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Software Defined Radio

Lec 3RF Front-End for SDR

Sajjad Hussain,

MCS-NUST

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Outline for Todays Lecture

RF implementation issues Purpose of RF Front End

Dynamic Range

RF Receiver Front-End Topologies

Enhanced Flexibility of RF Front-End with SDRs

Importance of Components in Over All performance

Noise and Distortion in RF

ADC/DAC distortionUse of MEMS for Flexible RF design

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Purpose of RF FrontEnd RX

Filtering of Unwanted Signals

Down-conversion

Amplification

ADC

TX DAC

Up-conversion

Power Amplification

Bandwidth Limitation

Need to have a balance between different componentsADC and

RF dynamic range DSP engr. must be aware of limitations of RF frontend to compensate

them in DSP

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SDRRF FrontEnd

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RF Front End- Basic Functions

Objectives of RF Frontend are Reject as many undesired signals as possible

Amplification of the deisred signal to the range of ADC with minimaldistortion

Minimize AWGN

Achieve a dynamic range which is compatible to that of ADC

Must separate the desired signal (-70 to -130 dBm) from the background RFenvironment (0 to -20 dBm) sets the system SNR

Achieving adequate dynamic range is one of the central issues in RF design

Overall sys. must have considerable dynamic range to accommodate high powerbackground signals to low power desired signals

The wider the BW, more the interference and noise and hence difficult to achievehigh dynamic range

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Dynamic Range Key design challenge in RF front-end

Measure of highest and lowest level signals that can be simultaneouslyaccommodated by radio

Strong relationship b/w battery consumption and dynamic range importanttradeoff for mobile systems

Limited by physical characteristics of various components

Improvements

Variable approaches to RF part design (proper selection of componentsand good circuit design techniques)

DSP algorithms after ADC

Good initial RF designs low interference and dynamic rangeconstraints in subsequent systems

Dynamic Range Lower bound -- AWGN sources (thermal noise, ADC quantization, jitter

etc)

Upper boundinterference (co-channel, adjacent channel, self-inducedetc.)

Attenuation of high-level interference signals to avoid non-linearities

presence of low-level desired signals Presence of DC bias

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Dynamic Range

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RF Receiver Front-End Topologies

A no. of different RF front-end topologies exist each withits own advantages and disadvantages

Most common Dual Conversion

Single Conversion

Tuned Radio Freq. receivers Selection of a topology depends on

Sensitivity

Selectivity

Stability

Dynamic Range Spurious Response

Scalability

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TRF

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RF RX Topologies - TRF

TRF (Tuned Radio Freq.) topology BPF, LNA, AGC

BPFfilter quality factor of 107for 30 kHz signal at 900MHz with 60dB attenuation for channel 60 kHz away

ADC directly samples RF input

Constraints in practical TRF transceiver ADC for high freq. signals

high power consumption with high sample-rate

Requires high dynamic range of 100 dB for wide BW

Extreme demands on tunable RF filter to remove interferences in

the dynamic range Advantageminimal no. of analog parts required

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Single-Mixer

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RF RX TopologiesDirect Conversion

Homodyne, Zero-IF Single mixing stagedirect conversion to baseband

Channel selection and ADC at baseband

Mixers have high power consumption

Low power consumption for 1-stage mixing can be traded-off forhigh dynamic range

Isolation b/w Local Oscillator (LO) and input ports isdesirable but difficult to achieve Capacitive coupling

Tracking and feedback of DC error Error due to non-matching of I and Q branch phase and

amplitudes Eliminate with DSP

Disadvantage as compared with TRF

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Homodyne

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RF RX TopologiesHeterodyne Receiver

Most common RF Front-end for radios Freq. translating the incoming signal to an IF that

is fixed and independent of Fc

BW

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Hetererodyne ReceiversImage Problem

Downconversion also

leads to upconversion

of a part of band

To mitigate this, an

image filter precedes

the mixer to suppress

the low-freq

interfering band60-80 dB attenuation

Careful freq. planning

to relieve filter req.

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RF RX TopologiesComparison

Tradeoff of sensitivity vs. selectivity TRF receivermore suitable for SDR

Filter requirements make the multi-mode operationdifficult

Retuning Complex interaction between multiple RF components

Simpler the RF chain more predictable response after re-tuning

Factors to consider : channel spacing, freq. plan,spurious response, total gain etc.

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Enhanced Flexibility of RF with SDR

Ways in which software based tuning of RFcomponents can be incorporated in classical RF

chain

MixersBiasing and phase distortion can be run-time

tuned by software

AmplifiersSophisticated power management

strategies in softwareCyclic On/Off for TDMA

systems

DSP based diversity combining

Software based IQ extraction and channelization

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Importance of Components on Overall

performance

Important for the DSP engineer tounderstand basic radio components andassociated distortiontradeoff exists incomplexity of RF and baseband chain

Antennas :

Underrated component in the overall link

Much of the link gain can be gained or lost in

the selection of antennaFor multi-mode support of SDR, antenna

design is of crucial importance

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performance

Antennas :Most antennas support BW of about 10% of Fc

Hard to support of multiple cellular (900 MHz and 2

GHz) with single antenna

Several antennas may be required increase

in size

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Components - Duplexers

Duplexers/DiplexersRF filters adding isolation between

transmitting and receiving bandseveral

orders of difference b/w power of TX and RX

expensive devices

Challenges for SDRduplexers + diplexers

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performance

RF Filters : Used for rejecting out-of-band interference

Also help isolate the receiver from transmitter

Should have small noise, low loss, provide selectivity

without compromise on BW Low Noise Amplifier : Boosts power in compatible range of other components

