Telecommunications – Receiver or analog modulated signal frequency converter – Local control of receiver operation
Reexamination Certificate
1996-01-30
2001-01-30
Nguyen, Lee (Department: 2746)
Telecommunications
Receiver or analog modulated signal frequency converter
Local control of receiver operation
C455S116000, C455S241100, C455S251100, C455S558000, C326S112000, C333S08100R, C333S08100R
Reexamination Certificate
active
06181922
ABSTRACT:
BACKGROUND OF THE INVENTION
1. Field of the Invention
The present invention generally relates to an attenuator unit and a step attenuator, and more particularly, to an attenuator unit and a step attenuator unit which attenuate a high-frequency signal in radio equipment. The present invention is also directed to an electronic apparatus including the step attenuator.
2. Description of the Related Art
A step attenuator is an attenuator whose attenuation value can flexibly be selected at a digital value. This step attenuator is widely used for controlling transmission power of radio equipment such as a portable telephone.
FIG. 1
shows a block diagram of the radio equipment to which the step attenuator is applied. A step attenuator
6
is connected between a transmission circuit
4
and a transmission power amplifier
8
. For example, when extremely large output power is transmitted from the radio equipment, the large output power may saturate a reception amplifier in radio equipment at a transmission destination, and may interfere with other radio equipment. In such a case, an attenuation value of the step attenuator
6
is selected to be large to reduce the output power of the radio equipment.
Since the step attenuator is widely used for portable apparatuses, miniaturization of the step attenuator is required. Further, for applying the step attenuator to the radio equipment, the step attenuator needs to have wide-band frequency performance.
FIG. 2
shows a typical configuration of the step attenuator. A step attenuator
20
is constructed with a plurality of (3 in this case) attenuator units
22
a,
22
b,
22
c
connected in series.
The attenuator units
22
a,
22
b,
22
c
respectively include two single-pole double-through (SPDT) switches
24
a
-
1
and
24
a
-
2
,
24
b
-
1
and
24
b
-
2
,
24
c
-
1
and
24
c
-
2
, and fixed attenuators
26
a,
26
b,
26
c.
In the SPDT switches, by selecting whether passing a supplied signal through the fixed attenuator or passing the supplied signal through the other path, an attenuation value of the attenuator
22
a,
22
b,
22
c
unit can be digitally controlled.
In a typical step attenuator, when attenuation values of the fixed attenuators are properly selected, by properly switching the SPDT switches of the attenuator units, a desired attenuation value can be digitally selected. In the step attenuator
20
shown in
FIG. 2
, the fixed attenuator
26
a
of the attenuator unit
22
a
has an attenuation value 1 dB, the fixed attenuator
26
b
of the attenuator unit
22
b
has an attenuation value 2 dB, and the fixed attenuator
26
c
of the attenuator unit
22
c
has an attenuation value 4 dB. Therefore, a total attenuation value of the step attenuator
20
can be varied from 0 to 7 dB by a 1-dB step by switching the SPDT switches of the attenuator units.
In each attenuator unit, for the fixed attenuator, a T-type attenuator and a &pgr;-type attenuator are commonly used.
FIG. 3
shows a schematic diagram of a prior-art attenuator unit using the T-type attenuator.
FIG. 4
shows a schematic diagram of a prior-art attenuator unit using the &pgr;-type attenuator.
An attenuator unit
30
shown in
FIG. 3
includes three resistors R
31
, R
32
, R
33
constituting the T-type attenuator, and two field-effect transistors (FETs)
32
,
34
operating as switches. When the FET
32
is non-conductive and the FET
34
is conductive, the attenuator unit
30
operates as the T-type attenuator and generates a large attenuation value. On the other hand, when the FET
32
is conductive and the FET
34
is non-conductive, the attenuation value of the attenuator unit
30
becomes small.
