JPS6131426B2 - - Google Patents

Description

DETAILED DESCRIPTION OF THE INVENTION The present invention relates to an intermodulation distortion analyzer (hereinafter abbreviated as IDA), and more particularly to an intermodulation distortion measuring instrument that is automatically adjusted to changing input levels.

The standardized intermodulation distortion measurement (SMPTE-IM) method can effectively measure the distortion characteristics of audio equipment. Intermodulation is the intermodulation of two
When transmitting the above frequencies, frequencies corresponding to the sum and difference of these two or more types of fundamental wave and harmonic frequencies are generated from this nonlinear circuit element. Thus, when two types of frequencies are applied to the input end of an audio frequency device, not only these two types of fundamental waves but also sum and difference frequencies, which generally have no harmonic relationship with the fundamental wave, appear at the output end. .

A typical IDA consists of two basic parts: a signal generator and an analyzer. The signal generator generates two types of signals, high and low frequency, and mixes these frequencies at a predetermined ratio to output the device under test (hereinafter referred to as DUT).
is applied to the input terminal of The analyzer is connected to the output end of the DUT and measures the intermodulation distortion generated by the DUT in percentage.

The amplitude ratio of the two signal frequencies supplied by the signal generator is usually set to 4:1, with the lower frequency having an amplitude 12 dB higher than the higher frequency. The above ratio can be varied depending on the use and may be, for example, a 1:1 ratio. The low frequencies generated are usually between 40 and 100 Hz, with 60 Hz being the most common. On the other hand, high frequencies
Any value between 4000 and 100000Hz is acceptable, but typically
It is 7000Hz.

However, the attenuation or gain of the amplifier under test, which is a function of frequency, will further change this ratio. Therefore, there is a drawback that the input level of the analysis section must be manually adjusted from time to time.

It is therefore an object of the present invention to provide an intermodulation distortion analyzer that is easy to operate.

Another object of the invention is to provide automatic level control means for an intermodulation distortion analyzer.

Other objects and effects of the present invention will become apparent from the following description with reference to the accompanying drawings.

The IDA of the present invention is a device that enables automatic measurement of intermodulation distortion using the SMPTE-IM method.
When measuring distortion using the SMPTE-IM method, a low frequency signal (60Hz) and a high frequency signal (7000Hz) are mixed at a predetermined ratio and applied to the DUT. Sidebands of the low frequency signal that occur near the high frequency signal represent distortion. The standard amplitude ratio of low frequency signals to high frequency signals is 4:1, although 1:1 ratios are often used for more rigorous testing. Since the DUT's attenuation or gain is a function of frequency, it changes the ratio. Therefore, the IDA of the present invention automatically adjusts the analyzer to changes in input signal level.

The automatic adjustment described above is achieved by detecting a carrier level equivalent to the DC level of the demodulated signal and forming an automatic level control loop using a voltage controlled amplifier at the beginning of the signal processing stage. The generated distortion output is automatically compared to the level of the high frequency signal used in the SMPTE-IM method.

Although IDA is well known to those skilled in the art, its basic analysis operations will be briefly described to aid in understanding the present invention. Figure 1 shows a typical IDA. FIG. 1a shows the intermodulation signal generator 10 and analyzer 3 of the IDA.
Two parts of 0 are shown. Although shown as an amplifier, DUT 20 may actually be any audio frequency device or circuit. The signal generator 10 generates signals of two frequencies, for example, 60 Hz and 7000 Hz, mixes these signals in a ratio of 4:1, and applies the mixed signal to the DUT 20 via a signal path 15. The distortion signal from the DUT 20 is applied to the analysis section 30 via the signal path 25. The analysis unit 30 analyzes the distortion signal and determines the distortion rate (%).
Determine.

FIG. 1b is a more detailed block diagram of the analysis section 30 of the conventional IDA shown in FIG. 1a. DUT2
A distortion output of 0 is applied via signal path 25 to high pass filter 31 . The output end of the filter 31 is connected to an amplifier 32, and the output end of the amplifier 32 is connected to a demodulator 33. The output of the demodulator 33 is applied to a low-pass filter 35 via a DC blocking capacitor 34, and then a digital voltmeter (hereinafter referred to as
DVM) 36. Demodulator 33
may be a conventional circuit such as a full-wave or half-wave rectifier or a synchronous rectifier.

In operation, the distorted output of the DUT 20 is applied to the input end of the analysis section via the signal path 25. A high-pass filter 31 is provided at the input end of the analysis section, and this filter removes the 60 Hz component and its harmonic components and passes only the 7000 Hz distortion component.The passed signal is amplified by the amplifier 32. Ru.

The output of amplifier 32 is then applied to a demodulator 33, such as a full wave rectifier as described above. Due to full-wave rectification, the high frequency component changes from 7000Hz to 14000Hz.

