Post on 14-Jan-2016
transcript
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