12_Mixers_1

Bhaskar Banerjee, EERF 6330, Sp2013, UTD

Mixers - I

Prof. Bhaskar Banerjee

EERF 6330- RF IC Design

Bhaskar Banerjee, EERF 6330, Sp2013, UTD 2

Outline

General Considerations Specifications

Performance parameters: Gain, Noise, Linearity. Port-port feedthrough/isolation.

Mixer Topologies Active Mixers Passive Mixers

Mixer Configurations Single Balanced Double Balanced Mixers

Reading: RF Microelectronics by Razavi The Design of CMOS RFIC by Thomas Lee


General Considerations Mixers perform frequency translation by multiplying two waveforms (and possibly

their harmonics).

The LO port of this mixer is very nonlinear. The RF port, of course, must remain sufficiently linear to satisfy the compression and/or intermodulation requirements.


Mixer operation

Translation of frequency Upconversion Downconversion

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Sidebands

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Multiplier

Multiplier Operation

SIF(t)SRF(t)

SLO(t)fRF

fLO

fRF-fLO fRF+fLOf

f

f


Mixer Operation Principles

Switch Operation RF signal appearing at the IF load is interrupted by the

switching action of the diode, which is caused by LO IF is the product of the switching waveform S(t) and the RF

input, making these mixers a type of multiplier


Specifications

Gain perspective Conversion gain Gain compression (P1dB)

Linearity perspective 3rd-order Intercept Point (IP3)

Noise perspective Noise figure (NF)

Dynamic range Spurious-free dynamic range (SFDR)

Port isolation Port return loss


Conversion Gain

Conversion gain Power Ratio of the IF output to the RF input

Down conversion mixer Should provide sufficient power gain to compensate for IF filter loss

and noise contribution from IF stage Too much gain may saturate mixer output

Power gain


Conversion Gain

Mixer Conversion Gain

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Noise in Mixers Noise figure (NF)

SSB NF Assumes signal input from only one sideband, but noise inputs from

both sidebands Applicable to heterodyne architecture

For simplicity, let us consider a noiseless mixer with unity gain:

The mixer exhibits a flat frequency response at its input from the image band to the signal band.

The noise figure of a noiseless mixer is 3 dB. This quantity is called the single-sideband (SSB) noise.


Noise in Mixers Noise figure (NF)

DSB NF Includes both signal and noise inputs from both sidebands Applicable to homodyne (direct conversion) architectures

In this case, only the noise in the signal band is translated to the baseband, thereby yielding equal input and output SNRs if the mixer is noiseless.

The noise figure is thus equal to 0 dB. This quantity is called the double-sideband (DSB) noise figure


Noise in Mixers

The mixer output noise is at a different frequency than the input noise

The mixer gain and noise are difficult to measure accurately SSB NF = DSB NF + 3dB (ideally) NF of a mixer tends to be higher than an amplifier because

noise from other frequencies mix down to the IF (in spite of filtering)

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Noise Behavior in Heterodyne Receiver ()

A student designs the heterodyne receiver shown below for two cases: (1) LO1 is far from RF ; (2) LO1 lies inside the band and so does the image. Study the noise behavior of the receiver in the two cases.

In the first case, the selectivity of the antenna, the BPF, and the LNA suppresses the thermal noise in the image band. Of course, the RF mixer still folds its own noise. The overall behavior is illustrated below, where SA denotes the noise spectrum at the output of the LNA and Smix the noise in the inputnetwork of the mixer itself. Thus, the mixer downconverts three significant noise components to IF: the amplified noise of the antenna and the LNA around RF , its own noise around RF , and its image noise around im.

Solution:


Noise Behavior in Heterodyne Receiver ()

A student designs the heterodyne receiver shown below for two cases: (1) LO1 is far from RF ; (2) LO1 lies inside the band and so does the image. Study the noise behavior of the receiver in the two cases.

In the second case, the noise produced by the antenna, the BPF, and the LNA exhibits a flat spectrum fromthe image frequency to the signal frequency. As shown on the right, the RF mixer now downconverts four significant noise components to IF: the output noise of the LNA around RF and im, and the input noise of the mixer around RF and im. We therefore conclude that the noise figure of the second frequency plan is substantially higher than that of the first. In fact, if the noise contributed by the mixer is much less than that contributed by the LNA, the noise figure penalty reaches 3 dB. The low-IF receivers of Chapter 4, on the other hand, do not suffer from this drawback because they employ image rejection.

