IC design for wireless system Lecture 6 Dr. Ahmed H. Madian Ahmed.madian@guc.edu.eg 1 outlines Introduction to mixers Mixer metrics Mixer topologies Mixer performance analysis Mixer design issues Dr. Ahmed H. Madian-IC for wireless systems 1
Mixer topologies Discrete implementations: Single-diode and diode-ring mixers IC implementations: MOSFET passive mixer Active mixers Gilbert-cell based mixer Square law mixer Sub-sampling mixer Harmonic mixer Dr. Ahmed H. Madian-IC for wireless systems 3 CMOS Passive Mixer R S V LO M1 M V LO V IF V LO M3 M4 V LO M1 through M4 act as switches Dr. Ahmed H. Madian-IC for wireless systems 4
CMOS Passive Mixer Use switches to perform the mixing operation No bias current required Allows low power operation to be achieved Dr. Ahmed H. Madian-IC for wireless systems 5 CMOS Passive Mixer RF- IF RF+ Same idea, redrawn RC filter not shown IF amplifier can be frequency selective [*] T. Lee Dr. Ahmed H. Madian-IC for wireless systems 6 3
CMOS Passive Mixer I M1 t V LO t LO RF V OUT t Vout IF GC V RF RF 4 4 4 4 Vout VRF. Cos RFt Cos LOt Cos 3 LOt Cos 5 LOt... 3 5 Dr. Ahmed H. Madian-IC for wireless systems 7 CMOS Passive Mixer Non-50% duty cycle of LO results in no DC offsets!! I M1 t DC-term of LO V LO t V OUT LO RF t 4 4 4 Vout VRF. Cos RFt DC Cos LOt Cos 3 LOt Cos 5 LOt... 3 5 Dr. Ahmed H. Madian-IC for wireless systems 8 4
CMOS Passive Mixer with Biasing V LO 00 V LO VLO V S R 00 S Cbias V gg 1nF R gg M1 V LO C V bias LO 1nF RL k C L M R sd V sd ' M ' M 1 R gg R sd Cbias 1nF Dr. Ahmed H. Madian-IC for wireless systems 9 A Highly Linear CMOS Mixer Transistors are alternated between the off and triode regions by the LO signal RF signal varies resistance of channel when in triode Large bias required on RF inputs to achieve triode operation High linearity achieved, but very poor noise figure Dr. Ahmed H. Madian-IC for wireless systems 10 5
Simple Switching Mixer (Single Balanced Mixer) The transistor M1 converts the RF voltage signal to the current signal. Transistors M and M3 commute the current between the two branches. V LO V RF M V out M1 I DC I M3 RF V LO Dr. Ahmed H. Madian-IC for wireless systems 11 Single balanced active mixer, BJT Single-ended input Differential LO Differential output provides gain for v in and steer the current back and forth at LO LOv in + DC v out = ±g m v in Dr. Ahmed H. Madian-IC for wireless systems 1 6
Double Balanced Mixer V OUT V LO M M3 V LO M M3 V LO I DC I I I RF DC RF V RF V RF Strong IF feed suppressed by double balanced mixer. All the even harmonics cancelled. All the odd harmonics doubled (including the signal). Dr. Ahmed H. Madian-IC for wireless systems 13 Gilbert Mixer Use a differential pair to achieve the transconductor implementation This is the preferred mixer implementation for most radio systems! Dr. Ahmed H. Madian-IC for wireless systems 14 7
Double balanced mixer, BJT Basically two SB mixers One gets +v in /, the other gets v in / Q 3 Q 4 1 + v in - Dr. Ahmed H. Madian-IC for wireless systems 15 Mixers based on MOS square law C large 0 I K. V V ds SQ GSQ T R b V LO V BB1 V RF I K. V V V V ds SQ bias RF LO T 0 K. V V V V V V. V V SQ bias T 0 RF LO bias T 0 RF LO ( V RF V cos( LO ) gives rise to ) t and cos( ) t RF LO RF LO Dr. Ahmed H. Madian-IC for wireless systems 16 8
Practical Square Law Mixers C large I. ds KSQ VGSQ VT 0 R b C large V RF V BB1 I BIAS V LO Dr. Ahmed H. Madian-IC for wireless systems 17 Practical Bipolar Mixer VBE C VT large IC ICO. e R b C large V RF V BB1 I BIAS V LO Dr. Ahmed H. Madian-IC for wireless systems 18 9
