Presentation on theme: "Communication Circuits Research Group"— Presentation transcript:
1 Communication Circuits Research Group Spring2014RF Systems and CircuitsMixersEmad HegaziProfessor, ECECommunication Circuits Research Group
2 Mixer Design Introduction to mixers Mixer metrics Mixer topologies Spring2014RF Systems and CircuitsMixer DesignIntroduction to mixersMixer metricsMixer topologiesMixer performance analysisMixer design issues
3 What is a mixer Frequency translation device Spring2014RF Systems and CircuitsWhat is a mixerFrequency translation deviceConvert RF frequency to a lower IF or base band for easy signal processing in receiversConvert base band signal or IF frequency to a higher IF or RF frequency for efficient transmission in transmittersCreative use of nonlinearity or time-varianceThese are usually harmful and unwantedThey generates frequencies not present at inputUsed together with appropriate filteringRemove unwanted frequencies
4 Two operation mechanisms Spring2014RF Systems and CircuitsTwo operation mechanismsNonlinear transfer functionUse device nonlinearities creatively!Intermodulation creates the desired frequency and unwanted frequenciesSwitching or samplingA time-varying processPreferred; fewer spursActive mixersPassive mixers
5 The Ideal Mixer If x(t)y(t) x(t) y(t) Then the output is down convert Spring2014RF Systems and CircuitsThe Ideal MixerIfx(t)x(t)y(t)y(t)Then the output isdown convertup convert
6 Is the mixer really a multiplier? Spring2014RF Systems and CircuitsIs the mixer really a multiplier?Not really, in fact, it should not.Why?
7 Commutating switch mixer Spring2014RF Systems and CircuitsCommutating switch mixer
8 Spring2014RF Systems and CircuitsA non-ideal mixer
9 Spring2014RF Systems and CircuitsMixer MetricsConversion gain – lowers noise impact of following stagesNoise Figure – impacts receiver sensitivityPort isolation – want to minimize interaction between the RF, IF, and LO portsLinearity (IIP3) – impacts receiver blocking performanceSpurious responsePower match – want max voltage gain rather than power match for integrated designsPower – want low power dissipationSensitivity to process/temp variations – need to make it manufacturable in high volume
10 Spring2014RF Systems and CircuitsConversion GainConversion gain or loss is the ratio of the desired IF output (voltage or power) to the RF input signal value ( voltage or power).If the input impedance and the load impedance of the mixer are both equal to the source impedance, then the voltage conversion gain and the power conversion gain of the mixer will be the same in dB’s.
11 Noise Figures: SSB vs DSB Spring2014RF Systems and CircuitsNoise Figures: SSB vs DSBSignalbandSignalbandImagebandThermalnoiseThermalnoiseLOLOIFSingle side bandDouble side band
12 Spring2014RF Systems and CircuitsSSB Noise FigureBroadband noise from mixer or front end filter will be located in both image and desired bandsNoise from both image and desired bands will combine in desired channel at IF outputChannel filter cannot remove this
13 DSB Noise Figure For zero IF, there is no image band Spring2014RF Systems and CircuitsDSB Noise FigureFor zero IF, there is no image bandNoise from positive and negative frequencies combine, but the signals combine as wellDSB noise figure is 3 dB lower than SSB noise figureDSB noise figure often quoted since it sounds better
14 Port-to-Port Isolations Spring2014RF Systems and CircuitsPort-to-Port IsolationsIsolationIsolation between RF, LO and IF portsLO/RF and LO/IF isolations are the most important features.Reducing LO leakage to other ports can be solved by filtering.IFRFLO
15 Spring2014RF Systems and CircuitsLO Feed throughFeed through from the LO port to IF output port due to parasitic capacitance, power supply coupling, etc.Often significant due to strong LO output signalIf large, can potentially desensitize the receiver due to the extra dynamic range consumed at the IF outputIf small, can generally be removed by filter at IF output
16 Reverse LO Feed through Spring2014RF Systems and CircuitsReverse LO Feed throughReverse feed through from the LO port to RF input port due to parasitic capacitance, etc.If large, and LNA doesn’t provide adequate isolation, then LO energy can leak out of antenna and violate emission standards for radioMust insure that isolation to antenna is adequate
17 Self-Mixing of Reverse LO Feedthrough Spring2014RF Systems and CircuitsSelf-Mixing of Reverse LO FeedthroughLO component in the RF input can pass back through the mixer and be modulated by the LO signalDC and 2fo component created at IF outputOf no consequence for a heterodyne system, but can cause problems for homodyne systems (i.e., zero IF)
18 Nonlinearity in Mixers Spring2014RF Systems and CircuitsNonlinearity in MixersIgnoring dynamic effects, three nonlinearities around an ideal mixerNonlinearity A: same impact as LNA nonlinearityNonlinearity B: change the spectrum of LO signalCause additional mixing that must be analyzedChange conversion gain somewhatNonlinearity C: cause self mixing of IF output
19 Focus on Nonlinearity in RF Input Path Spring2014RF Systems and CircuitsFocus on Nonlinearity in RF Input PathNonlinearity B not detrimental in most casesLO signal often a square wave anywayNonlinearity C avoidable with linear loadsNonlinearity A can hamper rejection of interferersCharacterize with IIP3 as with LNA designsUse two-tone test to measure (similar to LNA)
20 Mixer topologies Discrete implementations: IC implementations: Spring2014RF Systems and CircuitsMixer topologiesDiscrete implementations:Single-diode and diode-ring mixersIC implementations:MOSFET passive mixerActive mixersGilbert-cell based mixerSquare law mixerSub-sampling mixerHarmonic mixer
21 Single-diode passive mixer Spring2014RF Systems and CircuitsSingle-diode passive mixerSimplest and oldest passive mixerThe output RLC tank tuned to match IFInput = sum of RF, LO and DC biasNo port isolation and no conversion gain.Extremely useful at very high frequency (millimeter wave band)
22 Single-balanced diode mixer Spring2014RF Systems and CircuitsSingle-balanced diode mixerPoor gainGood LO-IF isolationGood LO-RF isolationPoor RF-IF isolationAttractive for very high frequency applications where transistors are slow.
