Miscellaneous active electrical nonlinear devices – circuits – and – Specific identifiable device – circuit – or system – Unwanted signal suppression
Reexamination Certificate
2002-06-05
2003-11-04
Cunningham, Terry D. (Department: 2816)
Miscellaneous active electrical nonlinear devices, circuits, and
Specific identifiable device, circuit, or system
Unwanted signal suppression
C327S558000
Reexamination Certificate
active
06642780
ABSTRACT:
FIELD OF THE INVENTION
The present invention relates to a variable frequency filter circuit for use, suitably as a base band LPF circuit, in an RF front-end section of a digital satellite broadcasting receiver of a direct conversion type, in which a received high-frequency signal is directly demodulated into a base band signal.
BACKGROUND OF THE INVENTION
In case an above-described digital satellite-broadcasting receiver is realized adopting a direct conversion method, it is necessary for a base band LPF (Low Pass Filter) to have a band sufficient for passing a data rate of a received signal, because an output from mixer is directly separated into desired quadrature modulation signals I and Q. On the other hand, as a band of a broadcasting signal is compressed while receiving channels are increased in number, it becomes more important to have a higher interference characteristic against an adjacent signal. In view of this, higher cut-off frequency accuracy ensures an amount of attenuation against adjacent channel interference, thereby improving reception quality (bit error rate). For this reason, the base band LPF circuit needs to accurately adjust a cut-off frequency in order to attenuate the adjacent signal by a sufficient amount within a most suitable bandwidth according to the data rate of the received signal.
FIG. 10
is a block diagram illustrating an electrical arrangement of a typical conventional LPF circuit
1
which is such a cut-off frequency variable LPF circuit realized on an integrated circuit. The LPF circuit
1
is a tertiary LPF. A base band signal from mixer, which is inputted from an input terminal
2
, is outputted from an output terminal
3
after passing through an LPF F
1
and an LPF F
2
connected in series. The LPF F
1
has primary cut-off frequency characteristics, whereas the LPF F
2
has secondary cut-off frequency characteristics. For example as shown in
FIG. 11
, the LPF F
1
is composed of a gm amplifier A
1
, and the LPF F
2
is composed of gm amplifiers A
21
and A
22
(hereinafter, the reference mark A is used to refer to all of them), and the LPF F
1
is provided with capacitor C
1
, and the LPF F
2
is provided with capacitors C
21
and C
22
. The capacitors C
1
, C
21
, and C
22
are for determining Q in connection with the respective gm amplifiers A
1
, A
21
, and A
22
.
Each of the gm amplifiers A is, as shown in
FIG. 12
, can be described as a circuit that is provided with a power supply
11
, constant current sources
12
to
15
, transistors Q
1
to Q
6
, and an emitter resistance RE.
The power supply
11
steps down a power supply voltage Vcc by a predetermined voltage. The transistor Q
1
, arranged as a diode with its collector connected with its base, supplies a constant current from the power supply
11
. A collector of the transistor Q
2
receives an emitter current of the transistor Q
1
. A base of the transistor Q
2
functions as a normal input IN. The transistor Q
2
receives a base band signal from the normal Input IN via an input terminal
2
. The constant current source
12
pulls out a constant current I
A
from an emitter of the transistor Q
2
. The transistors Q
1
and Q
2
, and the constant current source
12
constitute a series circuit.
The transistor Q
3
, arranged as a diode by connecting its collector with its base, supplies a constant current from the power supply
11
. A collector of the transistor Q
4
receives an emitter current of the transistor Q
3
. A base of transistor Q
4
functions as an inverting input XIN. A later-described output signal OUT is negatively fed back the inverting input XIN. The constant current source pulls out a constant current I
A
from an emitter of the transistor Q
4
. The transistors Q
3
and Q
4
, and the constant current source
13
constitute a series circuit similar to the above-described series circuit.
The emitter resistance RE is a resistance that connects the emitter of the transistor Q
2
and the emitter of the transistor Q
4
. Bases of the transistors Q
5
and Q
6
in pair respectively receive collector voltages of the transistors Q
2
and Q
4
. The output voltage OUT is derived from a collector of the transistor Q
5
, and negatively fed back into the base of the transistor Q
4
. The constant current source
14
pulls out a constant current I
B
mutually from emitters of the transistors Q
5
and Q
6
. The constant current source
15
supplies a constant current I
B
/2 to the collector of the transistor Q
5
.