Solution:


NF of Direct-Conversion ReceiversIt is difficult to define a noise figure for receivers that translate the signal to a zero IF.

This is the most common NF definition for direct-conversion receivers. The SNR in the final combined output would serve as a more accurate

measure of the noise performance, but it depends on the modulation scheme.


Distortion

Receiver: Characterized by both IM2 and IM3 IM3: Intermodulation between two undesired signals whose

amplitude is typically large due to 3rd order non-linearity in RF or baseband sections of the mixer creates direct interference with the desired signal.

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Distortion

IM2: In a zero-IF (DCR), the 2nd order non-linearity results in an envelope of multiple interferers to the baseband.

As the signal level rises, the non-linearity effects become a concern for all receiver blocks up to the channel select filter.

Transmitter: Nonlinearity in the baseband and RF sections of the upconversion mixer produces in-band spurious tones.

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Distortion

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Owing to device capacitances, mixers suffer from unwanted coupling (feedthrough) from one port to another.

In figure above, the gate-source and gate-drain capacitances create feedthrough fromthe LO port to the RF and IF ports.

In the direct-conversion receiver:LO-RF feedthrough is entirely determined by the symmetry of the mixer circuit and LO waveforms.

The LO-IF feedthrough is heavily suppressed by the baseband low-pass filter(s).

Port-to-Port Feedthrough/Isolation



Port isolation Amount of the leakage power between mixer ports (LO to IF, LO to RF, and

RF to IF) because mixer is not perfectly unilateral LO-to-RF feedthrough

LO signal leaking through antenna Should be small enough to avoid corrupting other RF systems

LO-to-IF Not important for Rx mixer High Q IF filter rejects it But large LO and RF signals at IF can saturate IF output port leading to

poor P1dB RF-to-IF isolation

Very critical for Rx Mixer (DCR) Sets linearity requirements (IM2) of the LNA!

LNA & Mixer on same package LO can feedthrough to RF input of LNA by passing RF filter and LNA


Port return loss RF and LO port

Typically, matched to 50 Typically, require more than 10dB return loss Necessary to avoid excessive signal reflection causing

Self-mixingDC offset

IF port Matched to that of IF filter Necessary to avoid excessive passband ripple in IF filter



Effect of Feedthrough in Direct-Conversion and Heterodyne RX

A large in-band interferer can couple to the LO and injection-pull it, thereby corrupting the LO spectrum.

The RF-IF feedthrough corrupts the baseband signal by the beat component resulting from even-order distortion in the RF path.

Here, the LO-RF feedthrough is relatively unimportant

The LO-IF feedthrough, becomes serious if IF and LO are too close to allow filtering of the latter.

Direct-Conversion RX:

Heterodyne RX:


Port-to-Port Feedthrough in Half-RF RX

Shown below is a receiver architecture wherein LO = RF /2 so that the RF channel is translated to an IF of RF - LO = LO and subsequently to zero. Study the effect of port-to-port feedthroughs in this architecture.

For the RF mixer, the LO-RF feedthrough is unimportant as it lies at RF/2 and is suppressed. Also, the RF-LO feedthrough is not critical because in-band interferers are far from the LO frequency, creating little injection pulling. The RF-IF feedthrough proves benign because low-frequency beat components appearing at the RF port can be removed by high-pass filtering.

The most critical feedthrough in this architecture is that from the LO port to the IF port of the RF mixer. Since IF = LO, this leakage lies in the center of the IF channel, potentially desensitizing the IF mixers (and producing dc offsets in the baseband.).

The IF mixers also suffer from port-to-port feedthroughs.


Mixer Topologies

Passive mixers Conversion loss, not gain High tolerance to IMD External baluns or transformers needed Low power consumption

Active mixers Conversion gain Active baluns - better for IC implementation More difficulty in achieving good IMD performance Higher power consumption


Mixer Configurations

Single Balanced Mixer Single ended RF signal Differential LO signal Lesser input referred noise for a given power dissipation More susceptible to noise in the LO path

Double balanced mixers Differential LO Differential RF Less even order distortion (ideally)

relaxes the half-IF issue in DCR generally RF is single ended (coming from LNA/Image-reject filter)

one of the RF inputs in the mixer connected directly to bias different propagation times (phase shifts) - finite even order distortions


Mixer Types

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