JP3111769B2 - A / D conversion circuit of optical pulse tester - Google Patents

Description

DETAILED DESCRIPTION OF THE INVENTION

[0001]

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to an A / D conversion method for an optical pulse tester (hereinafter referred to as OTDR).

[0002]

2. Description of the Related Art An OTDR transmits an optical pulse to an optical fiber through an optical directional coupler, and separates and detects return light by the optical directional coupler, thereby detecting a fault point, loss, and connection of the optical fiber. Measure the loss etc.

Next, the configuration of the OTDR according to the prior art will be described with reference to FIG. 4 is a timing generation circuit, 2
Is an optical pulse generator, 3 is an optical directional coupler, 4 is O / E
(Optoelectric) converter, 5 is an amplification circuit, 6 is an A / D conversion circuit, 10 is a display, and 11 is an optical fiber to be measured.

[0004] In FIG. 4, an optical pulse 2 A from an optical pulse generator 2 passes through an optical directional coupler 3 and enters an optical fiber 11 in synchronization with an electric pulse of a timing generation circuit 1. The backscattered light generated in the optical fiber 11 passes through the optical directional coupler 3 and is converted into an electric signal by the O / E converter 4. The electric signal is amplified by an amplifier circuit 5, converted into a digital signal by an A / D converter circuit 6, and displayed on a display 10.

[0005] The output of the amplifier circuit 5 is input to a signal input terminal 6A of the A / D conversion circuit 6, and a DC voltage source 16 is connected to a reference signal input terminal 6B of the A / D conversion circuit 6. A
As the / D conversion circuit 6, a parallel comparison type A / D conversion circuit (flash type) having a high conversion speed is used.

The quantization noise of the A / D conversion is determined by the word length (bit length) of the A / D conversion circuit 6, and it is necessary to lengthen the word tone in order to reduce the quantization noise. However, the flash type A / D conversion circuit requires 2 ⁿ (n is a word length) comparators. For example, a 16-bit A / D conversion circuit requires 2 ¹⁶ = 65536 comparators, which causes a problem that the circuit scale of the A / D conversion circuit becomes large.

In FIG. 4, it is necessary to increase the word length of the A / D conversion circuit 6 in order to reduce the quantization noise of the A / D conversion. To increase the word length, A /
The number of comparators used in the D conversion circuit 6 is increased, and the circuit scale is increased. On the other hand, there is a problem that the dynamic range is limited if the word length is small.

The present invention provides an A / D converter that reduces quantization noise without significantly increasing the number of comparators used in the A / D conversion circuit.
It is intended to provide a / D conversion circuit.

In order to achieve this object, the present invention provides an optical pulse generating circuit which synchronizes an electric pulse output from a timing generating circuit with an optical pulse.
And the optical pulse 2A is incident on the optical fiber 11 from the optical directional coupler 3, and the return light from the optical fiber 11 is sent from the optical directional coupler 3 to the O / E converter 4, and the O / E The output of the converter 4 is amplified by the amplifier circuit 5 and the A / D converter circuit 6
In the optical pulse tester displayed on the display 10 in synchronization with the electric pulse output from the timing generation circuit 1, f ₁ (t) = exp (−α)
an exponential function generating circuit 7 for outputting a voltage signal of t), a second digital signal output from the exponential function generating circuit 7 is converted into a first analog signal, and the first analog signal is converted to an A / D conversion circuit. Input to the reference signal input terminal 6B
The digital signals output from the A / D conversion circuit 8 and the A / D conversion circuit 6 are input to the multiplicand terminal 9A, and the branch output of the exponential function generation circuit 7 is input to the multiplier terminal 9B. A multiplication circuit 9 for outputting a signal;
A second analog signal attenuated by a function of ₂ (t) = exp (−βt) is input to the signal input terminal 6A of the A / D conversion circuit 6, and an exponent that satisfies f ₁ (t) ≦ f ₂ (t). An A / D conversion circuit for an optical pulse tester, wherein a reference signal of a function generation circuit 7 is input to a reference signal input terminal 6B of the A / D conversion circuit 6.

[0008]

Next, the configuration of the OTDR according to the present invention will be described with reference to FIG. In FIG. 1, 7 is an exponential function generation circuit, and 8 is D /
The A conversion circuit 9 is a multiplication circuit, and the other components are the same as those in FIG. That is, FIG. 1 is obtained by adding an exponential function generation circuit 7, a D / A conversion circuit 8, and a multiplication circuit 9 to the configuration of FIG.