An attenuator unit
40
shown in
FIG. 4
includes three resistors R
41
, R
42
, R
43
constituting the &pgr;-type attenuator, and three FETs
42
,
44
,
46
operating as switches. When the FET
42
is non-conductive and the FETs
44
,
46
are conductive, the attenuator unit
40
operates as the &pgr;-type attenuator and generates a large attenuation value. On the other hand, when the FET
42
is conductive and the FETs
44
,
46
are non-conductive, the attenuation value of the attenuator unit
40
becomes small.
In a shunt side of the attenuator unit
30
shown in
FIG. 3
, the FET
34
is connected, while in a shunt side of the attenuator unit
40
shown in
FIG. 4
, the FETs
44
,
46
are connected. In this way, since the shunt side of the T-type attenuator is constructed with a single FET and is in different from the &pgr;-type attenuator, the degree to which dispersion of frequency performance of resistance in a conductive condition of the FET has an influence on the attenuator unit
30
may be smaller than that in which the dispersion has an influence on the attenuator unit
40
.
However, when designing the attenuator unit
30
having the T-type attenuator to generate a large attenuation value, the resistance value in the shunt side needs to be extremely small. Such an extremely-small resistance needs a wide area and makes the design complex.
On the contrary, the attenuator unit
40
having the &pgr;-type attenuator can overcome the above-discussed problem, and, thus, the attenuator unit
40
is suitable for constructing the step attenuator.
However, the above-discussed prior-art step attenuator using the &pgr;-type attenuator has the following problems.
Since the attenuator unit
40
has two current paths in the shunt side, two FETs are required, and, thus, it is difficult to produce the step attenuator with small size and high density. As a result, the size of the step attenuator using the &pgr;-type attenuator is larger than that using the T-type attenuator.
Furthermore, the SPDT switches in the attenuator units need to be controlled to turn on and off. Namely, for the attenuator units, two signals of a control signal and an inverted control signal are required. As shown in
FIG. 2
, the inverted control signals can be generated by inverting the control signals in inverter circuits
28
a,
28
b,
28
c.
FIG. 5
shows a schematic diagram of a prior-art inverter circuit. An inverter circuit
50
shown in
FIG. 5
is constructed with a depletion-type FET (D-FET)
52
and an enhancement-type FET (E-FET)
54
. A signal supplied to a gate of the E-FET
54
is inverted and is produced from a drain of the E-FET
54
.
Since the inverter circuit
50
includes two FETs, electrical performance of the inverter circuit
50
is easily varied by dispersion in a process. Therefore, the inverter circuit
50
needs to be designed so as to absorb influences due to the dispersion.
Further, size of the two FETs is not negligible as compared with a circuit scale of the step attenuator, and, thus, the step attenuator size becomes large.
SUMMARY OF THE INVENTION
It is an object of the present invention to provide a step attenuator which can be produced with small size and high density. Further, the step attenuator may be easily designed. Also, the cost of the step attenuator can be reduced. This permits the disadvantages described above to be eliminated.
The object described above is achieved by an attenuator unit for attenuating a signal, the unit comprising: a &pgr;-type attenuator having a first resistor and second and third resistors which are arranged on both sides of the first resistor; a first transistor connected with the first resistor in parallel; and a second transistor connected between a joint node of the second resistor and the third resistor and a first voltage level; wherein by controlling a gate voltage of the first transistor and a gate voltage of the second transistor, an attenuation value of the attenuator unit is changed.
According to the above-mentioned attenuator unit, the number of transistors in a shunt side is reduced by one as compared to a prior-art attenuator unit having two FETs. Therefore, the number of components and a layout area for the attenuator unit may be reduced, and, thus, a simply-configurated attenuator unit may be realized. Furthermore, since the two FETs in the shunt side of the prior-art attenuator unit are replaced to a single FET in common
Iwai Kenji
Kobayashi Kazuhiko
Fujitsu Limited
Helfgott & Karas P.C.
Nguyen Lee
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