Electricity: measuring and testing – Measuring – testing – or sensing electricity – per se – Analysis of complex waves
Reexamination Certificate
1999-05-14
2001-08-14
Brown, Glenn W. (Department: 2858)
Electricity: measuring and testing
Measuring, testing, or sensing electricity, per se
Analysis of complex waves
C324S076190
Reexamination Certificate
active
06275020
ABSTRACT:
BACKGROUND OF THE INVENTION
1. Field of the Invention
The present invention relates to a frequency analysis method utilized in analyzing frequency components included in various kinds of signals and to a sweep type spectrum analyzer using this frequency analysis method, and more particularly, relates to a frequency analysis method which permits such spectrum analyzer to be swept at high or fast rates (speeds) even in a high resolution analysis and to a sweep type spectrum analyzer using such frequency analysis method.
2. Description of the Related Art
There have been two types of spectrum analyzers, one of which is referred to a sweep type spectrum analyzer and the other of which is an FFT (fast Fourier transform) type spectrum analyzer in correspondence to different methods of frequency analysis.
The sweep type spectrum analyzer means a spectrum analyzer of the type in which a local oscillator continuously performs a frequency sweep operation, a frequency spectrum component included in a signal to be measured is converted, by the frequency sweep operations, into an intermediate frequency signal consisting of a constant frequency component, and the power of the intermediate frequency signal is detected and displayed, as a spectrum component, on a screen of a cathode ray tube.
The FFT type spectrum analyzer means a spectrum analyzer of the type in which the oscillation frequency of a local oscillator is changed stepwise, the oscillation frequency in each step is resolved into a spectrum by the FFT transform means, and the Fourier transform results obtained in all of those steps are stored in a memory and are displayed on a display device.
The sweep type spectrum analyzer has a characteristic that all the frequency analysis results can be obtained by one frequency sweep operation. However on the other hand, there is a disadvantage in this type of spectrum analyzer that a time length required for one frequency analysis (time length of frequency sweep) must be longer as frequency resolution is made higher.
On the contrary, the time length required for a frequency analysis in the FFT type spectrum analyzer may be shorter than that required for the sweep type spectrum analyzer. However on the other hand, there is a disadvantage in this type of spectrum analyzer that since frequency analysis operation is performed stepwise, the frequency analysis results become discrete and hence all the spectrum components included in a signal to be measured cannot be extracted precisely.
As mentioned above, each of the sweep type spectrum analyzer and the FFT type spectrum analyzer has both merits and demerits. However, it can be said that if the sweep type spectrum analyzer can have a possibility of high rate sweep operations, the sweep type spectrum analyzer has a characteristic superior than that of the FFT type spectrum analyzer.
The reason why the sweep rates or speeds of the sweep type spectrum analyzer cannot be made higher is described in many technical books or journals (for example, “Spectrum Analyzer—Theory and Application” written by Morris Engelson and Fred Telewsky, translated by Kiyotaka Okada, and published by Nikkan Kogyo Shinbun Co., Ltd.; “Spectrum/Network Analyzer” written by Robert A. Witte, translated by Teruo Takeda and Nobutaka Arai, and published by Toppan Co., Ltd., and the like). Therefore, in this specification, such reason will be described very simply by concentrating on the items necessary for understanding the present invention.
First, a basic configuration of a sweep type spectrum analyzer will be described.
FIG. 13
shows a configuration of a conventional sweep type spectrum analyzer in a mostly simplified form. As shown, the spectrum analyzer can basically be configured by a mixer
12
, a local oscillator
13
, an intermediate frequency filter
14
, a sawtooth wave generator
15
, and a display device
16
. The mixer
12
, the local oscillator
13
and the intermediate frequency filter
14
constitute, as will be mentioned later, a time-to-frequency converting apparatus
18
.
The local oscillator
13
performs a frequency sweep over a preset frequency span or range f
LO
-f
HI
, and inputs the swept frequency signal LO to the mixer
12
. The mixer
12
mixes the swept frequency signal inputted thereto from the local oscillator
13
and a signal to be measured S
in
inputted to an input terminal
11
, and outputs, in this example, a difference signal between those two signals. Assuming that the center frequency of the passband of the intermediate frequency filter
14
is f
IF
, if the signal to be measured S
in
includes signals S
1
, S
2
and S
3
having frequencies f
1
, f
2
and f
3
(f
1
<f
2
<f
3
) respectively, intermediate frequency signals S
IF1
, S
IF2
and S
IF3
can be extracted through the intermediate frequency filter
14
every time the frequency f
LO
of the swept frequency signal LO satisfies conditions of f
LO
−f
1
=f
IF
, f
LO
−f
2
=f
IF
, and f
LO
−f
3
=f
IF
, respectively.
By supplying the intermediate frequency signals S
IF1
, S
IF2
and S
IF3
extracted from the intermediate frequency filter
14
to a vertical input terminal Y of the display device
16
, and by supplying a sawtooth wave signal S
W
outputted from the sawtooth wave generator
15
to a horizontal input terminal X of the display device
16
, the intermediate frequency signals S
IF1
, S
IF2
and S
IF3
are displayed on the display device
16
the abscissa X of which is made a frequency axis, in order (sequence) of the frequencies f
1
, f
2
and f
3
(f
1
<f
2
<f
3
) respectively.
The example shown in
FIG. 13
is a case in which the intermediate frequency signals S
IF1
, S
IF2
and S
IF3
extracted from the intermediate frequency filter
14
are directly inputted to a vertical input terminal Y of the display device
16
. However, there is another case in which, as shown in
FIG. 14
, a detector
17
is disposed at the output side of the intermediate frequency filter
14
, and the intermediate frequency signals S
IF1
, S
IF2
and S
IF3
are detected by the detector
17
, and thereafter, this detected signal is supplied to the vertical input terminal Y of the display device
16
to display frequency spectrums S
IF11
, S
IF12
and S
IF13
having a rectified and smoothed single polar envelope.
In a practical case, since the bandwidth of the intermediate frequency filter
14
is narrow as compared with the frequency sweeping range (span) or swept frequency bandwidth of the local oscillator
13
, the frequency spectrums S
IF11
, S
IF12
and S
IF13
are observed, as shown in
FIG. 15
, as line spectrums respectively if each of the signals S
1
, S
2
and S
3
included in the signal to be measured S
in
is a sine wave having single frequency.
From the above discussion, it could be understood that the signals S
1
, S
2
and S
3
included in the signal to be measured S
in
can be frequency-discriminated and can be converted, by the mixer
12
, the local oscillator
13
for generating a frequency sweep signal and the intermediate frequency filter
14
, to the intermediate frequency signals S
IF1
, S
IF2
and S
IF3
aligned on the time base in accordance with a lapse of time associated with the frequency sweep operation. Therefore, hereinafter, the frequency discriminating/converting means constituted by the mixer
12
, the local oscillator
13
and the intermediate frequency filter
14
will be referred to as time-to-frequency converting apparatus or converter
18
.
Here, attention is paid to the intermediate frequency signal S
IF1
shown in FIG.
13
.
FIG. 16
shows a behavior in which the intermediate frequency filter
14
responds to a signal Smix
1
having a difference frequency f
LO
−f
1
outputted from the mixer
12
.
Now, assuming that the center frequency of the intermediate frequency filter
14
is 10 MHz, the passband width of the intermediate frequency filter
14
defmed by −3 dB is ±1 MHz, the frequency f
1
of the signal S
1
is f
1
=100 MHz, if the oscillation frequency f
LO
of the local oscillator
Advantest Corporation
Brown Glenn W.
Gallagher & Lathrop
Lathrop David N.
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