MOSFET Mixer (with impedance matching) V DD C match IF Filter V BB R S C larg e L g I K. V V ds SQ GSQ T 0 R b O V RF V BB1 C larg e Matching Network L e V LO Dr. Ahmed H. Madian-IC for wireless systems 19 Sub-sampling Mixer Properly designed track-and-hold circuit works as subsampling mixer. The sampling clock s jitter must be very small Noise folding leads to large mixer noise figure. High linearity Dr. Ahmed H. Madian-IC for wireless systems 0 10
Harmonic Mixer Emitter-coupled BJTs work as two limiters. Odd symmetry suppress even order distortion eg LO selfmixing. Small RF signal modulates zero crossing of large LO signal. Harmonic mixer has low self-mixing DC offset, very attractive for direct conversion application. The RF signal will mix with the second harmonic of the LO. So the LO can run at half rate, which makes VCO design easier. Because of the harmonic mixing, conversion gain is usually small Dr. Ahmed H. Madian-IC for wireless systems 1 Features of Square Law Mixers Noise Figure: The square law MOSFET mixer can be designed to have very low noise figure. Linearity: true square law MOSFET mixer produces only DC, original tones, difference, and sum tones The corresponding BJT mixer produces a host of non-linear components due to the exponential function Power Dissipation: The square law mixer can be designed with very low power dissipation. Power Gain: Reasonable power gain can be achieved through the use of square law mixers. Isolation: Square law mixers offer poor isolation from LO to RF port. This is by far the biggest short coming of the square law mixers. Dr. Ahmed H. Madian-IC for wireless systems 11
Mixer performance analysis Analyze major metrics Conversion gain Port isolation Noise figure/factor Linearity, IIP3 Gain insights into design constraints and compromise Dr. Ahmed H. Madian-IC for wireless systems 3 Single-ended input Differential LO Differential output provides gain for v in and steer the current left and right at LO LOv in + DC Dr. Ahmed H. Madian-IC for wireless systems 4 1
Conversion gain Two output component: v out1 = ±g m v in v out = ±I QB DC IF signal is the RF LO component in v out1 v in + DC So gain =? Dr. Ahmed H. Madian-IC for wireless systems 5 Port isolation At what frequency is V out switching? v out = ±I QB DC v out = SW( LO )I QB DC v in + DC This is feed through from LO to output Dr. Ahmed H. Madian-IC for wireless systems 6 13
Port isolation How about LO to RF? This feed through is much smaller than LO to output LOv in + DC Dr. Ahmed H. Madian-IC for wireless systems 7 Port isolation How about RF to LO? If LO is generating a square wave signal, its output impedance is very small, resulting in small feed through from RF to LO to output. LOv in + DC Dr. Ahmed H. Madian-IC for wireless systems 8 14
Port isolation What about RF to output? Ideally, contribution to output is: SW( LO )*g m v in What can go wrong and cause an RF component at the output? v in + DC Dr. Ahmed H. Madian-IC for wireless systems 9 Noise Components: 1. Noise due to loads. Noise due to the input transistor ( ) 3. Noise due to switches ( and ) Dr. Ahmed H. Madian-IC for wireless systems 30 15
1. Noise due to loads: Each contributes v RL = 4kT f Since they are uncorrelated with each other, their noise power s add Total contribution of s: v orl = 8kT f Dr. Ahmed H. Madian-IC for wireless systems 31. Noise due input transistor (the transducer): From BJT device model, equivalent input noise voltage of a CE amplifier is: v in 1 g CE 4kT rb f m Dr. Ahmed H. Madian-IC for wireless systems 3 16
