RU2797660C1 - Threshold detection method for optical signals - Google Patents

Abstract

FIELD: reception of signals.

SUBSTANCE: invention is related, in particular, to the technique of separating signals from noise using avalanche photodiodes and can be used in location, communications and any field where the best real sensitivity is required. A method for threshold detection of optical signals using an avalanche photodiode, including threshold processing of signals and generation of output pulses using a threshold device when the signal from the output of the photodiode exceeds a predetermined response threshold. The avalanche-free mode of the photodiode is pre-set by applying a low bias voltage to it and the threshold level for noise frequency exceeding the threshold f is stabilized, then the threshold is fixed at this level, increased by x ₁ times and the bias voltage is increased until the frequency of noise exceeding the threshold reaches the initial value f, after which the bias voltage of the photodiode is fixed, the threshold level is increased by x ₂ times, moreover, the parameters of the threshold increase are set equal to x ₁ = x ₂ = x = √ (1+2/α), the frequency f is set in accordance with the ratio f = f ₀ ^(2/2+α) · f _p ^(2/2+α), where α is the coefficient determined by the material of the photodiode, f₀ is the frequency of crossing the zero level by noise, f_p is the maximum allowable frequency of exceeding the threshold by noise emissions in the operating mode, after which signal receipt is started.

EFFECT: simplification of the preparatory mode and provision of the optimal sensitivity in all operating conditions with a guaranteed probability of false positives and maximum performance.

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Description

The present invention relates to the reception of optical signals, in particular, to the technique of receiving signals using avalanche photodiodes, and can be used in location, communication and other photoelectronic systems.

A known method of receiving optical signals using avalanche photodiodes [1]. Known methods of stabilizing the avalanche mode of the photodiode, for example, by thermal compensation of the operating point of the bias voltage [2].

There is a method for threshold detection of optical signals using an avalanche photodiode, the bias voltage of which is maintained by stabilizing the frequency of noise pulses that occur during threshold processing of a mixture of signal and noise [3]. The disadvantage of this method is the dependence of the avalanche mode on the set threshold. This leads to an incorrect choice of the operating point of the photodiode and a deterioration in the threshold sensitivity.

раз и увеличивают напряжение смещения до тех пор, пока частота шумовых превышений порога не достигнет первоначальной величины f, после чего фиксируют напряжение смещения фотодиода, увеличивают пороговый уровень в

раз и приступают к приему сигналов [4].The closest to the proposed technical solution is the method of threshold detection of optical signals using an avalanche photodiode, which consists in receiving signals, converting them into electrical ones, amplifying them and comparing them with a threshold level, according to which the avalanche-free mode of the photodiode is preliminarily set by applying a low bias voltage to it and stabilizing threshold level by the frequency of noise exceeding the threshold f, then fix the threshold at this level, increase it in

times and increase the bias voltage until the frequency of noise exceeding the threshold reaches the initial value f, after which the bias voltage of the photodiode is fixed, the threshold level is increased by

times and start receiving signals [4].

The disadvantage of this method is the arbitrariness of the set parameters over a wide range. This can adversely affect the precise selection of the optimum operating point of the photodiode and the threshold level, and thus the threshold performance of the measurements.

The objective of the invention is to simplify the preparation mode and provide optimal sensitivity in all operating conditions with a guaranteed probability of false positives and maximum performance.

раз и увеличивают напряжение смещения до тех пор, пока частота шумовых превышений порога не достигнет первоначальной величины f, после чего фиксируют напряжение смещения фотодиода, увеличивают пороговый уровень в

