When uhf RFID reader receiver works, it also needs transmitter to send out unmodulated carrier wave.The receiver receives the label reflection signal, the antenna noise, the environment reflection, the transmitter direct coupling, and the receiver own noise and so on.DC offset is a kind of interference peculiar to the zero-intermediate frequency structure, which is caused by the local oscillator, transmitter leakage, environment reflection and other signals in the receiver coupled to the mixer input end. Due to the same frequency of reader-receiver transceiver, the dc deviation is much larger than that of conventional receivers. In addition, the common working distance is only 3-5 meters, and the carrier leakage is also affected by weather and environment, so the DC deviation is time-varying.DC offset not only destroys the dc working point of the rear circuit, but also affects the linearity of the amplifying and filtering circuit, making the signal-to-noise ratio worse. Using single antenna design, circulator circulator isolation degree is limited to launch leaked to receive the strength, the dc offset problem will be more serious, dc offset, fold reflection amplitude phase interference caused by environment, the local oscillator phase noise, ADC quantization noise can reduce the SNR of receiver, improving its performance in addition to the simulation of rf circuit was improved, also must take corresponding measures in the baseband signal processing algorithms.
Oversampling and filtering
According to Nyquist sampling theorem, in order to make sampling signals can be back to the original continuous signal, sampling frequency should be at least more than twice the highest frequency signal, sampling is on the basis of the Nyquist frequency sampling frequency had a sampling times the level of law, a sampling can reduce the effective bandwidth of quantization noise power, improve the signal-to-noise ratio, is to increase the resolution of the ADC, a sampling of data can be extracted by CIC filter, the data rate is back to normal levels, then cascaded FIR filter band-pass filtering, further reduce the noise power, improve signal-to-noise ratio.
DC offset correction
Circuit hardware way of dealing with the DC offset method includes: exchange coupling, carrier elimination, the harmonic mixing, self-correcting compensation, etc., the harmonic mixing processing, and self-correcting compensation methods are more complex, and the effect of the implementation has limitations, there is a processing method of eliminating the carrier, the method to increase the compensation circuit in analog rf and baseband unit and software, the increased complexity and costs, and debugging difficult. As mentioned earlier, the interference of dc offset can be alleviated by simply filtering the dc part of the signal by means of capacitive ac coupling, which is the simplest structure and the lowest cost among all schemes, and thus the most widely used.
The data decoding
Baseband data decoding method is divided into two kinds of zero crossing detection and coherent detection, working principle of the zero crossing detection is to set a threshold, the data buffer in each data samples are compared with the median, if the data sample and the value is greater than the threshold of the absolute value of the difference and is greater than the average, is judged to be 1, or are found to 0.Because the method is simple and easy to implement, and even can be decided by using comparator, it is widely used in mid - and low-end readers.
Many technologies are not 100% reliable, but do not affect our use of it, such as PC and so on. Even if the bar code is scanned one by one manually, it cannot be guaranteed that the worker will not make mistakes or miss scanning or reading. The advantage of RFID technology is more reflected in the group of reading, and the core of the advantage is the collision algorithm, can choose a good product when buy, a good process design, some necessary fault-tolerant mechanism, performance will influence a RFID products, speaking, reading and writing, and it also involves the RFID in the process of practical application to the recognition rate of 100%.