. Noise due to input transistor: If this is a differential amplifier, QB noise would be common mode But Q1 and Q just switching, the noise just appears at either terminal of out: gain vin v B v in(ce) out, Q CE Dr. Ahmed H. Madian-IC for wireless systems 33. Noise due to input transistor: Noise at the two terminals dependent? v v out, Q B out, Q B gain vince 1 g g R 4kT r f m L b m v in(ce) Dr. Ahmed H. Madian-IC for wireless systems 34 17
Total Noise due to RL and QB: If we assume rb is very small: vt g 8 1 mr ktrl f 4 L When: r b << 1/(g m ) and Dr. Ahmed H. Madian-IC for wireless systems 35 3. What about the noise due to switches? When Q is off and Q1 is on, acting like a cascode or more like a resistor if LO is strong Can show that Q1 s noise has little effect on v out V E1 ~V C1, V BE1 has similar noise as V C1, which cause jitter in the time for Q1 to turn off if the edges of LO are not infinitely steep Dr. Ahmed H. Madian-IC for wireless systems 36 18
3. What about the noise due to switches: Transition time jitter in the switching signal: no noise noise Effect is quite complex, quantitative analysis later Dr. Ahmed H. Madian-IC for wireless systems 37 How to improve Noise Figure of mixer: Reduce RL Increase gm and reduce r b of Faster switches Steeper rise or fall edge in LO Less jitter in LO Dr. Ahmed H. Madian-IC for wireless systems 38 19
IP3: The CE input transistor ( ) converts v in to I in BJTs cause 3 rd -order harmonics Multiplying by RL is linear operation Q1 & Q only modulate the frequency I IP3 mixer = IP3 CE s Vbe->I QB I e I 1 (1 v 1 1...) ( VBB vin)/ vt 3 s DC in in 3 in vt v 6v t t Dr. Ahmed H. Madian-IC for wireless systems 39 v v Double Balanced Mixer Basically two CE mixers One gets +v in /, the other gets v in / Q 3 Q 4 1 + v in - Dr. Ahmed H. Madian-IC for wireless systems 40 0
Double Balanced Mixer +1 V C C R R -1 Local Oscillator L L v out = g m v in Q1 Q Q3 Q4 v out = g m v in QB1 + v in - QB Dr. Ahmed H. Madian-IC for wireless systems 41 Double Balanced Mixer Benefits: Fully Differential No output signal at LO Three stages: CE input stages Switches Output load Dr. Ahmed H. Madian-IC for wireless systems 4 1
Double Balanced Mixer Noise: Suppose QB1 & QB give similar total gm Similar to CE Mixer IP3: Similar Taylor series expansion of transconductor transistors Vin split between two Q s, it can double before reaching the same level of nonlinearity IIP3 improved by 3 db Dr. Ahmed H. Madian-IC for wireless systems 43 1 + v in - Q 3 Q 4 Common Base Mixers Similar operation to CE mixers Different input stage is CB Slightly different output noise Different CB input noise Better linearity v in V Bias I DC Dr. Ahmed H. Madian-IC for wireless systems 44
Mixer Improvements De-biasing switches from input transistors: To lower NF we want high g m, but low and current Conflicting! We can set low I Switches and high I Qb using a current source c g R NF 4 m L 1 1 gmrlrs Dr. Ahmed H. Madian-IC for wireless systems 45 I difference LOv in + DC Q1 + out - QB Q I Switches I Qb MOS Single Balanced Mixer V LO The transistor M1 converts the RF voltage signal to the current signal. Transistors M and M3 commute the current between the two branches. V RF M V out M1 I DC M3 I RF V LO Dr. Ahmed H. Madian-IC for wireless systems 46 3