раз, причем, устанавливают параметры повышения порога равными

частоту f устанавливают в соответствии с соотношением

где α - коэффициент, определяемый материалом фотодиода, f₀ - частота пересечения шумом нулевого уровня, f_p - предельно допустимая частота превышения порога шумовыми выбросами в рабочем режиме, после чего приступают к приему сигналов.This problem is solved due to the fact that in the known method of threshold detection of optical signals using an avalanche photodiode, including threshold processing of signals and the formation of output pulses using a threshold device, when the signal from the output of the photodiode exceeds a predetermined response threshold, the avalanche-free mode of the photodiode is preliminarily set by applying to low bias voltage and stabilize the threshold level by the frequency of noise exceeding the threshold f, then fix the threshold at this level, increase it by

times and increase the bias voltage until the frequency of noise exceeding the threshold reaches the initial value f, after which the bias voltage of the photodiode is fixed, the threshold level is increased by

times, moreover, set the threshold increase parameters equal to

the frequency f is set in accordance with the ratio

where α is the coefficient determined by the material of the photodiode, f ₀ is the frequency of noise crossing the zero level, f _p is the maximum allowable frequency of exceeding the threshold by noise emissions in the operating mode, after which they start receiving signals.

и f устанавливают в качестве постоянных аппаратных параметров.The parameters α and f ₀ are determined during preliminary measurements in the design process, and the values f _p ,

and f are set as fixed hardware parameters.

Fig. 1 is a sequence diagram of the method. In FIG. Figure 2 shows the dependence of the frequency of noise emissions f _p in the operating mode on the frequency f of exceeding the threshold in the preparatory mode for different values of the parameter α. In FIG. 3 shows a typical dependence of the squared signal-to-noise ratio on the avalanche multiplication factor. In FIG. 4 shows a block diagram of the equipment that implements the method. In FIG. 1 shows the cyclogram of the method.

Preparation for receiving signals includes the first and second preparatory cycles (Fig. 1).

Τ ₁ - duration of the first preparatory cycle - setting the threshold U.

T ₂ - the duration of the transition process from the first to the second preparatory cycle, setting the threshold Um.

T ₃ - the duration of the second preparatory cycle - setting the optimal avalanche.

T ₄ - the duration of the transition process from the second preparatory cycle to the operating mode - setting the _threshold U work.

T ₅ - the duration of the operating mode.

It is known [5-8] that in the avalanche-free mode (M=1) the square of the root-mean-square noise σ at the output of the photodiode

where σ ₀ and σ ₁ are, respectively, the root-mean-square values of the non-multiplied (σ ₀ ) and multiplied (σ ₁ ) noise components.

The frequency f of crossing the threshold U by noise bursts in the avalanche-free regime [9]

- частота пересечения шумом нулевого порога; R’’(0) - вторая производная корреляционной функции шума на входе порогового устройства R(τ) при задержке τ=0 [9]. Зная частоты f и f₀ из (2) можно определить отношение порог/шумWhere

- frequency of noise crossing the zero threshold; R''(0) is the second derivative of the correlation function of the noise at the input of the threshold device R(τ) with a delay τ=0 [9]. Knowing the frequencies f and f ₀ from (2), we can determine the threshold/noise ratio

In avalanche mode [5]

where α is the parameter of the noise factor of avalanche multiplication F=M ^α determined by the material and structure of the photodiode [4-7].

Square signal-to-noise ratio

Inverse η ² value (square of noise/signal ratio)

The derivative of this quantity

The minimum noise/signal ratio is provided at dW/dM=0.

Condition (8) is satisfied when

Frequency of noise exceeding the threshold in avalanche mode

Substituting (9) into (10) gives the expression for the frequency of noise exceeding the threshold at Μ=M _opt . Taking into account the always present condition σ ₀ ² >> σ ₁ ²

From (2) and (11) the frequency ratio f(M=M _opt ) and f(M=1) is obtained.

Substituting (3) into (12) gives

As follows from (12) and (13), at constant values of the coefficient a, depending on the design of the photodiode, and U/σ, given by the frequency f, the ratio f(M _opt )/f is completely determined by these parameters and is also a constant parameter of the method.

According to [4], the frequency f can be any in the widest range provided that the conditions

And

The main design ratio of the proposed method follows from (2) and (11).

