Fundamentals of Communications (XE37ZKT), Part I. Superheterodyne, OFDM. 8th

Transcription

1 Page 1 of 17 Fundamentals of Communications (XE37ZKT), Part I Superheterodyne, OFDM Josef Dobeš 8th

2 1. The Simplest AM Crystal Radio Peak Detector Page 2 of 17

3 1. The Simplest AM Crystal Radio Peak Detector Superheterodyne With Single Conversion Fundamental Structure of the Superhet Mixing Process Image Frequency Page 2 of 17

4 1. The Simplest AM Crystal Radio Page 2 of 17 Peak Detector Superheterodyne With Single Conversion Fundamental Structure of the Superhet Mixing Process Image Frequency Downconverting or Upconverting Downconverting Example Upconverting Example

5 1. The Simplest AM Crystal Radio Page 2 of 17 Peak Detector Superheterodyne With Single Conversion Fundamental Structure of the Superhet Mixing Process Image Frequency Downconverting or Upconverting Downconverting Example Upconverting Example Superhet With Dual Conversion

6 Brick Wall Effect Multi Path Reception Typical Transmission Parameters Signal Spectrum in Adjacent Channels Multi Carrier System 64-QAM Used for One Carrier (Gray Code) Page 3 of 17

7 2. The Simplest AM Crystal Radio Page 4 of 17

8 2. The Simplest AM Crystal Radio A principle of the demodulation using the peak detector: Page 4 of Heriot-Watt University, Edinburgh

9 3. Superheterodyne (Superhet) With Single Conversion The diagram of the single conversion superheterodyne is the following: 2 Page 5 of 17 2 J.P. Silver ( Superheterodyne Receiver Tutorial

10 The frequencies produced in the superhet as a result of the mixing process (ω RF RF Signal, ω LO Local Oscillator): Page 6 of 17

11 However, unwanted signals at the RF image frequency being added to the IF and could increase the noise figure: Page 7 of 17

12 4. Downconverting or Upconverting Example: A terrestrial TV tuning receiver is designed to cover the RF frequency range of 45 to 860 MHz, with channel spacings of 8 MHz and an IF of 40 MHz. Assess the downconverting or upconverting receiver solutions relating to potential problems with the image frequency band. Page 8 of 17

13 4. Downconverting or Upconverting Example: A terrestrial TV tuning receiver is designed to cover the RF frequency range of 45 to 860 MHz, with channel spacings of 8 MHz and an IF of 40 MHz. Assess the downconverting or upconverting receiver solutions relating to potential problems with the image frequency band. Downconverting: The local oscillator minimum and maximum frequencies are given by the range and specified IF, i.e. f LOmin = f RFmin + f IF = 45 MHz + 40 MHz = 85 MHz, f LOmax = f RFmax + f IF = 860 MHz + 40 MHz = 900 MHz. Page 8 of 17

14 4. Downconverting or Upconverting Page 8 of 17 Example: A terrestrial TV tuning receiver is designed to cover the RF frequency range of 45 to 860 MHz, with channel spacings of 8 MHz and an IF of 40 MHz. Assess the downconverting or upconverting receiver solutions relating to potential problems with the image frequency band. Downconverting: The local oscillator minimum and maximum frequencies are given by the range and specified IF, i.e. f LOmin = f RFmin + f IF = 45 MHz + 40 MHz = 85 MHz, f LOmax = f RFmax + f IF = 860 MHz + 40 MHz = 900 MHz. Therefore, the resulting frequencies of the images are: f IMmin = f LOmin + f IF = 85 MHz + 40 MHz = 125 MHz, f IMmax = f LOmax + f IF = 900 MHz + 40 MHz = 940 MHz. The location of images is problematic see the diagram.

15 Upconverting: A better way consists in choosing a greater IF. Lets pick an IF frequency of say 1.5 GHz. The required LO will now be (the LO is below the IF now): 3 f LOmin = f IF f RFmax = 1500 MHz 860 MHz = 640 MHz, f LOmax = f IF f RFmin = 1500 MHz 45 MHz = 1455 MHz. Page 9 of 17

16 Upconverting: A better way consists in choosing a greater IF. Lets pick an IF frequency of say 1.5 GHz. The required LO will now be (the LO is below the IF now): 3 f LOmin = f IF f RFmax = 1500 MHz 860 MHz = 640 MHz, f LOmax = f IF f RFmin = 1500 MHz 45 MHz = 1455 MHz. Therefore, the resulting frequencies of the images are f IMmin = f LOmin + f IF = 640 MHz MHz = 2140 MHz, f IMmax = f LOmax + f IF = 1455 MHz MHz = 2955 MHz, which is not problematic now because the image band does not intersect the signal band see the diagram again. Page 9 of 17

17 Upconverting: A better way consists in choosing a greater IF. Lets pick an IF frequency of say 1.5 GHz. The required LO will now be (the LO is below the IF now): 3 Page 9 of 17 f LOmin = f IF f RFmax = 1500 MHz 860 MHz = 640 MHz, f LOmax = f IF f RFmin = 1500 MHz 45 MHz = 1455 MHz. Therefore, the resulting frequencies of the images are f IMmin = f LOmin + f IF = 640 MHz MHz = 2140 MHz, f IMmax = f LOmax + f IF = 1455 MHz MHz = 2955 MHz, which is not problematic now because the image band does not intersect the signal band see the diagram again. As shown, the greater IF causes the suppression of the images. However, the most selective filters can only be realized for lower frequencies. As a result, a superheterodyne with dual conversion can be considered an optimal solution see the superhet with the dual conversion. 3 J.P. Silver ( Superheterodyne Receiver Tutorial

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19 5. Superhet With Dual Conversion Page 11 of 17

20 6. Brick Wall Effect 4 Page 12 of 17 4 ntl:broadcast

21 Multi Path Reception Page 13 of 17

22 Typical Transmission Parameters Page 14 of 17

23 Signal Spectrum in Adjacent Channels Page 15 of 17

24 Multi Carrier System 5 Page 16 of

25 64-QAM Used for One Carrier (Gray Code) Page 17 of 17