CN104901747A - Adaptive gain adjustment method based on avalanche photo diode (APD) amplifier at front end of visible light receiver - Google Patents

Abstract

The invention discloses an adaptive gain adjustment method based on an avalanche photo diode (APD) amplifier at a front end of a visible light receiver. The method comprises the following steps of determining a feedback capacitance value; determining a feedback resistance value; amplifying the voltage of an output end of the APD amplifier, then converting the voltage by an A/D converter, and then gathering into a singlechip, wherein the A/D converter samples signals every t seconds; according to the voltage gathered by the singlechip, computing light current Is when APD gain is 1; computing the mean value of the light current Is through a moving average filter method, so as to obtain the value of direct component IL thereof; deducing a relation expression of the output voltage of the singlechip and IL, computing a series of value according to the expression, and making a data table; looking up the data table according to computed IL, and enabling the singlechip to output a control voltage to a voltage converter; and outputting a voltage by the voltage converter to a negative end of the APD to be used as a bias voltage of the APD. Through adoption of the method, according to different received light intensity, the bias voltage of the APD can be adaptively adjusted, so that the signal to noise ratio of the receiver is optimized.

Description

A kind of adaptive gain method of adjustment based on visible ray receiver front end APD amplifier

Technical field

The present invention relates to a kind of adaptive gain method of adjustment based on visible ray receiver front end APD amplifier, belong to visible light communication field.

Background technology

Visible light communication technology is the focus of current communications field research, possesses that transmission rate is high, frequency spectrum resource is sufficient, good confidentiality and the advantage such as harmless, and application prospect is very good, can complement one another with now widely used technology for radio frequency.From use transmitting antenna different with reception antenna in radio-frequency (RF) communication system, in visible light communication system, the front end of transmitter and receiver all uses photoelectric device.Wherein, transmitter front ends commonly uses white light LEDs, converts electrical signals to light signal; Receiver front end commonly uses APD, and light signal is converted to the signal of telecommunication.

Just based on transmitter front ends be white light LEDs, receiver front end is the visible light communication system of APD, we consider to study the adaptive gain adjustment technology in visible ray receiver.

The overall noise factor computing formula of n level cascade circuit is as follows:

$F=F_1+\frac{F_2-1}{G_1}+\frac{F_3-1}{G_1{G}_2}+...+\frac{F_n-1}{G_1{G}_2...G_{n-1}}$

Wherein, F is cascade overall noise factor, F ₁to F _nfor the first order is to the noise factor of n-th grade of circuit, G ₁to G _nfor the first order is to the power gain of n-th grade of circuit.

Obviously, the noise factor of front stage circuits on the impact of overall noise factor much larger than rear class.In general, the overall noise factor of cascade circuit depends primarily on the first order or front two-stage, and the noise factor of tertiary circuit is little of negligible on the impact of overall noise factor.Noise factor is defined as the ratio of input signal-to-noise ratio and output signal-to-noise ratio.When input signal-to-noise ratio one timing, the noise factor of circuit just determines the size of its output signal-to-noise ratio.

In visible ray receiver, the gain of first order avalanche photodide (APD) usually smaller (tens), and the gain of second level trans-impedance amplifier (TIA) larger (thousands of), therefore the signal-to-noise performance of visible ray receiver depends primarily on first order APD and second level TIA.Like this, just problem can be reduced to from optimizing whole receiver the front stage circuits that only needs optimization is made up of APD and TIA.For the ease of describing, by the front stage circuits that is made up of APD and TIA referred to as " APD amplifier ".

We carry out Accurate Model to the signal of APD amplifier and noise at consideration, and its model as shown in Figure 1.Analysis obtains the output signal-to-noise ratio of APD amplifier about APD gain M and TIA feedback resistance R ₂relational expression as follows:

$\mathrm{SNR}=\frac{V_{\mathrm{so}}^2}{E_{\mathrm{no}}^2}$

Wherein, represent the photoelectric signal of TIA output, its mean-square value is:

$V_{\mathrm{so}}^2=I_s^2{\left|A_s\right|}^2=\frac{1}{4}{m}^2{I}_L^2{M}^2{R}_2^2$

represent the total voltage noise of TIA output, its mean-square value is:

$\begin{array}{c}{E}_{\mathrm{no}}^2={\∫}_0^{\∞}{I}_n^2{\left|A_s\right|}^2\mathrm{df}+{\∫}_0^{\∞}{E}_{\mathrm{na}}^2{\left|A_n\right|}^2\mathrm{df}\\ =\frac{\mathrm{\π}{C}_2{f}_t}{2(C_1+C_2+2\mathrm{\π}{R}_2{C}_2^2{f}_t)}[2e(I_L+I_{\mathrm{dg}})M^{2}F{R}_2^2\\ +(2e{I}_{\mathrm{ds}}+I_{\mathrm{na}}^2+\frac{4\mathrm{kT}}{R_1}+\frac{E_{\mathrm{na}}^2}{R_1^2})R_2^2+\frac{2\mathrm{\π}{E}_{\mathrm{na}}^2(C_1+C_2)f_t{R}_1+4\mathrm{kT}{R}_1+2E_{\mathrm{na}}^2}{R_1}{R}_2\\ +E_{\mathrm{na}}^2]\end{array}$

