JPH08248078A - Jitter transfer characteristic measuring apparatus - Google Patents

Abstract

PURPOSE: To allow both the measuring efficiency and the measuring accuracy to be compatible even if a measuring signal having a high frequency is used. CONSTITUTION: If a signal of a predetermined level substantially corresponding to the modulation factor of the upper limit of the jitter withstanding standard characteristics of a circuit 1 to be measured is swept and output from the output terminal 23a of a network analyzer 23, a modulation signal in which the level is varied according to the level frequency characteristics similar to the jitter withstanding standard characteristics is input from level varying means 24 to jitter signal generating means 21, a measurement signal modulated by a large modulation factor is input to the circuit 1 in a range having high jitter withstand, or a measurement signal modulated by a small modulation factor is input to the circuit 1 in a range having low jitter withstand. The phase change component of the output signal of the circuit 1 is detected by a phase change detector 22, and the signal level of the same frequency as the modulation signal of the detected signal is detected by the analyzer 23.

Description

Detailed Description of the Invention

[0001]

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention, when an input signal subjected to jitter modulation (phase modulation) by a sine wave signal is applied to a circuit under test such as a digital transmission device, the amount of jitter of the output signal of the circuit under test is input. The present invention relates to a jitter transfer characteristic measuring device used to measure how much the signal jitter amount increases or decreases.

[0002]

2. Description of the Related Art In various digital transmission devices that perform relay transmission of digital signals, etc., input digital signals are subjected to signal processing and output.

In evaluating such a transmission device, an input signal phase-modulated by a sine wave signal is input to the transmission device, and the point at which the code error rate of the transmission data exceeds a reference value is measured. In addition to the measurement, it is necessary to measure the jitter transfer characteristic, which indicates how much the jitter increases by the transmission device itself, with respect to the input signal that is jitter-modulated by the sine wave signal below the jitter tolerance standard.

In order to measure the jitter transfer characteristic, the conventional jitter transfer characteristic measuring apparatus 10 having the configuration shown in FIG. 5 has been used.
Was used.

The jitter transfer characteristic measuring apparatus 10 obtains a modulation signal generator 11 which outputs a sine wave modulation signal and a signal of a predetermined frequency f ₀ which is phase-modulated with a modulation degree corresponding to the level of the modulation signal. Jitter signal generator 1 for outputting measurement signal
2, a phase change detector 13 that detects the phase change component of the output signal of the circuit under measurement 1 that has received the measurement signal, and a level detector 14 that detects the level of the signal detected by the phase change detector 13. It is configured.

Incidentally, the phase change detector 13 is generally shown in FIG.
, A phase comparator 13a for detecting the phase difference of the output signal of the circuit under test 1 with respect to the reference signal having the same frequency f _{0 as} the measurement signal, and the phase comparator 13a having the low-pass characteristic shown in FIG. And a carrier filter 13b that removes the carrier component of the frequency f ₀ of the measurement signal from the output signal of 13a (the frequency f _cut is the cutoff frequency of the carrier filter 13b).

When the circuit under test 1 is measured by the jitter transfer characteristic measuring apparatus having such a structure, it is correct unless the amount of jitter (modulation degree) of the measurement signal exceeds the jitter tolerance of the circuit under test 1. Cannot evaluate.

As shown in FIG. 8A, the standard characteristic of the jitter tolerance is 1.5 UI in the range of the frequency f ₁ to the frequency f ₂ of the modulated signal (unit UI is an abbreviation for unit interval, 1 UI is (Corresponding to one cycle time of the frequency f ₀ of the measurement signal), 0.15 UI in the range of frequencies f ₃ to f ₄ of the modulation signal, and 1.5 UI to 0 in the range of frequencies f ₂ to f ₃ of the modulation signal. -20 dB / up to 15 UI
It has the characteristic that it changes with the slope of dec. The frequencies f _{1 to} f ₄ are defined by the frequency f ₀ of the circuit under measurement and the measurement signal.

