1. ˜ SuperHeterodyne Rx ECE 4710: Lecture #18 fc + fLO fc – fLO -fc + fLO
Antenna
Mixer
fc + fLO
fc – fLO
-fc + fLO
-fc – fLO fc
Low Noise
RF Amp IF
FIlter IF
AMP
Demod /
Detector
fLO
fIF = -fc + fLO
LPF
Station Tuning Circuit ˜
Local
Oscillator
Digital or
Analog
Output
DSP
Baseband
Amplifier
ADC, Bit Detection,
Decoding, Adaptive Filter,
Error Correction, etc.
ECE 4710: Lecture #18

2. SuperHeterodyne Rx ECE 4710: Lecture #18 Station Tuner
May (optional) adjust center frequency of RF amplifier (LNA)
Adjusts local oscillator (LO) frequency so that fIF = fcfLO always (or fIF = -fc + fLO)
RF Low Noise Amplifier (LNA)
Provides preliminary amplification of signal (gain = 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. -fcfLO)
ECE 4710: Lecture #18

3. SuperHeterodyne Rx ECE 4710: Lecture #18 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 = dB)
Demodulator/Detector  shifts IF signal down to baseband for further processing
ECE 4710: Lecture #18

4. Common IF Frequencies ECE 4710: Lecture #18
fIF typically in 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 Primary Disadvantage
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
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 = = 1055 kHz
ECE 4710: Lecture #18

9. Desired WJHK Signal f (kHz) f (kHz) ECE 4710: Lecture #18 -fc1 = -600
WJHK Spectrum
@ Rx Input
f (kHz)
fIF = 455
f (kHz)
-fLO = -1055
fLO = 1055
-fc1 fLO
fc1 fLO
-fc1+ fLO
fc1+ fLO
-fIF = -455
fIF = 455
Desired
WJHK Spectrum
@ Mixer Output
-1655
1655
ECE 4710: Lecture #18

10. Undesired WTGR Signal ECE 4710: Lecture #18 -fc2 = -1510 -fc1 = -600
WJHK
WTGR
@ Rx Input
@ Rx Input
fIF = 455
fIF = 455
-fLO = -1055
fLO = 1055
-fc2+ fLO
fc2 fLO
-fIF = -455
fIF = 455
Undesired WTGR Spectrum +
Desired KJHK Spectrum BOTH
fIF !!!
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 fIF
For high side injection fIMAGE = fc1 + 2fIF and for this example fIMAGE =  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 RF before mixer if amplifier rejection is not sufficient
ECE 4710: Lecture #18

12. Station Tuning Circuit
Zero-IF Rx
Antenna
fIF = 0 !! fc
Low Noise
RF Amp
LPF
Baseband
Amplifier
Mixer
DSP
Station Tuning Circuit ˜
Local
Oscillator
fLO = fc
Digital or
Analog
Output
ECE 4710: Lecture #18

13. Zero-IF Rx fc = fLO so that fIF = 0  baseband Primary Advantages
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)
ECE 4710: Lecture #18

15. Rx Noise Power vs. Frequency
Baseband IF RF
Low Cost Amplifier ,
Low Cost Filter
Moderate Cost
Amplifier
Expensive Amplifier
Expensive Filter F l i c k e r N o s P w
Frequency f
noise 1
ECE 4710: Lecture #18