JPS61112972A - Method of measuring frequency characteristic of digital signal transmission system - Google Patents

Abstract

(57) [Summary] This bulletin contains application data before electronic filing, so abstract data is not recorded.

Description

DETAILED DESCRIPTION OF THE INVENTION Field of the Invention The present invention relates to a method for measuring frequency characteristics of a digital signal transmission system.

Prior Art In order to measure the frequency characteristics of a digital signal transmission system, the output signal of a swept frequency oscillator is supplied to the transmission system to be tested through an A/D converter, and after passing through the transmission system, the D/A
Methods of generating test signals by conversion and visual display using a spectrum analyzer are known. Furthermore, for testing transmission channels, in particular recording and/or playback transmission channels, a signal with a defined pulse shape is recorded and this signal is displayed on an oscilloscope after passing through the transmission system or after playback from a magnetic tape. The way to do it is
It is publicly known.

Problems to be Solved by the Invention Using this known method, the reproduced signal is very difficult to judge oscillographically, e.g. it is difficult to adjust the recording/reproducing channels, and it is difficult to do so without deep experience. I need.

The object of the invention is that it is inexpensive and easy to implement.
The purpose of this paper is to propose a method for measuring the frequency characteristics of digital signal transmission systems.

Means for solving the problem In order to solve the above-mentioned problem, the pulse width is equivalent to the reciprocal of the bit repetition frequency (bit period) of the digital signal, and the repetition frequency is smaller than the bit repetition frequency. A periodically repeated pulse is generated and supplied to the transmission system, and the frequency spectrum of the output signal of the transmission system is analyzed.

EXAMPLE FIG. 1 shows a test signal used in the present invention, which signal has a duration level B of a pit period,
It has level A for a time that is several times the bit period. The period of the test signal is NXT, where N is greater than 1.

If N increases to infinity (((), then the frequency spectrum of such a signal will consist of lines having the same magnitude of amplitude and repeating at 1 (NXT). In fact, since N is not infinite, the frequency spectrum of such a signal will be The spectrum shown in the figure results.

A similar spectrum is shown by a signal such as that in FIG. A meandering oscillation with each pit frequency in the signal is changed by suppressing one pulse at a time interval of NXT. Such a test signal results, for example, by a logical combination of the signal of FIG. 1 and a meandering oscillation having an A-bit frequency. The spectrum of the signal of FIG. 2 differs from that of the signal of FIG. 1 in that there is an additional strong spectral line at Z bit frequencies (shown by the dashed line in FIG. 4).

Since the DC' component relative to the AC voltage component of the signal of FIG. 2 is relatively small, this signal is particularly suitable for testing transmission channels that do not carry any DC voltage.

When determining N, on the other hand, if N is set to a small value, there is a very small spectrum within the frequency range that can be used for evaluation up to the first zero position ('/T in Figure 4). It must be taken into account that only lines are generated. On the other hand, N
is limited by the inability to distinguish the test signal from the noise, since the amount of energy in the test signal evaluated by the spectrum analyzer decreases as the spectrum analyzer increases. In practice, when testing a signal transmission channel with a 50MHz bit frequency, the values of N are 64 and 25.
6 was found to be advantageous.

FIG. 6 shows a block circuit diagram of an apparatus for implementing the method of the invention. A test signal generator 1 generates the signals shown in FIG. 1 or 2, which signals are supplied to a transmission system 2 to be tested. The test signal passing through the transmission system 2 reaches the D/A converter 4' via the switch 3. The analog signal is displayed as a spectrum using a commercially available spectrum analyzer 5.

The next point will be mentioned in connection with the D/A converter 4. That is, when transmitting a digital signal, it is important that the transmission channel for this signal can be used over a sufficient bandwidth. If the bandwidth is too small, the digital signal will be distorted such that fault-free reproduction and conversion of the transmitted signal is no longer possible.

To test such a channel using the method of the invention, the transmitted signal is fed directly to a spectrum analyzer without converting it into an analog signal. It is therefore important for this embodiment that the frequency characteristics of the transmission channel have an effect on the digital signal. However, there are also transmission channels (in a broad sense) in which the frequency characteristics of the analog signal represented by the digital signal are influenced by the transmission channel. In this case, the first problem is the digital filter. In order to measure such frequency characteristics, it is important to provide a D/A converter 4 in front of the spectrum analyzer 5, as shown in FIG. In the system of FIG. 6, digital signals are transmitted in parallel over, for example, eight parallel conductors. In order to be able to carry out comparative measurements,
The D/A converter 4 is directly connected to the test signal generator using the switch 3.

In FIG. 5, a spectrum with a frequency scale that has changed compared to FIG. 4 is shown. The curve marked a corresponds to the spectrum of the test signal. The curved line shows the vector of the output signal of the digital low-pass filter after being D/A converted.

FIG. 6 is a circuit diagram of an apparatus implementing the method of the present invention in the recording channel and reproduction channel of a magnetic recording apparatus, and FIG. 7 shows the frequency spectrum of the signal generated at the corresponding circuit point of the apparatus of FIG. FIG. In FIG. 6, 10.13 is a magnetic head, and 11 is a magnetic tape running between tape reels 12.12.

Effects of the Invention The method of the invention has the advantage that the test signal can be generated by simple digital circuit means and that the frequency characteristics can be directly indicated.

In an advantageous embodiment of the invention, the test signal of the invention is inserted into the recording signal during time float periods in which no information is recorded. This makes it possible to test recording or playback channels even during operation.

[Brief explanation of the drawing]

FIG. 1 is a diagram showing the voltage waveform of the first test signal of the present invention, FIG. 2 is a diagram showing the voltage waveform of the second test signal of the present invention, and FIG. FIG. 4 is a circuit diagram showing an embodiment of an apparatus for carrying out the method of the present invention, and FIG.
FIG. 5 is a diagram showing the frequency spectrum density of the test signal of the present invention, FIG. 5 is a frequency spectrum diagram showing the output of the digital low-pass filter after D/A conversion, and FIG. FIG. 7 is a circuit diagram of an apparatus implementing the method of the invention in a reproduction channel;
7 is a frequency spectrum diagram of a signal generated in the device of FIG. 6; FIG. 1...Test signal oscillator, 2...Transmission system, 3...Switcher, 5...Spectrum analyzer

Claims (1)

[Claims] 1. In a method for measuring frequency characteristics of a digital signal transmission system, the pulse width corresponds to the reciprocal value of the bit repetition frequency (bit period) of the digital signal, and the repetition frequency is equal to the bit repetition frequency. 1. A method for measuring the frequency characteristics of a digital signal transmission system, characterized by generating periodically repeated pulses smaller than the normal pulse, supplying the generated pulses to the transmission system, and analyzing the frequency spectrum of the output signal of the transmission system. 2. The pulse group of each period consists of a series of pulse trains having a width of a bit period and a time interval of the same size, and the interval from one pulse train to a secondary pulse train is one pulse and one period. A method for measuring the frequency of a digital signal transmission system according to item 1. 3. A frequency measuring method for a digital signal transmission system according to claim 1 or 2, wherein the output signal is D/A converted and then the spectrum is analyzed.