Should maximize gain -> tradeoff with power consumption

Induction of limited noise -> first stage in the RF chainultimately sets the noise performance of the system

Image Reject and IF Filters - Induction of low noise because of amplification in downstream

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performance

RF Mixer :

Used for down-conversion and can be a major source of inter-modulation

distortionNon-linear device

Increasing LO power can reduce non-linearity at the cost of increased powerconsumption

Local Oscillator :

Should have good tuning range and low phase noise

Multiplying a received signal by a noisy LO is equivalent in the frequency

domain of convolving their two spectra, producing a widened resulting signal

spectrum

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AGC

AGC

Ensures that signal has a voltage compatible with that of ADC inputrange

In some cases, it is advantageous to implement AGC as series of

amplifiers strategically placed in the circuit with gain that can be turned

on or off via software to keep circuit operating at ideal power levels for

variable range and types of signals Difficult to use in wide (multi-) band systems

Weak signals in noise and clipping of strong signals

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AGC Circuit

Compression Ratio (M)= change in input level in dB/ change in output level in dB

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Digital AGC and Operating Modes

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ADC

ADC

Most difficult component to select and places the most constraints on systemdesignbiggest power consumer in RX

If perfect ADCs available, TRF architecture would be chosen

Generally tradeoff b/w sampling-rate, dynamic-range, ADC resolution and

power consumption

Currently SDR implementations are for base-station applications because ofhigh power consumption of the ADC

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TX Architectures

Tends to be less complex than RX High power consumption in talk mode

Dual conversion TX is more practical due to betterisolation properties at cost of more expense and power

consumption Use of complex signal processing techs.

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TX Components on Overall performance

RF and IF VCOs : -- good phase noise characteristics +

los power consumption

Mixer / Upconverter : -- good linear characteristics to

reduce spurious products

IQ Modulators - should be well matched to avoid

constellation distortion

Power amplifier - should be wideband, linear and low

noise

tradeoff between linearity and power-efficiency

AMPS (Class B60%), IS-95 (Class AB- 30-45%), GSM (ClassBC40-50%)

In practice less than 25% battery power is effectively used during

transmission

In full duplex system, leakage of TX noise in RX circuitry

sensitivity reduced

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Noise and Distortion in RF Chain

limiting factors for a transceiver performancequantifying noise and distortion is

necessary to quantify transceiver performance

Noise Characterization

Source 1 - Thermal noise in resistive components

Antenna represents first source of noise

Source 2- ADC (thermal + quantization noise)

Error because of finite precision binary representation

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Noise Figure

Noise Figure (NF) describes how much noise isadded by different elements of receiver chain

Most common definition NF = SNRin/SNRout

NF provides indication how device degrades SNR.

Device manufacturers supplies the NF

NF total can be calculated by referring all the NFsback to the antenna

Once total NF is determined, sensitivity level of theRX can be determined for a minimal SNR

Keep analog components noise contribution less

than ADCs noise contribution is a good practice

S (dBm)=Noise floor (dBm)+SNRmin(dB)

Noise floor (dBm)=10log(kTeB)+NFtotaldB

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Noise FigureComponents Placement

Best to have LNA placed as early in the system as

possible because of its high gain

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Noise and Distortion in RF Chain

Distortion Characterization

Distortion occurs because of non-linearities in

system

It takes the form of harmonics

Cross-modulation distortion

Weak signal and strong interferer enters a non-linearity

amplitude variations

Inter- modulation distortion Multiple-signals in the non-linear device interact in a

mixing process to create signals at sum freqs.

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Distortion - ADC-DAC distortion

ADC/DAC introduce both noise (thermal/

quantization) and distortion

Distortion due to aperture jitter

If the signal level exceeds max level of ADC

results in non-linear distortion requirement

of AGC

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Distortion - Pre-distortion

Predistortion:

for good spectral efficiency, pulse shaping is used

non-constant envelope Class A amplifiers with high linear range but low efficiency

class AB,B,C amplifiers with better power efficiency bu non-

linearitiesspectral broadening

Predistortion is done before power amplification to

avoid the spectral broadening such that output of

amplifier is the ideal output

Analog /digital (better-tunable)

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Pre-distortion

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Use of MEMS in RF

Micro-mechanical components to add flexibility

and low loss at RF

Use of miniaturized/micro mechanical devices

Low loss wide bandwidth switches

Variable capacitors/inductors/varactors/ highquality factor filters/ tuners / reconfigurable

antennas

High level of sophistication and reliability with low

cost and power consumption because of IC

based fabrication

a solution for needed flexibility in RF

Reduces interference = dynamic range

requirements on componentslow power

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MEMS

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Conclusion

SDRs must consider the impact of imperfections in RF

Limitation removal by downstream DSP

Better interference removal filters

pre-distortion for non-linear power amps

software flexibility of gains

tradeoff of sampling-rate with resolution for ADC

etc.

Adaptive filtering of harmonics

Bottleneck of RF can be removed by early sampling

but constrained by ADC tech.cost/power

Documents

Lec3-SDR09-