Data processing: measuring – calibrating – or testing – Measurement system in a specific environment – Electrical signal parameter measurement system
Reexamination Certificate
1998-10-06
2001-05-15
Hoff, Marc S. (Department: 2857)
Data processing: measuring, calibrating, or testing
Measurement system in a specific environment
Electrical signal parameter measurement system
C702S077000, C324S076190, C324S076470, C331S017000
Reexamination Certificate
active
06233529
ABSTRACT:
FIELD OF THE INVENTION
This invention relates to a frequency spectrum analyzer for analyzing frequency spectrum of an incoming signal in a frequency domain, and more particularly, to a frequency spectrum analyzer having a time domain analysis function in which fast changes in frequency or time period of an incoming signal can be measured in the time domain with high accuracy and high resolution.
BACKGROUND OF THE INVENTION
Frequency spectrum analyzers are widely used for analyzing frequency spectrum of an incoming signal in a frequency domain. Typically in such a frequency spectrum analyzer, levels of frequency spectrum are displayed in a vertical direction with respect to a frequency range in a horizontal direction. A frequency spectrum analyzer may also include a function for displaying levels of the incoming signal in a time domain. A conventional example of such a frequency spectrum analyzer having frequency domain and time domain analysis capabilities is shown in FIG.
6
.
The conventional frequency spectrum analyzer of
FIG. 6
includes an RF section
10
, a detector
20
, an AD converter
30
, a microprocessor
50
, a display
60
, and a frequency reference source
70
. The RF section
10
is formed of an attenuator
11
, an amplifier
12
, a frequency mixer
13
, a local frequency oscillator
15
and an intermediate frequency (IF) filter
14
.
When the frequency spectrum analyzer of
FIG. 6
is used for analyzing frequency spectrum of an incoming signal in the frequency domain, the local frequency oscillator
15
linearly sweeps its frequency (sweep mode) for a specified frequency range. When the frequency spectrum analyzer is used for analyzing a time domain waveform of the incoming signal, the local frequency oscillator
15
is set a fixed frequency (zero span mode).
First, the basic operation of the frequency domain analysis is described in the following: An input RF signal provided to an RF terminal is adjusted its power level by the attenuator
11
and the amplifier
12
before being applied to the frequency mixer
13
in such a way that the maximum dynamic range is attained in the measured results. Typically, such a measurement dynamic range is determined by the maximum possible input power level to be applied to the frequency mixer
13
without distortion.
In the example of
FIG. 6
, the local frequency oscillator
15
generates a local signal whose frequency is swept linearly (sweep mode) with reference to the reference frequency source
70
. The RF signal frequency and the local signal frequency are mixed in the frequency mixer
13
, thereby creating IF signals having both sum and difference frequencies between the two frequencies. The IF filter
14
, which is a band pass filter, selects either one of the sum or difference signals from the frequency mixer
13
.
The detector
20
detects an amplitude of the IF signal from the output of the IF filter
14
. The AD converter
30
converts the amplitude of the IF signal to a digital signal. The resultant digital signal generated by the AD converter
30
is processed by the microprocessor
50
and is displayed on the display
60
as frequency spectrum with power levels. Typically, the vertical axis of the display
60
represents power levels of the spectrum while the horizontal axis represents frequencies of the spectrum. The microprocessor
50
also controls overall operation of the spectrum analyzer including that of the local oscillator
15
, detector
20
, display
60
and AD converter
30
via a system bus
80
.
Second, the basic operation of the time domain analysis in the frequency spectrum analyzer is described in the following: An input RF signal is provided to the frequency mixer
13
through the attenuator and amplifier in the same manner as in the frequency domain analysis. However, the local frequency oscillator
15
is tuned and fixed to an appropriate frequency (zero span mode) so that an IF signal which has a frequency equal to the center frequency of the band pass filter
14
is produced all the time by the frequency mixer
13
. The IF signal from the filter
14
is amplitude detected by the detector
20
and converted to a digital signal by the AD converter
30
. Therefore, on the display
60
, the power level of the IF signal, which is proportional to the input RF signal, is shown in the time domain.
Thus, in the conventional frequency spectrum analyzer, in the time domain analysis, it is possible to observe and analyze the changes in the power levels of the input RF signal with respect to the elapse of time. However, it is not possible to observe and analyze the changes in the frequency or time period of the input RF signal with respect to the elapse of time. This is because the input RF signal is displayed on the screen in the same manner as displayed by an oscilloscope where a waveform is shown in a format of power level versus time.
Thus, in the zero span mode of the conventional frequency spectrum analyzer, input RF signals whose time period or frequency changes quickly cannot be properly measured in the time domain. For example, settling times in a VCO (voltage controlled oscillator) or PLL (phase lock loop) circuit or a frequency deviation in an FM (frequency modulation) system is not effectively measured by the conventional spectrum analyzer.
SUMMARY OF THE INVENTION
Therefore, it is an object of the present invention to provide a frequency spectrum analyzer which is capable of measuring changes in the period or frequency in an input RF signal in the time domain.
It is another object of the present invention to provide a frequency spectrum analyzer which is capable of measuring small and fast changes in the period or frequency in an input RF signal in the time domain with high speed and accuracy.
It is a further object of the present invention to provide a frequency spectrum analyzer which is capable of measuring settling times in a VCO or PLL circuit or frequency deviation in an FM system in the time domain measurement with high speed and accuracy.
It is a further object of the present invention to provide a frequency spectrum analyzer which is capable of displaying results of time domain analysis with various dimensions and units including time, frequency, power level, data points and number of occurrence.
In the present invention, the spectrum analyzer is provided with a continuous period measurement function which continuously measures time periods of each and every cycle of the IF signal to analyze the changes in frequency and time period of the input signal in the time domain.
The frequency spectrum analyzer of the present invention having a sweep local oscillator is comprised of a continuous period measurement block for continuously measuring each time period of an IF signal produced by mixing the input signal and the local oscillator signal, and means for processing the data representing the continuous time period produced by the continuous period measurement block to analyze the input signal in a time domain.
An example of the continuous period measurement block employed in the frequency spectrum analyzer of the present invention is comprised of a divider for dividing a frequency of the IF signal by a predetermined ratio, a fractional time measurement circuit for measuring a time length of fractional time produced between an edge of the IF signal and a reference clock pulse by expanding the fractional time, a fractional time controller for controlling an operation of measuring the fractional time by the fractional time measurement circuit, a counter for counting the number of reference clock pulse during the time period of the IF signal including the fractional time expanded by the fractional time measurement circuit, a memory for storing data showing the time period obtained by the counter, and an address counter for generating address data for the memory.
According to the present invention, the frequency spectrum analyzer is capable of measuring changes in the period or frequency in an input RF signal in the time domain. The frequency spectrum analyzer can measure small and fas
Advantest Corp.
Bui Bryan
Hoff Marc S.
Muramatsu & Associates
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