Digital communication is widely used in wireless monitoring systems in municipal engineering and industrial fields such as water, electricity, gas, oil, and lamps. The core part is digital radio. The performance of digital radio directly affects the communication quality of the monitoring system. It is therefore a key device in the monitoring terminal. It is also a typical digital communication radio transceiver device. It has more digital communication interfaces than the most common walkie-talkie. Its internal circuitry contains the circuit and RF communication system solutions commonly used in almost all RF communication equipment. Therefore, digital radio stations are often used to build wireless RF monitoring systems. In communication systems, the most difficult part to design is the receiver. Because the receiver must have low noise figure, small group delay variation, small intermodulation distortion (IMD), large frequency dynamic range, stable automatic gain control (AGC), proper radio frequency (RF) and intermediate frequency (IF) Gain, good frequency stability, satisfactory gain stability (specifically, multiple channels), low phase noise, negligible in-band interference, sufficient selectivity, and appropriate bit error rate. At the same time, it may be subject to fees.
1 receiver solutionIn general, the design of the receiver mainly includes a superheterodyne receiver scheme, a direct down conversion scheme, and a digital zero intermediate frequency scheme. The superheterodyne receiver scheme has the advantages of easy implementation, high stability and less interference. It is easier to demodulate at a lower fixed intermediate frequency. The disadvantage is that the combined frequency interference is more serious; the direct down-conversion scheme does not have image interference. The disadvantages are local oscillator leakage, poor noise performance and DC offset; the digital zero-IF scheme is designed to digitize the process to the system's intermediate frequency.
In part, the signal-to-noise ratio (SNR) is improved and the reliability is improved. The disadvantage is that the complexity and cost of the system are high. Considering that this digital radio station mainly works in the field environment, it requires strong anti-interference and anti-noise ability, and must be miniaturized and low-cost. Therefore, the choice of superheterodyne receiver scheme is most suitable.
Superheterodyne receivers typically use an intermediate frequency in the processing of signals, and are therefore also referred to as intermediate frequency receivers. It has a mature theoretical basis and practical background, and has been widely used. This structure is more common in first generation cellular mobile phones, analog FM walkie-talkies, and FM radios. In normal applications, the first intermediate frequency is 10.7 MHz, 21.4 MHz, and 45 MHz, and the second intermediate frequency is 455 kHz. However, this structure requires the use of a high-order bandpass filter for suppressing the image signal and implementing the signal principle, and thus is difficult to integrate.
2 FM receiver compositionFigure 1 shows the basic structure of a typical superheterodyne receiver, the block diagram of the FM receiver. When the antenna receives the high frequency signal and passes through the input tuning loop, it is amplified by the RF amplification stage to enter the mixing stage. At this time, another high-frequency signal output by the local oscillator also enters the mixing stage, and the output of the mixing stage is a signal containing frequency components such as a sum frequency and a difference frequency. The output of the mixing stage is selected by the tuning circuit to select the intermediate frequency signal, and then amplified by the intermediate frequency amplifier to obtain a sufficiently high gain, and then the second frequency mixing is performed by the TA31136, and the low frequency signal is demodulated by the discriminator, and finally the low frequency power amplifier is used. Level amplification. Since the high frequency signal received by the antenna is mixed and then becomes a fixed intermediate frequency and then amplified, the sensitivity of the receiver is high.
The receiver uses superheterodyne reception and uses two downconversions. The first intermediate frequency is 21.4 MHz and the second intermediate frequency is 455 kHz. It has a receiving channel of 162.025 MHz, a signal bandwidth of 25 kHz, a demodulation method using FSK, a sensitivity of -110 dBm, and a wireless impedance of 50 Ω.
3 TA31166 IF demodulation chipThe key to this receiver module is the TA31136 IF demodulation chip, which is the second IF detector. TA31136 is mainly used for secondary mixing and demodulation of the fundamental frequency signal, and then the baseband signal is level-converted by the operational amplifier TL062 and sent to the communication board for processing.
3.1 Main features and pin functions of TA31166
TA31136 is a bipolar linear integrated chip from Toshiba Corporation, which is commonly used for FM IF demodulation of wireless phones. The chip is available in an SSOP package with low operating voltage. The characteristics of TA31136 are as follows:
Low operating voltage: Vcc = 1.8 ~ 5.5 V;
â—‡Good temperature characteristics;
◇ High sensitivity, 12 dB sensitivity is 11dBuV EMF (input 50Ω);
◇ intercept point high, 96dBuV (input 50Ω);
â—‡ Built-in 2nd-order mixing, working frequency is 10~100 MHz;
â—‡ Built-in noise detection circuit;
â—‡ has a signal strength indication function.
