Making the bias accurately track and approaching the breakdown voltage will enable the system to achieve higher detection sensitivity. To determine the breakdown voltage of the avalanche tube, a large amount of noise generated by avalanche breakdown can be converted into a false alarm pulse by a threshold detector. It is counted to obtain the average value of the noise over-threshold in each second, that is, the false alarm rate FAR. For a system with a bandwidth of 1/(2), the relationship between the noise in, the threshold it and the false alarm rate FAR is given by Rice. :FAR=123exp-i2t2i2n(1) The relationship between the bias voltage U and the false alarm rate can be approximated as: U=UB1-1nM+Rit2ln123FAR(2)
Where: UB is the breakdown voltage, n is a constant (n=254), M is the internal multiplication factor, R is the total equivalent resistance of the avalanche tube, and the first term on the right side of (2) is the actual addition to the avalanche tube. Bias, the second term is that the voltage drop across the equivalent resistance R is near the breakdown point. Because M is large, the first term of equation (2) is approximately equal to UB. If the bias voltage U is increased, it is mainly the second. The term increases, corresponding to a large false alarm rate FARmax, U is lowered, and FAR and M are also reduced. When M reaches the optimal value Mopt determined by the maximum signal-to-noise ratio, according to formula (1), let i2n= i2R+i2s0M25opt can obtain the corresponding FARmin (iR is thermal noise, is0 is the shot noise when there is no multiplication). Monitoring FAR not only knows the breakdown voltage of the avalanche tube, but also knows the breakdown degree of the false alarm digitally controlled bias source. In the design, the FAR is detected by the single-chip microcomputer and compared with the two reference false alarm rates FARmin and FARmax: the optimal bias voltage is discriminated by FARmin, the breakdown voltage is discriminated by FARmax, and the data is controlled by D/A according to the bias voltage given by the single chip microcomputer. Switching to control the bias voltage, so that the bias voltage automatically follows the breakdown voltage change, the temperature and light intensity can be compensated.
The design of the hardware design of the digitally controlled bias source is composed of the minimum system of the single-chip microcomputer, the D/A converter and the bias generator. The principle of the circuit false alarm digitally biased circuit sets the internal T0 and T1 of the 8031 ​​microcontroller separately. The 16-bit counter and the timer false alarm pulse are input by the P34 port, and the microcontroller is controlled by the program to make the T0 input the false alarm pulse in the T1 time to obtain the FAR8-bit D/A converter DAC0832 according to the first-level latch control. Connected, its output current is converted into analog voltage by the operational amplifier IC4, and is used as the biased digital control level to access the non-inverting input terminal of IC5. The inverting input terminal of IC5 is connected to the bias sampling signal. When the digital control level is 0, The output of IC5 is 0, the transistor V2 is cut off, the square wave of the primary input of the transformer TC does not constitute a path, the primary and secondary windings have no current, and the bias voltage is 0. With the bias voltage digital signal given by the single-chip computer software, the numerical control level is increased, IC5 Output the corresponding voltage, make V2 turn on, there is alternating current in TC, and the secondary bias also rises. Due to the negative feedback, the rise of the bias is restricted by the control level: when the digital control level remains unchanged When the bias voltage rises Therefore, the inverting input of IC5 is higher than the numerical control level of the non-inverting input, forcing V2 to be cut off, the current flowing into the primary of the transformer is reduced, the amplitude of the square wave is compressed, and the bias voltage is decreased, and vice versa, thus, the bias voltage will be It is always stable at the optimum working level corresponding to the digital control level.
Performance test and conclusion To test the control effect of the numerical control bias, the bias waveform obtained by numerical control and mode control for the avalanche tube bias at room temperature is as shown, when the static error is taken as 0015, the overshoot of the numerical control bias For 0042, the setup time is 78ms; the overshoot of the mode-controlled bias is 0153, and the setup time is 163ms. The same detector with two bias controls is used for ranging and contrast experiments under different temperature and light intensity conditions. The extinction ratio is about 13db higher than the mode-controlled bias. The test results show that the FAR digitally controlled bias technology can effectively improve the breakdown voltage tracking accuracy and greatly reduce the bias voltage set-up time. This technology overcomes the traditional mode-controlled bias voltage. The drawbacks of maintaining are suitable for continuous detection of photovoltaic systems with large changes in ambient temperature and background light intensity.
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