Spectrum analyzer

Details Spectrum analyzer Spectrum analyzer

1998-04-03

2001-05-15

Chin, Stephen (Department: 2734)

Pulse or digital communications

Receivers

C324S076240, C324S076270, C324S076350

Reexamination Certificate

active

06233288

ABSTRACT:

FIELD OF THE INVENTION
This invention relates to a wide band receiver such as a spectrum analyzer, and more particularly, to a local oscillator to be used in a frequency spectrum analyzer which is able to reduce dynamic spurious responses caused by a digital step sweep operation of the local oscillator.
BACKGROUND OF THE INVENTION
A wide band receiver such as a frequency spectrum analyzer is used to analyze frequency components of an incoming signal. An example of conventional spectrum analyzer utilizes a local oscillator whose frequency is digitally controlled by a step sweep signal through a direct digital synthesizer (DDS) technology. Such a conventional example of spectrum analyzer is explained with reference to
FIGS. 4
,
5
and
6
.
In the example of
FIG. 4
, the frequency spectrum analyzer is formed of a frequency converter
50
, a detector
62
, a display arithmetic unit
64
, and a display
68
. As also shown in
FIG. 4
, the frequency converter
50
includes an attenuator
51
, a first frequency mixer
52
, a local oscillator
30
, a second frequency mixer
53
, a fixed local oscillator
54
and a band pass filter (BPF)
55
.
The frequency converter
50
receives an input signal
100
to be analyzed and converts the input signal to an intermediate frequency signal when the local oscillator sweeps its frequency for a specified frequency range. In this example, the intermediate frequency signal is created by mixing the input signal
100
with a first local signal from the local oscillator
30
and a second local signal from the fixed local oscillator
54
. The intermediate frequency signal is filtered by the BPF
55
to a predetermined band width and is then provided to the detector
62
.
The BPF
55
may be formed of a plurality of band pass filters having a variety of resolution bandwidth which are set by a user. When two or more frequency spectrum in the input signal have small frequency differences, a band pass filter of sufficiently small bandwidth must be used to fully distinguish the frequency spectrum from the others.
The detector
62
detects DC voltages, i.e., envelope voltages, of the intermediate frequency signals from the BPF
55
. The detected voltages are provided to the display
68
through the display arithmetic unit
64
. The frequency spectrum contained in the input signal are displayed on the display
68
in a frequency domain wherein the horizontal axis is a frequency having a variable frequency span (frequency range on a full display) and the vertical axis is a power level.
The local oscillator
30
is a sweep oscillator which can sweep a desired frequency range in a step manner with use of the DDS (direct digital synthesizer) technology. As shown in
FIG. 5
, the local oscillator
30
includes a DDS time base
32
, a DDS
40
, a D/A (digital to analog) converter
34
, a LPF (low pass filter)
35
, a phase comparator
36
, a divider
37
, an integrator
38
, and a YTO (YIG-tuned oscillator)
39
. A phase lock loop (PLL) is formed of the YTO
39
, the divider
37
, the phase comparator
36
, and the integrator
38
.
The DDS time base
32
receives a reference clock
31
and sweep conditions
33
which includes a span (sweep frequency range) and a sweep time T
swp
and delivers a clock signal
32
ck
to the DDS
40
. The clock signal
32
ck
has a unit step time T
step
which is produced by dividing the reference clock
32
by a division factor Div, i.e., T
step
=(reference clock
31
)/Div. Thus, one clock time period of the clock signal
32
ck
is equal to the unit step time T
step
in the step sweep of the local oscillator
30
.
The DDS
40
is a synthesizer which generates digital data representing a digital sine wave of a desired frequency. As shown in
FIG. 6
, the DDS
40
is formed of a frequency register
42
, an adder
44
, and a ROM table memory
46
.
The frequency register
42
stores advance phase data
42
dt
and provides the phase data
42
dt
to one input of the adder
44
. The advance phase data is
32
bit data, for example, and defines a magnitude of phase advance of a sine wave to be generated by the local oscillator
