˜ 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
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 = 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
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 = 40-60 dB) Demodulator/Detector shifts IF signal down to baseband for further processing ECE 4710: Lecture #18
Common IF Frequencies ECE 4710: Lecture #18 fIF typically in 10-100 MHz range except for AM Radio ECE 4710: Lecture #18
Analog Filter Types ECE 4710: Lecture #18
Analog Filter Types ECE 4710: Lecture #18
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
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
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
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 Appear @ fIF !!! ECE 4710: Lecture #18
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
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
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
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
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 µ ECE 4710: Lecture #18