Modulation (5): FM receiver

Transcription

1 Modulation (5): FM receiver Luiz DaSilva Professor of Telecommunications Adapted from material by Dr Nicola Marchetti

2 Superheterodyne receiver q The term superheterodyne refers to a method of designing and building wireless communications or broadcast equipment, particularly radio receivers q To heterodyne means to mix two frequencies together so as to produce a beat frequency, namely the difference between the two q The term superheterodyne refers to creating a beat frequency that is lower than the one of the original signal 2

3 Mixing q The idea of the superhet revolves around the process of mixing, i.e., multiplying two signals together q When two signals are multiplied together the output contains signals at frequencies other than the two input frequencies q New signals are seen at frequencies that are the sum and difference of the two input signals, i.e. if the two input frequencies are f1 and f2, then new signals are seen at frequencies of (f1+f2) and (f1- f2) q We have already come across this in the context of AM modulation 3

4 Block diagram 4

5 RF amplifier q Signals enter the front end circuitry from the antenna. This contains the front end tuning for the superhet to amplify the signals before they enter the mixer q The level of this amplification is carefully calculated so that it does not overload the mixer when strong signals are present, but enables the signals to be amplified sufficiently to ensure a good signal to noise ratio is achieved 5

6 Local oscillator q The tuned and amplified signal then enters one port of the mixer. The local oscillator signal enters the other port q The local oscillator may consist of a variable frequency oscillator that can be tuned by altering the setting on a variable capacitor q Alternatively it may be a frequency synthesizer that will enable greater levels of stability and setting accuracy 6

7 Spectral diagram (after the mixer) That s why you have a bandpass filter a:er the mixer 7

8 IF stage q Superheterodyne receivers reduce the signal frequency by mixing in a signal from a local oscillator to produce the intermediate frequency (IF) q This stage contains most of the amplification in the receiver as well as the filtering that enables signals on one frequency to be separated from those on another q Filters may consist simply of LC transformers, or they may be much higher performance ceramic or even crystal filters, dependent upon what is required 8

9 Demodulation q Once the signals have passed through the IF stage of the superheterodyne receiver, they need to be demodulated q Different demodulators are required for different types of transmission, and as a result some receivers may have a variety of demodulators that can be switched in to accommodate the different types of transmission that are to be encountered q The output from the demodulator is the recovered signal (audio, but not only). If audio, it is amplified (and presented to the headphones or loudspeaker) 9

10 Frequency isolation q The ability to isolate signals, or reject unwanted ones, is a function of the receiver bandwidth. In real life, there are frequently sources that can interfere with your signal q Frequency regulators (e.g., ComReg) make frequency assignments that generally prevent this. Depending on the application, you might have a need for very narrow signal isolation. If the performance of your band-pass filter isn't sufficient to accomplish this, the performance can be improved by superheterodyning 10

11 Capture effect q In FM receivers, the demodulator will only extract zero axis crossings of the strongest of competing signals q e.g., if two signals have nearly equal strength, the stronger of the two will be captured" while rejecting the other q That is why when listening to a distant FM station and driving away from it and approaching another FM station on the same frequency, the stations will "bounce back and forth," hearing one then the other, never both at the same time q It can happen so rapidly that sometimes it sounds as if they are both being demodulated simultaneously 11

12 Intermodulation q Intermodulation occurs when the input to a nonlinear system contains two or more frequencies q Consider an input signal that contains frequency components f a, f b, and f c x( t) a ( 2π f t + ϕ ) + M sin( 2πf t + ϕ ) + M sin( πf t + ϕ ) = M sin 2 a a b q Suppose we obtain our output signal y(t) by passing the input through a non-linear function: b [ ( )] y ( t) = G x t b c c c 12

13 Intermodulation (cont d) q y(t) will contain components at frequencies f a, f b, and f c, known as the fundamental frequencies, as well as components at linear combinations of those frequencies, of the form k a f a + k b f b + k c f c q Here, k a, k b, and k c are arbitrary integers that can assume both positive and negative values q These are called intermodulation products (IMPs) 13

14 Intermodulation distortion q Intermodulation distortion (IMD) is a measure of the linearity of amplifiers, mixers, and other RF components q A way of measuring the intermodulation is by feeding two signals with a small frequency difference into the device-under-test q The second-order products fall at frequencies which can be easily removed by digital filters q However, some of the third-order products, e.g., 2f 2 f 1 and 2f 1 f 2 are close to the original signals and are therefore more difficult to filter 14

15 Intermodulation products 15