Microwave voltage-controlled-oscillator

Details Microwave voltage-controlled-oscillator Microwave voltage-controlled-oscillator

2001-03-23

2003-07-29

Pascal, Robert (Department: 2817)

Oscillators

With frequency adjusting means

With voltage sensitive capacitor

C331S1070SL

Reexamination Certificate

active

06600381

ABSTRACT:

FIELD OF THE INVENTION
The present invention in general relates to a microwave voltage controlled oscillator, such as the automobile radar, that performs narrow band modulation. More particularly, this invention relates to a microwave voltage controlled oscillator in which frequency modulation linearity is improved.
BACKGROUND OF THE INVENTION
A conventional microwave voltage controlled oscillator will be described.
FIG. 1
is a diagram showing a principle configuration of a typical microwave voltage controlled oscillator. Legend
61
denotes a negative resistance element (circuit), legend
62
denotes a resonator circuit, and legend
63
denotes a variable capacitance diode. This microwave voltage controlled oscillator is a well known combination of a variable capacitance diode and negative resistance.
It will be assumed here that, the impedance of the resonator circuit
62
which is to the left of the reference line A-A′ is Z
R
, and that the impedance of the negative resistance element
61
which is to the right of the reference line A-A′ is Z
N
. With this assumption in mind, the microwave voltage controlled oscillator oscillates stably when the following equation (1) holds true. That is,
Re{Z
R
+Z
N
}<0
Im{Z
R
+Z
N
}=0  (1)
d{Z
R
+Z
N
}/d&ohgr;>
0
where &ohgr; represents angular frequency.
It will be further assumed that the impedance of the variable capacitance diode
63
(capacitance C
V
) which is to the left of the reference line B-B′ is 1/j&ohgr;C
V
, and that the impedance of the resonator circuit
62
which is to the right of the reference line is R
T
+j&ohgr;L
T
. With this assumption in mind, the oscillation frequency f
osc
of the oscillator can be represented by the following equation (2).
f
osc
=1/[2&pgr;(
L
T
C
V
)
1/2
]  (2)
When the variable range of the frequency is very narrow and L
T
can be regarded as constant in the whole variable frequency range, an oscillation frequency proportionate to the control voltage can be obtained if the capacitance C
V
is in inverse proportion to the square of the control voltage.
However, a variable capacitance diode having a capacitance C
V
which is in inverse proportion to the square of the control voltage is not commercially available. Furthermore, as for a special diode capable of operating at a frequency equivalent to at least a microwave frequency, it is difficult to obtain even if it exists. Under a condition that L
T
is constant, therefore, a linear voltage controlled oscillator as described above has not been implemented.
FIG. 2
is a diagram showing a circuit configuration of the conventional microwave voltage controlled oscillator. Legend
64
denotes the above described shorted quarter—wavelength line as high impedance shunt stub. The quarter wavelength line becomes infinite in impedance at an oscillation frequency, and it does not affect the oscillation operation at the microwave frequencies. Therefore, the voltage-capacitance characteristic of the variable capacitance diode determines the linearity of frequency modulation.
FIG. 3
is a diagram showing a variable frequency characteristic and frequency modulation linearity (% indication) of the conventional microwave voltage controlled oscillator shown in FIG.
2
. Specifically,
FIG. 3A
represents the oscillation frequency—control voltage characteristic, and
FIG. 3B
represents the linearity of frequency modulation.
The linearity (i.e. the linearity index) of frequency modulation is defined by the oscillation frequency—control voltage characteristic shown in FIG.
3
A. For example, assuming that a maximum deviation between a straight line coupling both ends of a frequency modulation width W and illustrated frequencies is &Dgr;W, the linearity index can be represented by 100×&Dgr;W/W (%). To be concrete, the linearity index becomes at least 20% as shown in FIG.
3
B.
This does not exert influence upon fixed frequency oscillators for communication. For example, in devices for transmitting and receiving a frequency modulated signal and measuring a distance between cars, such as a crash prevention radar, however, this indicates a value remarkably degrading the precision of measurement results. In other words, even in the case where it is attempted to improve the precision by using some external frequency linearizer such as using ROM, the linearity index can not be suppressed enough. In the case of crash prevention radar, however, the frequency modulation width is negligibly small as compared with the oscillation frequency and a nearly fixed frequency can be considered.
A conventional linear microwave voltage controlled oscillator will now be described in detail by referring to a concrete example.
FIG. 4
is a diagram showing a configuration of a microwave voltage controlled oscillator (high frequency device) described in Japanese Patent Application Laid-Open No. 8-288715. Legend
31
denotes a resonator circuit, legend
32
denotes a capacitance (realized with a capacitor), and legend
33
denotes a negative resistance circuit (realized with amplifier circuit).
Furthermore, in the resonator circuit
31
, legend
47
denotes a main resonance line, legend
48
denotes a DC-cut capacitance, and legend
49
denotes a variable capacitance diode having a capacitance which is in inverse proportion to the square of voltage. Legend
52
denotes a series connection of a strip line
50
for correcting the square characteristic of the variable capacitance diode
49
to obtain a linear characteristic and a capacitor
51
. Legend
53
denotes a strip line for further improving the correction effect of the strip line
50
. Legend
54
denotes a terminal, and legend
55
denotes a capacitor. In the negative resistance circuit
33
, legend
34
denotes a transistor, legends
35
,
36
,
37
and
38
denote resistors, legends
39
,
40
,
41
,
42
and
43
denote capacitors, legend
44
denotes a coil, and legends
45
and
46
denote terminals.
In this microwave voltage controlled oscillator, for example, when a strip line is used as a distributed transmission line in the resonator circuit
31
and the variable frequency range is wide, L
T
changes linearly with the frequency. Even when the capacitance C
V
is inverse proportion to the square of the voltage, the oscillation frequency is not proportionate to the control voltage. Therefore, a correction circuit (corresponding to the strip line
50
and the strip line
53
) is incorporated so that the capacitance of the variable capacitance diode
49
will change linearly with the applied voltage. Furthermore, lengths of the strip lines
50
and
53
are defined as {fraction (1/10)} wavelength to {fraction (1/20)} wavelength and {fraction (3/16)} wavelength to {fraction (5/16)} wavelength, respectively, in order to modulate the oscillation frequency in a wide frequency band.
An explanation will be given about the strip line
50
. If the strip line
50
is not provided, changing the voltage only slightly causes an abrupt change of the above described variable capacitance when the control voltage is low, resulting in an abrupt change of the oscillation frequency. When the control voltage is high, the oscillation frequency changes gently as the voltage changes.
However, when strip line
50
is provided, the strip line
50
changes the impedance abruptly, and consequently the oscillation frequency changes gradually. In keeping therewith, the oscillation frequency changes linearly as the control voltage increases. In order to satisfy the expression “the strip line
50
changes the impedance abruptly, and consequently,” it is important that the impedance of the correction circuit has a great change at different frequencies. Especially here, the series connection
52
for generating a great impedance change with respect to a frequency is connected in parallel with the variable capacitance diode
49
.
In the conventional microwave voltage controlled oscillator, the correction circuit (the strip line
50
and th

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