When actually measuring the jitter transfer characteristic of the circuit under test 1, a measurement signal having a jitter amount smaller than the standard characteristic A is directly passed to the phase change detector 13 without passing through the circuit under test 1. When the output value R of the level detector 14 (jitter amount of the measurement signal itself) at the time of input and the measurement signal is input to the circuit under test 1 and the output of the circuit under test 1 is input to the phase change detector 14. Output value M of the level detector 14 of
The ratio to (the amount of jitter of the output signal of the circuit under test 1), that is, JT = 20log (M / R), is obtained, and this ratio is measured while changing the frequency of the modulation signal. As a result, for example, as shown in FIG. 9, the jitter transfer characteristic B of the circuit under test 1 with respect to the frequency (jitter frequency) of the modulated signal is measured.

Generally, in a digital transmission device or the like,
As shown in the characteristic B of Figure 9, as the higher beyond the frequency (f _c) with a modulation frequency of the measurement signal, the apparatus tends to reduce the amount of jitter.

When such a jitter transfer characteristic is obtained, the measurer determines whether or not the jitter transfer characteristic B is equal to or less than the standard characteristic C which is defined in advance as the upper limit value of the jitter transfer characteristic. The circuit under test 1 is evaluated. This standard characteristic C is in within the pass band (hereinafter frequency f _c) + 0.1 dB or less, in the range exceeding the passband +
The characteristic is such that the slope decreases from 0.1 dB to -20 dB / dec, and if the jitter transfer characteristic B of the circuit under test 1 is lower than the standard characteristic C, the jitter transfer characteristic of the circuit under test 1 satisfies the standard. It can be determined that they are doing. The frequency f _c is a frequency close to the frequency f ₃ defined by the standard characteristic A of the jitter tolerance of FIG.

However, in the conventional jitter transfer characteristic measuring apparatus 10 having such a configuration, the signal output from the circuit under test 1 passes through the carrier filter 13b of the phase change detector 13 other than the frequency component of the modulated signal. When the phase fluctuation component (for example, phase noise) is included, the level detector 14 also detects the signal level of the phase fluctuation component regardless of the frequency of the modulation signal. And
If the level of the signal of the phase fluctuation component is a value that cannot be ignored with respect to the increase in jitter of the circuit under test 1 in the frequency region above the pass band, the measurement result of the jitter transfer characteristic of the circuit under test 1 is With respect to the original jitter transfer characteristic B of the circuit 1, it becomes constant in a high frequency range like the characteristic B ′ and exceeds the standard characteristic C, and the evaluation of the circuit under test is erroneous.

To solve this, the level detector 14
A level detector with frequency selectivity is used instead of, and only the level of the signal component of the same frequency as the modulation signal is selectively detected while manually adjusting the selection frequency to be the same as the frequency of the modulation signal. However, this causes a problem that the frequency adjustment operation becomes complicated and the measurement efficiency is significantly reduced.

Therefore, for example, like a network analyzer, it has a function of sweeping while keeping the frequency of the output signal of a predetermined level and the received frequency of the input signal the same, and detecting the level of the input signal for each frequency. It is conceivable to use the device in place of the modulation signal generator 11 and the level detector 14 to make the measurement efficiently.

[0015]

However, as described above, in the measurement of the jitter transfer characteristic, the amount of jitter of the measurement signal (the level of the modulation signal) needs to be within the range of the standard characteristic C or less in FIG. , When using a device such as a network analyzer in which the level of the output signal during the sweep is constant, the jitter amount of the measurement signal is as shown in D of FIG.
The level of the modulation signal must be set so as to be lower than the lower value of the standard characteristic A (0.15 UI), which allows accurate jitter transfer characteristics to be obtained when the frequency of the measurement signal is high. There is a problem that measurement cannot be performed.

That is, for example, when the jitter transfer characteristic is measured by a measurement signal having a frequency f ₀ of 2.5 GHz, 2.
The amount of jitter of 0.15 UI for 5 GHz is 60 ps
(Pp), but as described above, the standard of the jitter transfer characteristic is specified to be +0.1 dB or less in the pass band range of FIG. 9, so it is equivalent to 0.1 dB of 60 ps. The jitter transfer characteristic must be measured with a resolution of 0.69 ps.

Therefore, unless the linearity error of the jitter signal generator 12 or the phase change detector 13 is 0.69 ps or less, highly accurate measurement cannot be performed. It is extremely difficult to configure the generator 12 and the phase change detector 14.

