JP3974880B2 - Jitter transfer characteristic measuring device - Google Patents

Description

[0001]
BACKGROUND OF THE INVENTION
The present invention relates to a technique for measuring jitter transfer characteristics of devices used in various networks with high accuracy and high reproducibility.
[0002]
[Prior art]
Regarding jitter characteristics of devices connected to networks such as SDH, SONET, and OTN, there is a jitter transfer characteristic as a typical characteristic defined in international standards such as ITU-T and Telcordia.
[0003]
Jitter transfer characteristics indicate how much the input signal jitter is suppressed and output when a signal having a predetermined amount of jitter is input to the device under test. By comparing the jitter transfer characteristic G and the standard characteristic R obtained for the device under test as shown in FIG. The quality of the jitter transfer characteristic of the device under test can be determined.
[0004]
As shown in FIG. 5, the standard characteristic R of the jitter transfer characteristic is defined such that the value of the jitter transfer characteristic in the flat portion should not exceed +0.1 dB.
[0005]
For example, if the bit rate is 9953 Mbps (1 UI = 100 ps) and the input jitter Ja is 1.5 UI, the 0.1 dB corresponds to only 0.017 UI, that is, 1.7 ps.
[0006]
In addition, regarding the measurement accuracy in the jitter measuring apparatus, ITU-T O.D. 172 specifies 0.05 dB or less at the highest accuracy.
[0007]
However, when measuring the jitter transfer characteristics, the output signal of the device under test has other frequencies generated by the device under test itself in addition to the frequency component fm of the jitter added to the input signal, as shown in FIG. A lot of jitter components are also included, and measurement errors occur due to the influence of the jitter components, and it is difficult to obtain the measurement accuracy described above.
[0008]
Therefore, it is necessary to extract only the frequency component equal to the jitter added to the input signal by the filter as shown in FIG. 6 from the jitter component of the output signal of the device under test.
[0009]
Moreover, it is necessary to change the frequency of the jitter given to the input signal over a wide range (for example, 100 Hz to several tens of MHz).
[0010]
In order to satisfy such a requirement, the conventional jitter transfer characteristic measuring device applies the jitter component signal detected from the output signal of the device under test and the input signal as described in the following Patent Document 1. A local oscillator signal with a frequency higher than the jitter frequency is input to the mixer and mixed, and only the jitter component of the predetermined frequency is extracted from the output of the mixer by a narrowband bandpass filter to detect its amplitude. It was.
[0011]
According to this configuration, even when the jitter frequency is varied in a wide range, the output jitter amount corresponding to the jitter added to the input signal can be detected by the bandpass filter having a fixed frequency and a narrow band.
[0012]
[Patent Document 1]
JP-A-8-220163
[Problems to be solved by the invention]
As described above, in a jitter transfer characteristic measurement device that extracts a jitter component corresponding to input jitter using a bandpass filter, the measurement accuracy is determined by the characteristics of the bandpass filter. The narrower the bandpass filter, the higher the measurement accuracy. In order to satisfy the above-mentioned ITU-T standard, the bandwidth of the bandpass filter is required to be several Hz or less.
[0014]
However, such a narrow-band filter generally has to be configured by connecting filter elements in multiple stages, and its characteristics are likely to fluctuate due to changes in temperature, and the measurement accuracy and reproducibility are significantly reduced due to the fluctuations in characteristics. There is a problem.
[0015]
In order to solve this, in the above-described Patent Document 1, the relationship between the change in temperature and the change in the characteristics of the bandpass filter is stored in advance, and the frequency of the local oscillation signal is changed following the change in temperature. Although the relative deviation of the characteristics of the pass filter is relatively compensated, there is also a variation in the characteristics of the individual band pass filters, so a complicated work of obtaining the relationship between the temperature and the pass characteristics of each measuring device is required. Moreover, it is difficult to perform temperature compensation completely.
