JPH04228195A - Sampling method - Google Patents

Abstract

PURPOSE:To widen the dynamic range of a sampling system. CONSTITUTION:An input signal (output of 10) and a reference level (output of 16) are sampled and are alternately supplied to an accumulation element 22. The signal amplitude accumulated in the accumulation element right after an input signal is supplied to the accumulation element is the function of the input signal amplitude and the signal amplitude accumulated in the accumulation element just before the supply of the input signal to the accumulation element is the function of the reference level. The accumulation level at the time of accumulating the input signals into the accumulation element is, therefore, always the reference level and is not affected by the level of the input signals accumulated by sampling just before the supply. The dynamic range is consequently widened.

Description

[Detailed description of the invention]

0001

[Industrial Application Field] The present invention relates to a method for operating a measuring device,
In particular, it relates to sampling methods.

0002

2. Description of the Related Art One application that uses sampling techniques is, for example, optical time domain reflectometry (OTDR). This OTDR
The laser diode is then activated periodically to deliver pulses of light to the fiber under test. Rayleigh backscattered light and Fresnel reflected light from the fiber under test are supplied to a photodetector. The photodetector generates a current depending on the brightness of the returned light incident on the photodetector. This current signal is applied to a transimpedance amplifier. A transimpedance amplifier converts a current signal into a voltage signal and performs amplification. At predetermined times relative to the activation of the laser diode, the amplified voltage signal is sampled and the sampled values are measured by an analog-to-digital (A/D) converter. This A/D converter processes the digital data values generated to interrogate the fiber under test (
(interrogation) storing the processed data in a memory location corresponding to the interval between the application of the pulse and the sampling of the output signal of the transimpedance amplifier; The accumulated sampling values are used to display the intensity of the return light received from the optical fiber as a function of distance from the laser diode to represent the attenuation of the emitted light.

Typically, a sampling gate and a sampling storage capacitor are used to sample a voltage signal. The sampling gate is opened (conducted) during a short sampling period to allow the storage capacitor to charge. Then, close the sampling gate (make it non-conductive). The voltage stored on the capacitor is converted to digital form and measured by controlling the conversion pulses, which occur at times related to the operation of the sampling gate.

The return optical signal received from the fiber under test consists of low-level backscattered signals and sometimes very large amplitude, short pulses caused by reflections. As a result, the return optical signal is dynamically
The range will be very wide. The dynamic range of the sampled electrical signal can be narrowed relative to the dynamic range of the return optical signal by transimpedance amplifier design, resulting in a linear transfer function for small signal levels and a linear transfer function for large signal levels. becomes the transfer function for However, even so,
The dynamic range of the signal provided to the sampling system is important. Additionally, the complex nature of the transimpedance amplifier's overall transfer function makes calibration difficult and limits accuracy somewhat.

The sampling efficiency of a sampling system is the relationship between the sampled value produced at the output of the sampling system and the value of the input signal during the sampling period. There are two aspects to the sampling efficiency of a sampling system consisting of a sampling gate and a storage capacitor: transfer efficiency and charging efficiency. The transfer efficiency of a sampling gate is the relationship between the voltage at the input of the sampling gate and the voltage at the output of the sampling gate during the sampling period. Charging efficiency is the relationship between the voltage at the output of the sampling gate during the sampling period and the voltage present on the storage capacitor when the sampling gate is closed at the end of the sampling period. The transfer efficiency is less than 100% due to the impedance of the sampling gate that constitutes the potential voltage divider. A transfer efficiency of less than 100% is
This can be corrected by providing a suitable gain stage after the storage capacitor. Charging efficiency is determined by the duration of the sampling interval. Since the duration of the sampling interval is related to the inverse of the input signal frequency, the duration of the sampling interval is limited by the bandwidth of the input signal. Charging efficiency is also determined by the amplitude of the input signal, since the rate of charge that can be delivered to the capacitor through the sampling gate is limited. Additionally, charging efficiency is determined by the direction of current flowing through the sampling gate. Also, if the amplitude of the input signal changes significantly between one sampling period and the next, in the case of OTDR, successive samples represent reflection phenomena and backscattering, and the measured value of the second sample is determined by the value of the first sample. The sampling system is thus signal dependent.

The brightness of the backscattered light received by the OTDR from the fiber under test is a function of the wavelength and intensity of the interrogation pulse. The intensity of the optical power produced by a laser diode is approximately a Gaussian function of wavelength, with an amplitude width of only a few microns. Therefore, to measure repetition within an OTDR, it is necessary to maintain the laser diode at a constant temperature. This can be achieved using a thermoelectric cooler operating in a feedback loop with a temperature sensor. The operating temperature of the laser is determined by a temperature level signal generated by the OTDR's internal controller. However, this does not compensate for measurements between different instruments.

