CN110852027B - Design method of laser receiving circuit coupling structure - Google Patents
Design method of laser receiving circuit coupling structure Download PDFInfo
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- CN110852027B CN110852027B CN201910791193.XA CN201910791193A CN110852027B CN 110852027 B CN110852027 B CN 110852027B CN 201910791193 A CN201910791193 A CN 201910791193A CN 110852027 B CN110852027 B CN 110852027B
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Abstract
The invention provides a design method of a coupling structure of a laser receiving circuit, which comprises the following steps of determining the input noise of the laser receiving circuit according to a laser receiver, a coupling capacitor and a first-stage amplifier model of the laser receiving circuit; determining the attenuation coefficient of the alternating current coupling structure to noise according to the input noise of the laser receiving circuit; and determining the high-frequency cut-off frequency formed by the coupling structure according to the attenuation coefficient and the cut-off frequency of the input signal, and determining the capacitance value in the coupling circuit according to a capacitance frequency formula. The invention reduces the noise of the circuit on the premise of ensuring the basic function of the circuit, and has universality.
Description
Technical Field
The invention belongs to the technical field of photoelectric detection circuits, and particularly relates to a design method of a coupling structure of a laser receiving circuit.
Background
The pulse laser range finder mainly comprises a laser transmitting module, an echo receiving module, a power supply module, a control module and the like. The optical system in the echo receiving module usually adopts a laser converging light path which is mature in the prior art, and the converging capability of the laser converging light path on light beams is strong. The time measuring circuit adopts a TDC-GP2 chip, and the time measuring precision can reach 6 ps. The power supply adopts a power supply module with mature technology. Therefore, the noise of the echo receiving circuit is a main factor influencing the ranging precision of the laser range finder. In order to reduce the noise of the echo receiving circuit, there are mainly the following methods: the noise of the input end of the laser receiving circuit is reduced, and the coupling structure is designed to inhibit the input noise of the circuit by analyzing the noise of the photoelectric detector; designing a low-noise amplifier, and inhibiting the amplification of circuit noise through an amplifying circuit; designing an impedance matching and digital filter network to reduce the noise of an echo circuit; and a high-precision threshold comparison circuit is adopted to reduce the shaking of the front edge of the echo and the error introduced by the circuit structure.
According to different coupling structures of laser receiving circuits, existing laser receiving circuits can be divided into alternating current coupling laser receiving circuits and direct current coupling laser receiving circuits. The dc-coupled laser receiver circuit shown in fig. 1 has the advantages of complete signal and excellent dynamic performance. But it transmits all the electrical signals generated by the photodetector to the amplifying circuit. The signal-to-noise ratio of the flow coupling structure outputting the amplified signal is 12 at this time, and the noise amplitude is 400mV, which greatly limits the detection capability of the system. The alternating current coupled laser receiving circuit can filter out direct current dark noise and low frequency noise generated by the photoelectric detector, so that the signal to noise ratio of the circuit is improved to 25. However, the existing ac coupling structure is designed only considering the requirements of the circuit on the cut-off frequency and the dynamic range, and does not consider the influence of the ac coupling structure on the circuit noise. However, the improper selection of the ac coupling structure may result in an increase in the input impedance of the circuit, which may introduce more input noise than the dc coupling, and may further reduce the dynamic range that the circuit can detect.
Disclosure of Invention
The invention aims to provide a design method of a coupling structure of a laser receiving circuit.
The technical solution for realizing the invention is as follows: a design method of a coupling structure of a laser receiving circuit comprises the following specific steps:
step 1, determining input noise of a laser receiving circuit according to a laser receiver, a coupling capacitor and a first-stage amplifier model of the laser receiving circuit;
and 3, determining the high-frequency cut-off frequency formed by the coupling structure according to the attenuation coefficient and the cut-off frequency of the input signal, and determining the capacitance value in the coupling circuit according to a capacitance frequency formula.
Preferably, the input noise of the laser receiving circuit determined in step 1 includes photodiode electronics noise, amplifier electronics noise, and noise introduced by the diode bias voltage.
Preferably, the photodiode electronics noise is:
in the formula, R d Is the junction resistance of the photodiode i k Is dark current, K is 1.38 × 10- 23 J/K represents Boltzmann constant, T represents operating temperature, q represents elementary charge, Z 1 Is the input impedance of the laser receiving circuit.
Preferably, the amplifier electronics noise calculation formula is:
in the formula, e n o p For amplifier electronics noise, i n Is the input noise current of the amplifier, e n Is the input noise voltage, Z, of the amplifier 1 Is the input impedance of the laser receiving circuit.
Preferably, the noise introduced by the diode bias voltage is specifically:
e g ＝i g (f)|Z 1 |
in the formula, e g Noise introduced for diode bias voltage, Z 1 Is the input impedance of the laser receiving circuit, i g Is a current source.
Preferably, the attenuation coefficient of the ac coupling structure to noise determined in step 2 is:
where K represents the noise e introduced by the diode bias voltage g Cut-off frequency f of medium and low frequency noise cg And cut-off frequency f of AC coupling circuit c Ratio of (a) to (b), f L The starting frequency of the first stage amplifier.
Compared with the prior art, the invention has the remarkable advantages that: the alternating current coupling structure is designed according to specific circuit parameters, the noise of the circuit is reduced on the premise of ensuring the basic function of the circuit, and the design method of the alternating current coupling structure has universality.
The present invention is described in further detail below with reference to the attached drawings.
Drawings
Fig. 1 is an input impedance model of a dc-coupled laser receiver circuit.
Fig. 2 is an input impedance model of an ac-coupled laser receiver circuit.
Fig. 3 is an input noise model of the receiving circuit.