следуетFrom the equality of these frequencies at

should

откуда отношение порогов во втором и первом подготовительных циклах

whence the ratio of thresholds in the second and first preparatory cycles

и

определяемые выражениями (16) и (17), могут отличаться; это усложняет конструкцию системы, ее отладку и метрологическое обеспечение, что ведет к увеличению трудоемкости и нестабильности характеристик в процессе эксплуатации.With an arbitrary choice of frequency f [4], the parameters

And

defined by expressions (16) and (17) may differ; this complicates the design of the system, its debugging and metrological support, which leads to an increase in labor intensity and instability of characteristics during operation.

Это, с одной стороны, позволяет унифицировать процесс изготовления и отладки, а, с другой стороны, однозначно определяет частоту f. Это обеспечивает однородность продукции и стабильность поддержания ее характеристик. При этом номинальная частота f определяется равенством выражений (16) и (17):According to the proposed solution, the coefficients for increasing the threshold are taken equal to:

This, on the one hand, makes it possible to unify the manufacturing and debugging process, and, on the other hand, it uniquely determines the frequency f. This ensures the homogeneity of the product and the stability of maintaining its characteristics. In this case, the nominal frequency f is determined by the equality of expressions (16) and (17):

откуда

where

where α is a factor determined by the material of the photodiode, f ₀ is the frequency of the noise crossing the zero level, f _p is the maximum allowable frequency of exceeding the threshold by noise emissions in the operating mode. All these parameters are determined by the design of the equipment and are set in advance during the design process.

Adjustment of the operating frequency f can be carried out according to the method of noise automatic threshold adjustment [8]. In FIG. 2 shows the adjustment characteristics for three types of avalanche photodiodes - germanium (α=1), silicon (α=0.5) and gallium arsenide (α=0.3). In the initial section, these characteristics are logarithmic-linear. This facilitates the procedure and metrology of adjustment.

As α decreases, the control characteristic becomes flatter.

Для α=0,5 и α=0,3 крутизна составляет соответственно 150 кГц/дек и 300 кГц/дек, а случайный разброс N за Τ=0,01 с не превышает 2%.For α=1, the slope is 25 kHz f per decade f _p. In this case, for the duration of the cycle Τ=0.1 s, an average of N=2500 noise emissions occurs, that is, the random deviation of N is only

For α=0.5 and α=0.3, the slope is 150 kHz/dec and 300 kHz/dec, respectively, and the random spread of N for Τ=0.01 s does not exceed 2%.

и α, известными с высокой точностью. Это позволяет поддерживать лавинный режим однозначно и устойчиво.The adjustment of the avalanche multiplication factor Μ is carried out in the first preparatory cycle and is completely determined by the parameters

and α known with high accuracy. This makes it possible to maintain the avalanche regime unambiguously and stably.

The peculiarity of the proposed method is the constancy of parameters (16) and (17) selected at the design stage and unchanged during operation under all conditions. The second significant feature - the same frequency f in the first and second preparatory modes allows you to choose it as close as possible to the limiting frequency f ₀ , which makes it possible to realize the minimum time to reach the operating mode and minimal random fluctuations in the hardware frequency estimate when implementing the method. The third important feature of this method is that in the region of the optimal avalanche, there is a weak dependence of the signal-to-noise ratio on the avalanche multiplication factor. In FIG. 3 shows a typical graph for this dependence. With the optimal value of the avalanche multiplication factor M _opt = 26, at the boundaries of a wide range of Μ from 18 to 46, only a five percent deterioration in the signal-to-noise ratio occurs, and in the range of Μ from 21 to 32, the signal-to-noise ratio deteriorates by only 1.5%.

In turn, with the considered technique for entering the operating mode, the nominal value of the frequency f can be maintained with high accuracy. It is easy to show that a 10% error in setting the frequency f leads to a deviation of Μ by only 5%. This makes it possible to achieve almost any accuracy M _opt at the minimum value of the parameter fT, which mainly determines the random deviation of f from the nominal value.