30
. By this data, as shown in a stepped ramp signal of
FIG. 7A
, a unit step frequency
92
is accumulated at every unit step time T
step
, which results in one sweep time T
swp
=M×T
step
. Here, M is a constant number of steps, such as 2,048 steps, in an overall sweep.
The adder
44
is for example a 32 bit accumulator to advance the unit phase of the above noted unit frequency
92
of the sine wave. At every clock signal
32
ck
from the DDS time base
32
, one input terminal of the adder
44
receives the advance phase data
42
dt
from the register
42
, while the other input terminal receives the data from a register
44
r
connected to the output of the adder
44
. The register
44
r
holds the output data of the adder
44
produced in the previous accumulation cycle. Thus, the adder
44
accumulates the data at the two input terminals and the result is latched in the register
44
r
for the next cycle.
The ROM table memory
46
converts the received data to step like sine wave data. For example, the ROM table memory
46
uses data in the upper 10 bit of 32 bit data from the adder
44
as address data to read 10 bit sine wave data
46
dt
from the table memory
46
. The sine wave data
46
dt
is supplied to the D/A converter
34
shown in FIG.
5
.
The D/A converter
34
in
FIG. 5
converts the 10 bit sine wave data
46
dt
to a step like analog signal. The LPF
35
removes frequency components of the clock signal
32
ck
in the step like analog signal to make a sine wave analog signal and provides the sine wave analog signal to one input terminal of the phase comparator
36
.
The phase comparator
36
detects phase differences between the two input signals and generates voltage signals representing the phase differences. Namely, the phase comparator
36
receives a reference phase signal from the DDS through the LPF
35
as well as an oscillation signal
39
osc
of the YTO
39
whose frequency is divided by 1/N at the divider
37
. The YTO
39
is a voltage controlled oscillator. The phase comparator
36
compares the phases of the two input signals and generates a voltage signal representing the phase differences between the two input signals. The voltage signal is applied to the integrator
38
which is typically a low pass filter. The integrator
38
integrates the voltage signals to produce an analog DC voltage which is supplied to an voltage control input of the YTO
39
.
The YTO
39
is a variable resonance oscillator in a microwave frequency band using, for example, a YIG (Yttrium Iron Garnet) crystal. The YTO
39
receives the analog DC voltage from the integrator
38
and generates the step sweep frequency signal
39
osc
which is phase locked by the PLL loop noted above. The sweep signal
39
osc
is supplied to the frequency mixer
52
in the frequency converter
50
to convert the frequency of the input signal
100
to the intermediate frequency signal through the first and second frequency mixers
52
and
53
.
As in the foregoing, the sweep operation of the local oscillator
30
is performed in the step manner. Therefore, as shown in
FIG. 7A
, the frequency of the local signal is also swept in the step like manner, and thus, the swept frequency varies discontinuously. As a result, a dynamic spurious response which is inverse proportional to the unit step time T
step
, i.e., &Dgr;f=1/T
step
is induced as shown in FIG.
7
B. Because the spurious frequency &Dgr;f is close to a center frequency f
o
of the input signal under measurement, it is difficult to remove this dynamic spurious by a filter circuit. Thus, the spurious will be displayed at the frequency positions f
o
±&Dgr;f on the display of the frequency spectrum analyzer even though the input signal does not have the frequency spectrum &Dgr;f.
SUMMARY OF THE INVENTION
Therefore, it is an object of the present invention to provide a frequency spectrum analyzer having a step sweep local oscillator which

Affiliated with

Also associated with

Rating

Spectrum analyzer does not yet have a rating. At this time, there are no reviews or comments for this patent.

If you have personal experience with Spectrum analyzer, we encourage you to share that experience with our LandOfFree.com community. Your opinion is very important and Spectrum analyzer will most certainly appreciate the feedback.

Rate now

Comments { 0 }

Profile ID: LFUS-PAI-O-2567118