Gains, gms, of the gm amplifiers A having the above arrangement can be represented by the following equation:
gm
=
1
{
(
RE
+
2
·
Vt
I
A
)
·
(
I
B
2
·
I
A
)
}
,
Vt
=
kT
q
(
1
)
where I
A
and I
B
are current values of the constant rent sources, RE is an emitter resistance value, k is the Boltzmann's constant (1.38×10
−23
), T is an absolute temperature (K), and q is an electrical charge of one electron (1.6×10
−19
).
Cut-off frequencies &ohgr;
1
and &ohgr;
2
of the LPFs F
1
and F
2
using the gm amplifiers A are represented as follows:
ω
1
=
gm
1
C
1
(
2
)
ω
2
=
gm
21
·
gm
22
C
21
·
C
22
(
3
)
where gm
1
is a gm of the am amplifier A
1
, gm
21
is a gm of the gm amplifier A
21
, and gm
22
is a gm of the gm amplifier A
22
.
Here, the cut-off frequency &ohgr;
1
of the LPF F
1
is in proportion with the gm
1
, and the cut-off frequency &ohgr;
2
of the LPF F
2
is in proportion with gm
21
and gm
22
. Meanwhile, the gm
1
, the gm
21
, and the gm
22
are in proportion with 1/I
B
. Therefore, when a value of the constant current I
B
is halved, the cut-off frequencies &ohgr;
1
and &ohgr;
2
are doubled. Thus, accurate control of the constant current I
B
enables accurate adjustment (varying) of the cut-off frequencies &ohgr;
1
and &ohgr;
2
. In response to cut-off frequency selection signals S
1
and S
2
inputted into a signal input terminal
4
, the value of the constant current I
B
is controlled by current value converting circuits B
1
and B
2
provided respectively in the LPF F
1
and LPF F
2
. The current value converting circuits B
1
and B
2
respectively control the constant current I
B
as reference currents IB
1
and IB
2
.
The LPF circuit
1
shown in
FIG. 10
is provided with a reference LPF F
2
a
(an LPF F
2
a
for reference), an input amplifier
5
, a phase comparator
6
, a high gain voltage amplifier
7
, a voltage-current converting circuit
8
, and constant current sources
9
and
10
, in order to control the value of the constant current I
B
accurately.
The input amplifier
5
adjusts a level of a reference frequency signal &ohgr;
0
inputted from an input terminal
2
a
. The phase comparator
6
detects a phase difference between (1) the reference frequency signal &ohgr;
0
whose level has been adjusted by the input amplifier
5
, and (2) an output signal of the LPF F
2
a
. The high gain voltage amplifier
7
amplifies the compared output signal, thereby ensuring sensitivity for the phase difference around 90°. The voltage-current converting circuit
8
converts, into a current value, a voltage value that is outputted from the high gain voltage amplifier
7
, then extracts the current value as a reference current I
0
for a current mirror circuit. The constant current sources
9
and
10
respectively generate constant currents IB
2
a
and IB
12
in accordance with the reference current I
0
, and supply the constant currents IB
2
a
and IB
12
to the reference LPF F
2
a
and the current value converting circuits B
1
and B
2
.
The reference LPF F
2
a
is a secondary LPF identical to the secondary LPF F
2
, which is an LPF of a main body, in terms of a circuit arrangement and a circuit element constant, so as to take an advantage of having, as shown in
FIG. 13
, a 90° phage difference between the input signal and the output signal for the cut-off frequency &ohgr;
0
at which a frequency property is dropped by −3 dB, th
Cunningham Terry D.
Harness & Dickey & Pierce P.L.C.
Sharp Kabushiki Kaisha
Tra Quan
LandOfFree
Variable frequency filter circuit does not yet have a rating. At this time, there are no reviews or comments for this patent.
If you have personal experience with Variable frequency filter circuit, we encourage you to share that experience with our LandOfFree.com community. Your opinion is very important and Variable frequency filter circuit will most certainly appreciate the feedback.
Profile ID: LFUS-PAI-O-3125077