DE3125749A1 - Method for testing analog/digital converters, particularly for testing codecs for PCM equipment, and device for carrying out the method - Google Patents

Abstract

In this method, a test signal, the amplitude and frequency of which can be programmed, is applied to the device under test. The response of the device under test is analysed in real time and processed in a microprocessor which, in turn, controls the generator for generating the test signal.

Description

Procedure for testing analog-digital converters, in particular

special for testing codecs for PCM devices, as well as device for Implementation of the method The invention relates to a method for testing Analog-to-digital converters, especially for testing codecs for PCM devices, as well as -a device for carrying out the method.

During the development, commissioning and testing of analog-to-digital converters, in particular of codecs or assemblies containing codecs, the Task, the digital output signals given in response to an analog input signal, e.g. analyze PCM words, in a way that allows an assessment of the transducer properties enables.

It is known to statically add PCM codecs in the analog-digital direction check by applying different DC voltage levels to the input and examine the corresponding PCM words. This known method therefore cannot be carried out in the majority of the given cases of application, because in the codecs commonly used in PCM switching technology, for example, the encoder input is not accessible from the outside in isolation. For assessing the behavior of the The overall arrangement containing the encoder and the necessary filters therefore many individual measurements are necessary in which the analog input signal is in its Amplitude and frequency is changed.

Another known method for adjusting codecs in the analog-digital direction consists of varying the channel level until it is a fixed sinusoidal level Input signal at the output the largest quantization interval with a given statistical probability is achieved.

The invention is based on the object of specifying a method which makes it possible to use all values as absolute values under operating conditions the measurement resolution and accuracy through the choice of the number of the evaluated PCM words.

This object is achieved by a method with the features of patent claim 1 solved.

Advantageous developments and refinements of the method according to of the invention are the subject of the subclaims, which are hereby to be shortened the description is expressly referred to.

An apparatus for carrying out the method according to the invention is the subject of further subclaims, which are hereby also expressly referred to will.

The invention provides for the measurement in analog-digital direction as follows to carry out that - the analog input of the test object from one in amplitude and Frequency programmable generator is acted upon by the digital controller of the test object is synchronized and with a microprocessor performing the evaluation forms a control loop, - the PCM words appearing serially at the output of the DUT be analyzed in the microprocessor under operating conditions, - by the synchronization of the generator from the digital talen control of the measurements necessary connection between analog input signal and digital response established becomes, - modulation limit and linearity - of the analog-digital converter in a few Measuring steps can be determined, - Linearity and distortions by comparing the can actually be determined with the theoretical frequency of the PCM words - and the dynamic behavior through defined activation of the analog input signal can be determined.

It is an advantage of the method according to the invention that all values are recorded under operating conditions and as absolute values and that on extensive Correlation calculations can be dispensed with, so that.

the analysis with the help of the microprocessor on the Sor-.

and adding limits while capturing the PCM words. By externally calculated comparison values that are stored in the microprocessor, short analysis times can be achieved. If necessary, the measuring time can be through the use of a plurality of microprocessors on the for real-time operation shorten the time required.

The invention is explained in more detail below with reference to the drawings: Fig. 1 shows an overview diagram of a measuring arrangement according to the invention, Fig. FIG. 2 shows a schematic representation of one that can be used in the arrangement according to FIG. 1 PCM word analyzer.

The measuring arrangement shown in Fig. 1 for quick testing and setting of analog-to-digital converters for PCM devices consists of a test signal generator 1, a matching circuit 2, an interface simulation gate 3 for controlling a test item 4, a real-time PCM word analyzer 5, a higher-level Microprocessor 6 for the control and evaluation as well as a display device 7th

The test signal generator generates a test signal whose statistical behavior is known, for example a sinusoidal signal with the probability density where x = u / A, that is to say the visual value u related to the amplitude 0, and z mean a normalization factor.

In response to this test signal, the device under test delivers a sequence of PCM words. yi, which have the probability density P (yi) = ni (yi) and in a fixed predetermined relationship to the probability density of the test signal st, ehen, if the test object works ideally.

It is assumed that the test unit is the encoder of a PCM codec and that the digitization is done with 8 bits, as it is in PCM telephone systems is common. The individual PCM values may have a time interval of 125 us and the bit rate within each PCM word is 2.048 MHz.

The following measurements are carried out for the statistical description of the PCM words: The value y is considered to be the effective zero error of the device under test. The one at the PCM output of the test item.

measured power level corresponds to 2 2 -2 s y -y.

This value is calculated by the microcomputer 6 and in the display device 7 - displayed. With its help, it is possible to adjust the encoder level in the transmission direction possible..

An approximate statement about the linearity of the encoder is obtained from the form factor By comparing the measured probability density p (Yi) with the (known) ideal distribution density.

one holds an exact statement about the behavior of the coder. The normalized target distribution is In Fig. 2 the scheme of a PCM word analyzer is shown. It essentially consists of one. Counters Z, memories SP1 to SP4, accumulators AK1 to AK4, a multiplexer MUX, a controller C and a series-parallel converter S / P.

