ENSC327 Communications Systems 5: Frequency Translation (3.6) and Superhet Receiver (3.9) Jie Liang School o Engineering Science Simon Fraser University 1
Outline Frequency translation (page 128) Superhet Receiver (Page 142) 2
Frequency Translation (Page 128) Frequency translation: Translate a signal centered around 1 to a signal centered around 2 (2 is usually lower than 1). Used in AM & FM radio (to bring signals rom dierent stations to the same intermediate req 2 or demodulation) Implementation by a mixer: The mixer includes a product modulator and a band-pass ilter. How to choose the local requency (lo) o the mixer? 1 lo 2 3
High-side tuning and low-side tuning Local req has two choices: Assume the message is: Product output: lo = 1 ± 2 (Assume 1 > 2) ( t) s1( t) = m( t)cos 2π 1 A BPF can be used to select the component with req 2. I I lo lo = = 1 1 + 2 2 : : The system is called high-side tuning or high-side injection. The system is called low-side tuning or low-side injection. However, this is not the end o the story, because another input requency can be translated to 2, which is called image signal. 4
Image Signal I lo = 1+ 2, then an signal at req 1+ 22 can also be moved to 2 : Example: i 1 = 1000 khz, 2 = 455 khz, Then or high-side tuning, lo = The image req is at Desired Input 1 Local req + 1 2 The undesired image signal at 1 + 22 beore the product modulator. should be iltered out 5
Image Signal I lo = 1 2, then an input at req 1 22 Case 1: 1 > 2 2 : can also be moved to 2 : Example: i 1 = 1000 khz, 2 = 455 khz, Then or low-side tuning, lo = Undesired image req Local req Desired Input The image req is at 1 2 2 1 2 1 The undesired image signal at 1 22 should be ilter out. 6
Image Signal I lo = 1 2, then an input with req 1 22 Case 2: 1 < 2 2 : will be moved to 2 : Example: i 1 = 555 khz, 2 = 455 khz, The or low-side tuning: lo = and the image req is at The undesired image signal at 22-1 should be ilter out. 7
How to Remove Image Signal Solution: Filter out the unwanted image signal beore the mixer. The center requency o the RF ilter is at the desired requency. The ilter is called RF ilter. The RF ilter and local requency are changed at the same time. 8
Outline Frequency translation (page 128) Superhet Receiver (Page 142) 9
Superheterodyne Receiver Early AM receivers: Tune the receiver to capture the desired input signal (a natural choice) Expensive to build narrow-band ilters at high requencies Even worse, this ilter must be tunable Superheterodyne: supersonic heterodyne receiver Heterodyne: mix or translate requency Short name: superhet A breakthrough in radio broadcast history Invented by Edwin H. Armstrong in 1918 The idea: Translate the input signal to a ixed requency at the receiver. The majority o the circuits can be ixed. 10
Superheterodyne Receiver Desired: RF Local Freq: = + lo RF Image Freq: RF+ 2 IF IF The input signal is translated to a ixed intermediate requency (IF) by a mixer. Only the Local Oscillator (LO) req is changed when we tune the radio. The ixed intermediate requency (IF): 455kHz or AM radio, 10.7MHz or FM radio Bandwidth: 10kHz in AM radio, and 200k Hz or FM radio. The IF ilter is not tunable, can be made to have good selectivity Usually made o quartz crystal. Example: Monolithic crystal ilters Can be implemented digitally 11 The RF ilter at the beginning can have wider bandwidth than IF ilter. IF
Superheterodyne Receiver Correction to the book: Equation (3.46) on Page 143 should be: = + lo RF IF Because high-side tuning is used in superheterodyne AM radio receiver. 12
Superheterodyne Receiver Why use high-side tuning? High-side tuning leads to smaller tuning ratio or local oscillator. This is easier to implement (by variable capacitor). AM station requency: 540 khz ~ 1600 khz (BW is 10 khz) The IF req: 455kHz The range o local requency or low-side tuning: Tuning ratio = max req / min req = The range o local requency or high-side tuning: Tuning ratio = Easier to implement. 13
Applications o Superhet Receiver The Spectrum Analyzer is essentially an electronically tuned Supherhet receiver http://cp.literature.agilent.com/litweb/pd/5952-0292.pd 14