Temperature compensated crystal oscillator

Details Temperature compensated crystal oscillator Temperature compensated crystal oscillator

2002-02-27

2003-11-04

Nguyen, Minh (Department: 2816)

Oscillators

With temperature modifier

C331S068000, C331S176000, C310S346000

Reexamination Certificate

active

06642803

ABSTRACT:

BACKGROUND OF THE INVENTION
1. Field of the Invention
The present invention relates generally to temperature compensated crystal oscillators, and more particularly to a miniaturized temperature compensated crystal oscillator, in which two cavities forming mounting structures for a crystal vibrating chip and an IC chip are arranged not to be vertically overlapped, thus allowing a separating layer between the two cavities to be removed.
2. Description of the Prior Art
Generally, crystal oscillators using crystal vibrating chips are essential parts to generate oscillation frequencies for controlling transmission and reception of signals between mobile communication terminals. The crystal oscillators have excellent frequency stability compared with other oscillators. However, the crystal vibrating chips are problematic in that their oscillation frequencies are varied due to ambient temperature, In order to solve the problem, the crystal oscillators have additional parts for compensating the frequency variation due to the temperature sensitivity of the crystal vibrating chips. Such oscillators are each so-called temperature compensated crystal oscillator (TCXO). The temperature compensated crystal oscillators are classified into two types according to the realization method of a temperature compensation circuit; one-chip type using an integrated circuit (IC) chip and a discrete type in which various parts such as a piezoelectric oscillating element, an integrated circuit, capacitors, inductors and resistors, are mounted. Hereinafter, various structures of temperature compensated crystal oscillators proposed in the prior art are described.
FIGS. 1
a
and
1
b
are a side sectional view and a plane view of a conventional discrete type TCXO
10
, respectively. As shown in
FIGS. 1
a
and
1
b
, the discrete type TCXO
10
has a structure in which a crystal oscillating unit
13
containing a crystal vibrating chip is arranged on the upper surface of a printed circuit layer (PCB)
11
, and a plurality of parts
15
for a temperature compensation circuit are arranged at both side portions of the crystal oscillating unit
13
The term “crystal oscillating unit” represents a surface mounted device type part constructed by packaging the crystal vibrating chip. The temperature compensation parts
15
generally occupy an area approximately 2 to 3 times as large as the crystal oscillating unit
13
with a size of 5.0×3.2 mm
2
or 4.7×2.9 mm
2
. Further, the printed circuit board
11
used in the TCXO
10
requires an area much larger than the crystal oscillating unit
13
, such that a final product installed with the crystal oscillating unit
13
is also increased in its size (for example, larger than 7.0×5.2 mm
2
).
As described above, the difficulty in the miniaturization of discrete type TCXO restricts the employment of the TCXO as parts of mobile communication terminals. On the other hand, the one-chip type TCXO is advantageous in that the final product can be miniaturized by the use of an IC chip in which a plurality of parts such as a temperature compensation circuit and etc. are integrated, although its phase noise characteristics are somewhat bad in comparison with the discrete type TCXO. As a result, one-chip type TCXO is widely used recently.
FIGS. 2
a
and
2
b
are side sectional views of two different conventional one-chip type TCXOs. Referring to
FIGS. 2
a
and
2
b,
one-chip type TCXOs
20
or
20
′ are comprised of a layered structure
21
or
21
′ having an IC chip
27
or
27
′ in which a plurality of parts are integrated.
Especially, the temperature compensated crystal oscillator (TCXO)
20
of
FIG. 2
a
is comprised of the layered structure
21
in which first to fourth layers
21
a
to
21
d
are stacked in turn and different cavities are formed in the third and fourth layers
21
c
and
21
d,
respectively. The layered structure
21
includes the IC chip
27
inserted into the cavity formed in the third layer
21
c,
wherein the IC chip
27
is bonded to connection pads
28
on the second layer
21
b.
Further, a crystal vibrating chip
23
a
is inserted into the cavity, which is formed in the fourth layer
21
d
and has an opening larger than the cavity of the third layer
21
c.
Finally, a metal cover
25
is mounted on the upper surface of the fourth layer
21
d.
The one-chip type TCXO
20
shown in
FIG. 2
a
is advantageous in the miniaturization aspect. However, if the crystal vibrating chip
23
a
is damaged in the mounting process, the IC chip
27
must be also discarded due to the difficulty in the separation of the crystal vibrating chip
23
a.
That is, when the crystal vibrating chip
23
a
is inserted after the IC chip
27
, most of the damage may occur in the process of inserting the crystal vibrating chip
23
a.
However, according to the process due to the structure, the crystal vibrating chip
23
a
is arranged after the IC chip
27
is mounted. Accordingly, even if damage of the crystal vibrating chip
23
a
is detected, it is difficult to reuse the already mounted IC chip
27
which is comparatively expensive, Therefore, this structure of
FIG. 2
a
unnecessarily increases costs in the process for producing high quality products. Further, this structure is problematic in that the performance of the TCXO
20
is decreased due to mutual electromagnetic interference, because the TCXO
20
does not have shielding means such as a layer for blocking electrical influence between the crystal vibrating chip
23
a
and the IC chip
27
, and so both of them directly influence each other.
In order to solve the problems, the surface mounted device-type crystal oscillator
20
′ of
FIG. 2
b
is proposed, wherein a crystal oscillating unit
23
′ is mounted on top. As already explained in
FIG. 1
a,
the term “crystal oscillating unit” represents a surface mounted device type part constructed by packaging the crystal vibrating chip. The TCXO
20
′ is constructed such that a cavity is formed in a layer
21
c
′, and an IC chip
27
′ is mounted on connection pads
28
′ formed on the upper surface of a layer
21
b
′ which forms the lower surface of the cavity using a flip chip bonding method, and finally the crystal oscillating unit
23
′ is mounted on the upper surface of the top layer
21
c
′, In the surface mounted device-type crystal oscillator
20
′, because the crystal vibrating chip is contained inside the crystal oscillating unit
23
′, the damage occurring in the inserting process of the crystal vibrating chip can be prevented, and further electromagnetic interference between the IC chip
27
′ and the crystal oscillator
23
′ can be effectively shielded by the package surrounding the crystal vibrating chip, thus maintaining the performance of good quality of the TCXO
20
′.
In order to basically solve the problems occurring in the TCXO
20
of
FIG. 2
a
compared with the crystal oscillator of
FIG. 2
b,
another structure of a temperature compensated crystal oscillator
30
of
FIG. 3
is proposed. Referring to
FIG. 3
, the temperature compensated crystal oscillator
30
has a TCXO structure in which an additional cavity is formed in a lower layer
31
b
to insert an IC chip
37
into the cavity. Referring to
FIGS. 3
a
and
3
b,
the TCXO
30
is comprised of upper layer regions
31
d
and
31
e
in which a cavity for mounting a crystal vibrating chip
33
is formed, and lower layer regions
31
a
and
31
b
in which a cavity for mounting the IC chip
37
is formed. In this case, the two cavities are separated by an additional layer
31
c.
Here, the upper layer regions
31
d
and
3
e
are elements corresponding to the surface mounted device-type crystal oscillating unit
23
′ shown in
FIG. 2
b.
According to the TCXO
30
, the mounting space of the IC chip
37
is separated from that of the crystal vibrating chip
33
by the layer
31
c
arranged between the two cavities, such that additional mounting spaces for the IC chip
37

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