Wherein, I _srepresent flashlight electric current, I _nsrepresent the shot noise of photoelectric current, I _ndrepresent the shot noise of dark current, R ₁and C ₁the inverting input representing APD and operational amplifier respectively combined resistance over the ground and complex capacitance, R ₂and C ₂represent feedback resistance and the feedback capacity of trans-impedance amplifier respectively, represent resistance R ₁thermal noise, represent resistance R ₂thermal noise, I _narepresent the equivalent inpnt current noise of amplifier, E _narepresent the equivalent inpnt voltage noise of amplifier.

For the ease of calculating, using 1/SNR as target function, namely might as well have:

$\begin{array}{c}1/\mathrm{SNR}=\frac{2\mathrm{\π}{C}_2{f}_t}{m^2{I}_L^2(C_1+C_2+2\mathrm{\π}{R}_2{C}_2^2{f}_t)}\{2e(I_L+I_{\mathrm{dg}})F\\ +[2e{I}_{\mathrm{ds}}+I_{\mathrm{na}}^2+\frac{4\mathrm{kT}}{R_1}+\frac{E_{\mathrm{na}}^2}{R_1^2}+\frac{2\mathrm{\π}{E}_{\mathrm{na}}^2(C_1+C_2)f_t{R}_1+4\mathrm{kT}{R}_1+2E_{\mathrm{na}}^2}{R_1{R}_2}\\ +\frac{E_{\mathrm{na}}^2}{R_2^2}\left]M^{-2}\right\}\end{array}$

Consider in side circuit application, the constraints that signal bandwidth and circuit stability are brought.

So-called signal bandwidth constraint, namely requires actual signal bandwidth f ₀can not more than the three dB bandwidth f of TIA _b, to ensure the high-gain of TIA to signal.Namely have:

$f_0^2\≤f_B^2=\frac{1}{2}[\sqrt{f_p^4+f_x^4+6f_p^2{f}_x^2}-(f_p^2+f_x^2)]$

Abbreviation obtains:

$R_2\≤\frac{1}{2\mathrm{\π}{f}_o{C}_2}\sqrt{\frac{C_2^2{f_t}^2-{(C_1+C_2)}^2{f}_0^2}{C_2^2{f_t}^2+{(C_1+C_2)}^2{f}_0^2}}$

So-called circuit stability constraint, namely requires the phase margin that design is certain, prevents circuit from occurring oscillatory occurences.Make phase margin be greater than 45 °, then must meet:

f _p≤f _x

Abbreviation obtains:

$R_2\≥\frac{C_1+C_2}{2\mathrm{\π}{f}_t{C}_2^2}$

Make optimization problem have solution, must meet:

$\frac{C_1+C_2}{2\mathrm{\π}{f}_t{C}_2^2}\≤\frac{1}{2\mathrm{\π}{f}_0{C}_2}\sqrt{\frac{C_2^2{f_t}^2-{(C_1+C_2)}^2{f}_0^2}{C_2^2{f_t}^2+{(C_1+C_2)}^2{f}_0^2}}$

Abbreviation obtains:

$C_2\≥\frac{C_1}{\sqrt{\sqrt{2}-1}{f}_t/f_0-1}$

Due to feedback capacity C ₂value larger, the three dB bandwidth of TIA is less, therefore C ₂value should be little as far as possible under the condition meeting above-mentioned inequality.

Analyze known, feedback resistance R ₂larger, 1/SNR is less.In order to make 1/SNR get minimum value, then R ₂maximum should be got, that is:

$R_{2_{\mathrm{opt}}}=\frac{1}{2\mathrm{\π}{f}_0{C}_2}\sqrt{\frac{C_2^2{f_t}^2-{(C_1+C_2)}^2{f}_0^2}{C_2^2{f_t}^2+{(C_1+C_2)}^2{f}_0^2}}$

1/SNR is non-monotonic with the change of APD gain M.In order to make 1/SNR get minimum value, the expression formula of 1/SNR should be differentiated to M, then get extreme value, abbreviation obtains:

$\begin{array}{c}{M}_{\mathrm{opt}}\\ ={\left[\frac{(2e{I}_{\mathrm{ds}}+I_{\mathrm{na}}^2)R_1^2{R}_2^2+2\mathrm{\π}{E}_{\mathrm{na}}^2(C_1+C_2)f_t{R}_1^2{R}_2+E_{\mathrm{na}}^2{(R_1+R_2)}^2+4\mathrm{kT}{R}_1{R}_2(R_1+R_2)}{e(I_L+I_{\mathrm{dg}})k_F{R}_1^2{R}_2^2}\right]}^{\frac{1}{3}}\end{array}$

R in this expression formula ₂i.e. optimal value

Summary of the invention

In order to improve the signal-to-noise performance of visible ray receiver, the invention provides a kind of adaptive gain method of adjustment based on visible ray receiver front end APD amplifier, it is characterized in that: design according to the joint optimization result to APD amplifier, the bias voltage of APD can be regulated according to different receiving light powers adaptively, thus make output signal-to-noise ratio reach optimum.