Therefore, when the frequency of the measurement signal is high, the network analyzer has the function of sweeping the frequency of the modulated signal and the function of detecting the level of the received signal while changing the received frequency in conjunction with this frequency sweep. It was not possible to carry out efficient measurement using such as.

As described above, according to the conventional technique, if a device such as a network analyzer is used in place of the modulation signal generator and the level detector in order to improve the measurement efficiency, the jitter transmission will occur when the frequency of the measurement signal is high. There has been a problem that the measurement accuracy in the frequency region having low characteristics is remarkably lowered, and if an attempt is made to improve the measurement accuracy, the measurement efficiency is remarkably lowered.

An object of the present invention is to solve this problem and to provide a jitter transmission characteristic measuring apparatus capable of achieving both measurement efficiency and measurement accuracy even when a high frequency measurement signal is used.

[0021]

In order to achieve the above object, the jitter transfer characteristic measuring apparatus of the present invention comprises a measuring signal of a predetermined frequency that is phase-modulated with a modulation degree proportional to the level of the input modulating signal. Between the jitter signal generating means (21) for inputting into the circuit under test, the phase change detecting means (22) for detecting the phase change component of the signal output from the circuit under measurement which has received the measurement signal, and a predetermined frequency range. Sweep signal generating means (2) for outputting a signal whose frequency is swept by
3c) and the output signal of the phase change detection means,
Selection level detecting means (23d) for detecting the level of a signal component having the same frequency as the output signal of the sweep signal generating means in conjunction with the frequency sweep of the sweep signal generating means, and sweep output from the sweep signal generating means. Level changing means (24) for receiving a signal and inputting to the jitter signal generating means a signal whose level changes in accordance with a level frequency characteristic approximated to a jitter tolerance standard characteristic of a circuit under test, which is defined in advance. I have it.

[0022]

Thus, in the jitter transfer characteristic measuring apparatus of the present invention, the signal output from the sweep signal generating means is swept to a level substantially equivalent to the upper limit modulation degree of the jitter tolerance standard characteristic of the circuit under test. When output, a modulation signal whose level changes according to the frequency characteristics close to the jitter tolerance standard characteristics is input from the level varying means to the jitter signal generating means, and in a high jitter tolerance area, modulation with a large modulation degree is performed. The measured signal thus obtained is input to the circuit under measurement, and in a region where the jitter tolerance is low, the measurement signal modulated with a small modulation factor is input to the circuit under measurement. The phase change component of the output signal of the circuit under measurement is detected by the phase change detecting means, and the signal level of the detected signal having the same frequency as the modulation signal is detected by the selection level detecting means.

[0023]

DESCRIPTION OF THE PREFERRED EMBODIMENTS An embodiment of the present invention will be described below with reference to the drawings. FIG. 1 shows a jitter transfer characteristic measuring device 2 of one embodiment.
0 configuration is shown. In this figure, a jitter signal generator 21 outputs a measurement signal of a predetermined frequency (for example, 2.5 GHz) phase-modulated with a modulation degree proportional to the level of a modulation signal output from a filter circuit 24 described later. Output to 1.

The signal output from the circuit under test 1 is input to the phase change detector 22. The phase change detector 22 has the same structure as the phase change detector 13 shown in FIG.
The phase difference between the reference signal having the same frequency as the measurement signal and the signal output from the circuit under test 1 is detected, and a signal having an amplitude proportional to the phase difference is output.

The signal output from the phase change detector 22 is input to the input terminal 23a of the network analyzer 23.

The network analyzer 23 serves both as the sweep signal generating means and the selection level detecting means of this embodiment, and has an output terminal 23a for outputting a frequency-swept signal and an output of the phase change detector 22. And an input terminal 23b for inputting a signal.

