Coded data generation or conversion – Analog to or from digital conversion – Analog to digital conversion
Reexamination Certificate
2000-11-01
2002-09-03
Young, Brian (Department: 2819)
Coded data generation or conversion
Analog to or from digital conversion
Analog to digital conversion
Reexamination Certificate
active
06445327
ABSTRACT:
CROSS-REFERENCE TO RELATED APPLICATIONS
This application is based upon claims the benefit of priority from the prior Japanese Patent Application No. 11-320453 filed Nov. 11, 1999, the entire contents of which are incorporated herein by reference.
BACKGROUND OF THE INVENTION
The present invention relates to a signal analyzing apparatus and, more particularly, to a signal analyzing apparatus that measures the frequency characteristics of signals used in mobile communication systems such as mobile telephones and cellular telephones and displays their waveforms.
For example, signals employed in mobile communication systems such as mobile telephones and cellular telephones are modulated by a number of modulation methods.
These mobile communication systems adopt TDMA (Time Division Multiple Access) as a communication method for using communication circuits efficiently.
The frequencies of carrier waves conveying signals used in those mobile communication systems are very high, ranging from several hundred MHz to several GHz.
Signal analyzing apparatuses such as spectrum analyzers are used to precisely measure various frequency components included in those signals.
FIG. 3
is a block diagram that illustrates a schematic configuration of the prior-art signal analyzing apparatus that measures the frequency characteristics of such signals of high frequencies.
In such a conventional signal analyzing apparatus
51
of
FIG. 3
, signal “a” of a high frequency that is entered through an input terminal
52
for decoding is controlled to have a predetermined level of intensity with an attenuator (ATT)
53
and then provided to a frequency conversion unit
54
.
The signal of a high frequency entered to the frequency conversion unit
54
for decoding is mixed in a signal mixer
55
with a local oscillation signal “b” transmitted from a local oscillator
56
and then converted to a signal of an intermediate frequency.
The signal of an intermediate frequency is narrowed in bandwidth with a BPF (Band Pass Filter)
57
and again mixed with the local oscillation signal transmitted from the local oscillator
59
in another signal mixer
58
in the subsequent part and sent out from the frequency conversion unit
54
.
The frequency provided by the local oscillator
56
in the frequency conversion unit
54
is swept over a predetermined frequency range by a sweep control unit
60
.
As a result, the frequency of the intermediate-frequency signal “c” sent out from the frequency conversion unit
54
changes in synchronization with the sweeping operation.
The intermediate-frequency signal “c” of which frequency was reduced and sent out from the frequency conversion unit
54
is entered to a Resolution BAND Width (RBW) filter
61
.
This RBW filter
61
is comprised of an analog band pass filter having frequency characteristics shown in
FIG. 4
, for example, and only selects necessary signals of intermediate frequencies with unnecessary frequency components being removed.
The bandwidth (RBW) 3 dB lower than the peak level of the central passing frequency fc in the frequency characteristics of the band pass filter represents the frequency resolution of this signal analyzing apparatus
51
.
Frequency f
1
of the intermediate-frequency signal “c” sent from the frequency conversion unit
54
changes in synchronization with the sweeping operation.
Therefore, the output signal sent out from the RBW filter
61
during a one sweeping period (sweeping interval) present the time-series waveform of each frequency component in the measurement signal “a” that has been converted to the intermediate-frequency signal “c”.
This output signal from the RBW filter
61
is log-converted by a logarithmic (LOG) converter
62
after a gain adjustment in an amplifier (not shown).
The output signal, of which signal level has been converted by the dB, is decoded in a detector (DET)
63
in the following part.
Consequently, the signals swept by DET
63
during the sweeping interval represent the amplitudes of the time-series waveforms of the swept frequencies.
Therefore, the output signals from the detector
63
represent frequency spectrum waveforms, when plotted along with a horizontal axis of frequency and a vertical axis of amplitude.
The signals provided from the detector
63
that represent frequency spectrum waveforms are entered to an analog Video BAND Width (VBW) filter (not shown).
This VBW filter is comprised of a low pass filter (LPF) that removes high-frequency components (noise) in the frequency spectrum waveforms to be shown in a display unit
64
that is installed on the front panel of the apparatus.
A peak detector (not shown) detects the peak of the analog frequency spectrum waveform provided from the VBW filter at each position on the time axis and the final frequency spectrum waveform is obtained that is envelope-detected.
Signals presenting the final frequency spectrum waveforms are converted into digital data in an A/D converter
65
that is installed in the following part.
These frequency spectrum waveforms converted into digital data are displayed on the screen of the display unit
64
mounted on the front panel.
Changing the display frequency range on the display unit
64
and sweeping frequency range, frequency spectra over a wide range of frequency and in an arbitrary frequency range can be provided for measurement.
Further, if the bandwidth (RBW) of the RBW filter
61
is changed, the frequency resolution of the signal analyzing apparatus
51
can be changed.
In general, a wider sweeping range leads to a wider bandwidth (RBW) and a lower wavenumber resolution.
Namely, the bandwidth (RBW) of the RBW filter
61
depends on changes in the sweeping frequency range.
For higher productivity in signal measurement, the throughput of tuning and inspection in such signal analyzing apparatuses should be raised.
One of the improvements is to reduce measurement time and another is to raise the sweeping seed.
In signal analyzing apparatuses such as conventional spectrum analyzers, since the minimum sweeping time is around 20 ms, the magnitude of delay is not important.
Signal analyzing apparatuses such as modern spectrum analyzers conduct high speed sweeping by raising the sweeping rate in the local oscillator.
However, in the prior-art signal analyzers shown in
FIG. 3
, a high-speed sweeping causes a delay in the response of signals upon data sampling in the A/D converter
65
because of a group delay time that is induced in devices ranging from the signal mixer
58
to the A/D converter
65
to which local oscillation signals are entered.
In particular, in the case of a high-speed sweeping conducted in the configuration of
FIG. 3
, delay occurs on the way to the A/D converter
65
due to the delay characteristics of the analog RBW filter
61
, and such delay increases if RBW (bandwidth) of the RBW filter
61
becomes narrow.
Such time delays appear as shifts in the frequency position when waveforms are shown in the display unit
64
.
Namely, as demonstrated in
FIG. 5
, the peak position that should be located at the central frequency fc shown by the solid line moves to a position, fc′, shown by the dotted line, causing an error display of frequency. Correct signal analysis thus becomes difficult to perform.
BRIEF SUMMARY OF THE INVENTION
This invention has been made to solve the above problems, and the object of the invention is, in particular, to provide a signal analyzing apparatus that can perform precise signal analysis by reducing frequency errors induced in high-speed sweeping.
To achieve the above object, according to the first aspect of the present invention, there is provided a signal analyzing apparatus comprising:
a frequency conversion unit (
4
) that converts input signals into those of intermediate-frequencies using a predetermined local oscillation frequency;
a sweeping unit (
12
) that sweeps the local oscillation frequency in a predetermined sweeping time;
a filter (
10
) that receives signals output from the frequency conversion unit and selectively passes signals having frequency components of
Anritsu Corporation
Frishauf Holtz Goodman & Chick P.C.
Nguyen John B
Young Brian
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