[0007] Accordingly, an object of the present invention is to solve the above-mentioned problems and provide a sampling method with a wide dynamic range.

[0008]

SUMMARY OF THE INVENTION The input signal sampling method of the present invention repeatedly couples an input signal to a sampling storage element. The amplitude of the signal stored in the storage element thus directly follows the input signal applied to the storage element and is a function of the input signal amplitude. Further, a reference level is applied to the sampling storage element while the input signal is continuously supplied to the storage element. Therefore, the amplitude of the signal stored by the storage element immediately before supplying the input signal to the storage element is a function of the reference level. Therefore, when charging the storage element to store the sampled input signal, the charging value of the storage element is always at the reference level. That is, the storage start level of the storage element is not affected by the value of the input signal stored immediately before. Therefore, the dynamic range of the sampling system is widened.

[0009] Furthermore, the sampling device according to the present invention includes:
It includes a sampling storage element and means for providing an input signal to the storage element. The magnitude of the value stored in a storage element immediately after the input signal is applied to the storage element is therefore a function of the signal amplitude. Furthermore, this sampling device immediately before supplying the input signal to the storage element,
Means are also provided for supplying a reference level to the storage element. The magnitude of the value stored in the storage element immediately before applying the input signal to the storage element is thus a function of the reference level.

Further in accordance with the invention, the input signal sampling apparatus includes a sampling storage element, a reference level source, and means for selectively supplying the input signal or reference level to the sampling storage element. The value stored in the storage element immediately before applying the input signal to the storage element is thus a function of the reference level. Also, the value stored in the storage element immediately after supplying the input signal to the storage element is a function of the input signal amplitude.

Further, according to the present invention, a method of operating a test equipment having a laser diode and a device for controlling the temperature of the laser diode in response to a temperature control signal includes operating the laser diode and controlling the temperature of the laser diode in response to a temperature control signal. Observe the wavelength distribution of the optical power emitted by the laser diode, compare the center wavelength of this wavelength with the desired value,
The temperature control signal is adjusted in response to the results of this comparison.

Also in accordance with the invention, the measurement instrument includes a signal acquisition device for ascertaining raw data values representative of measurable state measurements of the system under test. This method of operating the measuring instrument involves taking in a plurality of raw data values representing a plurality of measured values of the above-mentioned conditions and generating a plurality of calibration data values (calibrated data values) representing calibrated values of this condition; testing the raw data values and the calibration data values to generate data representing a predetermined relationship between the raw data values and a corrected data value that is at least approximately equal to the calibration data value; The accumulated data representing this predetermined relationship is sequentially used to generate a correction table that returns corrected data values according to the raw data values, and the raw data values are applied to the correction table.

Further, according to the invention, the measuring instrument includes a signal acquisition device for generating raw data values representative of measured values of measurable states of the system under test; A calibration memory containing data representing a predetermined relationship with substantially equal correction data values, a correction memory receiving as input the raw data values, and means for loading the correction memory into a table. Note that this table returns corrected data values in response to raw data values.

[0014]

DESCRIPTION OF THE PREFERRED EMBODIMENTS FIG. 1 shows an interrogation and data acquisition system employing the present invention, which is part of an OTDR. This system is activated intermittently by a laser diode driver 4 and sends interrogation pulses to the fiber under test 6.
It is equipped with a laser diode 2 that emits (supplies) to. A photodetector 8 detects the returned light portion received from the fiber under test.
supply to. The photodetector 8 generates a current signal proportional to the intensity of the returned light incident on the photodetector, and the transimpedance amplifier 10 converts this current signal into a voltage signal and amplifies it. This amplified voltage signal is supplied to one input terminal A of the analog multiplexer 12. The other input terminal B of this multiplexer 12 has a reference voltage source 1
receives a constant voltage level V from 6. Multiplexer 1
The output terminal of 2 is an analog-to-digital (A/D) converter 2.
Connected to 4. This A/D converter 24 includes a sampling gate 14, a storage capacitor 22, and an amplifier 18.
and a digitizer (digitization circuit) 20. The timing control circuit 26 includes a laser diode
It controls the operation of driver 4, multiplexer 12, sampling gate 14 and digitizer 20. The purpose of capacitor 28 will be discussed below with reference to FIG.