FIG. 2 is a diagram showing the internal configuration of the network analyzer 23. A sweep signal generator 23 for sweeping and outputting a sine wave signal of a predetermined level from the output terminal 23a.
c, the reception frequency is changed in association with the sweep signal generator 23c, and only the signal component having the same frequency as the frequency of the signal output from the output terminal 23a is received in a narrow band with a predetermined selectivity and the level thereof is changed. Selection level detection unit 23d for detection
And setting of the sweep frequency range of the sweep signal generator 23c and storage of level data detected by the selected level detector 23d,
The control unit 23e performs processing such as calculation, and the display unit 23f displays processing results of the control unit 23e.

The selection level detector 23d includes a mixer 23g, an intermediate frequency filter 23h and a level detector 2.
3i, and the sweep signal generator 23c
Inputs a signal having a frequency higher than the frequency fm of the signal output from the output terminal 23a by the central frequency α of the intermediate frequency filter 23h to the mixer 23g as a local oscillation signal. The mixer 23g frequency-synthesizes the local oscillation signal and the output signal (frequency fm) of the phase change detector 22 input to the input terminal 23b, and outputs the sum and difference components (2fm + α) and α. Therefore, from the intermediate frequency filter 23h, the signal component of the bandwidth of the intermediate frequency filter 23h centered on the frequency α, that is, the frequency of the signal output from the output terminal 23a among the signals output from the phase change detector 22 The same frequency component is output, and its level is detected by the level detector 23i.

The network analyzer 23 can store a plurality of sets of the levels of the input signal detected for each frequency for one sweep as the jitter amount, and the one set of the jitter amount is used as the reference amount. The ratio of the other set of jitter amounts to the amount can be displayed on the screen for each frequency.

The signal swept out from the network analyzer 23 is input to the level variable circuit 24 as shown in FIG. The level variable circuit 24 has a frequency characteristic in which the amplitude ratio of the output signal to the input signal is close to the standard characteristic of jitter tolerance (A in FIG. 8) which is defined in advance for the frequency of the circuit under test 1 and the measured signal. A plurality of lag lead filters 25 ₁ , 25 ₂ , ..., 25 having
_m and selection switches 26 and 27.

A plurality of lag lead filters 25 ₁ , 2
5 ₂ , ..., 25 _m may be a passive type composed only of a resistor and a capacitor or an active type composed of an amplifier, a resistor and a capacitor.

Among the lag lead filters, for example, 2.
The characteristic E of the lag lead filter corresponding to the measurement signal frequency of 5 GHz is as shown in FIG.
Against ₁ ~f _4, in the attenuation is small and constant in the low frequency range up to the frequency f ₁ ~f _2, at 20dB greater constant attenuation than the low frequency region in the high frequency region up to the frequency f ₃ ~f ₄ In the region of frequencies f _{2 to} f ₃ , the attenuation amount has a characteristic of increasing with a slope of approximately −20 dB / dec, and the attenuation amount in the low frequency region is a signal of a predetermined level input from the network analyzer 23. At this time, the jitter amount of the measurement signal is preset to a value that is slightly smaller than the standard characteristic A of FIG.

Since the standard characteristic of the jitter tolerance of the circuit under test differs depending on the frequency of the measurement signal, the selector switches 26, 27 are operated according to the frequency of the circuit under test and the frequency of the measurement signal to select which of the lag lead filters. Select or use.

Next, the measuring operation by the jitter transfer characteristic measuring apparatus 20 will be described. In advance, the plurality of lag lead filters 25 ₁ , 25 ₂ , ..., 25 _{m of the} level variable circuit 24
Of the measured circuit 1 and the frequency of the measurement signal (here,
A filter having characteristics close to the standard characteristics of jitter tolerance corresponding to 2.5 GHz) is selected, and the level of the output signal of the network analyzer 23 is set to a predetermined level.

Next, with the measurement signal output from the jitter signal generator 21 directly input to the phase change detector 22, the frequency of the output signal of the network analyzer 23 is swept from f ₁ or less to f ₄ or more.

By this sweeping, the level variable circuit 24 outputs a modulation signal whose amplitude changes according to the characteristic of FIG. 3, and the measurement signal phase-modulated with the modulation degree proportional to the amplitude of the modulation signal is the jitter signal. It is output from the generator 21. As shown in F of FIG. 3, the jitter amount of the measurement signal is always a value slightly lower than the standard according to the frequency characteristic of the level variable circuit 24, that is, the standard characteristic of the jitter tolerance (A of FIG. 8). Become.

