ScienTECH ST2115, Руководство пользователя

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ST2115
Scientech Technologies Pvt. Ltd. 32
Procedure:
1. Switch data switches to 1 or 0 as per your choice of binary data pattern.
2. Connect any two of the four taps viz. A, B, C or D to the inputs of EX-OR
gate of PN Sequence generator. Connect 240 KHz clock signal on board to the
clock input of the PN sequence generator.
3. Now switch ‘On’ the power supply and observe the output of Binary Data
generator and PN sequence generator. Since the data generator frequency used
here is 30 KHz and that of PN Sequence Generator is 240 KHz, and hence
there are 8 PN sequence bits per Data bits for spreading the binary signal.
4. Change the positions of taps for feedback in the PN Sequence Generator block
to obtain deferent patterns of the PN sequences. Switch ‘Off’ and then ‘On’
the power supply to reload the changes, if changes do not appear in the output
on changing the tap positions.
5. Connect output of binary data generator to one of the inputs of Direct
sequence spread spectrum generator input.
6. Connect output of PN sequence generator to the other input of DSSS Ex-OR
gate.
7. Now turn ‘On’ power supply and observe the output of DSSS generator block.
This is our DSSS signal.
8. Now connect output of this DSSS bock to the one of the input of Ex-OR gate
of Data Recovery block. Connect the same output of PN sequence generator,
which we have taken for spreading to the other input of this recovery gate for
despreading. Note that the PN sequence used for despreading is taken from
the same output pin where from the PN sequence is taken for spreading the
signal. This is because of the fact that there is complete synchronization
between the spreaded signal and PN sequence. In other words there is not any
significant delay involved in spreading process.
9. Observe the output of this data recovery block. This is recovered output
without almost any error.
10. Now change the tape positions of shift registers (A, B, C or D) to get a new
PN Sequence and repeat the above process again. Thus you will observe that
with each different sequence we are quit able to recover the original data. Also
with different PN sequences, the modulated (Spreaded) data looks different
i.e. we can recover the data if and only if we are using the same PN sequence
for both modulation and demodulation. This is the reason that this DSSS
technique has a large potential for being a multiple access technique. This
multiple access technique is known as “Code Division Multiple Access”
(CDMA) technique.
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