To achieve these goals, the technical solution used in the present invention is:

Based on an adaptive gain method of adjustment for visible ray receiver front end APD amplifier, it is characterized in that: according to right

The joint optimization result of APD amplifier and designing, can regulate according to different receiving light powers that APD's is inclined adaptively

Put voltage, thus make output signal-to-noise ratio reach optimum, specifically comprise the following steps:

1) feedback capacity C is determined ₂value, feedback capacity C ₂value must meet with lower inequality:

$C_2\≥\frac{C_1}{\sqrt{\sqrt{2}-1}{f}_t/f_0-1}$

Wherein, C ₁for the inverting input complex capacitance over the ground of APD and operational amplifier, f ₀for actual signal bandwidth, f _tfor the gain bandwidth product of operational amplifier;

Under the condition meeting above-mentioned inequality, get a little as far as possible electric capacity as feedback capacity C ₂.

2) feedback resistance R is determined ₂value, best feedback resistance expression formula as follows:

$R_{2_{\mathrm{opt}}}=\frac{1}{2\mathrm{\π}{f}_0{C}_2}\sqrt{\frac{C_2^2{f_t}^2-{(C_1+C_2)}^2{f}_0^2}{C_2^2{f_t}^2+{(C_1+C_2)}^2{f}_0^2}}$

Wherein, C ₂for step 1) in the feedback capacitance decided; By what calculate as feedback resistance R ₂value.

3) by the voltage signal V of APD amplifier out _o1first carry out voltage amplification, then collect in single-chip microcomputer after A/D conversion, wherein A/D once samples to signal every t second.

The voltage signal V of APD amplifier out _o1expression formula as follows:

V _o1＝I _sMR ₂

Wherein, M is APD gain, R ₂for step 2) in the feedback resistance value decided, I _sfor photoelectric current when APD gain is 1, its expression formula is as follows:

I _s＝I _L+mI _LS(t)

Wherein, I _lfor photoelectric current I _sdC component, mI _ls (t) is photoelectric current I _salternating current component, m is modulation depth, and S (t) is sinusoidal carrier signal.

Here, we will carry out n-th Gain tuning at hypothesis, and so the APD gain of current time is the result after last (namely (n-1)th time) Gain tuning, uses M _n-1represent; Similarly, photoelectric current I when being 1 by the APD gain of current time _{s (n-1)}represent; By APD amplifier output voltage V _{o1 (n-1)}represent.Now, V _{o1 (n-1)}expression formula can again be expressed as follows:

V _o1(n-1)＝I _s(n-1)M _n-1R ₂

In this formula, M _n-1and R ₂all known, as long as measure V _{o1 (n-1)}value, just can obtain I _{s (n-1)}.

But, due to V _o1usually smaller (millivolt magnitude), use gain is needed to be G ₁voltage amplifier, be enlarged into voltage signal V _o2(1 ~ 5V), then be input in A/D.V _o2expression formula as follows:

V _o2＝V _o1G ₁

In this formula, G ₁known, as long as measure V _o2value, just can obtain V _o1.

Finally, digital voltage V A/D is converted to _o3collect in single-chip microcomputer.Adopt the A/D of degree of precision, can be similar to and think V _o3=V _o2.

Therefore, as long as know the voltage V that single-chip microcomputer collects _o3value, just can according to V _o3→ V _o2→ V _o1→ I _sderivation path, draw I _svalue.

4) according to the voltage V that single-chip microcomputer collects _o3value, calculate photoelectric current I when APD gain is 1 _svalue.

From step 3), we know, can according to V _o3→ V _o2→ V _o1→ I _sderivation path, by V _o3release I _s.Due to V _o3→ V _o2→ V _o1→ I _srelation be all linear, we also directly can release V easily _o3with I _srelational expression, as follows:

$I_s=\frac{V_{o1}}{{\mathrm{MR}}_2}=\frac{V_{o2}}{{\mathrm{MR}}_2{G}_1}=\frac{V_{o3}}{{\mathrm{MR}}_2{G}_1}$

Wherein, R ₂and G ₁be all known quantitative, M represents current APD gain (i.e. the result of last Gain tuning), although can change along with each Gain tuning, is known.