In FIG. 1, the exponential function generation circuit 7 operates in synchronization with the electric pulse output from the timing generation circuit 1. The exponential function generating circuit 7 generates a voltage of f ₁ (t) = exp (−αt). The output of the exponential function generation circuit 7 is input to the D / A conversion circuit 8 and is converted into an analog signal. D /
The output of the A / D conversion circuit 8 is input to the reference input terminal 6B of the A / D conversion circuit 6.

A digital signal output from an output terminal 6 C of the A / D conversion circuit 6 is input to a multiplicand input terminal 9 A of the multiplication circuit 9. The output of the exponential function generator 7 is input to the multiplier input terminal 9B of the multiplier 9. Multiplication circuit 9
Is input to the display 10.

In FIG. 1, the input terminal 6 of the A / D conversion circuit 6
The analog signal input to A is digitally converted and output to an output terminal 9C of the multiplication circuit 9. That is, in FIG. 1, the exponential function generation circuit 7, the D / A conversion circuit 8, and the multiplication circuit 9
A / D conversion is performed.

Next, the operation of FIG. 1 will be described with reference to the time chart of FIG. FIG. 2A is an output waveform diagram of the timing generation circuit 1. FIG. 2A is an output waveform diagram of the optical pulse generator 2. The optical pulse 2A is transmitted to the optical fiber 11 in synchronization with the electric pulse shown in FIG. FIG. 2C shows that the return light of the optical fiber 11 is photoelectrically converted and amplified, and the A / D conversion circuit 6
7 is an analog waveform diagram input to an input terminal 6A of FIG. The input waveform in FIG. 2C decays exponentially.

In the OTDR, if the backscattered light generated in the optical fiber 11 has a uniform attenuation characteristic in the line length direction of the optical fiber 11, the backscattered light attenuates exponentially with time. Therefore, the return light of the optical fiber 11 can be represented by a function of f ₂ (t) = exp (−βt). Further, the loss of the optical fiber 11 is detected at the connection point from the return light, and Fresnel reflection occurs at the far end of the optical fiber 11.

In the section A of FIG. 2C, f ₂ (t) = exp (−β
t), points B and C are splice loss points,
Point D is a Fresnel reflection point.

In FIG. 1, when the same voltage as the voltage input to the reference signal input terminal 6B of the A / D conversion circuit 6 is input to the signal input terminal 6A, the A / D conversion circuit 6 outputs full scale. Works as follows.

FIG. 2D shows an input waveform input to the reference signal input terminal 6B of the A / D conversion circuit 6. The output signal of the exponential function generation circuit 7 is D / A converted by the D / A conversion circuit 8 and becomes a reference voltage signal of the A / D conversion circuit 6. Therefore, the output waveform output to the output terminal 6C of the A / D conversion circuit 6 is as shown in FIG.

The waveform in FIG. 2E is f ₃ (t) = f ₂ (t) / f ₁ (t)
Can be indicated by It is necessary to operate the exponential function generation circuit 7 so that f ₁ (t) ≦ f ₂ (t).

The waveform shown in FIG. 2D is input to the multiplier input terminal 9B of the multiplication circuit 9, and the waveform output to the output terminal 9C of the multiplication circuit 9 is as shown in FIG. Is A / D converted. In FIG. 2, the full scale of the A / D conversion circuit 6 is expressed as “1” and the quantization noise is expressed as “0” for ease of explanation.

FIG. 3 is a circuit diagram of the A / D conversion circuit 6. In FIG. 3, n-bit A / D conversion is performed, the first input terminals of 2 ⁿ comparators 61 are connected in parallel with the signal input terminal 6A, and the second input terminals of the comparators 61 The voltage divided by the 2 ⁿ voltage ^dividing resistors 62 is input. One end of the voltage dividing resistor 62 is connected to the reference signal input terminal 6B, and the other end of the voltage dividing resistor 62 is grounded. The output of the comparator 61 is input to an encoding circuit 63, which encodes the output and sends an n-bit digital signal to an output terminal 6C.

Next, the quantization noise of FIG. 1 will be described. Assuming that the word length of the A / D conversion circuit 6 is n, the maximum reference signal ± (0.5 /
Since ²ⁿ ) quantization noise is output, the conventional technique shown in FIG. 4 results in 1 / ²ⁿ noise.

