Electricity: measuring and testing – Measuring – testing – or sensing electricity – per se – Analysis of complex waves
Reexamination Certificate
1999-03-09
2001-11-13
Metjahic, Safet (Department: 2858)
Electricity: measuring and testing
Measuring, testing, or sensing electricity, per se
Analysis of complex waves
Reexamination Certificate
active
06316928
ABSTRACT:
FIELD OF THE INVENTION
This invention relates to a spectrum analyzer for analyzing frequency spectrum of an incoming signal and displaying the frequency spectrum on a frequency domain display. In particular, this invention relates to a spectrum analyzer that incorporates a YTO (YIG tuned oscillator) as a sweep frequency local oscillator and a YTF (YIG tuned filter) as a frequency preselector for an incoming signal.
BACKGROUND OF THE INVENTION
Frequency spectrum analyzers are frequently used for analyzing frequency spectrum in an input signal. Usually, such frequency spectrum is displayed on a frequency domain screen with a power level in a vertical direction while a frequency dispersion in a horizontal direction. Some spectrum analyzers, especially which can analyze higher frequency spectrum, incorporate a YIG tuned oscillator (YTO) as a local oscillator and a YIG tuned filter (YTF) as an input frequency preselector. Here, a YIG (Yttrium iron Garnet) is a high quality resonance element used in a high frequency band such as a microwave.
First, brief explanation is made regarding the YTO local oscillator and the YTF preselector. A YIG crystal is an element that magnetically resonates at a microwave frequency in response to intensity of a magnetic filed surrounding the crystal. By changing the direct current (DC) magnetic field, the resonance frequency of the YIG crystal changes linearly. The YIG crystal shows a wide range of frequency resonance which is controlled by a DC current (intensity of magnetic field) with excellent linearity. Further, the resonant characteristics of the YIG crystal has an excellent quality factor (Q) which is suitable for forming high quality oscillators and filters.
Therefore, a YTO is configured with use of a YIG crystal to establish a sweep frequency oscillator in a microwave frequency range. Since a frequency spectrum analyzer is required to analyze a wide frequency range of frequency spectrum, a YTO is advantageously incorporated as a local oscillator of the spectrum analyzer. Especially when a phase lock technology is applied to the high frequency oscillation signal of the YTO with use of a low frequency high stability crystal oscillator, such as a VCXO (voltage control crystal oscillator), it is possible to achieve a microwave sweep local oscillator of high signal purity.
As a frequency sweep range, a single octave bandwidth, such as from f
LO
to 2f
LO
Hz or from 2f
LO
to 4f
LO
Hz is common as YTO local oscillators. In a typical YTO, the frequency f
LO
is about 2 GHz. Although double octave bandwidth from f
LO
to 4f
LO
Hz is also available, such a YTO is usually expensive and difficult to manufacture with uniform quality.
A YTF is a sweep frequency band pass filter utilizing the property of the YIG crystal, in which a pass band frequency changes linearly in response to a DC voltage applied thereto. In the application of frequency preselectors in spectrum analyzers, it is relatively easy to achieve YTFs of double octave bandwidth from f
LO
to 4f
LO
Hz.
In the frequency spectrum analysis using a frequency spectrum analyzer, an input signal to be tested by the spectrum analyzer often has many different frequency components (spectrum). In addition, the signal power level of such frequency components ranges from a high level to a low level. Thus, in the case where analyzing a frequency component (spectrum) of a low power level, other frequency component of a high power level may be included in the input signal. In such a situation, an input frequency mixer of a spectrum analyzer will be saturated by the high power level component, which makes it impossible to analyze the spectrum having the small power level.
Accordingly, in the spectrum analyzer, it is desirable to perform a frequency selection of frequency spectrum in the incoming signal by placing a band pass filter before the first mixer. A YTF is advantageously incorporated as a frequency preselector in the spectrum analyzer since it can change the pass band frequency through a wide frequency range with high linearity and selectivity.
