0 foreword

The communication of digital transmission is employed in The wireless monitor systems of municipal works such as water, electricity, angry, oil, light and industrial field extensively, its key part is a radio station of digital transmission. The quality of the characteristic of radio station of digital transmission influences the communication quality of the monitor system directly, therefore is the key equipment in the monitor terminal, it is a wireless transceiver of radio frequency of the typical digital communication too, its most common intercommunication telephone set many digital communication interface. Its internal circuit includes commonly used circuit and radio frequency communication system solution of communicating device of nearly all radio frequencies, so, the radio station of digital transmission comes to construct The wireless RF monitoring system dailily. In the communication system, the part most difficult to design is the receiver. Because the receiver must possess low noise coefficient, small group delay variation, little inter-modulation distortion (IMD) , large dynamic range of frequency, stable automatic gain control (AGC) , appropriate radio frequency (RF) And intermediate frequency (IF) Gain to noise temperature ratio, good frequency stability, the satisfactory stability of gain (refer in particular to the multiway channel) , characteristics such as low phase noise, negligible inner band resistance, sufficient alternative, appropriate bit error rate,etc.. Meanwhile, may receive the restraint of the expenses.

A receiver scheme

Generally speaking, the cipher scheme of the receiver mainly includes superheterodyne receiver scheme, direct down-conversion scheme and digital zero intermediate frequency scheme. The superheterodyne receiver scheme realizes the easy, high and advantage interfered with few etc. of stability, have apter to realize demodulation in lower fixed intermediate frequency, it is comparatively serious that the shortcoming is the combination frequencies interfered with; There is no image interference in the direct down-conversion scheme, the shortcoming has local oscillation that is let out, the noise performance is discrepancy and produced direct current to bias; The digital zero intermediate frequency scheme has increased the digitized course to some of intermediate frequency of the system in the design, so can enable the signal to noise ratio (SNR) Improved, having improved reliability, the shortcoming is complexity and cost great of the system. On the rural atmosphere, require anti-interference, ability to resist noise to be strong in view of the groundwork of radio station of this digital transmission, also accomplish miniaturization and low cost, so, choose the superheterodyne receiver scheme to be most suitable.

The superheterodyne receivers generally all use intermediate frequency in the course of dealing with the signal, called the medium frequency receiver. It has mature theoretical basis and practice background, and already found extensive application. This structure is more common in first generation honeycomb cellular telephone, simulation FM intercommunication telephone set and FM radio. In conventional application, the first intermediate frequency is 10. 7 MHz, 21. 4 MHz and 45 MHz,etc., the second intermediate frequency is 455 kHz. However, this structure needs to use the high-order band-pass filter, in order to be used for inhibiting the image signal and realizing the signal principle, so is difficult to integrate.

Composition of 2 frequency-modulated receivers

It is a basic organization of a typical superheterodyne receiver, namely the composition block diagram of the frequency-modulated receiver that Fig. 1 shows. The line receives the high frequency signal and after inputting and tuning the loop on the same day, enlarge through the amplifier stage of the radio frequency, can enter mixer stage. Another high frequency signal that the local oscillatorLO outputs also enters mixer stage at this moment, but the output of the mixer stage is to contain the sum frequency, signal of the equifrequent weight of beat frequency. The output of the mixer stage selects the intermediate frequency signal through tuning the loop, and then amplified by the intermediate frequency amplifier, in order to obtain high gain to noise temperature ratio enough, then after TA31136 two-shot frequency mixing, and is published the low frequency signal by the demodulation of discriminator, enlarged by the work discharging of low frequency finally. The high frequency signal received because of the aerial will be enlarged as stationary intermediate frequency after frequency mixing, so, the great of degree of sensitivity of this receiver.

This receiver adopts the reception mode of superheterodyne, and use down-conversion two times, the first intermediate frequency is 21. 4 MHz, the second intermediate frequency is 455kHz. Its receiving channel is 162. 025 MHz, the signal bandwidth is 25 kHz, demodulation way adopts FSK way, the degree of sensitivity is - 110dBm, wireless impedance is 50 Ω.

3 TA31166 intermediate frequency demodulation chip

Originally receive in the module the more key one is TA31136 intermediate frequency demodulation chip, it is the second intermediate frequency detector. TA31136 is mainly used for carrying on two-shot frequency mixing and demodulating the fundamental signal out, then reuse the operation amplifier TL062 to send to the communication board and deal with after carrying on level switch of the fundamental signal.

3. Key feature and pin function of 1 TA31166

TA31136 the intersection of bipolar and linear integrated chip of Toshiba, often intermediate frequency demodulation of frequency modulation used for Cordless Phone. This chip adopts SSOP to capsulate, low working voltage. The characteristic of TA31136 is as follows:

Low working voltage: Vcc =1. 8- 5. 5 V;

Good temperature behavior;

High sensitivity, 12 dB degree of sensitivity is 11dBuV EMF (input 50Ω) ;

It is high to intercept some, 96dBuV (input 50Ω) ;

Built-in 2 stepses of frequency mixing, operating frequency is 10- 100 MHz;

Detection circuit of built-in noise;

Have directive function of signal intensity.