Post on 25-Sep-2019
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Thực tập chuyên đề
http://coltech.vnu.edu.vn/~tantd/chuyende.htm
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5 bài Viễn thông
• Fax
• Điều chế số
• PCM
• PAM-PPM-PWM
• PAM-4TDM
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C¸c §iÒu chÕ sè
ASK – FSK – PSK – QPSK – QAM
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Điều chế số (ASK,FSK,PSK...)
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• ASK kho¸ dÞch chuyÓn biªn ®é.
• FSK kho¸ dÞch chuyÓn tÇn sè.
• PSK kho¸ dÞch chuyÓn pha, víi nhiÒu lo¹i kh¸c nhau:– 2 pha hay c¬ sè 2 (2 PSK hay BPSK);
– 4 pha hay vu«ng pha (4 PSK hay QPSK) :
– 8 pha hay 16 pha (8 PSK hay 16 PSK)...
– TrÞ gi¸ tuyÖt ®èi hoÆc vi sai
• QAM §iÒu chÕ biªn ®é trùc giao
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ASK kho¸ dÞch chuyÓn biªn ®é.
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FSK kho¸ dÞch chuyÓn tÇn sè
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Kho¸ dÞch chuyÓn pha PSK
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4 -Pha (4 -PSK).
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H×nh 1. 8: S¬ ®å gi¶i ®iÒu chÕ QPSK
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kh«i phôc sãng mang tõ tÝn hiÖuQPSK
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§iÒu chÕ biªn ®é vu«ng pha (QAM).
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Kh«i phôc nhÞp cña d÷ liÖu
Kho¸ pha PLL
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M· ho¸ Manchester
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Bµi sè 2
®iÒu chÕ XUNG MÃ
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PCM
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Formatting and transmission of baseband signal
EncodeTransmit
PulsemodulateSample Quantize
Demodulate/Detect
Channel
Receive
Low-passfilter
Decode
PulsewaveformsBit stream
Format
Format
Digital info.
Textual info.
Analog info.
Textual info.
Analog info.
Digital info.
source
sink
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Sampling demo.(Speech properties and aliasing)
Voiced signal
Unvoiced signal
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• Giíi thiÖu nh÷ng kh¸i niÖm chung cña ®iÒu chÕPCM.
• M« t¶ c¸c lo¹i m· ho¸ PCM kh¸c nhau.
• M« t¶ viÖc truyÒn c¸c tÝn hiÖu xung trªn c¸c kªnh cã d¶i h¹n chÕ.
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• TÝn hiÖu analog lèi vµo ®i qua mét bé läc anti –aliasing th«ng thÊp vµ tíi bé lÊy mÉu.
• Bé lîng tö ho¸ tiÕp theo sÏ quy ®Þnh mét gi¸ trÞ ®iÖnthÕ tíi h¹n (clcar – cut) ®èi víi c¸c xung, mµ biªn ®écña chóng ®îc bao gåm trong mét kho¶ng ®· cho.
• Khi ®ã, tÝn hiÖu ®îc lîng tö ho¸ sÏ ®îc ®a tíi béchuyÓn ®æi A/ D ®Ó thùc hiÖn viÖc m· ho¸ nhÞ ph©ncña mçi mét xung.
• TÝn hiÖu ra A/ D song song sÏ ®îc chuyÓn ®æi thµnhnèi tiÕp nhê mét bé chuyÓn ®æi song song/ nèi tiÕptheo sau.
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• TÝn hiÖu chuçi PCM ®îc truyÒn qua kªnh truyÒn(c¸p kim lo¹i, sîi quang häc, hÖ tiÕp ©m radio) vµ ®Õnbé chuyÓn ®æi nèi tiÕp / song song mµ tõ (word) racña nã sÏ ®îc chuyÓn thµnh mét gi¸ trÞ analog nhêbé chuyÓn ®æi D/A sau ®ã.
• §Çu ra cña bé chuyÓn ®æi lµ mét tÝn hiÖu nh¶y bËcxÊp xØ víi tÝn hiÖu analog ban ®Çu. Bé läc th«ng thÊptiÕp theo sÏ “lµm s¹ch” tÝn hiÖu vµ cung cÊp d¹ng sãng ban ®Çu.
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Qunatization example
t
Ts: sampling time
x(nTs): sampled valuesxq(nTs): quantized values
boundaries
Quant. levels
111 3.1867
110 2.2762
101 1.3657
100 0.4552
011 -0.4552
010 -1.3657
001 -2.2762
000 -3.1867
PCMcodeword 110 110 111 110 100 010 011 100 100 011 PCM sequence
amplitudex(t)
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ån lîng tö hãa
• ån lîng tö hãa (hoÆc ®é lÖch) lµ sù kh¸c nhaugi÷a tÝn hiÖu analog vµ gi¸ trÞ lîng tö hãat¬ng øng
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Quantization error
• Quantizing error: The difference between the input and
output of a quantizer )()(ˆ)( txtxte −=
+
)(tx )(ˆ tx
)()(ˆ
)(
txtx
te
−
=
AGC
x
)(xqy =Qauntizer
Process of quantizing noise
)(tx )(ˆ tx
)(te
Model of quantizing noise
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NÐn t¬ng tù (Analog)
• Khi tÝn hiÖu tương tự ®îc truyÒn ®i, nã sÏ®îc nÐn tríc khi nã tíi ®îc bé m· hãa PCM thÝch hîp. Khi tÝn hiÖu ®îc thu nhËn, nã sÏ®îc gi·n ra trong bé gi¶i m· PCM lèi ra, tu©ntheo luËt phô cho luËt nÐn.
