Electricity: measuring and testing – Measuring – testing – or sensing electricity – per se – Analysis of complex waves
Reexamination Certificate
1998-12-14
2001-05-15
Metjahic, Safet (Department: 2858)
Electricity: measuring and testing
Measuring, testing, or sensing electricity, per se
Analysis of complex waves
C324S615000
Reexamination Certificate
active
06232760
ABSTRACT:
BACKGROUND OF THE INVENTION
The present invention relates to a method for determining and compensating a transmission function of a measurement apparatus, for example a spectrum analyzer or a vector analyzer.
Prior art methods of this type are disclosed in Hewlett Packard Journal, December 1993, p. 31 ff. or p. 47 ff. For the evaluation, the characteristic of the modulation signal used at the input side, and its exact time relation to the calibration signal, must be known. In many applications, this precondition is not given, for example when such a method is to be used in a measurement apparatus in which a single frequency calibration oscillator and an arbitrary sweep oscillator that increases linearly with frequency are indeed already present, as is the case for example in a spectrum analyzer. Since, however, particularly in such electronic measurement apparatuses with band-limiting filters, amplifiers or frequency-converting mixers, the frequency response of the measurement apparatus is influenced by these modules, (thus introducing a certain error to the measurement result), the determination of the transmission function is of particular importance in such measurement apparatuses. This is because when the amplitude and phase frequency response of the measurement apparatus are known, this apparatus can easily be compensated by a digital evaluation unit, thereby increasing measurement precision.
SUMMARY OF THE INVENTION
It is an object of the present invention to provide a method for determining the transmission function of a measurement apparatus that can also be used in measurement apparatuses that already contain essential components for this purpose.
In general terms the present invention is a method for determining the transmission function of a digital measurement apparatus, in particular of a spectrum analyzer. A calibration signal of the measurement apparatus is modulated such that within the useful frequency bandwidth a line spectrum arises. In a computer, the digital output signal which is digitized by an A/O converter that arises at the output of the measurement apparatus is evaluated.
In the computer, the modulation signal of the calibration signal is calculated from the known carrier frequency and the known sampling frequency of the A/D converter. The desired transmission function is then calculated therefrom according to absolute value and phase.
Advantageous developments of the present invention are as follows.
The calibration signal is produced in the measurement apparatus by multiplication of a single frequency carrier signal with an auxiliary signal whose frequency increases or decreases in a linear fashion.
The apparatus further has a signal frequency calibration oscillator and a sweep oscillator whose frequency increases or decreases in linear fashion. Before the circuit part whose transmission function is to be determined, the carrier signal of the calibration oscillator is multiplied, using a mixer, with the signal of the sweep oscillator.
The digitized output signal of the measurement apparatus is digitized in an A/D converter with a known sampling frequency, and, in the computer, the start frequency and the climb, as well as the beginning and ending of the ramp function of the modulation signal, are calculated therefrom. From this, as well as from the known carrier frequency of the calibration oscillator, the transmission function of the measurement apparatus is calculated.
In the computer, the function inverse to the calculated transmission function is calculated, and the transmission function of the measurement apparatus is compensated therewith, during operation of the measurement apparatus.
Using the determined inverse function, analog signal processing stages of the measurement apparatus are equalized with electronic circuitry.
In the inventive method, no immediate knowledge of the modulation signal used as the calibration signal is required, because according to the present invention the modulation parameters are calculated immediately in the evaluation unit, from the digitized output signal of the measurement apparatus. For this purpose, only knowledge of the carrier frequency of the calibration oscillator present at the input side and the sampling frequency of the A/D converter are required. The start frequency of the modulation signal and its slope can be calculated immediately from the output signal. The inventive method is thus advantageous for all measurement apparatuses having a standard single frequency calibration oscillator, a corresponding sweep oscillator, and an associated mixer. From the digitized output frequency, the start frequency of the sweep oscillator and its slope, and the resulting transmission function, can be calculated. For the compensation, it is then necessary only to calculate the corresponding inverse function, and to use it to correct the measurement results during operation of the measurement apparatus. In the same way, a compensation by means of an electronic equalization of analog signal processing stages is of course also possible. As a modulation signal, a linear sweep oscillator is particularly suitable, but other signals are also suitable for this, for example a pseudo-noise sequence. It is essential that only by this structure is a line spectrum generated within the employed frequency band of the measurement apparatus in which the transmission function is to be determined. A sweep with a linearly increasing or, respectively, decreasing frequency has the advantage that the line spectrum is also uniform. Thus, the spectral lines have the same frequency spacing with the same power.
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Hewlett—Packard Journal, Manfred Bartz et al, Baseband Vector Signal Analyzer Hardware Design, pp. 31-59. Dec. 1993.
Metjahic Safet
Nguyen Vincent Q.
Rohde & Schwarz GmbH & Co. KG
Schiff & Hardin & Waite
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