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PRESENTATION ONRF MIXERSBy:Zeeshan AkhtarM.tech 1st year Communication & Information14lecm-018Mixers are used for frequency conversion and is critical components in modern radio frequency (RF) systems.

A mixer converts RF power at one frequency into power at another frequency to make signal processing easier and also inexpensive.

A fundamental reason for frequency conversion is to allow amplification of the received signal at a frequency other than the RF, or the audio frequency. IntroductionMixer DefinitionsMixers are non-linear devices used in systems to translate one frequency to another. All mixer types work on the principle that a large Local Oscillator (LO) will cause switching/modulating the incoming Radio Frequency (RF) to the Intermediate Frequency (IF) The multiplication process begins by taking two signals:

a = A Sin(w1t + 1) and b= B Sin(w2t + 2) 3The resulting multiplied signal will be:

a.b = AB Sin(w1t + 1) Sin(w2t + 2) This can be multiplied out thus:

SinA SinB = -1/2 [cos(A+B) cos(A-B) ]Where A= (w1t + 1) and B= (w2t + 2)

Mixer Equations

High performance RF mixers use nonlinear characteristics to generate the multiplication. Thus, they also generate lots of undesired output frequencies.

Three techniques have proven to be effective in the implementation of mixers withhigh dynamic range:

1.Use a device that has a known and controlled nonlinearity.

2. Switch the RF signal path on and off at the LO frequency.

3. Sample the RF signal with a sample-hold function at the LO frequency.Image rejection Conversion gain: voltage or power Port-to-port isolationLarge signal performance: gain compression intermodulation distortion : third-orderintercept (TOI)Small signal performance: noise figureOperating range: Spurious-free dynamic rangeMixer Performance SpecificationsMaximize mixer performance by:

Maximize linearity in the signal path

2. Idealize switching: high slew rates

3. Minimize noise contributionsPassive or Active Mixers?

Passive nonlinear devices or switches conversion loss, not gain high tolerance to IMD external transformers needed

Active mixers can provide conversion gain better for IC implementation more difficulty in achieving good IMDperformanceMixer operating mechanisms

Nonlinear transfer function use device nonlinearities creatively! useful at mm-wave frequencies

Switching or sampling a time-varying process preferred; fewer spursNonlinear mixer

Non-Linear Mixer Operation

Mixer OutputWe can see that there are a lot of spurious outputs generated.

Ideally, we would like to see outputs only at10 MHz and 210 MHz.

So, we prefer the switching type mixer when the RF and LO frequencies are low enoughthat we can make decent switches.

Switching or Sampling mixersThis simple switch is operated by the LO. If the LO is a square wave with 50% duty cycle, it is easily represented by its Fourier Series.

The symmetry causes the even-order harmonics to drop out of the LO spectrum. When multiplied by a single frequency cosine at W(rf) the desired sum and difference outputs will be obtained as shown in the next slide.

Mixer OutputIdeal Single-balanced mixer

LO Switching Function T(t)

Output spectrum: SB mixer

The output spectrum of the single-balanced switching mixer is much less cluttered than the nonlinear mixer spectrum.

Ideal Double Balanced MixerAn ideal double balanced mixer consists of a switch driven by the local oscillator that reverses the polarity of the RF input at the LO frequency and a differential transconductance amplifier stage.

The polarity reversing switch and differential IF cancels any output at the RF input frequency since the DC term cancels as was the case for the single balanced design.

To get the highest performance from the mixer we must make the RF to IF path as linear as possible and minimize the switching time of the LO switch.

Double-balanced mixerOutput Spectrum: DB mixer

The polarity switching action can be clearly seen in the output voltage. There is no LO or RF feedthrough in this ideal DB mixer, even with a DC current in the signal path.Mixer circuit examples

Now lets look at some examplesDiode DB quad :familiar and widely usedwide bandwidth, limited by baluns

FET DB quad:not as well known, but good performance

Gilbert multiplier :Very widely used active mixerDiode DB quad mixer

The diode double-balanced quad mixer is a very popular design and available in a wide variety of frequency bandwidths and distortion specs.

The diodes act as a polarity reversing switch as seen in the bottom of the figure.

When the top of the LO transformer is positive, the blue path is conductive and will ground the top of the RF transformer.

When the top is negative, the red path is conducting and the RF polarity reverses.

The IMD performance is very poor with small LO power.DB Switching FET mixer

The channel resistance of a large FET when in its triode region (below saturation) can be quite low and is not as current dependent as the diode. Therefore, switching configurations using FETs can be more linear in the RF to IF path than diode switching mixers.

The conversion loss will be similar to the diode mixer.

Large LO drive voltage is needed (1 to 5 volts)Double-balanced Gilbert mixer

This double-balanced active mixer was first described in the 60s. Barrie Gilbert was awarded the patent.Some Advantages Easily integrated with other circuits

Better isolation than the DB diode or FET mixers

Requires less LO power than the passive mixers

Less fussy about IF termination impedance than the passive mixers due to the better isolation

Its main liability is large signal handling capabilityConclusion Understand operating principles of the mixer

What makes a good mixer?

Choices: nonlinear/switching mode; single/double balance; active/passive

Specify performance: Gain, NF, isolation etc.

Mixer examples - numerous other possibilitiesREFERENCESwww.radio-electronics.com

www.markimicrowave.com

B. Gilbert "A precise four quadrant multiplier with subnanosecond response",IEEE J. Solid-State Circuits, vol. SC-3, no. 12, pp.365 -373 1968

M. D. Jamal and A. F. TorCMOS RF Modeling, Characterization and Applications, 2002 :World Sci.

Thanku For Listening

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