Example 1

Initial data:

(σ ₀ /σ ₁ ) ² =900; α=0.5 (Si avalanche photodiode); f ₀ =10 ⁷ Hz; f=4⋅10 ⁵ Hz; f _pab =1 Hz.

Example 2 (the same at high temperature or with backlight, reducing the ratio σ ₀ /σ ₁ )

Initial data:

(σ ₀ /σ ₁ ) ² =100; α=0.5 (Si avalanche photodiode); f ₀ =10 ⁷ Hz; f=4⋅10 ⁵ Hz; f _pab =1 Hz.

The averaging time Τ of the frequency sensors is selected from the condition (14) taking into account the relationship Τ ~ _Tg [7], where _Tg is the time to enter the mode.

Example 3

Initial data:

(σ ₀ /σ ₁ ) ² =900; α=0.5 (Si avalanche photodiode); f ₀ =10 ⁷ Hz; f=4⋅10 ⁵ Hz; f _pab =1 Hz.

откуда Τ=100/f=2,5⋅10^-4 с.

whence Τ=100/f=2.5⋅10 ^-4 s.

Preparation time for work T _p ~ 2 Τ=5⋅10 ^-4 s.

In the known method, under the same assumptions, the time T _p ~ 1-3 s [8], that is, the gain compared to the analog is four orders of magnitude, which allows the proposed method to be used in high-speed frequency systems. At the same time, the methodological stability of the proposed method ensures its application in the widest operating conditions.

Example 4

Initial data:

f=4⋅10 ⁵ Γc. Τ=2.5⋅10 ^-4 s.

fT=100. M=26.

При этом относительное среднеквадратическое отклонение σ_М коэффициента лавинного умножения от номинального значения составит 5%. То есть в стандартных пределах ±3σ_М окажется диапазон Μ от 22 до 30, что, как видно из фиг. 3, может привести к ухудшению отношения сигнал/шум относительно потенциального значения не более чем на 1%.The relative standard deviation of the frequency setting error f is

In this case, the relative standard deviation σ _M of the avalanche multiplication coefficient from the nominal value will be 5%. That is, within the standard limits of ±3σ _M there will be a range of Μ from 22 to 30, which, as can be seen from Fig. 3 can lead to a deterioration in the signal-to-noise ratio relative to the potential value by no more than 1%.

A possible receiver according to the proposed method (Fig. 4) contains an avalanche photodiode 1, the output of which is connected through a matching amplifier 2 and controlled attenuators 3 and 4 to the input of a threshold pulse shaper 5. The output of the latter is connected to the inputs of frequency sensors 6 and 7. Sensor 6 is connected to the control input of the threshold shaper 5, and sensor 7 to the bias source of the photodiode 8. Sensors 6 and 7 and attenuators 3 and 4 are connected to the control unit 9. A key 10 is installed at the output of the threshold shaper, connected to the control unit.

The method is carried out as follows.

Previously (at the design stage) set: the frequency f ₀ determined by the bandwidth of the receiving path 1-4 to the input of the threshold shaper; frequency f, which satisfies the above limitations and features of the hardware used; frequency f _pab - according to the technical requirements; parameter a determined by the design of the photodiode; averaging interval Τ of the frequency encoder.

и тем самым поднимающий эквивалентный порог до уровня

Одновременно с помощью блока управления включают датчик частоты 7, управляющий коэффициентом лавинного умножения фотодиода путем подачи на него напряжения смещения, при котором частота шумовых срабатываний в лавинном режиме f_M снова станет равна частоте f, установленной в безлавинном режиме.Before receiving signals, a preparatory mode is switched on, during which the optimal parameters of the receiving path are set - the avalanche multiplication factor of the photodiode and the threshold of the threshold device. To this end, at the first stage, using the control unit 9, attenuators 3 and 4 are opened and a low bias voltage level is set on the bias source 8, corresponding to the avalanche multiplication factor Μ=1. At the same time, the response threshold U of the shaper 5 is set so that the frequency f of the noise exceeding the threshold corresponds to the nominal value. Upon reaching the steady value of the threshold U, using the control unit, turn on the attenuator 3, which introduces attenuation

and thereby raising the equivalent threshold to the level

At the same time, using the control unit, the frequency sensor 7 is turned on, which controls the avalanche multiplication factor of the photodiode by applying a bias voltage to it, at which the frequency of noise operations in the avalanche mode f _M will again become equal to the frequency f set in the avalanche-free mode.

