Frequency shooting of video access control intercom system and principle of prohibiting intercom host

What is the principle of 0 1.IF receiving part? Intermediate frequency? What is the second intermediate frequency? The second frequency conversion superheterodyne mode is adopted, with the first intermediate frequency of 49.95MHZ and the second intermediate frequency of 450KHZ.

02. What is the tuning principle of MPF for RF reception? How to adjust it? Function?

03. What is the principle of APC circuit and how to realize it? How to realize the high and low power when it happens? Automatic power control (APC) circuit can stabilize the emitted output power by detecting the drain current of FET Q5 19 of the final amplifier. The voltage comparison circuit U5 13 compares the voltage obtained from the last stage current with the set reference voltage. The automatic power control voltage is directly proportional to the difference between the automatic detection voltage output by U5 13 and the reference voltage. This output voltage controls the FET power amplifier and keeps the output power of the transmitting part constant. The output power of the transmitting part can be adjusted by controlling APC voltage by microprocessor.

04. How to realize the wide and narrow band of intermediate frequency? Why do you want to be wide and narrow? Through the surface acoustic wave filter, to meet the requirements of different channel spacing. Including 25k, 20k,12.5k. 。

05. What is the function of TA31136? What is its input signal? How many signals can it output? What's the role? What is the relationship between output and input? It is the second IF detector, which converts the 49.95MHZ signal output by the first IF into the 50.4MHZ audio signal for output. The input is a second intermediate frequency signal; The output signals are: inverse amplification output, demodulation AF signal output, intermediate frequency amplification signal output,

06. How to realize the squelch detection circuit? How did MCU detect this signal? And the signal-to-noise ratio? Silent circuit: When the signal is too weak to receive noise, the electronic circuit of the wireless receiver is cut off. After the interphone demodulates the received IF signal, it filters and shapes the sub-audio signal, inputs it into MCU, compares it with the CTCSS frequency set by this machine, and then generates a level to control AF mute and SP mute to decide whether to turn on mute. The MCU detects this signal: a part of the audio signal output by the intermediate frequency processing circuit TA3 1 136 enters the FM integrated circuit again, and its noise component is rectified by the filter and amplifier to generate a DC voltage corresponding to the noise component. Send it to the analog port of single chip microcomputer for testing. The input DC voltage is compared with the preset voltage value, and the output of the speaker is controlled to be turned on or off according to the comparison result.

07. How to realize RSSI detection circuit? How did MCU detect this signal? What is the correspondence between RSSI and reception level? The RSSI terminal of TA3 1 136 outputs the DC level of the IF amplifier according to the input signal level.

08. How are the pre-emphasis and de-emphasis circuits composed? What role does it play? What are the technical indicators? Composition: it is mainly realized by pre-emphasis and de-emphasis circuits in AK2346, and peripheral circuits can also realize pre-emphasis and de-emphasis functions. Pre-emphasis is realized by high-pass filter, and de-emphasis is realized by low-pass filter. Function: Sound quality mainly depends on the technical indexes of pre-emphasis and de-emphasis circuits.

09. How is the voltage expansion circuit formed? What role does it play? What are the technical indicators? Composition: It consists of compression and expansion circuits in AK2346. Function: The application of speech processing circuit, speech compression expansion circuit and low-level expansion circuit has a good effect on fidelity speech.

10, how about dual-tone, pentatonic, sub-audio, sub-audio digital, dual-tone multi-frequency signaling? What's the role? How is it generated and decoded in TC900? How is it modulated? A: A dual-tone signaling command consists of two audio signals, A tone +B tone. Pronounce a for a while and b for a while. Using dual tone signaling, you can choose to call the corresponding walkie-talkie; 5. The signaling command has the same function as the two signaling commands, but the difference is that it consists of five frequencies; CTCSS (continuous tone controlled squelching system) and continuous voice controlled squelching system, commonly known as sub-audio, is a technology of adding frequencies lower than audio (67Hz-250.3Hz) to audio signals and transmitting them together. Because its frequency range is lower than the standard audio, it is called secondary audio. After the interphone demodulates the received intermediate frequency signal, it filters and shapes the secondary audio signal, inputs it to the CPU, and compares it with the CTCSS frequency set by this machine, so as to decide whether to turn on the mute. Cd CSS (continuous digital control silencing system) is a kind of continuous digital control silencing system, which has the same function as CTCSS, but the difference is that it uses digital coding as the condition of whether the silencing is turned on or not. Using CTCSS/CDCSS function can avoid receiving irrelevant calls. DTMF (Dual Tone Multifrequency) is composed of a high-frequency group and a low-frequency group, and each group contains four frequencies. A high frequency signal and a low frequency signal are superimposed to form a combined signal, which represents a number. DTMF signaling has 16 codes. You can choose to use dual tone /DTMF signaling to call the corresponding interphone.

1 1. Can opening PA make any difference? How to reduce the impact?

What are the functions of 12 and AK2346? How to encode and decode MSK signaling? What is the optimal input level for MSK decoding? How to adjust the input level? What effect does the signal-to-noise ratio of the input signal have on the bit error rate? The functions of AK2346 are: audio processing; Transceiver amplification; Scrambling; Pre-emphasis and de-emphasis; MSK coding and decoding, the role of restriction; Audio compression and amplification, etc. After the arrival of the external analog sine signal, it is encoded and decoded by setting the control register of AK2346, 1 control register 2 and volume register 2. At the end of decoding and encoding, AK2346 will generate an interrupt signal to inform MCU to store data, and then MCU will perform operations through interleaving and inversion algorithms. Minimum shift keying pursues the continuity of signal phase path, which is a binary continuous phase FSK(CPFSK). This modulation method can obtain orthogonal modulation signal with minimum modulation index h=0.5.

13. What is the working principle of the chip? How to solve the data conflict between it and SDL and SCL of AT24C256? What is the relationship between RTC-INT and MCU? How does the charger charge the clock battery? Because * * * enjoys the I2C bus of cpu, there will be some resource conflicts, which can be solved by mutual exclusion signal management of OS system and time-sharing operation of cpu, that is, when sending R5VC387, mcu sends its startup address, and AT24C256 is not allowed to operate the data line during this time.