In FIG. 1, the output terminal 6 of the A / D conversion circuit 6
(F ₂ (t) / f ₁ (t)) ± (0.5 / 2 ⁿ ) is output to C, and ｛(f ₂ (t) / f) is output to the output terminal 9C of the multiplication circuit 9.
₁ (t)) ± (0.5 / 2 ⁿ )｝ · f ₁ (t) = f ₂ (t) ± f ₁ (t)
・ (0.5 / 2 ⁿ ) is output. That is, f ₁ (t) /
2 ⁿ noise.

Since f ₁ (t) ≧ 1 and n is an integer, f ₁ (t)
/ ²ⁿ ≦ 1 / ²ⁿ . That is, the quantization noise is f ₁ (t) times higher than the conventional technology. For example, at t = t ₁ , f
_{If 1} (t ₁ ) = 1/2 ⁸ , the quantization noise becomes 1/256. If n = 8, the quantization noise at this time is (1/2)
56) × (1/2 ⁸ ) = 1/2 ¹⁶ which is 16
It is equivalent to a bit conversion circuit. At this time, since the A / D conversion circuit 6 has 8 bits, the number of the comparators 61 used in the A / D conversion circuit 6 may be 2 ⁸ = 256, which is 1/256 of the 16-bit conversion circuit. .

In the OTDR, a signal to be A / D converted is backscattered light, and the signal is expected to attenuate exponentially. Therefore, an exponential function generating circuit is used in the present invention. As described above, if the signal to be A / D converted can be predicted to some extent, the present invention can be applied. To apply,
The exponential function generating circuit may be a circuit that generates a predicted function. In the case of a repetitive phenomenon, the signal may be roughly grasped in the first cycle to generate a similar signal.

[0024]

According to the present invention, the first exponentially attenuating function is provided.
Is input to the signal input terminal of the A / D conversion circuit, and the second analog signal that is attenuated exponentially and smaller than the first analog signal in synchronization with the first analog signal is A
Input to the reference signal input terminal of the / D conversion circuit,
The number of comparators used in the A / D conversion circuit can be reduced as compared with the conventional case.

[Brief description of the drawings]

FIG. 1 is a configuration diagram of an OTDR according to the present invention.

FIG. 2 is a time chart of FIG.

FIG. 3 is a circuit diagram of an A / D conversion circuit 6;

FIG. 4 is a configuration diagram of an OTDR according to the related art.

[Explanation of symbols]

 DESCRIPTION OF SYMBOLS 1 Timing generation circuit 2 Optical pulse generator 2A Optical pulse 3 Optical directional coupler 4 O / E converter 5 Amplification circuit 6 A / D conversion circuit 6A Signal input terminal 6B Reference signal input terminal 7 Exponential function generation circuit 8 D / A conversion circuit 9 Multiplication circuit 9A Multiplicand terminal 9B Multiplier terminal 10 Display 11 Optical fiber

Claims (1)

(57) [Claims] An optical pulse generation circuit (2) synchronizes with an electric pulse output from a timing generation circuit (1) to generate an optical pulse (2).
A), the optical pulse (2A) is incident on the optical fiber (11) from the optical directional coupler (3), and the return light from the optical fiber (11) is transmitted from the optical directional coupler (3). To the O / E converter (4)
The output of the O / E converter (4) is amplified by the amplifier circuit (5),
The optical signal is converted into a first digital signal by a D conversion circuit (6) and displayed on a display (10). In an optical pulse tester, the signal is synchronized with an electric pulse output from a timing generation circuit (1), and f ₁ (t) An exponential function generating circuit (7) for outputting a voltage signal of = exp (-αt), and a second digital signal output from the exponential function generating circuit (7) is converted into a first analog signal. A D / A conversion circuit (8) for inputting an analog signal to a reference signal input terminal (6B) of the A / D conversion circuit (6), and a digital signal output from the A / D conversion circuit (6) for a multiplicand terminal (9A entered), the branch output of the exponential function generator (7) inputted to the multiplier terminal (9B), and a display unit (10) to a third multiplier circuit for outputting a digital signal (9), f ₂ A second analog signal attenuated by the function of (t) = exp (−βt) is input to the signal input terminal (6A) of the A / D conversion circuit (6), and f ₁ The reference signal of the exponential function generating circuit (7) satisfying (t) ≦ f ₂ (t) is converted to the reference signal input terminal (6B) of the A / D conversion circuit (6).
An A / D conversion circuit for an optical pulse tester, wherein the A / D conversion circuit inputs a signal to the A / D converter.