1 Overview MC3372/MC3372 is a narrowband frequency modulation (FM) communication chip produced by Motorola. The chip integrates common communication circuit units such as oscillators, mixers, intermediate frequency limiting amplifiers, integral frequency detectors, active filters, noise suppression switches, and signal strength measurement circuits. MC3371 and MC3372 can be used to design filters for narrowband FM reception. Its operating frequency can reach up to 60MHz. Its low voltage design makes it power-saving and highly sensitive, and it can achieve the best performance in narrowband voice or data communication. In addition, when using MC3371/MC3372 to design dual-frequency communication filters, only a small number of external components are needed to achieve complete functions. MC3372/MC7732 has the following characteristics: ● Wide voltage range: 2.0～9.0V; ● Low power consumption, in the state of squelch off when Vcc is 4.0V, Icc=3.2mA; when squelch, the minimum current will increase; ● Signal strength indication can be performed, with a dynamic range of 60dB; ● Application bandwidth is 0.1～100MHz; ● High sensitivity, the input voltage can be as low as 5μV.
2 Internal structure MC3371 integrates a mixer, a signal strength detection intermediate frequency detector with logarithmic response, an integral frequency detector, an active filter and a squelch trigger circuit. The specific internal structure is shown in Figure 1. When applied, the mixer amplifier converts the radio frequency (RF) signal to a 455kHz frequency signal first, and then inputs it into the limiting filter and signal detection circuit through an external bandpass filter to restore the audio signal. The active filter and frequency detector in the MC3371/MC3372 chip can also be used to monitor the "noise band". If noise higher than the normal audio band is detected, it means that there is no useful signal. In this case, the noise circuit switch can be used for mute processing. Whether the strength of the input signal reaches the normal level is monitored by the intermediate frequency signal strength detection circuit in the limiting amplifier. 3 Principle and Application The filter composed of MC3371/MC3372 can be used in power line carrier communication systems. Due to the shortcomings of high noise and high loss in the power line communication channel, the useful signal is often submerged in the noise within a very short distance, and sometimes the signal-to-noise ratio is even lower than 0dB. In this way, simply increasing the transmission power can no longer achieve significant results, but can only start from the reception aspect. The receiving sensitivity of MC3371 is 5μV, and there are two mixers and an active filter inside, so it can be used with a small number of external components to form a highly sensitive filter circuit. When using MC3371/MC3372 to design a filter circuit, the ADW input signal is first sent to MC3371 after simple filtering, and then the signal is output after two mixing and two filtering. The purpose of the two mixing is to effectively remove the interference of out-of-band noise and spike noise, and at the same time to ensure the convenience of post-reading processing of this system. After two mixing, the signal can be moved from a relatively high frequency to a low frequency band.
The input filter in Figure 2 uses a simple LC to select the frequency, and its frequency is determined by C8 and L2. Since the maximum input voltage requirement of MC3371 is 1V, it is also necessary to limit the input to prevent chip damage. In the figure, two diodes are used to limit the amplitude, and the follower amplifier of the transistor is input into the chip. FILT is a ceramic filter used to complete the second-level filtering. The active filter composed of R16, R17, C13, C14 and the on-chip amplifier is used to complete the third-level filtering, and finally output from pin 11 of MC3371. The two mixing operations are performed inside MC3371, and the mixer and demodulator (the demodulator also acts as a mixer) in MC3371 are used for mixing. Two reference frequencies are used for mixing. MC3371 has an oscillator (pins 1 and 2) built in, which can be connected to an external oscillator circuit to generate reference frequency 1. In this system, both reference frequencies are input from the outside. MC3371/MC3372 chips are designed for FM wireless communication applications (such as FM radio), but since the chip has many commonly used communication unit circuits built in and has quite good performance, it can also be widely used in other applications if it can be flexibly and cleverly combined. 4 Conclusion This filter circuit makes full use of the internal unit circuit of MC3371 and gives full play to the high receiving sensitivity of MC3371. The filter circuit composed of MC3371/MC3372 has high sensitivity. When applied to power line carrier communication systems, it can effectively distinguish the carrier signal that is completely submerged in the noise background. Since the output filter in the application of this design is directly digitally processed, the output waveform is basically a square wave. If the design of the output filter amplifier is changed, other waveforms can also be obtained. From this, we can see that many special chips are not necessarily limited to specific tasks. Some of the circuits in them may be the parts that are difficult to design in conventional circuit design. If these functions can be fully explored, the design and product costs can be greatly saved, and it may produce twice the result with half the effort.
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