The operation of the system shown in FIG. 1 will be explained. When the timing control circuit 26 supplies a pulse to the laser diode driver 4, the laser diode 2
fires an interrogation pulse into the fiber under test. The timing control circuit 26 sends the pulses to the multiplexer 1 during a sampling period that is a predetermined time relation to the firing of the interrogation pulse.
2 and sampling gate 14, multiplexer 12 connects its output to input A and sampling gate 14 is open. In this operation, storage capacitor 22 is charged to a voltage determined by the amplitude of the voltage signal generated by amplifier 10. Next, sampling
Gate 14 is closed and a conversion pulse is provided to digitizer 20, which measures the voltage stored on capacitor 22 by converting the voltage level to digital form.

As shown in FIG. 2, the timing control circuit 26
connects the reference voltage source 16 to the capacitor 22 before connecting the amplifier 10 to the capacitor 22. Therefore, before the output signal of the transimpedance amplifier 10 is sampled, the timing control circuit 26 opens the sampling gate 14 and connects the output of the multiplexer 12 to its input B so that the capacitor 22 is at the reference potential level. Charged to V. In this operation, capacitor 22 is reset to a constant reference level before sampling the signal. The interrogation and sampling system described with reference to FIG. 1 thus eliminates or significantly reduces signal dependence.

A disadvantage of the system shown in FIG.
Multiplexer 12 must be fast and accurate, and sampling gate 14 must operate at twice the effective sampling frequency. In high resolution systems, the interrogation pulses are very short and the bandwidth of the sampling device is high. Also, the sampling period is very short and the charging efficiency is less than 100%. Even if the reset voltage is constant and the sampling gate is open for a relatively long period of time when sampling the reference potential,
It is difficult to provide different sampling periods depending on whether the multiplexer 12 selects input A or B.

The system shown in FIG. 3 eliminates these drawbacks. In this FIG. 3, two sampling gates 14A and 14B are used in place of multiplexer 12 and a single sampling gate 14. By using independent reset sampling gates, 100% accuracy can be achieved over long sampling periods without compromising signal sampling bandwidth or having to operate a single sampling gate with two different sampling periods. Charging efficiency can be achieved. Each sampling gate operates only at valid signal sampling frequencies. FIG. 4 shows the operation of the device shown in FIG. Figure 4
, TS represents the period between signal samplings,
TR represents the period between reset and sampling, and ts
represents the signal sampling period, and tR is the reset period.
Represents the sampling period. × represents an analog/digital conversion point. relative length of the signal sampling period ts
is shown in FIG. 4, but this ts must be short enough to keep the signal level essentially constant.

[0019] Sampling with charging efficiency less than 100%
When the system is used to sample a signal whose potential has changed stepwise, the first sampling to be performed after the step change is determined by both the potential after the step change and the potential before the step change. Sampling shown in Figure 3
Since the system alternates between the reset level and the input signal, after a step change, every signal sampling is actually the first sampling unless the signal level is equal to the reset level. A capacitor 28 connected to amplifier 18 provides positive feedback to storage capacitor 22. By proper selection of capacitor 28, 10
0% charging efficiency can be achieved. Then, the reset signal is applied to the voltage charged in the capacitor 22 during the signal sampling period.
Sampling has no effect.

By setting the reset level to the middle of the signal sampling level range, the sampling
Minimize the maximum current to be provided through gate 14A. However, the current to be provided through sampling gate 14A for low level signals is almost the maximum current and may be inaccurate. Furthermore, having the reset level in the middle of the signal sampling level range is a precursor to unipolar sampling. To perform unipolar sampling, the reset level V is set outside the signal sampling level range. In practice for the embodiment shown in FIG. 3, the output signal range of amplifier 10 is +/-0.7V and the transmission efficiency of signal sampling gate 14A is approximately 6.
Since it is set to 0%, the sampling level range is +/-
It becomes 0.42V. Reset sampling gate 1
The transmission efficiency of 4B is 100%. Since the reset level V was set to approximately -0.5V, the storage capacitor is already driven positive when the input signal is sampled. This is the OTD to charge the storage capacitor.
For low level signals of R, the sampling gate 14
Errors are eliminated by minimizing the current that must be sourced through A and by limiting the current that must be sourced through the capacitance of sampling gate 14A. To improve the accuracy of sampling this low level signal, the accuracy of high level sampling must be sacrificed. This is because the maximum change in the potential of the capacitor is approximately 0.9V instead of 0.42V. However, the precision of high-level sampling is
Even if this is not a critical state, it can be corrected by using the configuration shown in FIG.

FIG. 5 shows how OTDR
FIG.