Therefore, according to this expression formula, write the division routine of single-chip microcomputer, can directly by V _o3calculate I _s.

5) moving average filter method is utilized to obtain I _saverage, thus obtain I _lvalue.

By step 3) known, A/D once samples to signal every t second.That is, every t second, single-chip microcomputer just can collect a voltage signal V _o3.According to step 4), for each voltage signal V _o3, corresponding I can be calculated _s.That is, every t second, we just can calculate an I _s.

Owing to being optimized the result that obtains to APD amplifier only and photoelectric current DC component I _lrelevant, therefore need according to I _svalue, obtain its DC component I _lvalue.According to step 3), photoelectric current I _sexpression formula as follows:

I _s＝I _L+mI _LS(t)

Wherein, the alternating current component mI of photoelectric current _lthe average of S (t) is 0, and DC component I _laverage be exactly I _l, so photoelectric current I _saverage just equal its DC component I _l.Namely have:

$\stackrel{\‾}{I_s}=I_L$

Therefore, as long as obtain photoelectric current I _saverage just can obtain its DC component I _lvalue.

So-called moving average filter method, the N number of I will obtained continuously exactly _sregard a queue as, the length of queue is fixed as N.According to the first in first out of queue, calculate a new I at every turn _sdata, all put it into tail of the queue, and give up to fall the data of original head of the queue.Then often obtain a new data, all the N number of data in queue are done an arithmetic mean.Like this, the mean value of photoelectric current can just be obtained thus obtain I _l.

Particularly, for n-th Gain tuning, single-chip microcomputer collects a voltage signal V _{o3 (n-1)}, calculate corresponding photoelectric current I _{s (n-1)}.Originally data I was had in the long queue for N _{s (n-1-N)}i _{s (n-2)}, these data in front N Gain tuning stored in; By new data I _{s (n-1)}put into tail of the queue, and give up to fall the data of original head of the queue, the data namely in current queue are I _{s (n-N)}i _{s (n-1)}; Do sums on average to the N number of data in current queue, just can obtain I _{l (n-1)}value, as follows:

$I_{L(n-1)}=\stackrel{\‾}{I_{s(n-1)}}=\frac{I_{s(n-N)}+I_{s(n-N+1)}+...+I_{s(n-1)}}{N}$

6) according to the output voltage V of single-chip microcomputer _o4with photoelectric current DC component I _lrelation, a series of value computed in advance, and make tables of data.

By step 5) can I be obtained _lvalue.

And I _lwith best APD gain M _optrelational expression as follows:

$\begin{array}{c}{M}_{\mathrm{opt}}\\ ={\left[\frac{(2e{I}_{\mathrm{ds}}+I_{\mathrm{na}}^2)R_1^2{R}_2^2+2\mathrm{\π}{E}_{\mathrm{na}}^2(C_1+C_2)f_t{R}_1^2{R}_2+E_{\mathrm{na}}^2{(R_1+R_2)}^2+4\mathrm{kT}{R}_1{R}_2(R_1+R_2)}{e(I_L+I_{\mathrm{dg}})k_F{R}_1^2{R}_2^2}\right]}^{\frac{1}{3}}\end{array}$

Wherein, e is electron charge, and k is Boltzmann constant, and T is absolute temperature, I _dgfor participating in the body leakage current doubled in APD, I _dsfor not participating in the face leakage current doubled in APD, R ₁for APD and amplifier inverting input combined resistance over the ground, I _nafor the equivalent inpnt current noise of amplifier, E _nafor the equivalent inpnt voltage noise of amplifier, k _ffor the charge carrier ionization ratio in APD, be normally defined the ratio of the ionization probability of hole and electronics.C ₂for step 1) in the feedback capacitance decided, R ₂for step 2) in the feedback resistance value decided.

Therefore, can according to I _lrelease best APD gain M _opt, derivation path is I _l→ M _opt

The gain M of APD is relevant with its bias voltage V, and relation is as follows:

$M=\frac{1}{1-{\left(\frac{V}{V_B}\right)}^n}$

In this formula, V _bfor the puncture voltage of APD, n is the amount relevant with APD material, structure, can obtain according to V-M curve on APD databook.

Therefore, if known best APD gain M _opt, the just bias voltage V of APD can be released _opt, derivation path is M _opt→ V _opt

Due to the bias voltage V usual larger (a few hectovolt) of APD, single-chip microcomputer cannot directly export so large voltage, therefore needs after single-chip microcomputer, to connect an electric pressure converter, by the control voltage V that single-chip microcomputer exports _o4convert voltage V to.The input voltage V of voltage conversion chip _o4and be linear relationship between output voltage V, that is:

V＝k _VV _o4

Wherein, k _vfor voltage transitions coefficient, can obtain according to the input voltage provided in the databook of voltage conversion chip-output voltage curve chart matching.

