Electricity: measuring and testing – Impedance – admittance or other quantities representative of... – Parameter related to the reproduction or fidelity of a...
Reexamination Certificate
1996-09-03
2001-05-08
Ballato, Josie (Department: 2858)
Electricity: measuring and testing
Impedance, admittance or other quantities representative of...
Parameter related to the reproduction or fidelity of a...
C324S076270, C324S076460, C324S076230
Reexamination Certificate
active
06229316
ABSTRACT:
BACKGROUND OF THE INVENTION
The present invention relates to a method for measuring an S/N (signal to noise ratio), a C/N (carrier wave level to specific frequency noise level), a third order distortion and a fifth order distortion in an input signal etc. using a spectrum analyzer.
FIG. 1
shows a general configuration example of a spectrum analyzer. An output signal of an object
11
is inputted to a spectrum analyzer
15
as an input signal. In the spectrum analyzer
15
, an input signal is supplied to a frequency mixer
17
via an input variable attenuator
16
and then the frequency of the input signal is mixed with the frequency of a local signal from a frequency sweep generator
18
. Then the mixed output is supplied to a band pass filter
19
and the output of the filter
19
is amplified by an amplifier
21
and then the frequency of the amplified output is mixed with the frequency of a local signal from a local oscillator
23
by a frequency mixer
22
. And then, the intermediate frequency signal is taken out by a band pass filter
24
and the output of the filter
24
is detected by a detector
26
. The detected output is converted into a digital signal by an AID converter
28
after passing through a low pass filter
27
and is stored in a buffer memory
29
. A control part
31
is a so called CPU and performs a setting of attenuation amount for the attenuator
16
in accordance with a parameter set by parameter setting means
32
, a control of the frequency sweep generator
18
by controlling a ramp voltage generator
34
through a timing controller
33
. That is, the control part
31
(CPU) performs a setting of a sweep frequency band, a setting of a band width RBW for each of the filters
19
and
24
, a setting of a band width VBW for the filter
27
and a setting of a sampling period for the A/D converter
28
, as well as a display control of the data stored in the buffer memory
29
on a display unit
35
.
In a conventional system, for example when a C/N of a continuous wave input signal is measured, a carrier wave frequency of a signal to be measured and a noise frequency f
N
(is usually prescribed in accordance with, for example, a modulation mode of an input signal) whose noise level is to be measured against the signal are set by the parameter setting means
32
. Then data are taken out from the memory
29
and are displayed on the screen of the display unit
35
as shown in, for example, FIG.
2
A. In addition, a ratio L
C
/L
N
of a carrier wave frequency data (level) L
C
to a data (level) L
N
of the noise frequency is displayed on a part of the screen like, for example, C/N—53 dBc/Hz. In this case, since the noise level L
N
changes at random, the band width VBW of the low pass filter
27
is usually set to relatively narrow band, i.e., to a level of {fraction (1/10+L )} of the band width RBW of the band pass filters
19
and
24
so that the measured noise levels are averaged.
In the case where a C/N of an input signal is measured using this spectrum analyzer, the following operations are performed in conventional system.
1. A central frequency is set by pushing a frequency button.
2. A frequency interval (an offset value) between a signal to be measured (a carrier wave) and a noise is set.
3. A frequency span (a frequency interval to be displayed on a display screen) is set by pushing a frequency span button.
4. A peak of the carrier wave is coincided with the central frequency of the screen (FIG.
2
A).
5. The carrier wave level is coincided with a reference level.
6. A marker is put on a peak point of the signal through a peak search process.
7. A delta marker is used as a marker.
8. The delta marker
38
is coincided with the noise frequency to be measured (FIG.
2
A).
9. A noise measurement is selected.
10. An indicated value of a noise level is read.
In these operations, when a frequency span is set, a band width RBW (usually, the settable width is predetermined) of the band pass filters
19
and
24
is set by trial and error so that the wave forms of the signal portion and the noise portion are accurately displayed.
When a intermodulation distortion is measured by a spectrum analyzer
15
, as indicated by a dashed line in
FIG. 1
, test signals of the same amplitude having frequencies f
1
and f
2
from signal generators
12
and
13
respectively are combined by a power combiner
14
and are supplied to the object
11
, and then a intermodulation distortion generated by the object
11
is measured.
Formerly, the measurement of a intermodulation distortion has been performed in the following sequence.
1. A central frequency is set to the frequency of one of the input signals, namely, the frequency of one of the two fundamental waves on which a intermodulation distortion is based, i.e., a signal having a frequency f
1
and a signal having a frequency f
2
.
2. An input signal frequency range, i.e., a displayed frequency range is manually set by pushing a span button.
3. A resolution band width is set by pushing a band width button. That is, the setting of each band width RBW of the band pass filters
19
and
24
is switched from an automatic operation to a manual operation, and then a resolution band width RBW is set. The reason for the manual operation is that when a resolution band width RBW is automatically set, a spectrum of a mutual intermodulation is hidden at the bottom portion of the input signal due to a low level of a intermodulation distortion or an influence of the band pass filters
19
and
24
. In such a case, a spectrum of a intermodulation distortion may not be observed.
4. A third order distortion is measured by pushing a TOI button.
5. The above operations 1-3 are repeated by changing the setting of the resolution band width RBW until four clear peaks appear on the display screen, i.e., as shown, for example, in
FIG. 2B
, until four clear peaks, the spectrums
41
and
42
of two input signals (fundamental waves) having frequencies f
1
and f
2
respectively, a third order distortion
43
of a frequency (2f
1
−f
2
) generated by a intermodulation of these two input signals and a third order distortion
44
of a frequency (2f
2
−f
1
) appear on the display screen.
Incidentally, when a level of the fundamental waves
41
and
42
at the input side of the input attenuator
16
is L as shown in
FIG. 3A and a
level difference between the third order distortions
43
,
44
and the respective fundamental waves
41
,
42
is &Dgr;L, a distortion amount (level) of the third order distortions
43
and
44
is L−&Dgr;L. When an attenuation amount in the input attenuator
16
is ATT, as shown in
FIG. 3B
, the level of the fundamental waves
41
and
42
at the output side of the input attenuator
16
is L−ATT and the level of the third order distortions
43
and
44
is L−(&Dgr;L+ATT). In a spectrum analyzer
15
, it is clearly stated in the specifications that when a fundamental wave having a level of X dBm is inputted to the mixer
17
, a third order distortion of (X−Y) dBm is generated. From a generation characteristic (a generation principle) of a third order distortion, when the input fundamental wave level is X+&Dgr;X, the third order distortion level is Y+&Dgr;Y, where &Dgr;Y=3&Dgr;X. That is, a third order distortion generated in the mixer
17
is increased by three times of the input fundamental wave level increment &Dgr;X, i.e., 3&Dgr;X.
Therefore, when the fundamental wave level is attenuated by &Dgr;ATT in the input attenuator
16
, each of the third order distortions is decreased by &Dgr;ATT. However, the third order distortion generated in the mixer
17
is decreased by 3&Dgr;ATT. From such a relationship, when the attenuation amount of the input attenuator
16
is large, the third order distortion generated in the mixer
17
is greatly reduced to be neglected. The input/output characteristic of the mixer
17
for the fundamental waves
41
and
42
is indicated by a linear line
45
. When the level of the fundamental waves
41
and
42
is small and the third
Aoyama Osamu
Fukui Takayoshi
Kasahara Toshiharu
Kosuge Takashi
Miyamae Yoshiaki
Advantest Corporation
Ballato Josie
Solis Jose M.
Staas & Halsey , LLP
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