Electricity: measuring and testing – Measuring – testing – or sensing electricity – per se – Analysis of complex waves
Reexamination Certificate
2000-01-13
2001-07-24
Metjahic, Safet (Department: 2858)
Electricity: measuring and testing
Measuring, testing, or sensing electricity, per se
Analysis of complex waves
C324S076270
Reexamination Certificate
active
06265861
ABSTRACT:
FIELD OF THE INVENTION
This invention relates to a frequency spectrum analyzer for analyzing frequency spectrum of an input signal, and more particularly, to a frequency spectrum analyzer having a high carrier wave to noise (C/N) ratio for analyzing frequency spectrum of an input signal with a wide dynamic range, high resolution, and low noise.
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 includes three or more frequency converters connected in series each of which is formed of a frequency mixer, a local oscillator and a band pass filter to produce intermediate frequency (IF) signals without image (spurious) responses.
An example of conventional frequency spectrum analyzer is shown in FIG.
3
. The frequency spectrum analyzer of
FIG. 3
includes three frequency converters. The first frequency converter is formed of a first frequency mixer
11
, a first IF filter
21
and a first local oscillator
31
. The second frequency converter is formed of a second frequency mixer
12
, a second IF filter
22
and a second local oscillator
32
. The third frequency converter is formed of a third frequency mixer
13
, a third IF filter
23
and a third local oscillator
33
. The frequency spectrum analyzer further includes a ramp wave generator
50
, a detector
60
and a display
70
.
Typically, the first local oscillator
31
is a sweep frequency oscillator whose frequency is linearly swept by a ramp wave from the ramp wave generator. The second and third local oscillators are fixed frequency oscillators. The frequency of the first local signal is higher than that of the second and third local signals.
An input signal F
1
to be analyzed is mixed with the first local signal by the first frequency mixer
11
, thereby producing first IF signals having both sum and difference frequencies between the input and first local signals. The first IF filter
21
, which is a band pass filter, selects either one of the sum or difference IF signals from the first frequency mixer
11
.
Thus, the first IF signal is provided to the second frequency mixer
12
where it is mixed with the second local signal from the second local oscillator
32
. The second frequency mixer produces second IF signals having both sum and difference frequencies between the first IF signal and the second local signal. The second IF filter
22
, which is a band pass filter, selects either one of the sum or difference IF signals from the second frequency mixer
12
.
Similarly, the second IF signal is provided to the third frequency mixer
13
where it is mixed with the third local signal from the third local oscillator
33
. The third frequency mixer produces third IF signals having both sum and difference frequencies between the second IF signal and the third local signal. The third IF filter
23
, which is a band pass filter, selects either one of the sum or difference IF signals from the third frequency mixer
13
.
The third IF signal from the third IF filter
23
is provided to the detector
60
where a DC voltage proportional to the AC power level of the third IF signal is produced. The DC voltage is provided to the display
70
where it is displayed in a vertical axis as a power level. Since the ramp wave is also applied to the display
70
for driving a horizontal axis thereof, the display screen shows frequency spectrum of the input signal F
1
in a frequency domain. In such a frequency domain analysis, the power level is shown in the vertical direction while the frequency range (span) is shown in the horizontal direction.
As briefly mentioned above, a frequency spectrum analyzer employs such multiple stages of frequency converters for eliminating image frequencies (spurious responses) by selecting appropriate frequencies in the local signals and IF signals. Further to eliminating the spurious responses, it is also important for a frequency spectrum analyzer to have a high carrier wave to noise (C/N) ratio to analyze an input signal with high sensitivity and resolution.
As is well known in the art, a C/N ratio of a spectrum analyzer is determined by C/N ratios (purity) of local signals used therein. This is because phase noise of local oscillators is usually larger than noise floors of other components in the spectrum analyzer. It is also known in the art that a C/N ratio of a fixed frequency oscillator is higher than that of a sweep frequency oscillator. Further, an oscillator having a highly selective resonant circuit such as a crystal oscillator has a higher C/N ratio than other types of oscillators.
In the arrangement of
FIG. 3
, the first local oscillator
31
is a wide range sweep oscillator typically using a YIG resonator. The second and third local oscillators
32
and
33
are fixed frequency oscillators. A crystal oscillator with high stability is usually used as the third local oscillator
33
. Thus, generally, degrees of phase noise in the first to third local oscillators will be expressed in the following order:
&phgr;
LO1
>&phgr;
LO2
>&phgr;
LO3
(1)
where &phgr;
LO1
denotes the phase noise of the first local oscillator
31
, &phgr;
LO2
denotes the phase noise of the second local oscillator
32
, and &phgr;
LO3
denotes the phase noise of the third local oscillator
33
.
When the noise floor of other components in the spectrum analyzer is lower than the phase noise of local oscillators, and the phase noise of the local oscillators is random noise, overall phase noise &phgr;
N
of the frequency spectrum analyzer will be expressed as follows:
&phgr;
N
=((&phgr;
LO1
)
2
+(&phgr;
LO2
)
2
+(&phgr;
LO3
)
2
)
½
(2)
Since the phase noise of the first local oscillator
31
is the largest, the equation (2) is written to:
&phgr;
N
≈&phgr;
LO1
(3)
Thus, the C/N ratio of the frequency spectrum analyzer is almost equal to the C/N ratio of the first local oscillator
31
. Since the first local oscillator
31
is a wide range sweep oscillator, typically a YIG tuned voltage controlled oscillator, which is expensive and is difficult to further decrease its phase noise.
SUMMARY OF THE INVENTION
Therefore, it is an object of the present invention to provide a frequency spectrum analyzer having an overall noise level which is significantly lower than the noise level in the conventional spectrum analyzer.
It is another object of the present invention to provide a frequency spectrum analyzer which is capable of improving the carrier to noise (C/N) ratio by employing a relatively simple and small circuit.
It is a further object of the present invention to provide a frequency spectrum analyzer which is capable of analyzing frequency spectrum of an input signal with a wide dynamic range and high resolution.
It is a further object of the present invention to provide a frequency spectrum analyzer which is capable of switching between a high C/N ratio mode with a relatively small frequency range and a conventional frequency spectrum analyzer mode.
In the frequency spectrum analyzer of the present invention, the frequency of the first local oscillator is divided by the ratio of N before being mixed with the input signal to be analyzed. Accordingly, the overall carrier wave to noise (C/N) ratio of the spectrum analyzer is improved by the factor of division ratio N.
In the first embodiment of the present invention, the frequency spectrum analyzer for analyzing frequency spectrum of an input signal is comprised of: a first frequency converter formed of a first frequency mixer, a first IF (intermediate frequency) filter and a first local signal oscillator to produce a first IF signal; a second frequency converter which receives the first IF signal and formed of a second frequency mixer, a second IF filter and a second local signal oscillator to produce a seco
Advantest Corp.
Metjahic Safet
Muramatsu & Associates
Nguyen Vincent Q.
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