This measurement signal is a frequency phase change detector 22.
, The phase change component of the measurement signal itself is detected by the phase change detector 22, and the levels R ₁ , R ₂ , R ₃ , ..., R _N of each frequency of the detection signal are the jitter of the measurement signal itself. The quantity is stored in the network analyzer 23.

Next, the measurement signal output from the jitter signal generator 21 is input to the circuit under test 1, the output signal of the measurement circuit 1 is input to the phase change detector 22, and the output of the network analyzer 23 is output. The frequency of the signal is swept in the same range as above.

As a result of this sweeping, the level variable circuit 24 outputs a modulation signal whose amplitude changes in accordance with the characteristics shown in FIG. 3 as described above, and the measurement signal phase-modulated with a modulation degree proportional to the amplitude of the modulation signal. Is input to the circuit under test 1.

[0040] Then, the phase fluctuation component of the output signal of the measuring circuit 1 is detected by the phase change detector 22, the level L ₁ of each frequency of the detection signal, L _2, L _3, ......, is L _N The jitter amount of the output signal of the circuit under test 1 is stored in the network analyzer 23.

At this time, the amount of jitter of the measurement signal itself is slightly smaller than the standard characteristic of jitter tolerance in the range of the modulation signal frequency f _{1 to} f ₄ due to the frequency characteristic of the level variable circuit 24 as described above. It has become. That is, the amount of jitter in the low frequency region from frequencies f _{1 to} f ₄ is almost 1.
5UI (pp), that is, 1.5UI / 2.5GHz =
It becomes 600 ps (pp), and this jitter amount is 0.1d.
B becomes 6.9 ps (pp).

The value of 6.9 ps (pp) is 0.1d of the jitter amount of 0.15 UI from the frequencies f _{3 to} f _4.
20 dB for 0.69 ps (pp) corresponding to B
Is also big.

Therefore, the linearity error of the jitter signal generator 21 and the phase change detector 22 is 6.9 ps (p-
If it can be ignored for p), the transfer characteristic in this low frequency region can be measured with high accuracy. This is equivalent to the apparent linearity error reduction of 20 dB in the jitter signal generator 21 and the phase change detector 22 in the low frequency range.

Further, even in the frequency region where the modulation signal frequency is f _{2 to} f ₃ , the measurement is performed with a jitter amount of 0.15 UI or more, so that the linearity error of the jitter signal generator 21 and the phase change detector 22 is increased. Apparently decreases.

Therefore, the jitter amount of the output signal of the circuit under test 1 is changed from the low frequency band to the middle frequency band of the modulated signal,
It can be sufficiently detected with a resolution of 0.1 dB which is the standard value of the transfer characteristic. As described above, since the frequency f ₃ and the upper limit frequency f _c of the pass band of the standard characteristic (C in FIG. 4) for evaluating the jitter transfer characteristic are close to each other, the measurement result in this pass band region is sufficient. Has accuracy.

When the sweep with the circuit under test 1 connected is completed, the network analyzer 23 determines the amount of jitter (R) for each frequency of the measurement signal itself obtained when the measurement is performed without connecting the circuit under test 1. ₁ , R ₂ , ... _RN ),
Ratio of jitter amounts (L ₁ , L ₂ , ... L _N ) of signals output from the circuit under test 1 for each frequency, that is, JT ₁ = 20 log (L ₁ / R ₁ ), JT ₂ = 20 log ( _{L 2 / R 2), ............} JT N = 20log (L N / R N) to seek, and the results, a jitter transfer characteristic G of the circuit under test 1
Is displayed on the screen for each frequency as shown in FIG.
The calculation process and the display process may be performed each time the frequency of the modulated signal changes by one step.

The measurer determines whether or not the characteristic G exceeds the standard characteristic C, and evaluates the characteristic of the circuit under test 1.

Since the jitter amount of the measurement signal itself is stored in the network analyzer 23, when the next circuit under test is measured, the measurement signal is immediately input to the circuit under test and the same as above. Take a measurement.

