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Now if you want to read a distance of 10 meters stably, make the RFID reader’s recognition rate reach 100%, and be able to adapt to changes in weather and environment, the reading effect is better than common products, what should be done?
The recognition rate of rfid readers reaches 100%?
1. Analysis: First of all, what are the factors that affect the performance of the receiver?
When the UHF RFID reader/writer receiver works, the transmitter also needs to send out an unmodulated carrier wave. What the receiver receives includes tag reflection signals, antenna noise, environmental reflections, direct transmitter coupling, and the receiver’s own noise. DC offset is a unique interference of the zero-IF structure. It is formed by the coupling of signals such as the local oscillator in the receiver, transmitter leakage, and environmental reflections to the mixer input. The same frequency of the reader’s transceiver causes the DC offset to be much larger than that of the conventional receiver. In addition, the common working distance is only 3-5 meters. The carrier leakage is also affected by the antenna feeder and the environment, and the DC offset is time-varying. The DC offset not only destroys the DC operating point of the subsequent circuit, but also affects the linearity performance of the amplifying and filtering circuit, and makes the signal-to-noise ratio worse. In a single antenna design using a circulator, the limited isolation of the circulator leads to a large transmission leakage to the receiving end, and the DC offset problem will be more serious. DC offset, amplitude and phase interference caused by environmental reflections, local oscillator phase noise, ADC The quantization noise can reduce the signal-to-noise ratio of the receiver, and to improve its performance, in addition to improving the analog radio frequency circuit, corresponding measures must be taken in the baseband signal processing algorithm.
2. Practice: Process the baseband digital signal
(1) Oversampling and filtering
According to the Nyquist sampling theorem, in order to restore the sampling signal to the original continuous signal, the sampling frequency should be at least twice the highest frequency of the signal. Oversampling is based on the Nyquist frequency by increasing the sampling frequency by one. The level of the sampling multiple law, oversampling can reduce the power of quantization noise in the effective bandwidth and improve the signal-to-noise ratio, which is equivalent to increasing the resolution of the ADC. The data obtained by oversampling can be extracted with a CIC filter to bring the data rate back to Normal level, and then cascade FIR filters for band-pass filtering to further reduce the noise power and improve the signal-to-noise ratio.
(2) DC offset correction
The methods to deal with DC offset by circuit hardware include: AC coupling, carrier elimination, harmonic mixing, self-correction compensation, etc., among which harmonic mixing processing and self-correction compensation methods are more complicated, but the effects achieved are limited , There is a processing method for carrier elimination. This method needs to add compensation circuits and software to the analog radio frequency and baseband unit at the same time, which increases the complexity and cost, and is difficult to debug. As mentioned earlier, the DC part of the signal can be filtered out simply by capacitive AC coupling to reduce the interference of DC offset. This method has the simplest structure and the lowest cost among all the schemes, so it is the most widely used.
(3) Data decoding
Baseband data decoding methods are divided into two types: zero-crossing detection and coherent detection. The working principle of zero-crossing detection is to set a threshold, and compare each data sample in the data buffer with the median value. The absolute value of the difference value is greater than the threshold and greater than the average value, it is judged as 1, otherwise it is judged as 0. Because this method is simple and easy to implement, and even a comparator can be used to realize the judgment, it is widely used in low-end reader products.
3. Final summary:
Many technologies are not 100% reliable, but it does not affect our use of it, such as PCs. Even for barcodes, we use manual methods to scan them one by one, but there is no guarantee that the worker will not make mistakes, and there will be no missed scans and missed readings. The advantages of RFID technology are more reflected in group reading, and the core of this advantage lies in the anti-collision algorithm. You can choose a good product when buying, a good process design, and some necessary fault tolerance mechanisms. The read and write performance of an rfid product is also related to the 100% recognition rate of RFID in the actual application process.
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