Thus, the incoming signal can be preselected by the YTF when the frequency pass band of the YTF sweeps linearly in the microwave frequency range. The pass band frequency of the YTF preselector is the YTO frequency less an intermediate frequency f
IF
Hz. Here, the frequency f
IF
Hz is an intermediate frequency of an input frequency mixer, i.e., a sum or difference between the input signal frequency and the local oscillator (YTO) frequency. In the analysis of the low frequency range of the input signal, high frequency components are removed by a low pass filter provided at the input of the spectrum analyzer, thereby preventing the saturation of the frequency mixer.
A basic configuration of this type of frequency spectrum analyzer in the conventional technology is explained with reference to FIG.
3
. In the example of
FIG. 3
, the spectrum analyzer includes a base band block
10
and a high band bock
20
which are selected by an input switch
6
and a local oscillator switch
8
. A YTO (YIG tune oscillator)
7
is a first local sweep oscillator for the base band block
10
and the high band block
20
connected through the switch
8
. The base band block
10
is comprised of a low pass filter
10
, a first frequency mixer
12
, an intermediate frequency (IF) band pass filter
13
, a second frequency mixer
14
, and a second local oscillator
15
. The high band block
20
includes a YTF (YIG tune filter)
21
and a high band frequency mixer
22
.
The spectrum analyzer of
FIG. 3
further includes an IF switch
25
, a third frequency mixer
27
, a third local oscillator
27
, a detector
28
, an A/D (analog to digital) converter
29
, a display
31
, a ramp generator
30
, DAC (digital to analog converters)
36
and
38
, analog voltage adders
37
and
39
, a span data generator
32
, offset voltage generators
33
and
35
, and a gain data generator
34
.
In the base band block
10
, the first frequency mixer
12
produces a first IF signal f
53
having a frequency f
IF
1
and the second frequency mixer
14
produces a second IF signal f
54
having a frequency f
IF
2
. In the high band block
20
, the high band frequency mixer
22
produces the second IF signal f
54
with the IF frequency f
IF
2
. The second IF signal f
54
(frequency f
IF
2
) from either the base band block
10
or the high band block
20
is received by the third frequency mixer
26
through the IF switch
25
.
In this example, a frequency range of an input signal that can be tested by the spectrum analyzer is about 0 Hz to 16f
LO
Hz by using a harmonics waves of the oscillation frequency of the YTO. However, for the simplicity of explanation, the operation concerning the input frequency ranging 0 Hz to (4f
LO
minus f
IF
2
)Hz using a fundamental frequency of the local oscillator (YTO) is described in the following. In this situation, the sweep frequency pass band of the YTF preselector 21 ranges from (f
LO
minus F
IF
2
) Hz to (4F
LO
minus f
IF
2
) Hz, and the sweep oscillator frequency of the local oscillator YTO 7 ranges from f
LO
Hz to 4f
LO
Hz. An example of the first local frequency f
LO
is 2 GHz, and the first IF frequency f
IF
1
is 2 GHz, and the second IF frequency f
IF
2
is 0.4 GHz.
In order to drive the local oscillator YTO
7
and the preselector YTF
21
, a ramp signal of, for example, ±5V is generated by the ramp generator (saw-tooth wave generator)
30
. The ramp signal is branched out, and one is used for an X axis sweep of the display
31
and the other is supplied to a reference voltage (V
REF
) terminal of the multiplying type DAC (digital to analog converter)
36
. The frequency range (span) data from the span data generator (frequency range data generator)
32
is input to the digital data terminal of the multiplying type DAC
36
.
The ramp wave and the frequency range data are multiplied, and a resultant analog ramp signal is produced at the output of the DAC
36
. Thus, assuming that the input ramp signal is x, the frequency range data is a, and t
Advantest Corp.
Leroux E P
Metjahic Safet
Muramatsu & Associates
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