Circuit Design and Simulation of Ultra-Wideband (UWB) Receiver

introduction

At present, the demodulation methods of ultra-wideband (UWB) can be summarized into the following three methods: one is to construct a template signal similar to the received signal and then use the correlation method; the other is to demodulate the UWB signal by using the integral detection method; and the other is to perform A/D conversion on the received signal and identify the signal by sampling value. The first solution is more difficult to implement, because if the ns-level pulse is to achieve correlation reception, the synchronization time must be accurate to the ps level, which is a challenge for both the receiver and the transmitter. The simple integral detection method has poor anti-interference performance. The sampling method is used to receive UWB signals, but the premise is that a high-speed ADC must be used, and the complexity of the receiver will be greatly increased. This paper proposes to apply a synchronously controlled gating pulse in the RF front end, gating when the UWB signal arrives, and controlling the integral signal to enter the subsequent decision circuit. The advantage of this method is that the signal-to-noise ratio of the received signal is improved, and multiple access communication is realized at the same time.

Figure 1 Receiver block diagram

Figure 2 Circuit simulation diagram

The structure of the new receiver

The front end of a traditional receiver has a bandpass filter to prevent out-of-band interference signals and noise from entering, thereby improving the signal-to-noise ratio. However, the frequency band of the UWB signal is relatively wide, and there are a lot of noise and other narrowband interference signals in this frequency band. The effect of using the method of adding a bandpass filter to the front end of the receiver to improve the signal-to-noise ratio of the received signal is not very obvious. The UWB signal is different from the traditional narrowband signal. It is a signal that is discontinuous in time and has a relatively narrow pulse width. Therefore, the signal is detected by integration when the signal arrives, and noise and external interference signals are shielded at other times. That is, the signal-to-noise ratio of the UWB receiver is improved by isolating noise in the time domain. This paper adopts a simple square detection method to realize the integral detection of UWB signals. The block diagram of the receiver is shown in Figure 1.

The UWB signal received by the antenna passes through a broadband amplifier and enters the integral detection circuit. When the UWB signal pulse arrives, the strobe pulse controls the UWB integral signal to enter the comparison judgment circuit to realize the detection and judgment of the pseudo code signal. After the pseudo code signal after the judgment is multiplied with the local PN code, it enters the threshold judgment circuit through the integration of the low-pass filter to realize the correct demodulation of the data. Here, the capture and synchronization of the UWB integral signal by the strobe pulse is the key part of this system. The capture process is as follows:

1. Synchronous capture of pulse waveform integration signal.

As shown in Figure 1, first, the integrated signal of UWB is controlled by the strobe pulse to enter the threshold decision circuit, where the repetition rate of the strobe pulse is the same as the rate of the pseudo code; then the integrated signal level entering the threshold decision circuit is judged, and it is judged as '1' when it exceeds the decision threshold, otherwise it is '0'; the signal after the judgment is sent to the shift register, and the length of the shift register is set according to the characteristics of the PN code used. In the case of correct reception, the pseudo code that the shift register can store must contain two '1's, in order to prevent the system from entering the out-of-step state due to misjudgment and misoperation when noise enters the system. The data of the shift register is read once every Tf time. If the data stored in it contains the symbol of '1', the strobe pulse stops shifting and it is considered to be synchronized; if it is all '0', the strobe pulse is controlled to continue shifting and searching.

2. Synchronous capture of PN code.

Because the period of the PN code is pTf, where Tf is the period of a single pseudo code, and the p value is relatively large, in order to obtain a more accurate correlation, generally nTf[td]

Performance Analysis

Compared with the waveform correlation method, synchronous capture is easier to achieve. In the same search algorithm, assuming that the received signal period is 1ns, if the waveform correlation method is used to search for the shift time is 1/4 of the period, that is, 0.25ns, then the shift of the envelope is also set to 1/4 of the integrated signal. Assuming that the integration time of the envelope is 80ns, the step shift time is 20ns. In terms of synchronous tracking, the waveform correlation method requires clock synchronization to be accurate to tens of ps, but the synchronous envelope detection method only needs to be accurate to a few ns, and the hardware is easier to implement.

