Signal generator

Details Signal generator Signal generator

1999-12-22

2001-11-13

Karlsen, Ernest (Department: 2858)

Electricity: measuring and testing

Impedance, admittance or other quantities representative of...

With auxiliary means to condition stimulus/response signals

C324S120000, C327S117000

Reexamination Certificate

active

06316943

ABSTRACT:

FIELD OF THE INVENTION
This invention relates to signal sources in general and, in particular, it concerns the generation of measurement signals and local signals in network analyzers.
BACKGROUND OF THE ART
Network analyzers measure frequency characteristics of circuit networks or circuit components by sweeping a measurement signal over a wide range. In order to measure frequency characteristics, a vector voltmeter which measures the amplitude and phase of the measurement signal, must be highly accurate over such a wide range. However, since it is difficult to make vector voltmeters with high accuracies over the entire range of measured frequencies, the measurement signal is generally converted to a constant frequency by a frequency conversion means (i.e., through a heterodyne method) and is then measured by the vector voltmeter.
FIG. 5
shows the most basic structure of a network analyzer which uses the heterodyne method. A signal source
10
comprises a measurement signal source
19
and a local signal source
20
. Measurement signal source
19
generates a measurement signal and applies it to an object of measurement
15
. Local signal source
20
generates a signal with a frequency that is separated from the measurement frequency by an intermediate frequency. The response signal, resulting from application of a signal to measurement object
15
, and the signal from local signal source
20
, are mixed by a mixer
16
, converted to an intermediate frequency and output to a filter
17
. Filter
17
enables passage of signal frequency components at the intermediate frequency. Vector voltmeter
18
measures the amplitude and phase of the measurement signal that has been converted to the intermediate frequency. Since vector voltmeter
18
performs measurements at the constant intermediate frequency, highly accurate measurements are achieved.
In applying the heterodyne method to a network analyzer, it is important to generate a local frequency that is accurately displaced with respect to the sweep measurement frequency by an amount of the intermediate frequency.
FIG. 6
shows a prior art example of a method of generating the measurement frequency and the local frequency.
FIG. 6
shows only signal source
10
part of FIG.
5
.
FIG. 6
illustrates a first fixed-frequency signal source
21
, a sweepable variable-frequency signal source
22
, a second fixed-frequency signal source
23
, a first mixer
24
, a second mixer
25
, a first filter
26
, and a second filter
27
. The signals of first fixed-frequency signal source
21
, with a frequency fc, and variable-frequency signal source
22
, with a frequency fd, are mixed by first mixer
24
, and signals with the frequencies fd±fc are output. The fd+fc signal is removed by first filter
26
, and the signal with frequency fd−fc is output as the measurement signal. Therefore, the frequency of the measurement signal varies according to changes in the frequency fd of variable-frequency signal source
22
.
In the same manner, a frequency fd of variable-frequency signal source
22
and a frequency fe of second fixed-frequency signal source
23
are mixed by second mixer
25
, and a frequency fd±fe is output. Second filter
27
removes the component with the frequency fd+fe, in the same manner as the first filter, and the signal with the frequency fd−fe is output as the local signal. The frequency of the local signal also varies according to the changes in the frequency fd of variable-frequency signal source
22
.
The difference in the frequencies of the output signal of first filter
26
and second filter
27
is fc−fe; this is a constant frequency, without relation to the frequency of the measurement signal. Therefore, if the measurement signal and the local signal are mixed by mixer
16
of
FIG. 5
, an intermediate frequency signal with a constant frequency is output.
The prior art shown in
FIG. 6
is an ingenious method for generating two sweep frequencies with a constant frequency difference, but it has the following drawbacks. First, a plurality of high-frequency signal sources, composed of complex circuits, are required. Further, since the desired frequencies are generated by mixing frequencies using a plurality of mixers, many frequency components other than the desired frequencies are generated. As a result, a complex means of removing errors due to these frequency components is required. Because of this complex circuit assembly, the prior art has the problem of high cost.
Accordingly, it is an object of this invention to provide a simple signal generation method, in order to reduce the cost of a network analyzer.
SUMMARY OF THE INVENTION
This invention employs a simple structure to generate a measurement signal and a local signal with a frequency that is displaced by an intermediate frequency from the frequency of the measurement signal. The structure includes one high-frequency variable-frequency signal source, one low-frequency fixed-frequency signal source, and two frequency dividers.


REFERENCES:
patent: 1684403 (1928-09-01), Mason
patent: 2143094 (1939-01-01), Swift
patent: 4733167 (1988-03-01), Tamamura
patent: 4780890 (1988-10-01), Kane
patent: 5056108 (1991-10-01), Lund

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