[0016]
An object of the present invention is to further improve this point and provide a jitter transfer characteristic measuring apparatus that does not require compensation for a temperature change or the like, has high reproducibility, and can perform highly accurate measurement.
[0017]
[Means for Solving the Problems]
In order to achieve the above object, a jitter transfer characteristic measuring apparatus according to claim 1 of the present invention comprises:
A clock signal generator (21) for generating a clock signal;
A signal generator (22) for outputting a sinusoidal modulation signal having a predetermined amplitude at a specified modulation frequency and outputting a local oscillation signal having a difference by a predetermined frequency with respect to the modulation signal;
A jitter signal generator (23) that phase-modulates the clock signal with the modulation signal and inputs the phase-modulated clock signal or a pattern signal synchronized with the clock signal as an input jitter signal to the device under test;
A jitter demodulator (24) for demodulating the jitter of the signal output from the device under test that has received the input jitter signal;
A mixer (25) for mixing the output signal of the jitter demodulator and the local signal;
A low-pass filter (26) for extracting a signal in a band including the predetermined frequency from the output signal of the mixer;
An A / D converter (27) for sampling the output signal of the low-pass filter, digitally converting the sampled value, and outputting it;
Amplitude calculation means (30) for performing a Fourier transform process on the sample value output from the A / D converter to obtain the amplitude value of the signal component of the predetermined frequency;
Transfer characteristic calculation means (31) for calculating jitter transfer characteristics of the device under test based on the amplitude value of the modulation signal and the amplitude value calculated by the amplitude calculation means ;
The signal generator is configured to change a frequency difference of the local oscillation signal with respect to the specified modulation frequency;
The amplitude calculation means changes the frequency of the signal component for which the amplitude is obtained by the Fourier transform process in conjunction with the change of the frequency difference.
[0019]
DETAILED DESCRIPTION OF THE INVENTION
Hereinafter, embodiments of the present invention will be described with reference to the drawings.
FIG. 1 shows the configuration of a jitter transfer characteristic measuring apparatus 20 according to an embodiment of the present invention.
[0020]
In FIG. 1, a clock signal generator 21 outputs a clock signal C having a frequency fc corresponding to the data transmission rate of the device under test 1.
[0021]
The signal generator 22 outputs a sine wave modulation signal M having a predetermined amplitude Vm and a local signal L having a difference from the modulation signal M by a predetermined frequency fi.
[0022]
The signal generator 22 outputs a modulation signal M at a modulation frequency fm designated by a measurement control unit 32 to be described later, and outputs a local oscillation signal L at a frequency fa of fm ± fi. The designated modulation frequency fm is a jitter frequency, for example, designated in the range of 100 Hz to 80 MHz, and the predetermined frequency fi is set to 60 Hz, for example.
[0023]
Further, the amplitude Vm of the modulation signal M output from the signal generator 22 is normally set in a range that does not exceed the jitter tolerance of the device under test 1, and depends on the modulation frequency fm along the frequency characteristics of the jitter tolerance. It may be changed or may be a constant value.
[0024]
The clock signal C and the modulation signal M are input to the jitter signal generator 23.
The jitter signal generation unit 23 includes a phase modulator, phase-modulates the clock signal C with the modulation signal M, and uses a signal C ′ to which an amount of jitter corresponding to the amplitude Vm of the modulation signal M is given as an input jitter signal. Generate and input to the device under test 1.
[0025]
Here, it is assumed that the device under test 1 is configured to perform waveform shaping processing or the like on the signal input from the clock input terminal and output the processed signal from the output terminal. The case where the input jitter signal C ′ is input to the input terminal will be described. However, when the device under test 1 has only an input / output terminal for data, the jitter signal generator 23 is phase-modulated. Alternatively, a pattern signal synchronized with the clock signal C ′ may be generated and input to the device under test 1 as an input jitter signal.
[0026]
Upon receiving the input jitter signal C ′, the device under measurement 1 outputs a signal C ″ generated internally based on the signal C ′.