Detailed Description
A design method of a coupling structure of a laser receiving circuit comprises the following specific steps:
step 1, determining the input noise of a laser receiving circuit according to a laser receiver, a coupling capacitor and a first-stage amplifier model of the laser receiving circuit, specifically:
input impedance of the laser receiving circuit as shown in fig. 2, the input impedance of the laser receiving circuit includes three parts, namely, impedance of a laser receiver (APD), coupling capacitance, and input resistance of a first-stage amplifier. Wherein the impedance of the laser receiver is determined by the junction capacitance C d And junction resistance R d Two parts are formed. Calculating the input impedance Z of the laser receiving circuit according to the parallel and series connection rules of the input impedance of the laser receiving circuit 1 。
The noise model of the ac-coupled laser receiver circuit is shown in fig. 3, where the letter e represents voltage noise and the letter i represents current noise.
The electronic noise at the input of the laser receiver circuit is mainly composed of photodiode electronic noise, amplifier electronic noise and noise introduced by diode bias voltage, and can be expressed as
Electronic noise e inside the photodiode PD The method comprises the following steps: thermal noise of junction resistance and random noise caused by dark current existing in the photodiodeJunction resistance R of fixed photodiode d Dark current is i k Electronic noise e of photodiode PD Expressed as:
wherein K is 1.38X 10 -23 J/K represents Boltzmann constant; t represents the working temperature and is 25 ℃; q is 1.6 × 10 -19 C represents a primary charge.
Electronic noise e of amplifier n o p The calculation formula is as follows:
in the formula i n Is the input noise current of the amplifier, e n Is the input noise voltage of the amplifier.
Noise e introduced by diode bias voltage g Due to the introduction of the bias voltage of the photodetector, it is mainly due to the input noise generated by the voltage fluctuation caused by the switching frequency and the temperature characteristic of the bias voltage, and it is referred to as voltage noise herein for short. The voltage noise is mainly reflected in the fluctuation of the input current of the photodiode, so that the current source i is used g Formally express it, whereby the noise e of the power supply g Comprises the following steps:
e g ＝i g (f)|Z 1 |
the total input noise power of the laser receiving circuit is as follows:
wherein R is 1 Is the input resistance of the operational amplifier; k represents Boltzmann constant, and K is 1.38 × 10 -23 J/K; t represents the working temperature and is 25 ℃; q is 1.6 × 10 -19 C represents a primary charge; i.e. i k Is the dark current of the photodetector; i.e. i n Representing input noise of an amplifierA stream; i.e. i g Representing a noise current generated by a photodetector bias voltage; f. of cg Represents i g (f) Cut-off frequency of medium and low frequency noise; f. of c Expressed as the cut-off frequency of the ac-coupled circuit; f. of d Expressed as the cut-off frequency of the photodetector.
And 2, determining the attenuation coefficient of the alternating current coupling structure to noise according to the input noise of the laser receiving circuit. The method specifically comprises the following steps:
wherein K represents e g Cut-off frequency f of medium and low frequency noise cg And cut-off frequency f of AC coupling circuit c Ratio of (a) to (b), f L The starting frequency of the amplifying circuit.
And 3, determining the high-frequency cut-off frequency of the alternating current coupling circuit according to the attenuation coefficient and the cut-off frequency of the low-frequency noise, and determining the capacitance value in the coupling circuit according to a capacitance frequency formula.
In designing an AC coupling structure, first, an attenuation coefficient ∈ necessary for an AC coupling circuit is determined, and then ∈ is added to f obtained by analyzing an input signal cg 、f L Bringing into the above formula to obtain K, and further simplifying to obtain f c Finally, by the capacitance frequency formula f c 1/2 pi RC, the coupling capacitance C is obtained.
The result shows that in the 90m distance measurement, the signal-to-noise ratio of the ^ 0.1 alternating-current coupling structure echo signal is 25, the distance measurement precision is 7.5cm, and the signal-to-noise ratio and the distance measurement precision are obviously improved.
Claims (2)
1. A design method of a coupling structure of a laser receiving circuit is characterized by comprising the following specific steps:
step 1, determining input noise of a laser receiving circuit according to a laser receiver, a coupling capacitor and a first-stage amplifier model of the laser receiving circuit, wherein the input noise comprises photodiode electronic noise, amplifier electronic noise and noise introduced by diode bias voltage, and the photodiode electronic noise is as follows:
in the formula, R d Is the junction resistance of the photodiode i k For dark current, K is 1.38X 10 23 J/K represents Boltzmann constant, T represents operating temperature, q represents elementary charge, Z 1 Is the input impedance of the laser receiving circuit;
the amplifier electronics noise calculation formula is:
in the formula, e nop For amplifier electronics noise, i n Is the input noise current of the amplifier, e n Is the input noise voltage of the amplifier, Z 1 Is the input impedance of the laser receiving circuit;
the noise introduced by the diode bias voltage is specifically:
e g ＝i g (f)|Z 1 |
in the formula, e g Noise introduced for diode bias voltage, Z 1 Is the input impedance of the laser receiving circuit, i g Is a current source;
step 2, determining the attenuation coefficient of the alternating current coupling structure to noise according to the input noise of the laser receiving circuit, wherein the attenuation coefficient is as follows:
where K represents the noise e introduced by the diode bias voltage g Cut-off frequency f of medium and low frequency noise cg And the cut-off frequency f of the AC coupling circuit c Ratio of (a) to (b), f L The starting frequency of the first-stage amplifier;
and 3, determining the high-frequency cut-off frequency formed by the coupling structure according to the attenuation coefficient and the cut-off frequency of the input signal, and determining the capacitance value in the coupling circuit according to a capacitance frequency formula.
2. The method of claim 1, wherein the input impedance Z of the laser receiver circuit is 1 The method specifically comprises the following steps:
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