ECE 4710: Lecture #18 1

SuperHeterodyne Rx

IFFIlter

˜

Antenna

Low NoiseRF Amp

LPF

BasebandAmplifier

Digital orAnalogOutput

LocalOscillator

Mixer

Station Tuning Circuit

IFAMP

Demod /Detector

DSP

ADC, Bit Detection, Decoding, Adaptive Filter,

Error Correction, etc.

fc

fLO

fc + fLO

fc – fLO

-fc + fLO

-fc – fLO

fIF = -fc + fLO

ECE 4710: Lecture #18 2

SuperHeterodyne Rx

Station Tuner May (optional) adjust center frequency of RF amplifier (LNA)

Adjusts local oscillator (LO) frequency so that fIF = fcfLO always (or fIF

= -fc + fLO)

RF Low Noise Amplifier (LNA) Provides preliminary amplification of signal (gain = 15-25 dB) Bandpass frequency response provides some filtering of adjacent

channels and noise moderate passband >> signal BW Has low noise characteristics so that very little additional noise is

added to received signal by the amplifier

Mixer translates incoming signal from fc to fIF

Note that additional mixer product f ’s also present (e.g. -fcfLO)

ECE 4710: Lecture #18 3

SuperHeterodyne Rx

Intermediate Frequency = fIF

Fixed frequency does NOT depend on fc

IF frequency must be low enough so that high-performance filters and amplifiers can be economically built

IF Filter Fixed BW centered on fIF with very sharp rolloff

Eliminates 1) channels adjacent to fc, 2) higher order mixer products like fc+fLO, and 3) noise outside of desired signal BW

IF Amplifier Provides high gain signal amplification (gain = 40-60 dB)

Demodulator/Detector shifts IF signal down to baseband for further processing

ECE 4710: Lecture #18 4

Common IF Frequencies

fIF typically in 10-100 MHz range except for AM Radio

ECE 4710: Lecture #18 5

Analog Filter Types

ECE 4710: Lecture #18 6

Analog Filter Types

ECE 4710: Lecture #18 7

SuperHeterodyne Rx

Primary Advantages Majority of amplification and filtering performed at fixed

frequency regardless of selected channel Easier and less expensive to design high gain amplifiers

and sharp rolloff filters at IF rather than RF Primary Disadvantage

By introducing IF stage the possibility exists that unwanted signal spectrums will also be shifted to fIF

Image Frequency

injection) side-(low if,2

injection) side-(high if,2

cLOIFc

cLOIFcimage ffff

fffff

ECE 4710: Lecture #18 8

Image Frequency Example

Broadcast AM Radio DSB-LC Station channels from fc = 540–1600 kHz with BW=10 kHz

fIF = 455 kHZ

Local Oscillator frequency range fLO = fc ± fIF

High side injection used so fLO = fc + fIF 1 – 2.1 MHz

Desired Station WJHK has fc1 = 600 kHz

Undesired Station WTGR has fc2 = 1510 kHz

Both stations operating in same area

For desired station fLO = fc1 + fIF = 455 + 600 = 1055 kHz

ECE 4710: Lecture #18 9

Desired WJHK Signal

f (kHz)

fc1 = 600-fc1 = -600 0

WJHK Spectrum@ Rx Input

f (kHz)

fLO = 10550-fLO = -1055

fIF = 455

fIF = 455-fIF = -455 0

DesiredWJHK Spectrum@ Mixer Output

-fc1+ fLO fc1 fLO

-1655 1655

fc1+ fLO -fc1 fLO

ECE 4710: Lecture #18 10

Undesired WTGR Signal

fc1 = 600-fc1 = -600 0

WJHK

fLO = 10550-fLO = -1055

fIF = 455

fIF = 455-fIF = -455 0

Undesired WTGR Spectrum +Desired KJHK Spectrum BOTH

Appear @ fIF !!!

fc2 fLO -fc2+ fLO

fc2 = 1510

fIF = 455

WTGR@ Rx Input @ Rx Input

-fc2 = -1510

ECE 4710: Lecture #18 11

Image Frequency Example

If fc2 station (WTGR) is present along with fc1 station (WJHK) then superheterodyne Rx will receive both simultaneously at mixer output @ fIF

For high side injection fIMAGE = fc1 + 2fIF and for this example fIMAGE = 600 + 2 455 = 1510 = fc2 !!

How is image frequency problem minimized? Choose fIF as large as possible so fIMAGE > largest expected fc

Attenuate image frequency before mixer» Bandpass response of RF amplifier attenuates image frequency a modest

amount

» Image frequency rejection filter sometimes added @ RF before mixer if amplifier rejection is not sufficient

ECE 4710: Lecture #18 12

Zero-IF Rx

˜

Antenna

Low NoiseRF Amp

LPFBasebandAmplifier

Digital orAnalogOutput

LocalOscillator

Mixer

Station Tuning Circuit

DSP

fc

fLO = fc

fIF = 0 !!

ECE 4710: Lecture #18 13

Zero-IF Rx

fc = fLO so that fIF = 0 baseband Direct conversion from RF to baseband Direct conversion Rx or Homodyne Rx alternate names

Primary Advantages No image frequency Baseband hardware is normally all digital with software

control» Easy algorithm update + multiple application use

All baseband signal processing on single DSP chip “system on chip”

ECE 4710: Lecture #18 14

Zero-IF Rx

Primary Disadvantages Poorer noise performance than SuperH Rx

» Semiconductor noise (transistors, diodes, op-amps, etc.) is 1 / f larger noise power at lower frequencies

Less dynamic range than SuperH Rx (ability to detect weak signals in presence of noise)

High performance mixer and DSP required for good performance

Becoming more widely used in wireless applications for mobile units System on chip means low power consumption, mass

production (low manufacturing cost), & small size (good for mobile units)

Rx Noise Power vs. Frequency

ECE 4710: Lecture #18 15

Baseband IF RF

Low Cost Amplifier, Low Cost Filter

Moderate Cost Amplifier,

Low Cost Filter

Expensive Amplifier, Expensive Filter

Fli c

ker

Nois

e P

ower

Frequency

fPnoise

1