[0027]
The jitter demodulator 24 includes a phase detector, demodulates the jitter component of the output signal C ″ of the device under test 1, and outputs the demodulated signal M ′ to the mixer 25.
[0028]
In this jitter demodulation, for example, a phase comparison between the reference clock signal C and the output signal C ″ of the device under test 1 is performed, a pulse signal having a width equal to the phase difference is generated, and a low-pass signal is generated from the pulse signal. It is obtained by removing the clock frequency component with a filter.
[0029]
However, in this demodulated signal M ′, as shown in FIG. 5, in addition to the frequency component of the modulated signal M given to the input jitter signal C ′, other frequencies generated by the device under test 1 itself are included. Contains ingredients.
[0030]
The mixer 25 mixes the demodulated signal M ′ with the local signal L output from the signal generator 22 and outputs the sum and difference frequency components.
[0031]
The low-pass filter 26 extracts a signal in a band including the predetermined frequency fi from the output of the mixer 25.
[0032]
That is, if the frequency component of the demodulated signal M ′ is only fm and the frequency fa of the local oscillation signal L is fm + fi, the frequency of the sum component output from the mixer 25 is
(Fm + fi) + fm = 2fm + fi
It becomes.
[0033]
The frequency of the difference component is
(Fm + fi) −fm = fi
It becomes.
[0034]
If the frequency fa of the local oscillation signal L is fm-fi, the frequency of the sum component is
(Fm−fi) + fm = 2fm−fi
The frequency of the difference component is
fm− (fm−fi) = fi
It becomes.
[0035]
Therefore, if the lower limit value fma of the modulation frequency fm is higher than the predetermined frequency fi, the cutoff frequency fb of the low-pass filter 26 is set between 2 fma-fi and fi as shown in FIG. As shown, a signal in a band including the predetermined frequency fi can be extracted.
[0036]
As will be described later, since this apparatus obtains the amplitude value of the signal component of the predetermined frequency fi by Fourier transform, the cutoff frequency fb of the low-pass filter 26 is set so that the component of the predetermined frequency fi can be passed without loss. If set, there is no problem, and it may be higher than 2 fma-fi, and particularly severe characteristics are not required.
[0037]
The output signal u of the low-pass filter 26 is sampled by the A / D converter 27 for a predetermined time with a predetermined period Ts (period of 1/2 fb or less), and the sample value is digitally converted and output to the amplitude calculation means 30. Is done.
[0038]
The amplitude calculation means 30 performs DFT (Discrete Fourier Transform) or FFT (Fast Fourier Transform) processing on the sample value U output in time series from the A / D converter 27, and a signal component of a predetermined frequency fi. Find the amplitude of.
[0039]
For example, in the case of DFT processing, if the sample value obtained in time series is U (p) (p = 0 to N−1), the frequency point k (= 0) obtained by dividing the frequency range of ± fb by N. ˜N−1) can be obtained by the following DFT calculation.
[0040]
_{X (k) = p ΣU (} p) exp (-j2πpk / N)
However the symbol _p sigma represents the sum of up to p = 0~N-1.
[0041]
The above calculation is performed for the predetermined frequency fi, that is, by selecting the predetermined frequency fi so as to coincide with the frequency point and performing the calculation, the signal of the frequency fi is obtained from the signal component of each frequency shown in FIG. Only the component amplitude Vx can be determined.
[0042]
The amplitude value Vx obtained by the amplitude calculating means 30 is output to the transfer characteristic calculating means 31.
[0043]
The transfer characteristic calculation means 31 calculates the jitter transfer characteristic of the device under measurement 1 based on the amplitude value Vm of the modulation signal M and the amplitude value Vx calculated by the amplitude calculation means 30.
[0044]
That is, the jitter transfer characteristic is expressed by the following equation, where Ja is the input jitter amount (unit UI) and Jb is the output jitter amount.
[0045]
G (dB) = 20 · log (Jb / Ja)
[0046]
Here, the input jitter amount Ja is proportional to the amplitude value Vm of the modulation signal M, and the output jitter amount Jb is proportional to the amplitude value Vx calculated by the amplitude calculator 30.
