# A method of measuring jitter

(57) Abstract:

A method of measuring jitter relates to the field of digital communication systems with pulse-code modulation, in particular to methods of control of these systems. The proposed solution is aimed at improving the measurement accuracy and functionality. This problem is solved in the proposed method, due to the selection of the measured time interval between the fronts of the primary pulse signal, the nominal time interval between which corresponds to an integer number of periods of the line frequency, in contrast to the known, where the primary signal is first converted using WH. 1 Il. The invention relates to the field of digital communication systems with pulse-code modulation (PCM), in particular to methods of control of these systems.Known methods of measuring jitter, based on direct measurement of the phase difference using the phase detector. One such method is implemented in the device and may be similar to the proposed technical solutions. When measuring jitter in a specified way at first with the help of selector clock frequency (WH), a signal is generated clock frequency,you comes to two channel reference and measurement. In the reference channel jitter is suppressed narrowband quartz filter, and measuring the channel filter is missing. The outputs of the two channels connected to the phase detector, and with it the signal proportional to the phase jitter, is supplied to the device functional processing for obtaining the desired characteristics of jitter, such as dispersion.However, closer to the proposed method to the technical essence is the one that is implemented in the device. This technical solution is chosen as the prototype. In this device are sequentially enabled, the block matching, the rectifier, the selector clock frequency, the frequency detector, an integrator, a quadratic detector and indicator. The signal from the communication line is received on block matching, where it is amplified and then rectified by rectifier and fed to the WH. The selected clock frequency supplied to the frequency detector, with which jitter is converted into an equivalent frequency deviations. The integration of these deviations in the integrator allows you to receive jitter, which are further functionally converted using a quadratic detector, and the result is displayed on the display.a

_{1}=

_{3}f

_{n}(1),

where a

_{1}the first auxiliary value;

_{3}t the th value to an integer, thus receiving the second auxiliary value, calculate the third auxiliary value according to the formula

< / BR>

where a

_{2}the second auxiliary value;

and

_{3}the third auxiliary value,

calculate the deviation from the nominal frequency by the formula

f = f

_{n}(a

_{3}-1) (3),

where f is the deviation from the nominal frequency,Hz,

measurement of the third time interval, and calculating the first, second, third auxiliary variables and deviations from the nominal frequency conducted periodically receiving a set of values of the deviations of the frequencies for discrete moments of time increments equal to the second time interval, the total number of these values is determined by the formula, rounding the result to an integer

< / BR>

where m is the total number of reference points (integer);

_{1}the first interval of time,

_{2}the second time interval,

then make the integration of the deviations of the frequency by the formula

< / BR>

where

_{n}deviation phase (rad) at the discrete time n;

n the number of discrete point in time from 1 to m;

3,14.i index, varying from 1 to n;

Df

_{i}frequency deviation (Hz) in discrete time, the line is the t according to the formula

< / BR>

where K auxiliary coefficient, glad;

i index, varying from 1 to m;

_{i}the deviation of the phase in discrete time corresponding to the index i;

calculate the amount of jitter in the shares of the nominal frequency for each discrete point in time according to the formula

< / BR>

where g

_{n}the magnitude of the jitter in discrete time n;

the first, second and third time intervals chosen based on the frequency range of the phase jitter by formulas

< / BR>

where f

_{min}the lower frequency jitter;

< / BR>

where f

_{max}the upper frequency jitter;

_{3}

_{2}(10)

In the proposed method the main measuring operation is the measurement time interval, namely the third time interval. Measurement time is widely known, tested, provide digital result with high accuracy. However, the selection of the measured interval between the fronts of the primary pulse signal, the nominal time interval between which corresponds to an integer number of periods of the line frequency, are new, as in the known methods of measuring jitter of the primary signal at the beginning preassure. Let us consider in more detail these operations.First, according to the formula (1) calculates the first auxiliary value and

_{1}, which in its physical sense indicates the number of periods of the nominal line frequency f

_{n}in the first time interval

_{1}. Since the real linear frequency differs from the nominal, the number of a

_{1}will not be whole. However, deviations from a whole will not exceed units percent, since these deviations are caused by instability of the linear frequency, which for systems with PCM does not exceed 0.01% and phase jitter, which at short intervals of time does not exceed a few percent from the period of the line frequency. Therefore, when rounding numbers and

