Today's wireless remote control and wireless data transmission system, the amplitude modification mode is still firmly occupied by the half-wural river, and the modulation method refers to the modulation signal to control the amplitude of the high-frequency carrier, so that the amplitude of the carrier is changed by the modulation signal. Change, the generally used modem receiving system can be divided into straightforward, super reproducing and super-extraction, etc., and ultra-extra-difference reception is the best, stable, product consistency, best super external Differential modem receiving system includes high places, local oscillation, mixing, multi-stage medium places, detection, and low-input circuits, and rough functional block diagrams such as right:
For product design and producers, complete a complete ultra-extraortary difference AM receiving system requires calculation of antenna circuit impedance matching, high-placed / horn coil or transformer design and adjustment, multi-stage intermediate frequency coupling transformer cooperation and whole machine Adjustment steps, not only a small challenge for design engineers, and due to the presence of multi-stage coupling transformers, the product cost cannot be lowered, and especially for the production line workers require a certain level of operation and proficiency, otherwise it will not be toned. Excellent performance. In view of this, the author introduces a complete AM receiving scheme that does not require production line adjustment.
PT8R2012W is a integrated AM receiving IC designed as remote control toys, civil safety alarm systems, interactive messages, and civilian remote measuring systems, including all circuitry required to demodulate from high frequency input to demodulation output. . The IC has a very small peripheral component, low operating voltage, small power consumption, high integration, free adjustment, and high sensitivity. Its operating voltage is low to 1.8-3.0V. It is recommended to have a nominal operating frequency range of 10-60 MHz, covering a common SW band (frequency point) such as 27 MHz / 35MHz / 40MHz / 49MHz / 55MHz / 64MHz. Typical reception sensitivity is about -85 ~ -90dBm when using basic working lines (see Figure 3). The comparison test is not affected by the performance of IC when the signal frequency is not higher than 75MHz. If the user is used for higher operating frequencies, the receiving sensitivity of the IC will decrease, for example, the sensitivity of 80-90 MHz will might reduce 5-8 dBm, which is more sharply reduced after 100MHz. In the test, when the 1kHz signal is also employed, 90% of the modulation is adjusted, the chip has excellent dynamic range of 85 dB. This IC is an inexpensive 8-pin packet package (SOP-8), and its pin allocation and internal function block diagrams are shown in Fig. 2 and Figure 4, respectively.
The significant advantage of this IC is to make some corresponding optimizations in IC for design, production, and commissioning difficulties. The external components of the AM receiving circuit are greatly simplified without reducing product performance, and is exempted Adjust production. That is, as long as the components are intact, there is no need for any adjustments, and the assembly is completed. As shown in Figure 3 and Figure 4, the RF signal is input from 1 foot (RFI), enlarged by the internal low noise amplifier to the appropriate amplitude oscillation signal mixed with 2 foot (OSC), and the medium frequency generated after mixing The signal is sent to the intermediate frequency amplifier after the external 455kHz ceramic filter is saved, then decoded the output, the intermediate frequency amplifier is controlled by the previous low noise high frequency amplifier controlled by the AGC (Automatic Gain Control) circuit, so the appropriate place can be automatically set. A large amount, try to ensure that the output is a suitable intermediate frequency voltage supply mixing circuit. The approximate process is basically the same as the standard ultra-horizontal difference AM receiving circuit, but each unit in which it has a unique advantage, and we will discuss one by one.
First, look at the low noise high frequency amplifier in the inside, the amplifier uses broadband inputs, and the selection circuit is canceled. And has the advantage of low input impedance, high output impedance, and measured at a typical input impedance of about 60W at 27 MHz frequencies. The advantage of extremely low input impedance is to directly match the antenna loop, without the usual antenna impedance conversion circuit, and even directly connect the rod antenna. This bypass the debugging process of the antenna loop. Second, the IC is used to use a new type of native oscillator circuit, which can be connected to external crystals in a single pin (another pin directly on the crystal oscillator), which can greatly saving the auxiliary part and the PCB space. Further, the frequency amplifier itself has a certain frequency of restriction, so sufficient selectivity can be obtained as long as a 455kHz ceramic filter is used. IC built-in medium frequency amplification circuit mating automatic gain control (AGC) circuit, which is in an automatic adjustment within 0-80 dB, ensuring that the whole machine has excellent 35dB. When a normal 455kHz ceramic filter is adopted, its -3db bandwidth is about 7.5 kHz. It is worth noting that because the bandwidth is only 7.5 kHz, in addition to a RC filtering in the decoded output, the modulation signal cannot be improved, and the practical data rate is generally not higher than 2 kHz, and the IC internal decoding circuit has been optimized for data transmission. The linearity of the signal that causes the decoded signal will not be too good, so this IC is not ideal for voice transmission effect. The main application goals are still low-speed data and coding, buttons control signals, etc. If you want to try with low-end voice toys or only those who deliver vocals, the RC filter can be canceled, and the 455kHz filter is replaced with a slightly wide bandwidth of the 455kHz ceramic filter. Extreme application can even cancel it, just use a capacitance communication circuit for tens to hundreds of skin method.
Please see the recommended simple application line (Figure 4), which is due to an antenna matching loop and a high-frequency amplifier, so the overall sensitivity is up to -95 ~ -100DBM. Where C1 / C2 / L2 make up the matching circuit, Q1 / R1 / L3 constitutes a broadband amplifier, which can additionally provide an additional 20 dB amplification amount, and do not need to increase adjustment steps during production. The zoomed high-frequency signal is sent to the local oscillation signal of the IC and 2 pin. After the mixing signal mixed, the oscillate is output from the 3 pin, and the 讯 讯讯 is filtered through the X2, and then pass through the multi-stage medium frequency detection by the internal product. The modulated signal is output, and the demodulated signal is given after the external P-shaped filter is filtered. 5 feet are the control terminal of the internal AGC circuit, think that this pin level can be appropriately improved, reverse (increasing a resistor), can reduce AGC amplification. Although the overall circuit is already very concise, if it is not particularly paying special attention, there is also room for reducing, for example, the additional high-frequency amplifier composed of Q1 / R1 / L3, so although the overall sensitivity is 15-20 dB, but if With a single tube oscillation, the primary tripode is high frequently amplified, or the transmission distance other than twentieth meters can be guaranteed. In addition, the external P-shaped filter composed of R4 / C6 / C7 can also be omitted as appropriate. This makes it up to the simplest use line as shown in Figure 3.
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