[0047]
Therefore, the jitter transfer characteristic G can be obtained by performing the following calculation.
[0048]
G (dB) = 20 · log (Vx / Vm)
[0049]
In practice, however, the amplitude accuracy of the modulation signal M output from the signal generator 22, the phase modulation characteristic of the jitter signal generation unit 23, the phase detection characteristic of the jitter demodulation unit 24, the frequency characteristic of the mixer 25, etc. are affected. In the above theoretical calculation, accurate jitter transfer characteristics cannot be obtained.
[0050]
Therefore, prior to the measurement of the device under test 1 (or later), the input jitter signal C ′ output from the jitter signal generator 23 is sent to the jitter demodulator 24 as shown by the dotted line in FIG. Directly input, the amplitude value Vm ′ for each modulation frequency is obtained by the amplitude calculation means 30 and stored, and the amplitude value Vx obtained for each modulation frequency when the device under test 1 is connected is obtained. On the other hand, an accurate jitter transfer characteristic G is obtained by performing the following calculation.
[0051]
G (dB) = 20 · log (Vx / Vm ′)
[0052]
The measurement control unit 32 assigns the modulation frequency fm to the signal generator 22 and stores the jitter transfer characteristic G calculated by the transfer characteristic calculation means 31 in the internal memory in association with the modulation frequency fm. The processing is sequentially performed for a predetermined modulation frequency range, the jitter transfer characteristic for each modulation frequency is acquired, and the acquired characteristic data and data of the standard characteristic R designated in advance are output to the characteristic display means 33.
[0053]
The characteristic display means 33 receives the jitter transfer characteristic G and the standard characteristic R obtained for the device under test 1, and displays both characteristics on the same coordinate plane as shown in FIG. Is displayed in an identifiable manner.
[0054]
From this display, if the jitter transfer characteristic G of the device under measurement 1 is lower than the standard characteristic R over the entire modulation frequency range, it can be determined that the jitter transfer characteristic of the device under measurement 1 satisfies the standard.
[0055]
If even a part of the jitter transfer characteristic G of the device under test 1 is higher than the standard characteristic R, it can be determined that the jitter transfer characteristic of the device under test 1 does not satisfy the standard.
[0056]
Note that the jitter transfer characteristics are evaluated not only by the measurer on the display screen as described above, but also by comparing the measured jitter transfer characteristics G with the standard characteristics R for each frequency so as to satisfy the standards. It may be configured to automatically determine whether or not it is output and output the result by display or the like.
[0057]
Since the jitter transfer characteristic obtained in this way is based on the amplitude component of only the point frequency fi calculated by the Fourier transform process as described above, it satisfies the accuracy specified by ITU-T. Can do.
[0058]
In addition, since the amplitude value of the frequency fi is obtained by digital arithmetic processing as described above, an error due to a temperature change does not occur in principle, and measurement with high reproducibility can be performed.
[0059]
Note that the accuracy of amplitude calculation by the Fourier transform described above increases as the number of periods of the acquired signal increases, that is, as the predetermined frequency fi increases.
[0060]
Further, if it is sufficient to acquire sample values for the same period for each modulation frequency, the number of sample values to be acquired can be reduced as the predetermined frequency fi is increased, and the measurement time can be shortened.
[0061]
That is, the higher the predetermined frequency fi in terms of accuracy and measurement time, the more advantageous. However, when the predetermined frequency fi is set high (for example, 1 kHz) in this way, it enters the frequency region (100 Hz to 80 MHz) of the modulation signal M. Thus, a state occurs in which the modulation frequency fm coincides with the predetermined frequency fi, and the difference frequency component cannot be correctly extracted from the output of the mixer 25.
[0062]
In such a case, the predetermined frequency fi may be changed to a frequency that does not coincide with the modulation frequency fm (for example, 400 Hz), and the amplitude value of the changed frequency component may be obtained.