_{1}to an integer, rounding is much less than 0.5, i.e. 50% After rounding the number of turns of the second auxiliary value and

_{2}, which in its physical sense shows the number of periods measured line frequency in the first time interval

_{1}.By the formula (2) calculate the third auxiliary value and

_{3}, which in its physical sense shows the ratio of the measured line frequency to the nominal. Further, according to the formula (3) calculate the difference this is Yong be taken into account.Measurement of the third time interval to produce periodically with a period equal to the second time interval within the first time interval. Determining first and second time intervals produced by the formulas (8),(9) prior to direct measurement. These formulas are based on the Nyquist theorem, the condition (10) must be provided with consideration of the peculiarities of the code. The number of measurements in one series is determined by the formula (4). Almost can initially be done periodically in m dimensions of the third time interval, storing them in a register, then perform computations on the results of these measurements, and after computing accumulate m results. When sufficient speed microprocessor to carry out measurements and calculations in parallel.When processing a series of m measurements first calculate the deviation of the phase at discrete points in time according to the formula (5). Due to the discrete nature of the measurements in this equation the integration is performed by summing, which is quite acceptable. Received rejection phase contain a linear component due to deviation of the measured line from the nominal frequency, and phase Trojan caracterizado the slope of the linear component, by the formula (6). At the conclusion of formulas taken into account the following circumstances. If you build the dependence of the deviation of the phase from sample number, representing the deviation is constant on each interval, you will get some size, limited number of rectangles. Since the phase deviation can have different signs, then there is no linear component of the total area considering the sign of elementary squares must be equal to zero. If the area is not equal to zero, it can be equated to the area of a triangle formed by the linear component of the phase and the x-axis. From the equality of these areas should the formula (6). It should be noted that the auxiliary coefficient can be both positive and negative, or zero when no linear component of the phase.The method of linearization, which is based on the formula (6), is widely used in processing of experimental data. Experimental points always have some variation, so connecting line will be broken. When replacing this broken linear dependence of the direct line of conduct so that the square of the deviations on both sides of the line are equal.The amount of jitter is usually expressed in fractions of the period of lizania for each discrete point in time is calculated by the formula (7). Having obtained the values of jitter in digital form in discrete moments of time, it is possible to calculate various parameters such as RMS jitter by the usual formulas for determining the RMS value. Using the fast Fourier transform, it is possible to find the frequency spectrum of the phase jitter.Thus, the proposed method for the measurement of phase jitter solves the problem. This increase in accuracy is achieved by selecting as the primary measuring operation of the measuring time interval from the reception of the digital result. In addition, the proposed method allows to extend the functionality by determining the amount of jitter at discrete points in time,i.e., by obtaining initial values of jitter, knowing that,you can calculate any features: integral, frequency, etc. of the Invention corresponds to the modern level of technology, because it is based on the digital measurement methods and microprocessors.The device carrying out the proposed method,shown in the drawing. The following notation: shaper fronts 1,shaper schetchik 8,the first trigger 9, scheme OR 10, the second trigger 11.The input device is connected to the input of the shaper fronts 1. The log of formation of fronts 1 connected to the first input of forming a delay of 2, with the first input of the first system And 3 and with a second input of the second circuit And 4. The second input of the shaper delay 2 is connected to the generator output quantizing pulses 5 and with a second input of the third circuit And 6. The reset input of the shaper delay 2 is connected to the output of the microprocessor 7 and the reset input of the counter 8, a second input circuit OR 9. The output driver delay 2 connected to the first input of the second circuit And 4. The second input of the first circuit And 3 connected to the inverse output of the first flip-flop 10. The output of the first circuit And 3 is connected to the input of the first flip-flop 10, and also to the input of the second trigger 11. The output of the second circuit And 4 connected to the first input circuit OR 9. The first input of the third circuit And 6 is connected to the output of the second trigger 11 and to the first input of the microprocessor 7. The output of the third circuit And 6 is connected to the input of the counter 8. The second input of the processor 7 is connected with the output of the counter 8. The output of the circuit OR connected to the reset input of the second trigger 11.Device that the proposed method works as follows. The signal I from the code pulses, corresponding to the edges of the signal, the distance between them along the time axis corresponds to an integer number of periods of the line frequency. The allocation of such fronts is provided in any system and the PCM, because they are necessary for the operation of the selector clock frequency (WH), therefore, the scheme of such a driver known.The operation of the device occurs in cycles. The cycle begins upon receipt of the reset pulse from the microprocessor 7. This pulse sets in original condition shaper delay 2, both of the trigger 10, 11 and the counter 8. The first came after the pulse shaper fronts 1 starts shaper delay 2 and passing through the first circuit And 3, the tilt of the first and second triggers 10, 11. Through a third scheme, And 6 on the counter 8 begin counting pulses with a frequency f

