noes X an Y for small relative phase noise V cont value is limite, resulting in small tuning range Smaller inuctance gives larger tuning range. But, lower voltage swing an thus larger relative phase noise 3

24 Interpolative () s) α ( s) + α ( s) ( For stable oscillation, (s). GmRL s GmRLs α + α R L C s + L s + R R L C s + L s + R This circuit oscillates at a single frequency if the above equation has a unique set of conjugate roots on the imaginary axis or in the right half plane. 4

25 Interpolative () If the resonant frequencies of two tanks, an, are apart ; Two frequencies where the total phase shift is zero an the gain is greater than unity, meaning the circuit may oscillate at more than one frequency. 5 The maximum spacing between an that avois this effect is ; + Q Interpolative oscillators also have trae-off between the phase noise an tuning range. Main concern of interpolative oscillator Noise contribution from large number of active evices But, combination of capacitor tuning an interpolation may provie a wie tuning range.

26 Monolithic Inuctors Critical parameters of monolithic inuctors Q (~), self-resonant frequency, an silicon area Loss mechanisms in monolithic inuctors Discontinuous not to increase magnetic coupling Conuctive plate reuces capacitive coupling. 6

27 RC-CR Network Quarature Signal Generation Phase ifference is 9 o for all frequencies. Output amplitues are equal only at /RC. In LO path, information lies in only the zero crossing points an the amplitues can be equalize by limiting amplifier ; Amplitue limiting is not easy at Gz. Phase (an gain) mismatch of the chains of limiters in I an Q paths becomes critical. AM-PM istortion ; unequal input amplitue exhibits unequal phase shift. 7

28 Frequency Division Quarature Signal Generation LO with 5% uty cycle LO LO quarature in phase phase LO Generation of requires substantial power or may be simply impossible ue to technology limitations. Input uty shoul be 5%. 8

29 Quarature Oscillator Quarature Signal Generation 9 Q φ φ In single phase oscillator, is maximum at zero phase shift an thus Q is maximum at oscillation frequency. In couple oscillator, the phase shift in each oscillator is nonzero, meaning smaller an thus smaller Q. φ

Understanding VCO Concepts

Understanding VCO Concepts OSCILLATOR FUNDAMENTALS An oscillator circuit can be modeled as shown in Figure 1 as the combination of an amplifier with gain A (jω) and a feedback network β (jω), having frequency-dependent