ELEN 701 RF & Microwave Systems Engineering

Lecture 2September 27, 2006Dr. Michael ThorburnSanta Clara University

Lecture 2 – Radio Architecture and Design Considerations, Part I

• Architecture– Superheterodyne Architecture

• Configuration• RF/IF/BB Sections

– Direct Conversion Architecture• Configuration

• Functional Description of Fundamental Elements – Duplexers,LNAs,Filters, Mixers, LO Synthesizers

• Key Performance Parameters and Analyses– Frequency Planning – Spurious Analysis– Receiver Sensitivity, Linearity and Selectivity

• Analysis Techniques – Nonlinear Systems– Noise Figure

Superheterodyne Full-Duplex Architecture Configuration

Mixing incoming signal with an offset LO in a nonlinear device to generate an intermediate IF signal

Key Nonlinear device: Frequency Mixer or Frequency Converter

Major Elements and Performance Parameters

• Receiver – Sensitivity– Linearity– Selectivity

• Transmitter (next week)– Output Power– Spectrum– Modulation Accuracy

Antenna & Duplexer

Transmit Signal869-894 MHz

Receive Signal824-849 MHz

Band Pass Filter – Receive Band

Band Pass Filter – Transmit Band

Antenna

RF Front End

Receive Signal824-849 MHz

LNA is important in achieving good reception sensitivity – gain is step controlled to cope with receiver dynamic range

Bandpass Filter - High Edge to further suppress transmit; Low Edge for image rejection

UHF Synthesizer provides LO power to RF converters in RCV and TX. It also has role of channel tuning for the transceiver

Nonlinear characteristics of mixer generates spurious frequencies which must be managed

RF Amp establishes NF of Receiver

IF Section

IF Section@ IF Frequency

DeMod

IF Amp provides front end gain for IF Section

VGA establishes signal level for DeMod

BasebandSection

IF SAW Filter has high selectivity for channel filtering to separate desired from unwanted channels

Direct Conversion Architecture Configuration

Comparison of Architectures -Partitioned by Function

• Superheterodyne– RF Section

• LNA• RF BPF• RFA

– Frequency Conversion• RF Downconverter (Mixer)• LO

– IF Section• IFA• IF SAW• VGA

– Frequency Conversion• IQ Demod

– Baseband (I & Q channels)• LPF• BBA• ADC

• Direct Conversion– RF Section

• LNA• RF BPF• RFA

– Frequency Conversion• I/Q Demod/ Downconverter• UHF Synthesizer

– Baseband (I & Q channels)• BBA• LPF• BBA• ADC

Comparison of Architectures –Partitioned by Units

• Superheterodyne– Receiver

• LNA• RF BPF• RFA• RF Mixer• LO• IFA• IF SAW

– Demod• VGA• I/Q Demodulator• BB LPF

– Baseband (I & Q channels)• BBA• ADC

• Direct Conversion– RF Section

• LNA• RF BPF

– RF Demod• RFA• I/Q Demod/ Downconverter• UHF Synthesizer• BBA• LPF

– Baseband (I & Q channels)• BBA• ADC

Next Steps: Design Process• Architecture - Functional Block Diagram• Design/Performance

– Frequency Planning• Selection of IF frequency to minimize self interference

– Spurious Analysis

– Line-up Analysis• Signal Strength (Gain/Loss) Budgets• Determine Required Gains

– Receiver Sensitivity• Minimum detectable desired signal strength to obtain a

certain BER or frame error rate (FER)• Noise Figure

Design Process

– Receiver Linearity– Receiver Selectivity

• Characteristic of the receiver that allows it to identify the desired signal at one frequency apart from those at all other frequencies.

– Mainly determined by RF, IF and BB filters!

