1、1JH5001 communication principle, integrated experiment system220MHz Double trace oscilloscope4The principle and circuit experimentIn the actual baseband transmission system, not all code word can be found in the channel transmission. For example, contains rich dc and low-frequency components of base
2、band signal is not fit in the channel transmission, because it has the potential to cause serious distortion signal. At the same time, general baseband transmission system is extracted from the received baseband signal flow timing signal, and the received timing signal is depends on the type of tran
3、smission, if type appeared for a long period of time 0 or even 1 sign, the baseband signal may appear zero potential for very long periods of time, so that the take-up, timing recovery system is difficult to guarantee the accuracy of the timing signal. The actual baseband transmission system may als
4、o put forward other requirements, thus the baseband signal is also a variety of possible requirements. Sum up, the transmission in baseband signal there are two main requirements:Transfer code （transmission code is also known as line code） will depend on the actual channel characteristic of structur
5、e and the system working condition. In a more complex baseband transmission system, the structure of the transmission code should have the following main features: 1. Can get from its corresponding baseband signal timing information2. The corresponding baseband signal no dc component, and only a sma
6、ll low frequency components3. Is not affected by information source statistical properties, can be adapted to the changes in the source4. As much as possible to improve the transmission efficiency of transmission type5. Have internal error detection ability, and so on.Satisfaction or part of the abo
7、ve characteristics of transmission type variety, mainly include: the CMI code, AMI, HDB3, and so on, the following will mainly introduce the CMI code.In the CMI code module, complete the CMI encoding and decoding function.The CMI encoding rules are shown in table 1:Table 1 the CMI encoding rulesThe
8、input code wordCoding results01100/11 Thus in CMI encoding, input code word 0 directly output 01 type, relatively simple. For input code word is 1, the output the CMI code word 00 or 11 yards there are two kinds of results, thus the input 1 state must be memory. At the same time, the coding rate dou
9、bled, thus shaping the output clock must have 2 times the input stream. Here the CMI code first called the CMI code, the second is called the CMI code of low.In the CMI decoding side, there are two kinds of synchronous and asynchronous, so need to synchronize. The design of the synchronization proce
10、ss can be: according to the state of the code word for 10 type does not exist in the input code word, if there are 10 yards, you must adjust the sync. In this function module, can be observed in the CMI in the synchronization process in the process of decoding. The CMI code has the following feature
11、s:1. There is no dc component.2. In the CMI code flow, have very strong clock component, for at the receiving end to restore the clock signal.3. Have error detection ability, this is because 1 code expressed with 00 or 11, and a 0 01 code, said in the CMI code stream does not exist 10 yards, and not
12、 with 11 yards group 00 continuously appear, this feature can be used to test some of the wrong words.The CMI encoding module composition block diagram is shown in figure1.CMI encoder by: 1 code encoder, 0 code encoder, the output of the selector.1 Encoder: because in the CMI encoding rules, require
13、s the input code to 1, 00 appear alternately, 11 yards, and therefore you must set up a state in the circuit to confirm the last coding input bits to 1 state. This mechanism is done by a D flip-flop, every time when the input code flow in 1 yd, turn on a D flip-flop state, so as to complete the 1 co
14、de coding state memory （1） state memory. D flip-flop, meanwhile, the Q output will also as input bits to 1 encoding output （TPX03 test point）.0 encoder: when the input code flow is 0, the clock signal output do 01 code.Selector: output from the input stream buffer output Q encoder output 1 or 0 enco
15、der is used to select the output.After input code encoded on the test point TPX05 can measure the CMI encoding output results.M sequence generator: M sequence generator output controlled by type choice jumpers KX02, produce different special code sequence （111100010011010 or 111100010011010）. When s
16、electing the input data set jumpers KX01 in M position （right side）, the CMI encoder input for M sequence generator output data, can use the oscilloscope observation the CMI encoding output signal at this time, verify the CMI encoding rules.Figure 2 CMI encoding module composition block diagram Wron
17、g words.if generator: in order to verify the CMI decoder system capable to detect wrong words.if, insert wrong words.if can artificially in CMI encoder. Set KX03 in E_EN position （left side）, insert the wrong words.if, otherwise set in NO_N position （right side）, there is no wrong words.if inserted.
