D1-04_Detection with the DECISION MAKER.pdf - DETECTION WITH THE DECISION MAKER PREPARATION 2 EXPERIMENT 4 the test signal 4 signal regeneration 5 using

D1-04_Detection with the DECISION MAKER.pdf - DETECTION...

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Copyright © 2005 Emona Instruments Pty Ltd D1-04-rev 2.0 - 1 DETECTION WITH THE DETECTION WITH THE DETECTION WITH THE DETECTION WITH THE DECISION MAKER DECISION MAKER DECISION MAKER PREPARATION ................................................................................ 2 EXPERIMENT .................................................................................. 4 the test signal ............................................................................ 4 signal regeneration .................................................................... 5 using the COMPARATOR .................................................................... 6 signal levels ........................................................................................... 6 DC threshold ......................................................................................... 6 using the DECISION MAKER: ............................................................. 6 error counting ........................................................................... 8 summing up .............................................................................. 8 TUTORIAL QUESTIONS ................................................................. 8
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D1-04 - 2 Copyright © 2005 Emona Instruments Pty Ltd DETECTION WITH THE DETECTION WITH THE DETECTION WITH THE DETECTION WITH THE DECISION MAKER DECISION MAKER DECISION MAKER ACHIEVEMENTS: familiarization with the DECISION MAKER module, to be used in later experiments. Demonstration of the superiority of a gated detector compared with a simple comparator. PREREQUISITES: familiarity with the SEQUENCE GENERATOR and eye patterns; completion of the experiment entitled PRBS generation in this Volume. EXTRA MODULES : DECISION MAKER. PREPARAT PREPARATION ION ION The shape of a binary sequence waveform is affected by transmission through a noisy, bandlimited channel. Since the aim of a transmission system is to deliver a perfect copy of the input to the output, the received sequence needs to be restored to its original shape, or regenerated . The regeneration process is performed by what we will call a detector . You should refer to your text book to see the wide range of circuits which has been devised for performing this regeneration process, from the simple amplitude limiter, or the comparator, to extremely sophisticated and intelligent schemes. As an example of moderate wave shaping, refer to Figure 1, which shows the waveshape changes suffered by a sequence after passage through a bandlimited channel. time BANDLIMITED SEQUENCE OUTPUT (showing delay) SEQUENCE BIT CLOCK 0 + - BIPOLAR INPUT Figure 1: waveforms before and after moderate bandlimiting
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Detection with the DECISION MAKER Copyright © 2005 Emona Instruments Pty Ltd D1-04 - 3 If the bandlimited bipolar output waveform of Figure 1 is connected to a comparator, whose reference is zero volts, then the output will be HI (say +V volt) whilst the waveform is above zero volts, and LO (say -V volt) when it is below. Visual inspection leads us to believe that it is unlikely the comparator would make any mistakes. Examination of the eye pattern of the same waveform would confirm this. If there was noise, and perhaps further bandlimiting, the comparator would eventually start to fail. But other, more sophisticated circuits, operating on the same waveform, might still succeed in regenerating a perfect copy of the original. To be fair to the comparator, it is competent to decide whether the signal is above (a HI), or below (a LO) a reference. Provided the signal-to-noise ratio (SNR) is not too low then this will be during the HI and LO bits. As the SNR reduces the comparator output may not match favourably the input, especially as regards pulse width. Finally its performance as a detector will deteriorate, and extra pulses (or ‘bits’) will appear, as indicated in Figure 2 below. But note that it is still operating
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  • Spring '17
  • karish
  • Clock signal, ........., decision maker, Oscilloscope

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