• C¸c luËt nÐn sö dông trong ®iÖn tho¹i lµ luËt A vµ luËt µ. LuËt A thêng ®îc dïng nh lµchuÈn mực ch©u ¢u, cßn luËt µ lµ chuÈn Mü.
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Non-uniform quantization
• It is done by uniformly quantizing the “compressed” signal.
• At the receiver, an inverse compression characteristic, called “expansion” is employed to avoid signal distortion.
compression+expansion companding
)(ty)(tx )(ˆ ty )(ˆ tx
x
)(xCy = x
yCompress Qauntize
ChannelExpand
Transmitter Receiver
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M· hãa PCM vi ph©n
• Khi c¸c tÝn hiÖu analog ®îc m· hãa PCM (©m thanh, h×nh ¶nh,...) rÊt nhiÒu mÉu trong mét hµng cã thÓ chØcïng mét møc lîng tö hãa. Sau ®ã, nhiÒu m· PCM ®ång nhÊt sÏ ®îc truyÒn ®i, ®iÒu nµy lµ thõa ®èi víi viÖc t¸i t¹o tÝn hiÖu nhËn ®îc.
• DPCM cho phÐp truyÒn m· sè liªn quan ®Õn sù sai kh¸c gi÷a hai mÉu liªn tiÕp vµ kh«ng ph¶i lµ m· sèliªn quan ®Õn mÉu hiÖn t¹i. V× biªn ®é cña sù sai kh¸c nµy gi÷a c¸c mÉu lµ nhá h¬n biªn ®é cña b¶n th©n mÉu cho nªn DPCM cÇn cã sè c¸c bit nhá h¬n PCM th«ng thêng.
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Gi¶n ®å m¾t
• MÆt giao cña c¸c tÝn hiÖu cã thÓ thu nhËn ®îcnhê mét dao ®éng ký sÏ t¹o nªn c¸c h×nh ¶nh®îc gäi lµ gi¶n ®å m¾t. Trôc thêi gian ®îc®ång bé víi tÇn sè tÝn hiÖu trong khi tÝn hiÖu sÏ®îc ®Æt cho trôc tung. Do tÝnh lu ¶nh, métlîng lín c¸c xung thªm vµo ®îc nhËn biÕttrªn mµn h×nh
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Bµi 3
Thùc tËp Tele - FAX
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Fax
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• FAX - Simile lµ thiÕt bÞ sö dông m¹ng ®iÖntho¹i c«ng céng, cho phÐp truyÒn ®i xa métv¨n b¶n, hoÆc h×nh ¶nh tÜnh, t¹o b¶n sao copy cña tµi liÖu chÝnh t¹i bé thu.
• C¸c giai ®o¹n chÝnh lµ:
– QuÐt h×nh ¶nh ®Ó truyÒn.
– TruyÒn
– T¸i t¹o h×nh ¶nh nhËn ®îc
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Bé§iÒu khiÓn
BéghÐp nèi víi
®êng d©yModem
vµxö lý tiÕng
nãi
Bé
nguånBé ghi
Bé§iÒu khiÓn vµo/
raBé ®äc
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Khèi ®äc
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S¬ ®å chøc n¨ng truyÒn dÉn
Tµi liÖu
biÕn ®æi nèitiÕp / song
song
sensor CCD
biÕn ®æi A/ D
§ÖmRAM
M· ho¸ Modem ®êng
®iÖn tho¹i
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S¬ ®å chøc n¨ng thu
§êng ®iÖntho¹i Modem
Bé®iÖm thu
Gi¶i m·BiÕn ®æi song song/ nèi tiÕp IN
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S¬ ®å chøc n¨ng viÖc sao chÐp nguyªn b¶n
CCD
Bé®iÖm ¶nh
BiÕn ®æi song song/ nèi tiÕp IN
BiÕn ®æinèi tiÕp/ song
song
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Bµi 4
Bé truyÒn dÉn PAM 4 - kªnh
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PAM-4TDM
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Bé truyÒn dÉn PAM 4 - kªnh
• §a vµo nh÷ng kh¸i niÖm chÝnh vÒ sù dånkªnh chia thêi gian.
• KiÓm tra sù ho¹t ®éng cña bé biÕn ®iÖu PAM.
• KiÓm tra sù ho¹t ®éng cña bé truyÒn dÉn PAM 4 - kªnh.
• KiÓm tra d¹ng sãng cña c¸c tÝn hiÖu.
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Bµi 5
§iÒu chÕ xung
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PAM-PPM-PWM
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QUANTIZERSAMPLERS(t)
CODEC
S(nT) S’(nT)
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Bé ®iÒu chÕ PAM
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H×nh 2.2: Bé ®iÒu chÕ PAM lÊy mÉu tù nhiªn
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Bé ®iÒu chÕ PAM lÊy mÉu b»ng
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H×nh 2.4: TÝn hiÖu PAM lÊy mÉu b»ng
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Bé thu PAM
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HÖ thèng truyÒn th«ng PAM víi ®êng d©y vµ nhiÔu
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TÝn hiÖu t¬ng tùC¸c xung lÊy mÉuTÝn hiÖu PAMTÝn hiÖu PWMTÝn hiÖu PPM
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Bé ®iÒu chÕ PWM.
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