The counter Z records the number of scanning periods examined. He is switched on by a sync signal SYNC, which also the Seri.en-P-arallelwandle'r S / P controls. The memories SP1 to SP4 are assigned the 8-bit PCM word fed. The memory location in SP1 addressed in the respective sampling interval is in each case increased by one, so that the memory SP1- the amplitude density after the end of the observation time p (Yi)).

Before the start of the measurement, for each of the 28th = 256 quantization levels, the associated linearized amplitude values are saved. The PCM word issued by the DUT serves as an address to control the Memory SP2. In the accumulator AK2, the sum of all recorded during the measuring time is recorded Samples formed. If the test object is working correctly, its content must - (ideally Coding) become practically zero after the end of the measuring time.

Before the start of the measurement, the size of the ~ A sampling interval is stored in the memory SP3 saved. The accumulator A-K3 thus contains the after the end of the observation period Rectification mean value of the signal at the output of the test object.

Before the start of the Measurement of the square value belonging to the respective quantization interval Stored amplitude. The accumulator thus contains AK4 after the observation time has ended the square of the rms value of the signal emitted by the test object.

Provided that the additional non-linear distortion of the test specimen are negligible, can be determined according to the previously described Already carry out a level setting of the encoder.

If the additional non-linear distortions are taken into account should be, the signal at the output of the device under test must also be digital Be subjected to pre-filtering before it is fed to the PCM word analyzer.

With a PCM word analyzer enhanced in this way, you can also selective level measurements can be carried out in the analog-digital direction. From the Quotient Sy - Sy (fM) k = -----------, Sy (where 5y (fM) is the selectively measured Rms value after the aforementioned digital pre-filtering allows conclusions to be drawn can be drawn on the quantization distortion.

Ideally, the RMS value of the unfiltered signal must match the one of the filtered signal match.

5 claims 2 figures L e r s e i t e

Claims (5)

Claims method for examination. of analog-to-digital converters, especially for testing codecs for PCM devices where the analog input of the DUT is acted upon with a changeable measurement signal and the digital response of the test object processed on the measurement signal in real time and to the. ideal Converter behavior to be expected digital signal is related, d a d u r c h g ek e n n z e i c h n e t 'that the measurement signal (p (x)) from one in amplitude and frequency programmable generator (1) generated and via a matching circuit (2) the DUT (4) is fed that the digital response signal (P (Yj)) over a converter (3) of a device (5) for analyzing the response signal (P (yi)) is fed and that a 'control loop is formed by the device (5) for analysis via a microprocessor (6) on the one hand to this device (5) back and on the other hand to the programming input of the generator mentioned. (1) runs. 2. The method according to claim 1, d a d u r c h g e k e n nz e i c h n e t that for calculating the mean value of the response signal (P (yi)) of the device under test (4) A memory with the digital response signal emitted by the test item (4) (SP2) is addressed in which the quanti sierungsinterval len of the analog-to-digital conversion corresponding linearized amplitude values are stored and that in one with the storage (SP2) connected accumulator (AK2) the sum of all during the Measurement time formed samples is stored. 3. The method according to claim 1, d a d u r c h g e k e n nz e i c h n e t that to determine the effective value of the response signal (P (Yi)) another Memory (SP4), in its memory locations before the start of the measurement the relevant Amplitude square belonging to the quantization interval saved is addressed by the response signal and that in one with the other Storage (SP4) connected AkkumulatoT (AK4) after the end of the observation time corresponding to the square of the rms value of the signal emitted by the device under test Information is supplied to said microprocessor (6). 4. Device for performing the Verfahr.ns according to one of the claims 1 to 3, g e k e'n n z e i c h n e t by - a generator (1) for generating a Measurement signal (p (x)) programmable in amplitude and frequency, - one in the connection matching circuit (2) inserted between the generator (1) and the test item (4), - A device (3) for synchronizing the test item (4) and for adaptation of the response signal (P (yi)))) 'of the test object (4), - a device (5) for analysis of the response signal (p (yg)) of the device under test (4) in real time with the said Device ~ (3) for synchronization and adaptation is in connection, - one with the said device (5) for analysis connected microprocessor (6), the on the one hand the device (5) for analysis and on the other hand the generator (1) to generate. of the measuring signal (-p (x)) controls - as well as by one with the microprocessor (6) connected 'display device. 5. Apparatus according to claim 4, d a d u r c h g e k e n nz e i c h n e t that the device (5) for analyzing from a plurality of by the Response signals (P (yi)) addressable memories (SP1 to SP4) exist on their Storage locations from the quantization intervals of the analog-digital conversion corresponding Variables derived from amplitude values (e.g. amplitude value itself, square of the amplitude value etc.), as well as from accumulators connected to the memories' (SP1 to SP4) (AK1 to AK4), the outputs of which with the named microprocessor (6) are connected.