The signal-to-noise ratio of the signal is improved by using the time domain filtering method. This article uses an extremely narrow pulse signal, which is a communication method that transmits pulse signals instantly. The switch method can be used to shield noise and external signal interference in the time domain. When the strobe pulse does not arrive, the integral signal is grounded to prevent external interference signals and noise from entering the subsequent decision circuit. Only when the strobe pulse arrives can the external noise and interference signals enter the decision circuit with the integral signal of the UWB signal. If the noise influence of the system itself is not considered, the signal-to-noise ratio is Eb/No in the normal state, where Eb is the energy of the signal and N0 is the energy of the noise. When an electronic switch is added, the signal-to-noise ratio will become EbTf/NoTon, where Tf is the pulse period and Ton is the opening time of the electronic switch. The characteristic of the UWB signal is that the Tf/Ton value is relatively large at low rates, which greatly improves the receiving sensitivity of UWB. At the same time, other UWB signals using different pseudo-random codes can be shielded to achieve multi-address communication. Of course, it is inevitable that the electronic switch will affect the entire receiving system, such as the influence of the selection pulse on the UWB integrated signal. If the interference is too large, it will affect the decision output of the subsequent stage.

Circuit Simulation

The circuit of system simulation is shown in Figure 2. For simplicity, only the front-stage pseudo code output circuit is simulated. The '1010' signal with a code rate of 2MHz is used in the simulation. The UWB signal is coupled by an inductor to become two signals with opposite polarities. It is detected by the detector diode and enters the subsequent integration circuit. When the gate pulse is synchronized with the received UWB signal, the gate pulse controls the electronic switch to turn on when the signal arrives. After the signal is integrated, it enters the low-pass amplifier circuit behind, and then outputs the pseudo code signal through the hysteresis comparator. When there is no UWB signal, it is connected to the ground, so that the signal input to the low-pass filter is zero, preventing the external signal and noise from entering the subsequent amplification and comparison circuit after integration. In fact, most of the signal energy is concentrated between 80ns and 100ns, so the conduction time of the electronic switch is set to 80ns. In the system simulation, the parameters of the electronic switch are set to open and close at 10ns, and the signal integration time is between 80ns and 100ns, so the bandwidth of the active low-pass filter is designed to be 10MHz and the cut-off frequency is 20MHz.

The simulation software used in this paper is Multisim7. The input UWB signal is delayed and superimposed by several signal sources to simulate the received multipath signal, and a large-amplitude Gaussian white noise signal is added at the same time. When there is no strobe pulse input, the signal enters the subsequent integration circuit through the detector diode. The integrated signal waveform shows that the noise of the integrated signal waveform is very large. Some random noise exceeds the amplitude of the signal and enters the low-pass filter to output the low-frequency component after integration. However, the amplitude of some noise items is similar to that of the signal, and an incorrect judgment occurs after the hysteresis comparator output.

When the strobe pulse controlled by the pseudo code is synchronized with the input signal, it can be integrated through the diode. When the switch is turned on, the external noise is completely shielded, and the noise can enter the detection circuit only when the strobe pulse arrives. When there is no UWB pulse in the strobe, the integral amplitude of the noise is also large, but it is mainly high-frequency components. Therefore, after passing through the low-pass filter, the high-frequency components can be filtered out, and the correct detection of the pseudo code signal can be achieved through the hysteresis comparator.

Circuit Implementation

In the actual circuit, the detector diode uses the zero-bias detector diode of Chengdu Yaguang, model 2H10673A, with a turn-on voltage of 200 mV and an operating frequency greater than 3GHz. The gate pulse device uses the PIN diode 2K60840, with a breakdown voltage greater than 20V, a forward differential resistance of 1.55, and a junction capacitance of 0.3pF. The received signal is an irregular signal with a pulse period of 3ns after antenna widening, so the operating frequency of the detector tube can fully meet the requirements. Due to limited conditions, the turn-on and turn-off time of the PIN tube cannot be tested, but the system requirements can be met through actual testing. In the actual test, a 256kb/s TH-PPM modulated ultra-wideband signal was added as an external multiple access interference signal.

Conclusion

This paper realizes the synchronous detection of UWB signals by adding a synchronous strobe pulse after integration, and shields external interference signals and noise, thereby realizing multi-access communication. This paper is a detection method under the assumption that the transmission rate is not very high, the overlap of multi-access signals is not much, and there is no obvious inter-code interference. Through experiments, this scheme is a better detection method under low rate conditions.