23 Double-balanced diode mixer Spring2014RF Systems and CircuitsDouble-balanced diode mixerPoor gain (typically -6dB)Good LO-IF LO-RF RF-IF isolationGood linearity and dynamic rangeAttractive for very high frequency applications where transistors are slow.
24 CMOS Passive Mixer Use switches to perform the mixing operation Spring2014RF Systems and CircuitsCMOS Passive MixerUse switches to perform the mixing operationNo bias current requiredAllows low power operation to be achieved
25 CMOS Passive Mixer Non-50% duty cycle of LO results in no DC offsets!! Spring2014RF Systems and CircuitsCMOS Passive MixerNon-50% duty cycle of LO results in no DC offsets!!DC-term of LO
26 A Highly Linear CMOS Mixer Spring2014RF Systems and CircuitsA Highly Linear CMOS MixerTransistors are alternated between the off and triode regions by the LO signalRF signal varies resistance of channel when in triodeLarge bias required on RF inputs to achieve triode operationHigh linearity achieved, but very poor noise figure
27 Simple Switching Mixer (Single Balanced Mixer) Spring2014RF Systems and CircuitsSimple Switching Mixer (Single Balanced Mixer)The transistor M1 converts the RF voltage signal to the current signal.Transistors M2 and M3 commute the current between the two branches.
28 MOS Single Balanced Mixer Spring2014RF Systems and CircuitsMOS Single Balanced MixerThe transistor M1 converts the RF voltage signal to the current signal.Transistors M2 and M3 commute the current between the two branches.
29 MOS Single Balanced Mixer Spring2014RF Systems and CircuitsMOS Single Balanced Mixer
30 MOS Single Balanced Mixer Spring2014RF Systems and CircuitsMOS Single Balanced MixerIF Filter
31 MOS Single Balanced Mixer IF FilterwLORF-wLORF+wLORF-
32 MOS Single Balanced Mixer Spring2014RF Systems and CircuitsMOS Single Balanced MixerRFwSLOwwLORF-
33 Single Balanced Mixer Analysis: Linearity Linearity of the Mixer primarily depends on the linearity of the transducer (I_tail=Gm*V_rf). Inductor Ls helps improve linearity of the transducer.The transducer transistor M1 can be biased in the linear law region to improve the linearity of the Mixer. Unfortunately this results in increasing the noise figure of the mixer (as discussed in LNA design).
34 Single Balanced Mixer Analysis: Linearity Spring2014RF Systems and CircuitsSingle Balanced Mixer Analysis: LinearityUsing the common gate stage as the transducer improves the linearity of the mixer. Unfortunately the approach reduces the gain and increases the noise figure of the mixer.
35 Single Balanced Mixer Analysis: Isolation LO-RF Feed throughThe strong LO easily feeds through and ends up at the RF port in the above architecture especially if the LO does not have a 50% duty cycle. Why?
36 Weak LO-RF Feed through Single Balanced Mixer Analysis: IsolationWeak LO-RF Feed throughThe amplified RF signal from the transducer is passed to the commuting switches through use of a common gate stage ensuring that the mixer operation is unaffected. Adding the common gate stage suppresses the LO-RF feed through.
37 Single Balanced Mixer Analysis: Isolation Spring2014RF Systems and CircuitsSingle Balanced Mixer Analysis: IsolationLO-IF Feed throughThe strong LO-IF feed-through may cause the mixer or the amplifier following the mixer to saturate. It is therefore important to minimize the LO-IF feed-through.
38 Double Balanced Mixer RF Systems and Circuits Spring2014RF Systems and CircuitsDouble Balanced MixerStrong LO-IF feed suppressed by double balanced mixer.All the even harmonics cancelled.All the odd harmonics doubled (including the signal).
39 Double Balanced Mixer The LO feed through cancels. The output voltage due to RF signal doubles.
40 Spring2014RF Systems and CircuitsGilbert MixerUse a differential pair to achieve the transconductor implementationThis is the preferred mixer implementation for most radio systems!
41 Mixers based on MOS square law Spring2014RF Systems and CircuitsMixers based on MOS square law
42 Practical Square Law Mixers Spring2014RF Systems and CircuitsPractical Square Law Mixers
43 Features of CMOS Square Law Mixers Spring2014RF Systems and CircuitsFeatures of CMOS Square Law MixersNoise Figure: very low.Linearity: produces only DC, original tones, difference, and sum tonesThe corresponding BJT mixer produces a lot of non-linear components due to the exponential functionPower Dissipation: The square law mixer can be designed with very low power dissipation.Power Gain: Reasonable power gain can be achieved.Isolation: poor isolation from LO to RF port. This is by far the biggest short coming of the square law mixers.
44 Spring2014RF Systems and CircuitsSub-sampling MixerProperly designed track-and-hold circuit works as sub-sampling mixer.The sampling clock’s jitter must be very smallNoise folding leads to large mixer noise figure.High linearity