The 14000 Hz component is removed when passing through the low-pass filter 35, and only the demodulation envelope generated by the 60 Hz component passes. The output of filter 35 is a ripple voltage or pulsating DC voltage signal. Even if this signal has a constant DC level, it is removed by the DC blocking capacitor 34. DVM 36 measures and displays the amplitude of the ripple voltage. DVM36
can be calibrated so that the distortion factor or dB can be read directly.

Furthermore, in order to explain the operation of the analysis section 30, the following conditions are assumed. That is, assume that a distortion-free amplifier is used as the DUT 20, the high and low frequencies and amplitude ratios are the same as in the above example, and the output of the DUT 20 is distortion-free and approximately the same as the output of the signal generator 10. .

The undistorted signal passes through a high-pass filter 31.
After removing the 60Hz component, the amplifier 32 and demodulator 33
is applied to Since the DUT 20 does not generate distortion based on the above assumption, the 7000Hz component is not modulated by the 60Hz component. Therefore, the output of the demodulator is a 14000 Hz signal with equal peak amplitude.

The DC blocking capacitor 34 removes the DC component of the signal and passes only the high frequency component which is later removed by the low pass filter 35. Therefore, there is no output from the filter 35, and no distortion is displayed on the DVM 36.

The procedure for measuring intermodulation distortion of a typical audio frequency device using the IDA described above is as follows. Connect the output end of the intermodulation signal generator to the input end of the DUT. Connect the output end of the DUT to the input end of the intermodulation analysis section. Set the signal generator to the desired frequency combination. Adjust the combined signal to a level that produces the desired output level.
Applied to the input end of the DUT. Manually adjust the analyzer gain to obtain the appropriate carrier current level.
Measure the intermodulation distortion rate using DVM and read the display.

The IDA of the present invention improves on prior devices by eliminating manual gain adjustment operations at the above steps. A second preferred embodiment of the invention
This is shown in the block diagram in Figure.

Comparing FIG. 2 with FIG. 1, amplifier 32 has been replaced by voltage controlled amplifier 32'. The amplifier 32' may be a conventional voltage controlled amplifier as described in the "Guidebook of Electronic Circuits" by John Marcus, published by McGraw-Hill Book Company in 1974.
Furthermore, an integrator 40 is added that integrates the difference between the output of the demodulator 33 and the reference voltage V _REF to generate a control voltage 42 . V _REF represents the level of high frequency components in the output signal of the signal generating section 10. The DC level of the output of the demodulator 33 is equal to the carrier wave, that is, the high frequency component level.

Since the operation of the IDA of the present invention is similar to that described above, only the improvements will be described. As mentioned above, the level ratio of the high frequency and low frequency components of the test signal varies, but the present invention constantly monitors the carrier wave level so as to maintain the ratio accurately, and adjusts the high frequency of the signal from the signal generator. Compare with ingredient levels.
The difference between these signals is integrated by an integrator 40 and applied as a control voltage 42 to the gain adjustment section of the amplifier 32' to increase or decrease the gain of the amplifier 32'. If the two compared signals are equal,
The control voltage is zero and the gain of amplifier 32' remains unchanged.

As explained above, the present invention applies the output signals and reference signals of the voltage controlled amplifier 32', the demodulator 33, and the demodulator 33, and uses the output signals to
Since a servo loop is formed with the integrator 40 that controls the gain of the demodulator 33, the carrier wave level at the output end of the demodulator 33 is automatically maintained constant, and there is no need to manually adjust the carrier wave level one by one. The intermodulation distortion factor can always be displayed accurately as a ratio (%) to the test high frequency signal level.

Although preferred embodiments of the present invention have been described above, circuits,
It will be obvious to those skilled in the art that various changes and modifications can be made to timing and the like. Therefore, the present invention includes such various changes and modifications.

[Brief explanation of the drawing]

FIG. 1 is a block diagram showing a conventional example of the present invention, and FIG. 2 is a block diagram showing a preferred embodiment of the present invention. 20...Device under test, 31...High pass filter, 32'...Variable gain amplifier, 33...Demodulator, 34...Capacitor, 35...Low pass filter, 36...Display means, V _REF ...Reference voltage,
40... Integrator.

Claims (1)

[Claims] 1. A high-pass filter to which an output signal obtained by supplying high- and low-frequency signals having a predetermined amplitude ratio relationship to the device under test is supplied, and a variable gain amplifier to which the output signal of the high-pass filter is supplied. a demodulator to which the output signal of the variable gain amplifier is supplied; a low pass filter to which the output signal of the demodulator is supplied via a capacitor; and a voltage of the output signal of the demodulator and the high frequency signal. an integrator for integrating a voltage difference between reference voltages equal to the level, and display means for displaying the output signal level of the low-pass filter, and controlling the gain of the variable gain amplifier by the output signal of the integrator; An intermodulation distortion analyzer characterized in that the DC level of the output signal of the demodulator is made approximately equal to the reference voltage.