Therefore, according to the just bias voltage V of APD _opt, the control voltage V that single-chip microcomputer needs to export can be released _o4, derivation path is V _opt→ V _o4

In sum, if photoelectric current DC component I when known APD gain is 1 _l, just can by a series of derivation, when drawing to make the output signal-to-noise ratio of APD amplifier maximum, single-chip microcomputer needs the control voltage V exported _o4, derivation path is: I _l→ M _opt→ V _opt→ V _o4

Due to I _l→ M _opt→ V _opt→ V _o4in there is non-linear relation, cannot directly write Single Chip Microcomputer (SCM) program and calculate.Therefore, we can only according to some given I _l, corresponding V computed in advance _o4value, and make a tables of data, stored in for subsequent use in single-chip microcomputer.Need according to I _lcalculate corresponding V _o4time, table look-at.

7) according to step 5) in the I that obtains _l, finding step 6) in the tables of data that makes, allow single-chip microcomputer export a control voltage V _o4to in electric pressure converter.

8) electric pressure converter exports the negative terminal of voltage V a to APD, as the bias voltage of APD.

Accompanying drawing explanation

Fig. 1 is FB(flow block) of the present invention;

Fig. 2 is the equivalent noise model of APD amplifier;

Fig. 3 is under different response light electric currents, to the optimum results of TIA feedback resistance;

Fig. 4 is under different response light electric currents, to the optimum results of gain in APD;

Fig. 5 is under different response light electric currents, to the optimum results of APD amplifier output signal-to-noise ratio;

Fig. 6 is adaptive gain Circuit tuning figure of the present invention.

Embodiment

Below in conjunction with the drawings and specific embodiments, illustrate the present invention further, these embodiments should be understood only be not used in for illustration of the present invention and limit the scope of the invention, after having read the present invention, the amendment of those skilled in the art to the various equivalent form of value of the present invention has all fallen within the application's claims limited range.

As shown in Figure 1, specific embodiment of the invention step is illustrated.It proposes based on to the optimum results of APD amplifier.

As shown in Figure 2, the equivalent noise model of APD amplifier is illustrated.The present invention carries out combined optimization based on this model to APD and TIA just, thus makes the output signal-to-noise ratio of APD amplifier optimum.

In an embodiment, the APD that we adopt is S5343, and uses OPA657 amplifier to realize TIA, and voltage conversion chip adopts GFHV-201P.In addition, also setting actual signal bandwidth is 50MHz, and modulation depth is 1, and room temperature is 290K etc.In practice, photoelectric current DC component I _lbe unknown, need the voltage signal V gathered according to single-chip microcomputer _o3release.Herein, we directly suppose photoelectric current I _lwhen illuminance for room lighting is 1000lux, the response current of APD (is 1.08 × 10 as calculated ^-6a), be used for implementation step of demonstrating, and verify the reasonability of optimum results.In addition, the gain M=50 of APD after last Gain tuning will also be supposed.Each parameter value is as following table:

When each parameter value is as this table, optimum results is:

TIA feedback resistance:

$R_{2_{\mathrm{opt}}}=\frac{1}{2\mathrm{\π}{f}_0{C}_2}\sqrt{\frac{C_2^2{f_t}^2-{(C_1+C_2)}^2{f}_0^2}{C_2^2{f_t}^2+{(C_1+C_2)}^2{f}_0^2}}=1.41\×{10}^3\mathrm{\Ω}$

Gain in APD:

$\begin{array}{c}{M}_{\mathrm{opt}}\\ ={\left[\frac{(2e{I}_{\mathrm{ds}}+I_{\mathrm{na}}^2)R_1^2{R}_2^2+2\mathrm{\π}{E}_{\mathrm{na}}^2(C_1+C_2)f_t{R}_1^2{R}_2+E_{\mathrm{na}}^2{(R_1+R_2)}^2+4\mathrm{kT}{R}_1{R}_2(R_1+R_2)}{e(I_L+I_{\mathrm{dg}})k_F{R}_1^2{R}_2^2}\right]}^{\frac{1}{3}}\\ =70.71\end{array}$

APD amplifier output signal-to-noise ratio:

${\mathrm{SNR}}_{\mathrm{opt}}=10\log \left(\frac{V_{\mathrm{so}}^2}{E_{\mathrm{no}}^2}\right)=31.96\mathrm{dB}$

The concrete steps of the adaptive gain method of adjustment that the present invention proposes are as follows:

1) feedback capacity C is determined ₂value.Feedback capacity C ₂value must meet with lower inequality:

$C_2\≥\frac{C_1}{\sqrt{\sqrt{2}-1}{f}_t/f_0-1}=1.03\mathrm{pF}$

Under the condition meeting inequality, a little as far as possible electric capacity should be got as feedback capacity C ₂.Therefore, we get feedback capacity C ₂=2pF.