As shown in FIG. 5, the OTDR front panel connector 50 connects optical fibers 54, 5
8 and a programmable optical attenuator 62 to an external laser source 66 . This external laser source 6
6 contains a laser diode that generates a pulse of light of known center wavelength and intensity. The laser source 66 is
In a pre-calibration operation, this attenuated wavelength dependence and non-linearity is compensated for since it was previously used to determine the characteristics of the attenuator. The programmable system controller 72 is an OTD.
R, connect to attenuator and laser source. The controller 72 is
For example, it may be a personal computer with a GPIB card that can communicate with the OTDR, laser source 66 and attenuator 62. This system controller disables the OTDR laser diode driver and
A digital output value from D converter 24' is received via switch 70. The controller 72 also adjusts the attenuator to a selected attenuation level, for example 0 dB, and stores this value;
An external laser source is triggered to generate a pulse of light. This light pulse is detected by a photodetector 8. Additionally, controller 72 stores the data values generated by analog-to-digital converter 24' and repeats these operations for a number of attenuation levels within a predetermined range. For example, this behavior:
Repeated over a 100 dB range in 1 dB steps. A desired transfer function of the attenuator for the calibrated data values is defined and controller 72 stores this function. This desired transfer function may be logarithmic in the range of 0 to 50 dB attenuation and linear in the range of 50 to 100 dB attenuation. By repeating the measurement operation for each attenuation level as the A/D converter 24' stores the values generated, a raw data value corresponding to each calibrated data value can be captured.

If the receiver transfer function is the same as the desired transfer function, then the raw data value at a given attenuation level is equal to the corresponding calibration data value. However, in general these two transfer functions are not equal.

The raw data values and the calibrated data values are shown as a graph of data points in a two-dimensional orthogonal coordinate system, as shown at point P in FIG. To simplify the discussion below, the calibrated data values are shown as having a minimum increment corresponding to a 1 dB increment of attenuation. However, in reality, the calibration data value is a 16-bit number, so the minimum increment corresponds to an attenuation increment of much less than 1 dB. Similarly, the smallest increment in raw data value corresponds to an attenuation change of much less than 1 dB. Therefore, in FIG. 6, raw data is shown as non-integer values. Starting at one end of the data value range, eg, one end corresponding to 0 dB attenuation, controller 72 tests the data value pairs and determines a partial linear approximation of a function that relates the calibration data values to the raw data values. In FIG. 6, the controller tests the first two data points P0, P1 (for 0 and 1 dB attenuation, respectively) and calculates the linear function associated with these data points. This linear function is represented by line 82 in FIG. The controller tests the raw data value corresponding to the setting of 1 dB, ie point P1. The difference between the expected data value and the raw data value based on the linear function is the calibration value for that point. The controller then advances to the next data corresponding to point P2. This calibration value is determined from the difference between the expected data value based on the linear function and the raw data value for point P2. The controller then calculates the difference between the calibration value for point P1 and the calibration value for point P2. If this difference is greater than a preset threshold, typically 1 dB, the area between P1 and P2 is further calibrated. In this operation, the region between P1 and P2 is divided into two parts and the calibration is performed at the midpoint between P1 and P2. This threshold test continues until the difference between two adjacent calibration values in the region between P1 and P2 is less than a preset threshold. The controller continues to calculate calibration values for all measured data points. Also,
This divides the range between adjacent points into many segments until the difference between adjacent data values over the entire calibration range is less than a preset threshold. Each of the two endpoints of the data segment making up the set of calibration data corresponds to a pair of data representing a raw attenuation setting and a calibration value. The calibration value for any intermediate raw data value within the segment is
Raw data values are obtained from a partially linear function relating calibration values for segment endpoints. For each portion of the partially linear function, the controller determines the calibration data value for the start of the segment, the increment size, or the change in the calibration data value per unit change in the raw data value, and the calibration data value corresponding to that segment. The number of units during the data range is loaded into the calibration ROM 74.

The linear part of the desired transfer function range, ie 50
For a 100 dB attenuation range from
Compute a linear function that is very close to these data points. The controller loads the calibration data value for the beginning of the linear portion of the range, the calibrated data increment size, and the size of the raw data range into the calibration ROM.

Typical OTDRs are 830nm and 130nm
It has at least two laser diodes operating at 0 nm, and several other operating conditions govern the relationship between the brightness of the returned light and the attenuation affected by the light transmitted by the fiber under test. This calibration procedure is repeated for each other combination of operating conditions and the corresponding set of calibration data is loaded into ROM 74.