Одновременно открывают ключ 10, пропускающий выходные импульсы формирователя 5 на выход. Таким образом, осуществляется переход в рабочий режим приема оптических сигналов. При этом частота шумовых срабатываний на выходе не превышает заданного допустимого значения f_p, а коэффициент лавинного умножения фотодиода Μ=М_опт обеспечивает максимальное отношение сигнал/шум.After entering the mode f _M =f, the bias voltage of the photodiode is fixed at the achieved level with the help of the control unit and the attenuator 4 is switched on by the command from the control unit, introducing attenuation

At the same time, the key 10 is opened, which passes the output pulses of the shaper 5 to the output. Thus, the transition to the operating mode of receiving optical signals is carried out. In this case, the frequency of noise operations at the output does not exceed the specified allowable value f _p , and the avalanche multiplication factor of the photodiode Μ=M _opt provides the maximum signal-to-noise ratio.

Thus, the described method solves the problem - simplifying the preparatory mode and ensuring optimal sensitivity in all operating conditions with a guaranteed probability of false positives and maximum speed.

Information sources

1. Ross M. Laser receivers. - M.: Mir., 1969. - 520 p.

2. RF patent No. 2248670. The device for switching on the avalanche photodiode in the receiver of optical radiation. 2005

3 US Pat. 4,077,718. Receiver for optical radar. 1978.

4. RF patent No. 2755602. Method for threshold detection of optical signals. - prototype.

5. Vilner V.G., Leichenko Yu.A., Motenko B.N. Analysis of the input circuit of a photodetector with an avalanche photodiode and anti-noise correction. Optical-mechanical industry, 1981, No. 9, - S. 59.

6. Anisimova I.D. and others. Semiconductor photodetectors: Ultraviolet, visible and near infrared ranges of the spectrum. Ed. IN AND. Stafeev. - M.: Radio and communication, 1984. - 216 p.

7. A. M. Filachev, I. I. Taubkin, and M. A. Trishenkov, Solid State Photoelectronics. Photodiodes. - M.: Fizmatkniga, 2011. - 448 p.

8. Vilner V.G. Design of threshold devices with noise threshold stabilization. - Optical-mechanical industry, 1984, No. 5, S. 39-41.

9. Tikhonov V.I. Emissions of random processes. Ch. ed. Phys.-Math. lit., 1970, p. 392.

Claims (2)

1. A method for threshold detection of optical signals using an avalanche photodiode, including threshold processing of signals and the formation of output pulses using a threshold device when the signal from the output of the photodiode exceeds a predetermined response threshold, characterized in that the avalanche-free mode of the photodiode is preliminarily set by applying a low bias voltage to it and stabilize the threshold level by the frequency of noise exceeding the threshold f, then fix the threshold at this level, increase it by

times and increase the bias voltage until the frequency of noise exceeding the threshold reaches the initial value f, after which the bias voltage of the photodiode is fixed, the threshold level is increased by

times, moreover, set the threshold increase parameters equal to

and the frequency f is set in accordance with the relation

where α is the coefficient determined by the material of the photodiode, f ₀ is the frequency of noise crossing the zero level, f _p is the maximum allowable frequency of exceeding the threshold by noise emissions in the operating mode, after which they start receiving signals. 2. The method according to claim 1, characterized in that the parameters α and f ₀ are determined during preliminary measurements in the design process, and the values f _p ,

and f are set as fixed hardware parameters.

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