2) the best feedback resistance R of TIA is calculated ₂value as follows:

$R_{2_{\mathrm{opt}}}=\frac{1}{2\mathrm{\π}{f}_0{C}_2}\sqrt{\frac{C_2^2{f_t}^2-{(C_1+C_2)}^2{f}_0^2}{C_2^2{f_t}^2+{(C_1+C_2)}^2{f}_0^2}}=1.41\×{10}^3\mathrm{\Ω}$

3) by the voltage signal V of APD amplifier out _o1first carry out voltage amplification, then collect in single-chip microcomputer after A/D conversion, wherein A/D once samples to signal every t second.

The voltage signal V of APD amplifier out _o1=76.14 × [1+sin (10 ⁸π t)] mV

The gain of getting voltage amplifier is G ₁after=20, V _o1through the voltage signal V that this amplifier exports _o2for:

V _o2＝V _o1G ₁＝3.81×[1+sin(10 ⁸πt)]V

V _o2voltage V after A/D conversion _o3digital voltage, the analogue value of its representative and V _o2equal, be 3.81 × [1+sin (10 ⁸π t)] V.

4) according to the voltage V that single-chip microcomputer collects _o3value, photoelectric current I when APD gain is 1 can be calculated _svalue.Computing formula is as follows:

$I_s=\frac{V_{o3}}{{\mathrm{MR}}_2{G}_1}=1.08\×[1+\sin \left({10}^8\mathrm{\πt}\right)]\×{10}^{-6}A$

5) moving average filter method is utilized to obtain I _saverage, thus obtain I _lvalue.

$I_L=\stackrel{\‾}{I_s}=1.08\×{10}^{-6}A$

6) according to the output voltage V of single-chip microcomputer _o4photoelectric current direct current component I when being 1 with APD gain _lrelation, a series of value computed in advance, and make tables of data.

By step 5), obtain I _l=1.08 × 10 ^-6a

Then according to I _lwith best APD gain M relational expression, best APD gain M can be calculated as follows:

$\begin{array}{c}{M}_{\mathrm{opt}}\\ ={\left[\frac{(2e{I}_{\mathrm{ds}}+I_{\mathrm{na}}^2)R_1^2{R}_2^2+2\mathrm{\π}{E}_{\mathrm{na}}^2(C_1+C_2)f_t{R}_1^2{R}_2+E_{\mathrm{na}}^2{(R_1+R_2)}^2+4\mathrm{kT}{R}_1{R}_2(R_1+R_2)}{e(I_L+I_{\mathrm{dg}})k_F{R}_1^2{R}_2^2}\right]}^{\frac{1}{3}}\\ =70.71\end{array}$

In APD, the relational expression of gain M and bias voltage V is as follows:

$M=\frac{1}{1-{\left(\frac{V}{V_B}\right)}^n}$

According to the V-M curve provided in S5343 databook, matching obtains n=2.

Now the bias voltage V of APD should be:

$V=V_B{(1-\frac{1}{M})}^{\frac{1}{n}}=148.94V$

And the input voltage V of electric pressure converter _o4and be linear relationship between output voltage V, that is:

V＝k _VV _o4

According to the V in GFHV-201P databook _in~ V _outcurve, matching obtains k _v=100/3

Then now control voltage V _o4should be:

V _o4＝V/k _V＝4.47V

7) according to the input voltage signal V of single-chip microcomputer ₁, finding step 4) in the form that makes, allow single-chip microcomputer export a control voltage V _o4to in electric pressure converter.

According to 6) the middle result calculated, the control voltage V that single-chip microcomputer exports _o4=4.47V

8) electric pressure converter exports the negative terminal of voltage V a to APD, as the bias voltage of APD.

According to 6) the middle result calculated, the output voltage V=148.94V of electric pressure converter

As shown in Fig. 3,4,5, under illustrating different response light electric currents, to the optimum results of the output signal-to-noise ratio of gain and APD amplifier in TIA feedback resistance, APD.Except photoelectric current I _lother outer parameter value is still as shown in table 1.

As shown in Figure 6, the schematic diagram of adaptive gain Circuit tuning of the present invention is illustrated.It is that the adaptive gain method of adjustment proposed according to the present invention designs.

Claims (2)

1. the adaptive gain method of adjustment based on visible ray receiver front end APD amplifier, it is characterized in that: design according to the optimum results of APD amplifier, the bias voltage of APD can be regulated according to different receiving light powers adaptively, thus make output signal-to-noise ratio reach optimum.

