Coded data generation or conversion – Analog to or from digital conversion – Using optical device
Reexamination Certificate
1999-09-28
2002-03-12
Jeanpierre, Peguy (Department: 2819)
Coded data generation or conversion
Analog to or from digital conversion
Using optical device,
C341S159000
Reexamination Certificate
active
06356220
ABSTRACT:
BACKGROUND OF THE INVENTION
1. Field of the Invention
The present invention relates to an A/D converting device, an A/D converting method and an optical pulse testing apparatus, which are suitable for an optical test.
2. Description of the Related Art
The OTDR (Optical Time Domain Reflectometry) method for detecting a breaking point of an optical cable and for measuring a loss or the like, which is caused by a connecting point thereof has been developed. In the OTDR method, the breaking point of the optical cable is detected and the loss or the like, which is caused by the connecting point of the optical cable is measured by measuring a pulse height of a light returned back to the incident end of the optical cable.
An optical pulse testing apparatus using the OTDR method has been used. The optical pulse testing apparatus comprises a pulse generator, a connector, a fiber to be measured, a coupler, an optical fiber, an APD (Avalanche Photo Diode), an OP-amp (operational amplifier), an A/D (Analog to Digital) converter, a signal processing part or the like.
In the optical pulse testing apparatus, the optical pulse is generated by the pulse generator to transmit it to the coupler. The optical pulse passes through the coupler to transmit it to the optical fiber. While the optical pulse transmitted to the optical fiber from the coupler passes through the optical fiber, (that is, in proportion to the length of the optical fiber from the incident end thereof,) the loss which the optical fiber has causes the power of the optical pulse to be damped.
Because backward scattering lights are caused by the characteristics of the optical fiber at each point of the optical fiber, the backward scattering lights are transmitted to the APD through the coupler and the optical fiber. The backward scattering lights passing through the optical fiber and received by the APD are converted into electric current waveform signals corresponding to the intensities of the received lights by the APD to output them to the OP-amp. The electric current waveform signals outputted from the APD are amplified by a predetermined gain with the OP-amp and are converted into the voltage waveform signals. Then, the voltage waveform signals are converted into digital signals having a predetermined number of bits, which are sampled by a predetermined sampling frequency with the A/D converter, to output the digital signals to the CPU circuit in the signal processing unit. In the CPU circuit, the level of the backward scattering light is identified on the basis of the digital signal outputted from the A/D converter to display the waveform of the backward scattering light received by the APD or that of the Fresnel reflection light received by the APD on the display.
In the A/D converter used in the optical pulse testing apparatus, because the frequency band width of the voltage waveform signal inputted into the A/D converter is several hundreds MHz, in order to convert the inputted voltage waveform signal into the digital signal having a low distortion by a predetermined resolution (number of bits), the sampling frequency having not less than several hundreds MHz and the resolution enabling the change of the voltage to be detected more particularly are required.
When it is attempted that these requirements are satisfied by using one A/D converter, the cost of the one A/D converter is higher than that of the A/D converter for covering several tens MHz band, which is used in signal processing systems, such as broadcasting media or the like. For this reason, according to an earlier development, when the A/D converter for covering several tens MHz band is used, the following method is applied. Each sampling timing is changed in each sweeping operation in order to reconfigure the bit data obtained by carrying out the sweeping operations a plurality of times. Thereby, the same digital data having a desirable resolution can be obtained as the data to be obtained by using the expensive A/D converter in one sweeping operation. As a result, the A/D converter for covering several hundreds MHz band can be realized by a low cost.
However, it is necessary that the sweeping operations are carried out a plurality of times to obtain the desirable resolution by using the above method. It is thought that the following method is carried out. A plurality of A/D converters are assigned to each sampling timing. As a result, the digital signal having a desirable resolution can be obtained by carrying out only one sweeping operation.
In the earlier optical pulse testing apparatus, when the method in which a plurality of A/D converters are used is carried out, the following problems are caused.
FIG. 7A
shows an output characteristic of an A/D converter
21
.
FIG. 7B
shows an output characteristic of an A/D converter
22
. When the A/D converters
21
and
22
convert the same analog signal into a digital signal, the output level of the digital signal into which the analog signal is converted by the A/D converter
21
is different from that of the digital signal into which the analog signal is converted by the A/D converter
22
in the vicinity of the reference level shown by the dashed line, as shown in
FIGS. 7A and 7B
.
FIG. 8
shows a circuit construction of an A/D converting unit using the A/D converters
21
and
22
in the optical pulse testing apparatus.
In
FIG. 8
, each digital signal outputted from the A/D converters
21
and
22
is outputted to a CPU circuit
24
through a switch
23
. The switch
23
is controlled by a switching signal outputted from the CPU circuit
24
so that the digital signal outputted from the A/D converter
21
and the digital signal outputted from the A/D converter
22
are outputted to the CPU circuit
24
alternately.
In case that the digital signal outputted from the A/D converter
21
is outputted to the CPU circuit
24
through the switch
23
at the first sampling timing T
1
, the digital signal outputted from the A/D converter
22
is outputted to the CPU circuit
24
through the switch
23
at the next sampling timing T
2
.
In the A/D converting unit, when the voltage waveform signal of which the level is reduced with the lapse of time as shown in
FIG. 9A
is inputted into the A/D converters
21
and
22
, there is a problem that the digital signal into which the voltage waveform signal is converted has noises which do not correspond to the reduction of the input level of the voltage waveform signal.
Because of the difference between the output characteristic of the A/D converter
21
and that of the A/D converter
22
, which are shown in
FIGS. 7A and 7B
, even though the input level of the voltage waveform signal inputted into the A/D converter
21
is the same as that of the A/D converter
22
, the output level of the digital signal outputted from the A/D converter
21
is different from that of the A/D converter
22
. As a result, when the two digital signals into which the voltage waveform signal is converted by the A/D converters
21
and
22
are outputted alternately, the output level is not constant in the vicinity of the reference level as shown in FIG.
9
B.
In case of using a plurality of A/D converters, because of quantization errors caused by an irregularity of a gain of the A/D converter and that of linearity of the converted digital signal, the output value of the digital signal into which the A/D converter
21
converts the analog signal is not coincident with that of the A/D converter
22
, even though the analog signal inputted into the A/D converter
21
is the same as that of the A/D converter
22
. The problem that the waveform measurement value is not constant in each sampling when the two A/D converters
21
and
22
are used is caused by the above non-coincidence of the output values. In particular, when the data to be analyzed by a bit resolution which is much higher than that of the A/D converter is processed by using an A/D convert with dither method like an optical pulse testing apparatus, the above non-coincidence of the measurement values causes serious problems.
SUMMAR
Ando Electric Co., LTD
Jeanpierre Peguy
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