ENSC327 Communications Systems 5: Frequency Translation (3.6) and Superhet Receiver (3.9)
Jie Liang School of 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 f 1 to a signal centered around f 2 (f 2 is usually lower than f 1). Used in AM & FM radio (to bring signals from different stations to the same intermediate freq f2 for demodulation) Implementation by a mixer : The mixer includes a product modulator and a band-pass filter. How to choose the local frequency ( flo ) of the mixer?
f1 f2 lo f 3 High-side tuning and low-side tuning = ± Local freq has two choices: flo f1 f2 (Assume f1 > f2)
Assume the message is: s1(t) = m(t)cos(2π 1tf ) Product output:
A BPF can be used to select the component with freq f2.
If flo = f1 + f2 : The system is called high-side tuning or high-side injection .
If flo = f1 − f2 : The system is called low-side tuning or low-side injection .
However, this is not the end of the story, because another input
frequency can be translated to f2 , which is called image signal. 4 Image Signal
If flo = f1 + f2 , then an signal at freq f1 + 2 f2 can also be moved to f2:
Example : if f1 = 1000 kHz, f2 = 455 kHz,
Then for high-side tuning, Desired Local Input flo = freq The image freq is at f + f1 f1 f2
The undesired image signal at f 1 + 2f 2 should be filtered out
before the product modulator. 5 Image Signal
If flo = f1 − f2 , then an input at freq f1 − 2 f2 can also be moved to f2:
Case 1: f1 > 2 f2 :
Example : if f1 = 1000 kHz, f2 = 455 kHz,
Then for low-side tuning, Undesired Local Desired image freq freq Input flo = The image freq is at f − 2 − f1 f2 f1 f2 f1
The undesired image signal at f 1 – 2f 2 should be filter out. 6 Image Signal
If flo = f1 − f2 , then an input with freq f1 − 2 f2 will be moved to f2:
Case 2: f1 < 2 f2 :
Example : if f1 = 555 kHz, f2 = 455 kHz,
The for low-side tuning: flo = and the image freq is at
The undesired image signal at 2f 2 - f1 should be filter out. 7 How to Remove Image Signal
Solution: Filter out the unwanted image signal before the mixer. The center frequency of the RF filter is at the desired frequency. The filter is called RF filter . The RF filter and local frequency are changed at the same time.
8 Outline
Frequency translation (page 128) Superhet Receiver (Page 142)
Early AM receivers: Tune the receiver to capture the desired input signal (a natural choice) Expensive to build narrow-band filters at high frequencies Even worse, this filter must be tunable Superheterodyne : supersonic heterodyne receiver Heterodyne: mix or translate frequency Short name: superhet A breakthrough in radio broadcast history Invented by Edwin H. Armstrong in 1918 The idea: Translate the input signal to a fixed frequency at the receiver. The majority of the circuits can be fixed. 10 Superheterodyne Receiver
Desired: f f RF IF Local Freq:
flo = fRF + f IF Image Freq:
fRF + 2 fIF The input signal is translated to a fixed intermediate frequency (IF) by a mixer . Only the Local Oscillator (LO) freq is changed when we tune the radio. The fixed intermediate frequency (IF): 455kHz for AM radio , 10.7MHz for FM radio Bandwidth: 10kHz in AM radio, and 200k Hz for FM radio. The IF filter is not tunable , can be made to have good selectivity Usually made of quartz crystal. Example: Monolithic crystal filters Can be implemented digitally The RF filter at the beginning can have wider bandwidth than IF filter. 11 Superheterodyne Receiver Correction to the book: Equation (3.46) on Page 143 should be:
flo = f RF + f 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 for local oscillator. This is easier to implement (by variable capacitor). AM station frequency: 540 kHz ~ 1600 kHz (BW is 10 kHz) The IF freq: 455kHz The range of local frequency for low-side tuning:
Tuning ratio = max freq / min freq = The range of local frequency for high-side tuning:
Tuning ratio = Easier to implement. 13 Applications of Superhet Receiver The Spectrum Analyzer is essentially an electronically tuned Supherhet receiver
http://cp.literature.agilent.com/litweb/pdf/5952-0292.pdf 14