Table 1 lists the pin function descriptions for the TA31166.
3.2 Typical application circuit of TA31166
Figure 2 shows the typical application connection diagram of the TA31136. The first IF signal (21.7 MHz) is used to filter the adjacent clutter signal through the crystal filter, and then amplified by the resonant amplifier and input through the 16-pin and The 2-pin local oscillator signal (21.245 MHz) is internally mixed, and the two intermediate frequencies of the difference frequency 455 kHz are obtained. The output of the 3-pin output is filtered by the 455 kHz ceramic filter to remove the unwanted signal. After the 5-pin input, the signal is passed. The IF is amplified to the integral detection, and the audio signal is demodulated and output by the nine legs. The output signal of TA31136 has inverting amplification output (7 feet), demodulated AF signal output (9 feet), and intermediate frequency amplification signal output (11 feet). Pin 10 is a phase-shifting network that can be connected to a mid-cycle with a resonant frequency of 455 kHz. The TA31136 has high sensitivity and built-in noise detection circuit and RSSI (Received Signal Strength Indicator) function, and can output DC level for the IF amplifier according to the input signal level. In order to ensure the uniformity of the signal size before demodulation under different received signals, the circuit also has an automatic gain control (AGC) circuit. The amplification factor of the AGC is linearly proportional to the RSSI, so the AGC is calibrated. Magnification, the output of the AGC can be guaranteed to accurately calibrate the RSSI. The voltage of pin 9 of TA31136 is variable, and the voltage is 1.11 V at the frequency of the receiving path. When the frequency is changed, the voltage will drop.
When the squelch detection is implemented, a part of the audio signal output by the TA31136 can be divided into resistors 7 and 8 by the resistor, and the noise component can be amplified and rectified by the chip internal filter and amplifier to generate a The DC voltage corresponding to the noise component is sent to the analog port of the MCU for detection. The input DC voltage can be compared with a preset voltage value, and then the output of the speaker can be turned on or off according to the comparison result. In order to ensure that the receiver has no noise when there is no input signal, the MCU outputs high and low voltage signals according to the comparison result to control the 14 pin of 31136 to realize the opening and closing of the squelch when receiving the signal. When the 14 pin receives the signal, it is 1.7V high level, and when there is no signal, it is 0 V low level.
4 Radio receiving circuit based on TA31136Figure 3 shows the application circuit diagram of the TA31136 in this receiver. The Vcc of this circuit is powered by 5 V. The received signal becomes 21.7 MHz after the first conversion and then passes through two 21.7 MHz crystal filters to enhance the selectivity of the input signal. T10 is a mid-cycle with a resonant frequency of 21.7 MHz, and a frequency selective amplifier is formed with a transistor Q2. VCC2 is 5 V and provides a bias voltage to the triode. U3 (TA31136) mixes the 21.7 MHz signal from the 16-pin input with the 21.245 MHz 1-pin local oscillator signal to form a two-IF 455 kHz. After the intermediate frequency amplification, the demodulated signal from the middle of the 455 kHz frequency discrimination is passed through a low-pass filter composed of an internal resistor R64 and an external capacitor C116, and the intermediate frequency and the higher harmonics are filtered to enter the low-level circuit. The program used the mid-week before the frequency-detection part of the 10th and 11th feet. It was found that the middle-circle coil was difficult to debug, and later changed to the ceramic discriminator CDB450C24 of Murata Manufacturing Co., Ltd. Because CDB450C24 is simple in structure and easy to implement, the frequency is accurate and negative 1%, the bandwidth is about 10 kHz, and no load capacitance is needed.
Using the secondary frequency conversion scheme, not only can the total gain of the receiver be dispersed to three amplification stages of high frequency, intermediate frequency and baseband, but the local oscillator signal can be more effectively isolated from the received signal. Since the secondary frequency conversion has more filtering than the single frequency conversion, the anti-interference of the relative frequency is stronger, so it has better anti-interference than the single frequency conversion. However, the superheterodyne circuit also has shortcomings, and image frequency interference may occur. Because the image frequency is within the passband of the input loop, the frequency shifting action of the heterodyne will shift the image frequency position and nearby station signals into the middle frequency band, thereby forming interference to the received signal. In order to improve the image frequency suppression capability and sensitivity, and to make the input loop maintain a relatively uniform sensitivity throughout the entire band, the image frequency can be completely suppressed by the notch or absorption loop.
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