_{0}with generator quantizing pulses 5 and the input of the first circuit And 3 is blocked by the first trigger 9. At the end of the delay time t

_{ass}from the output of the shaper delay 2 receives permission for the second circuit And 4, and the first coming after the pulse from the output of the shaper fronts 1, passes through the second scheme And 4, scheme OR 9 and is supplied to the reset input of the second trigger 11.The formation of what is written is some number after the arrival of the pulse from the shaper fronts 1 is formed the resolution to the account and subtraction of pulses with a frequency f

_{0}coming from the generator quantizing pulses 5. When zeroing subtractive counter is formed by the output enable signal.After a reset of the second trigger 11 with its output to the third circuit And 6 enters the ban, the passage of the counting pulses to the counter 8 is stopped. Re-launch of the second trigger 11 during one cycle is blocked by the first trigger 10 through the first circuit And 3. From the output of the second trigger 11 to the input of the microprocessor 7 receives the signal, allowing the reading of information from the counter 8.The cycle time is set by the microprocessor 7, and is equal to the duration of the second time interval. The duration of the third time interval is determined by the time during which a pulse frequency f

_{0}come to the counter 8. This time can vary from cycle to cycle. This irregularity is determined by the irregularity of the pulse from the output of forming fronts 1 and depends on the type of code, adopted in the communication system. The delay time determines the lower boundary of the third time interval, and the duration of the cycle of vernut cycle time the shorter the cycle, the greater the range of measured frequencies, but at high frequencies limitation is the lack of processor speed. In principle the combined system, when the low-frequency portion of the phase jitter is measured using a microprocessor, and high-frequency any analog method.Sources of information:

1. Author's certificate N 902272, MKI H 04 B 3/46, 1982.2. Author's certificate N 696617, MKI H 04 B 3/46, 1979 (prototype). A method of measuring jitter, comprising detecting phase by integrating the deviation frequency during the measurement time, wherein prior to the integration of the deviation frequency select the measurement time equal to the first time interval, break the first time interval to an integer number of equal intervals, equal to the second time interval, and with a period equal to the second time interval during the first time interval is measured a third time interval between the wavefronts, the nominal time interval between which corresponds to an integer number of periods of the line frequency, calculate the first auxiliary value according to the formula

a

_{1}=

_{3}>f

_{n}nominal line frequency, Hz,

round the first auxiliary value to an integer, thus obtaining a second auxiliary value, calculate the third auxiliary value according to the formula

< / BR>

where a

_{2}the second auxiliary value;

and

_{3}the third auxiliary value, calculate the deviation from the nominal frequency by the formula

f = f

_{n}(a

_{3}-1),

where f is the deviation from the nominal frequency, Hz,

measurement of the third time interval, and calculating the first, second, third auxiliary variables and deviations from the nominal frequency conducted periodically receiving a set of values of the deviations of the frequencies for discrete moments of time increments equal to the second time interval, the total number of these values is determined by the formula, rounding the result to an integer

< / BR>

where m is the total number of reference points (integer);

_{1}the first time interval;

_{2}- second time interval,

then make the integration of the deviations of the frequency by the formula

< / BR>

where

_{n}- deviation phase, glad, at the discrete time n;

n the number of discrete point in time from 1 to m;

i index, varying>/BR>calculate m values of the deviations of the phase, and then compute the auxiliary coefficient according to the formula

< / BR>

where To auxiliary coefficient, glad;

i index, varying from 1 to m;

_{i}- deviation phase in discrete time corresponding to the index i;

calculate the amount of jitter in fractions of the period of the rated frequency for each discrete point in time according to the formula

< / BR>

where g

_{n}the magnitude of the jitter in discrete time n,

the first, second and third time intervals chosen based on the frequency range of the phase jitter by formulas

< / BR>

where f

_{m}

_{i}

_{n}the lower frequency jitter,

< / BR>

where f

_{m}

_{a}

_{x}the upper frequency jitter,

_{3}

_{2}.with

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