[0063]
The jitter transfer measurement / measurement apparatus 20 ′ shown in FIG. 4 takes the above-mentioned points into consideration, and configures the signal generator 22 so that the predetermined frequency fi can be changed, that is, with respect to the designated modulation frequency fm. It is configured so that the frequency of the local oscillation signal L can be changed, and in conjunction with the change of the predetermined frequency fi of the signal generator 22, the amplitude calculation means 30 can change the frequency fi of the signal for obtaining the amplitude by Fourier transform. With this configuration, it is possible to perform highly accurate measurement at a high predetermined frequency fi or a short-time measurement.
[0064]
【The invention's effect】
As described above, the jitter transfer characteristic measuring apparatus of the present invention uses a low-pass filter to obtain a band component including a predetermined frequency corresponding to the input jitter frequency from the mixed component of the output signal of the jitter demodulator and the local oscillation signal. The jitter transfer characteristic is calculated by obtaining the amplitude of jitter of a predetermined frequency by performing Fourier transform processing on the sample value sequence obtained by extracting and A / D converting the output signal.
[0065]
Therefore, the jitter transfer characteristic can be measured with high accuracy and high reproducibility without performing temperature compensation or the like.
[0066]
Further, by configuring the signal generator so that the predetermined frequency can be changed, and by configuring the signal generator so that the frequency of the amplitude value calculation target by the amplitude calculating means can be changed, more accurate measurement can be performed.
[Brief description of the drawings]
FIG. 1 is a diagram illustrating a configuration of an embodiment of the present invention. FIG. 2 is a diagram illustrating a characteristic example of a main part of the embodiment. FIG. 3 is a diagram illustrating a jitter component included in an output signal of the main part of the embodiment. FIG. 4 is a diagram showing a modification of the embodiment. FIG. 5 is a diagram showing an example of jitter transfer characteristics. FIG. 6 is a diagram showing jitter components included in the output signal of the device under test.
DESCRIPTION OF SYMBOLS 1 ... Device to be measured, 20, 20 '... Jitter transfer characteristic measuring device, 21 ... Clock signal generator, 22 ... Signal generator, 23 ... Jitter signal generator, 24 ... Jitter demodulator, 25 ...... Mixer, 26 ... Low-pass filter, 27 ... A / D converter, 30 ... Amplitude calculation means, 31 ... Transfer characteristic calculation means, 32 ... Measurement control section, 33 ... Characteristic display means, 34 …… Display

Claims (1)

A clock signal generator (21) for generating a clock signal;
A signal generator (22) for outputting a sine wave modulation signal having a predetermined amplitude at a specified modulation frequency and outputting a local oscillation signal having a difference by a predetermined frequency with respect to the modulation signal;
A jitter signal generator (23) that phase-modulates the clock signal with the modulation signal and inputs the phase-modulated clock signal or a pattern signal synchronized with the clock signal as an input jitter signal to the device under test;
A jitter demodulator (24) for demodulating the jitter of the signal output from the device under test that has received the input jitter signal;
A mixer (25) for mixing the output signal of the jitter demodulator and the local signal;
A low-pass filter (26) for extracting a signal in a band including the predetermined frequency from the output signal of the mixer;
An A / D converter (27) that samples an output signal of the low-pass filter, digitally converts the sample value, and outputs the sample value;
Amplitude calculation means (30) for performing a Fourier transform process on the sample value output from the A / D converter to obtain the amplitude value of the signal component of the predetermined frequency;
Transfer characteristic calculation means (31) for calculating jitter transfer characteristics of the device under test based on the amplitude value of the modulation signal and the amplitude value calculated by the amplitude calculation means ;
The signal generator is configured to change a frequency difference of the local oscillation signal with respect to the designated modulation frequency;
The jitter transfer characteristic measuring apparatus , wherein the amplitude calculating means changes a frequency of a signal component for obtaining an amplitude by the Fourier transform process in conjunction with the change of the frequency difference .

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