2. the adaptive gain method of adjustment based on visible ray receiver front end APD amplifier according to claim 1, comprises the following steps:

1) feedback capacity C is determined ₂value, feedback capacity C ₂value must meet with lower inequality:

$C_2\≥\frac{C_1}{\sqrt{\sqrt{2}-1}{f}_t/f_0-1}$

Wherein, C ₁for the inverting input complex capacitance over the ground of APD and operational amplifier, f ₀for actual signal bandwidth, f _tfor the gain bandwidth product of operational amplifier;

Under the condition meeting above-mentioned inequality, get a little as far as possible electric capacity as feedback capacity C ₂.

2) feedback resistance R is determined ₂value, best feedback resistance expression formula as follows:

$R_{2_{\mathrm{opt}}}=\frac{1}{2\mathrm{\π}{f}_0{C}_2}\sqrt{\frac{C_2^2{f}_t^2-{(C_1+C_2)}^2{f}_0^2}{C_2^2{f}_t^2+{(C_1+C_2)}^2{f}_0^2}}$

Wherein, C ₂for step 1) in the feedback capacitance decided; By what calculate as feedback resistance R ₂value.

3) by the voltage signal V of APD amplifier out _o1first carry out voltage amplification, then collect in single-chip microcomputer after A/D conversion, wherein A/D once samples to signal every t second.

The voltage signal V of APD amplifier out _o1expression formula as follows:

V _o1＝I _sMR ₂

Wherein, M is APD gain, R ₂for step 2) in the feedback resistance value decided, I _sfor photoelectric current when APD gain is 1, its expression formula is as follows:

I _s＝I _L+mI _LS(t)

Wherein, I _lfor photoelectric current I _sdC component, mI _ls (t) is photoelectric current I _salternating current component, m is modulation depth, and S (t) is sinusoidal carrier signal.

Here, we will carry out n-th Gain tuning at hypothesis, and so the APD gain of current time is the result after last (namely (n-1)th time) Gain tuning, uses M _n-1represent; Similarly, photoelectric current I when being 1 by the APD gain of current time _{s (n-1)}represent; By APD amplifier output voltage V _{o1 (n-1)}represent.Now, V _{o1 (n-1)}expression formula can again be expressed as follows:

V _o1(n-1)＝I _s(n-1)M _n-1R ₂

In this formula, M _n-1and R ₂all known, as long as measure V _{o1 (n-1)}value, just can obtain I _{s (n-1)}.

But, due to V _o1usually smaller (millivolt magnitude), use gain is needed to be G ₁voltage amplifier, be enlarged into voltage signal V _o2(1 ~ 5V), then be input in A/D.V _o2expression formula as follows:

V _o2＝V _o1G ₁

In this formula, G ₁known, as long as measure V _o2value, just can obtain V _o1.

Finally, digital voltage V A/D is converted to _o3collect in single-chip microcomputer.Adopt the A/D of degree of precision, can be similar to and think V _o3=V _o2.

Therefore, as long as know the voltage V that single-chip microcomputer collects _o3value, just can according to V _o3→ V _o2→ V _o1→ I _sderivation path, draw I _svalue.

4) according to the voltage V that single-chip microcomputer collects _o3value, calculate photoelectric current I when APD gain is 1 _svalue.

From step 3), we know, can according to V _o3→ V _o2→ V _o1→ I _sderivation path, by V _o3release I _s.Due to V _o3→ V _o2→ V _o1→ I _srelation be all linear, we also directly can release V easily _o3with I _srelational expression, as follows:

$I_s=\frac{V_{o1}}{{\mathrm{MR}}_2}=\frac{V_o}{{\mathrm{MR}}_2{G}_1}=\frac{V_{o3}}{{\mathrm{MR}}_2{G}_1}$

Wherein, R ₂and G ₁be all known quantitative, M represents current APD gain (i.e. the result of last Gain tuning), although can change along with each Gain tuning, is known.

Therefore, according to this expression formula, write the division routine of single-chip microcomputer, can directly by V _o3calculate I _s.

5) moving average filter method is utilized to obtain I _saverage, thus obtain I _lvalue.

By step 3) known, A/D once samples to signal every t second.That is, every t second, single-chip microcomputer just can collect a voltage signal V _o3.According to step 4), for each voltage signal V _o3, corresponding I can be calculated _s.That is, every t second, we just can calculate an I _s.

Owing to being optimized the result that obtains to APD amplifier only and photoelectric current DC component I _lrelevant, therefore need according to I _svalue, obtain its DC component I _lvalue.According to step 3), photoelectric current I _sexpression formula as follows:

I _s＝I _L+mI _LS(t)

Wherein, the alternating current component mI of photoelectric current _lthe average of S (t) is 0, and DC component I _laverage be exactly I _l, so photoelectric current I _saverage just equal its DC component I _l.Namely have:

$\stackrel{\‾}{I_s}=I_L$

Therefore, as long as obtain photoelectric current I _saverage just can obtain its DC component I _lvalue.

