In the OSNR model for EDFA amplifiers， SNR OUT = 1/（1/SNR IN + FhvB/P IN ），  what does SNR IN represent？（） A:  the signal to noise of the current optical amplifier B:  the signal to noise of the previous amplifier or source C:  the signal to noise introduced by the optical multiplexer D:  the accumulated noise to signal ratio of the optical channel

下列标识符号中,()是不合法的标识符。 A: 9moon B: State0 C: Not_ack_0 D: signal1

_____, the train started. A: The signal given B: Giving the signal C: The signal being given D: The signal giving

What is the coding method which represents the digital signal 1, 0 by changing the phase value of the carrier signal? A: ASK B: FSK C: PSK D: NRZ

2. Railway signals can be divided into home signal, starting signal, distant signal, repeating signal and shunting signal.

下面一段VHDL编程语句是正确的。（） signal a : std_logic; a <= “1”;

what is an analog signal? A: Analog signal is a continous values B: Analog signal is a discrete values C: Analog signal is sometimes continous,sometimes discrete

________, the train started. A: The signal given B: After the signal was given C: Given the signal D: A and B

According to the meaning of stop signal, it is divided into absolute signal and _____.

According to the meaning of stop signal, it is divided into absolute signal and （ ）. A: clea B: permissive signal C: estricting D: approach