Once the calibration is complete, switch 70 connects the digital output of analog-to-digital converter 24' to calibration RA.
Connect to M78 address line. The data line of RAM 78 is connected to the OTDR storage and display device 80. In the measurement mode, the front panel controls of the OTDR are adjusted to the predetermined combination of operating conditions. The internal controller 76 of the measuring instrument reads the corresponding calibration data from the calibration RAM, generates a correction table, and stores it in the correction RAM.
Load it into M78. A/D converter 24' generates a range of raw data values by starting with a correction data value for the beginning of the period and incrementing that starting value by an increment size for each unit within the raw data range. Calibration data values are calculated for each raw data value within the range to generate a correction table. Correction RAM 7 whose address is the corresponding raw data value
Load the correction data value into storage location 8. Thus, in response to a given raw data value, RAM 78 generates a corrected data value that is the same as the calculated data value, within the accuracy of the procedure used to derive the corrected data. By generating a partially linear function that approximates the calibration function, storing calibration data that defines the partially linear function, and using the calibration data to generate a lookup table, each raw data generated by the A/D converter is There is no need to calibrate the correction data value in response to the value. This allows many large calibration tables to be provided without having to accumulate all the data points.

By using the calibration method described with reference to FIGS. 5 and 6, it is possible to compensate for accuracy errors in samples acquired using the systems shown in FIGS. 1 and 3.

The OTDR shown in FIG.
4 and a temperature sensor 86. These and laser
Thermal coupling with diode 2 is good. The temperature sensor generates a measurement signal representative of the temperature of the laser diode 2. The front panel connector of the OTDR connects to an optical spectrum analyzer 88. The spectrum analyzer generates data values representing received optical power as a function of wavelength. These data values are provided to system controller 72. This system controller connects to a laser driver (not shown in FIG. 7) that continuously triggers the laser driver so that the laser diode generates a fast sequence of light pulses. An optical spectrum analyzer measures the wavelength distribution of optical power received from a laser diode. The system controller compares the center wavelength of the optical power emitted by the laser diode to the desired wavelength. If the center wavelength differs from the desired wavelength by more than a predetermined tolerance, the system controller generates a digital desired temperature signal. This signal is supplied to the OTDR. In the OTDR, a digital-to-analog (D/A) converter 90 converts the desired temperature signal to analog format. The comparator 92 is the detector 86
The measured value signal generated by the D/A converter 90 is compared with the desired value signal from the D/A converter 90, and current is supplied to the thermoelectric cooler so as to maintain a constant between these two signals. Thus, the laser diode 2 is maintained at a temperature at which the center wavelength is equal to the desired wavelength. Controller 72 loads this desired temperature signal into ROM 94. During normal operation of the OTDR, the digital values stored in the ROM 94 are transferred to the D/A converter 9.
0 converts to analog format. This analog signal is used to control the operation of cooler 84 to maintain diode 2 at a temperature where the center wavelength is equal to the desired wavelength.

In some cases, the temperature range achievable with thermoelectric cooler 84 is not sufficient to achieve the desired center wavelength of laser diode 2. In this case, a temperature control data value is selected to bring the center wavelength of laser diode 2 as close as possible to the desired value, and the error value is
Supplies R's internal controller. This internal controller uses the error value described above with a known function relating backscattered power to wavelength and compensates for that error within the linear range of the transfer function.

[0031]

As described above, according to the present invention, before sampling an input signal and storing it in the storage element, the accumulated value of the storage element is always set as the reference level. Therefore, when charging the storage element to store the sampled input signal, the charging value of the storage element is always at the reference level. That is, the storage start level of the storage element is not affected by the value of the input signal stored immediately before. Therefore, the dynamic range of the sampling system is widened.

[Brief explanation of the drawing]

FIG. 1 is a block diagram of an interrogation and data capture system employing the present invention.

FIG. 2 is a timing diagram of the operation of the system shown in FIG. 1;

FIG. 3 is a block diagram of another interrogation and data capture system employing the present invention.

FIG. 4 is a waveform diagram illustrating the operation of the system shown in FIG. 3;

FIG. 5 is a block diagram of an apparatus for calibrating an OTDR.

FIG. 6 is a graph explaining the operation of the device shown in FIG. 5;

FIG. 7 is a block diagram of an apparatus for controlling the operating temperature of a laser diode in an OTDR.

[Explanation of symbols]

10 Transimpedance amplifier (for input signal) 12 Multiplexer 14 Sampling gate 16 Reference voltage source 22 Storage capacitor (storage element)

Claims (1)

[Claims] 1. An input signal is sampled and repeatedly supplied to a storage element, and the value stored in the storage element immediately after supplying the input signal to the storage element is a function of the amplitude of the input signal; supplying a reference level to the storage element during an interval for supplying the input signal to the storage element;
A sampling method characterized in that the value stored in the storage element immediately before supplying the input signal to the storage element is made a function of the reference level.