So-called moving average filter method, the N number of I will obtained continuously exactly _sregard a queue as, the length of queue is fixed as N.According to the first in first out of queue, calculate a new I at every turn _sdata, all put it into tail of the queue, and give up to fall the data of original head of the queue.Then often obtain a new data, all the N number of data in queue are done an arithmetic mean.Like this, the mean value of photoelectric current can just be obtained thus obtain I _l.

Particularly, for n-th Gain tuning, single-chip microcomputer collects a voltage signal V _{o3 (n-1)}, calculate corresponding photoelectric current I _{s (n-1)}.Originally data I was had in the long queue for N _{s (n-1-N)}i _{s (n-2)}, these data in front N Gain tuning stored in; By new data I _{s (n-1)}put into tail of the queue, and give up to fall the data of original head of the queue, the data namely in current queue are I _{s (n-N)}i _{s (n-1)}; Do sums on average to the N number of data in current queue, just can obtain I _{l (n-1)}value, as follows:

$I_{L(n-1)}=\stackrel{\‾}{I_{s(n-1)}}=\frac{I_{s(n-N)}+I_{s(n-N+1)}+...+I_{s(n-1)}}{N}$

6) according to the output voltage V of single-chip microcomputer _o4with photoelectric current DC component I _lrelation, a series of value computed in advance, and make tables of data.

By step 5) can I be obtained _lvalue.

And I _lwith best APD gain M _optrelational expression as follows:

$\begin{array}{c}{M}_{\mathrm{opt}}\\ ={\left[\frac{({2\mathrm{eI}}_{\mathrm{ds}}+I_{\mathrm{na}}^2)R_1^2{R}_2^2+2\mathrm{\π}{E}_{\mathrm{na}}^2(C_1+C_2)f_t{R}_1^2{R}_2+E_{\mathrm{na}}^2{(R_1+R_2)}^2+4\mathrm{kT}{R}_1{R}_2(R_1+R_2)}{e(I_L+I_{\mathrm{dg}})k_F{R}_1^2{R}_2^2}\right]}^{\frac{1}{3}}\end{array}$

Wherein, e is electron charge, and k is Boltzmann constant, and T is absolute temperature, I _dgfor participating in the body leakage current doubled in APD, I _dsfor not participating in the face leakage current doubled in APD, R ₁for APD and amplifier inverting input combined resistance over the ground, I _nafor the equivalent inpnt current noise of amplifier, E _nafor the equivalent inpnt voltage noise of amplifier, k _ffor the charge carrier ionization ratio in APD, be normally defined the ratio of the ionization probability of hole and electronics.C ₂for step 1) in the feedback capacitance decided, R ₂for step 2) in the feedback resistance value decided.

Therefore, can according to I _lrelease best APD gain M _opt, derivation path is I _l→ M _opt

The gain M of APD is relevant with its bias voltage V, and relation is as follows:

$M=\frac{1}{1-{\left(\frac{V}{V_B}\right)}^n}$

In this formula, V _bfor the puncture voltage of APD, n is the amount relevant with APD material, structure, can obtain according to V-M curve on APD databook.

Therefore, if known best APD gain M _opt, the just bias voltage V of APD can be released _opt, derivation path is M _opt→ V _opt

Due to the bias voltage V usual larger (a few hectovolt) of APD, single-chip microcomputer cannot directly export so large voltage, therefore needs after single-chip microcomputer, to connect an electric pressure converter, by the control voltage V that single-chip microcomputer exports _o4convert voltage V to.The input voltage V of voltage conversion chip _o4and be linear relationship between output voltage V, that is:

V＝k _VV _o4

Wherein, k _vfor voltage transitions coefficient, can obtain according to the input voltage provided in the databook of voltage conversion chip-output voltage curve chart matching.

Therefore, according to the just bias voltage V of APD _opt, the control voltage V that single-chip microcomputer needs to export can be released _o4, derivation path is V _opt→ V _o4

In sum, if photoelectric current DC component I when known APD gain is 1 _l, just can by a series of derivation, when drawing to make the output signal-to-noise ratio of APD amplifier maximum, single-chip microcomputer needs the control voltage V exported _o4, derivation path is: I _l→ M _opt→ V _opt→ V _o4

Due to I _l→ M _opt→ V _opt→ V _o4in there is non-linear relation, cannot directly write Single Chip Microcomputer (SCM) program and calculate.Therefore, we can only according to some given I _l, corresponding V computed in advance _o4value, and make a tables of data, stored in for subsequent use in single-chip microcomputer.Need according to I _lcalculate corresponding V _o4time, table look-at.

7) according to step 5) in the I that obtains _l, finding step 6) in the tables of data that makes, allow single-chip microcomputer export a control voltage V _o4to in electric pressure converter.

8) electric pressure converter exports the negative terminal of voltage V a to APD, as the bias voltage of APD.