Key Functional Blocks

• Duplexer• Low Noise Amplifier• Band Pass Filters• Mixer (Downconverter)• LO Synthesizer• IF Section• Demodulator

Duplexer

• Key Functions– Connects Receive (Rx) from Transmit (Tx)

signal paths to antenna– Provides separation of power in Rx and Tx

paths – in particular providing rejection of Txin Rx path

Low Noise Amplifier (LNA)

• Key Function– First stage of signal amplification– Establishes Noise Figure (NF) of System

• Primary Factor• Receiver and Demod may degrade NF and should

be tracked– Thereby establishing Transceiver Sensitivity

RF Band Pass Filter

• Key Functions– Provides additional rejection of transmit band

(high band edge)– Provides rejection of mixer image frequencies

and other spurious signals (low band edge)

• Note: LNA is typically very broad band. Pre-LNA filtering required to protect LNA from overdrive. Pre-Receiver filtering is required to protect Receiver from other unwanted signals – Spurs (Spurious Signals)

Mixer/Downconverter• Key Function

– Converts from receive RF frequency to IF frequency• Input to Mixer is RF signal and LO signal• Output is IF signal

• General Concept:– In frequency domain: RF – LO = IF– Spurs include:

• Image Frequency: RF + LO• Mixing Spurs: m x RF +/- n x LO• Harmonics: m x RF• LO Harmonics: n x LO

Mixer

( )))cos(())cos((21)cos()cos( tttt LORFLORFLORF ωωωωωω −++=×

Nonlinear Device:

Desired Function:

Actual Function:

...)()()()()()()( 52

42

321 +++++= txtxatxatxatxatxaty LORFLORFLORF

LO Synthsizer

• Key Function:– Provides LO– Via LO selection (Superheterodyne) – selects

channels

IF Section

• Key Function:– Establishes power level and filtering for

Demod and Baseband Section

Demodulator

• Demodulates IF signal in preparation of Analog-to-Digital Converter

Receiver Sensitivity

• Figure of Merit is G/T• Antenna Gain - Gives G• T has an Antenna Component and a Transceiver

Component

Feeder Input Loss,1/LF

+

Equivalent InputNoise Temp, TF

Feeder Network

TF

Noise-freesystem, Gain GRX

+

Equivalent InputNoise Temp, TRx

Receiver

TRx

Antenna NoiseTemp TA

T1 = TA + TF (LF -1) + [ TRx / (1/LF) ]

= TA + TF (LF -1) + [TRx * LF ]

T2T1

Note: LF > 1

T2 = TA / LF+ TF (LF -1) / LF + TRx

System Noise Figure & Noise Temperature

LNA Noise Figure and Gain are critically important to System Noise Temperature

output

input

SNRSNR

F =Noise Figure:

GAIN = GSignal S1

Noise N1Signal S2Noise N2

System Noise Figure

GAIN = G

Signal S1

Noise N1

Self Noise No

Signal G x S1Noise G x (N1+No)

1

011N

NNF ++=

System Noise Figure and Noise Temperature

Feeder Input Loss,1/LF

+

Equivalent InputNoise Temp, TF

Feeder Network

TF

Noise-freesystem, Gain GRX

+

Equivalent InputNoise Temp, TRx

Receiver

TRx

Antenna NoiseTemp TA

T1 = TA + TF (LF -1) + [ TRx / (1/LF) ]

= TA + TF (LF -1) + [TRx * LF ]

T2T1

Note: LF > 1

T2 = TA / LF+ TF (LF -1) / LF + TRx

LNA Noise Figure and Gain are critically important to System Noise Temperature ( ) 01 TFTe −=

Trades between Noise Figure and Linearity

• Best Noise Figure– Lump Gain in Front End

• Best Linearity– Distribute Gain Evenly

Cellular Spectrum Example

• Uplink (Forward): 824 – 849 MHz• Downlink (Return): 869 – 894 MHz

• Band Separation = 20 MHz (=869-849)• Channel Spacing = 30 kHz (CDMA)

Uplink Channel Downlink Channel869 MHz 894 MHz849 MHz824 MHz

Frequency Plan

Uplink Channel Downlink Channel869 MHz 894 MHz849 MHz824 MHz

MHzFMHzIF

MHzIF

RxTx

Tx

Rx

4536.130

36.85

=Δ==

−