18、Random sequence generator: to observed the CMI decoder step function, can generate a random data into the CMI decoder, make them unable to sync. To set the input data to choose jumpers KX01 in Dt position （left side）, and then set the jumpers KX04 in 2 _3 position （right side）, the CMI encoder will
19、choose random signal sequence data output. Normal work, jumpers KX04 set in 1 _2 position （left side）.In this module, test point arrangement is as follows:1. TPX01：The input data（256Kbps）2. TPX02：Input clock（256KHz）3.TPX03：1 state memory output4.TPX04：The output clock（512KHz）5.TPX05：The CMI encoding
20、 output（512Kbps）6.TPX06：Add wrong output instructions The CMI decoding module composition block diagram is shown in figure 2.Figure 2 CMI decoding module composition block diagramThe CMI by the string and converter, decoder, decoder circuit synchronous detector circuit, pulse circuit composition and
21、 so on.1. String and converter, the input of a 512 KBPS CMI code flow into a string and converter, first in the role of the clock will highs and lows of the CMI code code words size shunt output.2. The CMI decoder: when the CMI code the highs and lows of the CMI was achieved by xor gate code decodin
22、g. The differences due to the delay in the circuit, the output end there may be a burr, and output of plastic. After decoding results can be measured on TPY07, its waveform with TPX01 should agree, there is only a delay.3. Synchronous detector: according to the principle of the CMI encoding, wont ap
23、pear when the CMI code synchronization 10 yards （regardless of the channel transfer wrong words.if）; If the CMI code no synchronous （i.e. the highs and lows of the CMI wrong lock）, there will be more groups of 10 code word, at this time will not be the correct decoding. Synchronous detector principl
24、e is: when in a certain period of time （1024 - bit）, such as appear more groups of 10 yards, thinks the CMI decoder synchronization. The synchronous detection circuit output a control signal to the buckle to deduct a clock pulse circuit, adjust a bit delay, make the CMI decoder synchronization. The
25、CMI decoder is detected 10 yards, will output the wrong words.if instructions （TPY05）.4. Test point TPY03 is adjust the observation time period （1024 - bit）.1. TPY01：The CMI encoding input data2. TPY02：512KHz input clock3. TPY03：adjust the observation time period （1024 - bit）4. TPY04：buckle pulse in
26、dicating5. TPY05：wrong words.if output instructions6. TPY06：256KHz clock output7. TPY07：CMI decoding data output5The experimental stepsFirst the input signal choose jumpers KX01 set up in the M location （right side）; Add wrong can make jumpers KX03 set in error-free NO_E position （right side）; M seq
27、uence type selector switch KX02 set in 2 _3 position （right side）, produced seven cycle m sequence; The output data selector switch KX04 set in 1 _2 position, select the CMI encoding data output.用示波器同时观测CMI编码器输入数据（TPX01）和输出编码数据（TPX05）。观测时用TPX01同步，仔细调整示波器同步。找出并画下一个m序列周期输入数据和对应编码输出数据波形。根据观测结果，分析编码输出数据
28、是否与编码理论一致。将KX02设置在1_2位置（左端），产生15位周期m序列，重复上一步骤测量。画下测量波形，分析测量结果。2. 1码状态记忆测量（1） 用示波器同时观测CMI编码器输入数据（TPX01）和1码状态记忆输出（TPX03）。画下一个m序列周期输入数据和对应1码状态记忆输出数据波形。根据观测结果，分析是否符合相互关系。（2） 将KX02设置在2_3位置，重复上述测量。3. CMI码解码波形测试用示波器同时观测CMI编码器输入数据（TPX01）和CMI解码器输出数据（TPY07）。观测时用TPX01同步。验证CMI译码器能否正常译码，两者波形除时延外应一一对应。4. CMI码编码加错
29、波形观测跳线开关KX03是加错控制开关，当KX03设置在E_EN位置时（左端），将在输出编码数据流中每隔一定时间插入1个错码。TPX06是发端加错指示测试点，用示波器同时观测加错指示点TPX06和输出编码数据TPX05的波形，观测时用TPX06同步。画下有错码时的输出编码数据，并分析接收端CMI译码器可否检测出。5. CMI码检错功能测试首先将输入信号选择跳线开关KX01设置在Dt位置（左端）；将加错跳线开关KX03设置在E_EN位置，人为插入错码，模拟数据经信道传输误码。（1） 用示波器同时测量加错指示点TPX06和CMI译码模块中检测错码指示点TPY05波形。（2） 将输入信号选择跳线开关
30、KX01设置在M位置（右端），将m序列码型选择开关KX02设置在1_2位置（或2_3），重复（1）试验。观测测量结果有何变化。（3） 关机5秒钟后再开机，重复（2）试验。认真观测测试结果有何变化（注：可以重复多测试几次关机后再开机）。问题与思考：为什么有时检测错码检测点输出波形与加错指示波形不一致？6. CMI译码同步观测CMI译码器是否同步可以通过检测错码检测电路输出反映出。从当CMI译码器未同步时，错码将连续的检测出。观测时，将输入信号选择跳线开关KX01设置在Dt位置（左端），输出数据选择开关KX04设置在2_3位置（输出不经CMI编码，使接收端无法同步）。（1） 用示波器测量失步时的检测错码检测点（TPY05）波形。（2） 将KX04设置在1_2位置，检测错码检测点波形应立刻同步。7. 抗连0码性能测试（1） 将输入信号选择跳线开关KX01拔去，使CMI编码输入数据悬空（全0码）。用示波器测量输出编码数据（TPX05）。输出数据为01码，说明具有丰富的时钟信息。（2） 测量CMI译码输