[0049]

Other Embodiments In the above embodiments, the network analyzer in which the sweep signal generating means and the selection level detecting means are integrally formed is used, but the frequency of the output signal and the selection frequency are the same. If it can be interlocked so that the frequency can be swept,
The sweep signal generating means and the selection level detecting means may be formed separately.

In the above embodiment, the modulation signal is swept while the measurement signal output from the jitter signal generator is directly input to the phase change detector, and then the jitter transfer characteristic of the circuit under test 1 is measured. However, the jitter amount of the measurement signal itself and the jitter amount of the output signal of the circuit under measurement 1 may be detected and the ratio thereof may be detected each time the frequency of the modulation signal is changed.

Further, the level variable circuit 24 of the above-mentioned embodiment is constituted by the lag lead filter and the switch, but a wide band amplifier or attenuator which does not affect the frequency characteristic of the filter is provided in the preceding stage or the latter stage of this filter. Then, the amplification degree or the attenuation amount of this amplifier or attenuator may be set in advance so that the level of the modulation signal for the jitter signal generator 21 is slightly lower than the jitter tolerance standard characteristic of the circuit under test 1. Good. By doing so, it is possible to deal with the case where the level of the signal swept out from the network analyzer 23 is low or too high.

[0052]

As described above, according to the jitter transfer characteristic measuring apparatus of the present invention, the signal swept and output from the sweep signal generating means at a predetermined level is approximated to the standard frequency characteristic of the jitter tolerance of the circuit under test. Since the signal is input to the jitter signal generating means through the level variable circuit having characteristics, in the low frequency region where the jitter tolerance is high, the measurement signal modulated with a large modulation factor is input to the circuit under test and the jitter tolerance is low. In the high frequency region, the measurement signal modulated with a small modulation factor is input to the circuit under measurement.

Therefore, it is possible to use a network analyzer which sweeps and outputs a signal having a constant level, and if the level is adjusted to a predetermined level corresponding to the jitter tolerance of the circuit under test in the low frequency region, this It is possible to perform measurement with very little influence of the linearity error of the jitter signal generating means and the phase change detecting means in the low frequency region
Even if the frequency of the measurement signal is high, it is possible to achieve both measurement efficiency and measurement accuracy.

[Brief description of drawings]

FIG. 1 is a block diagram showing the configuration of an embodiment of the present invention.

FIG. 2 is a block diagram showing a configuration of a main part of one embodiment.

FIG. 3 is a characteristic diagram of a level variable circuit according to an embodiment.

FIG. 4 is a diagram showing measurement results according to an example.

FIG. 5 is a diagram showing a configuration of a conventional device.

FIG. 6 is a block diagram showing a configuration of a main part of a conventional device.

FIG. 7 is a characteristic diagram of a main part of a conventional device.

FIG. 8 is a diagram showing a relationship between a standard characteristic of jitter tolerance and a jitter amount of a measurement signal.

FIG. 9 is a diagram showing measurement results by a conventional device.

[Explanation of symbols]

DESCRIPTION OF SYMBOLS 1 Circuit under test 20 Jitter transfer characteristic measuring device 21 Jitter signal generator 22 Phase change detector 23 Network analyzer 23c Sweep signal generator 23d Selection level detector 24 Level variable circuit 25 _{1 to} 25 _m Lag lead filter

Claims (1)

[Claims] 1. A jitter signal generating means (21) for inputting to a circuit under test a measurement signal of a predetermined frequency that is phase-modulated with a modulation factor proportional to the level of the input modulation signal, and a circuit for receiving the measurement signal. A phase change detecting means (22) for detecting a phase change component of a signal output from the measuring circuit; a sweep signal generating means (23c) for outputting a signal whose frequency is swept in a predetermined frequency range at a predetermined level; Selection level detecting means (23d) for receiving the output signal of the phase change detecting means and interlocking with the frequency sweep of the sweep signal generating means to detect the level of the signal component of the same frequency as the output signal of the sweep signal generating means. Receiving a signal output from the sweep signal generating means,
Jitter transfer provided with level varying means (24) for inputting to the jitter signal generating means a signal whose level changes according to the level frequency characteristic approximated to the jitter tolerance standard characteristic of the circuit under test defined in advance. Characteristic measuring device.