Notch filter and method

Details Notch filter and method Notch filter and method

2001-05-25

2003-07-01

Ham, Seungsook (Department: 2817)

Wave transmission lines and networks

Coupling networks

Frequency domain filters utilizing only lumped parameters

C333S176000

Reexamination Certificate

active

06587018

ABSTRACT:

BACKGROUND OF THE INVENTION
1. Field of the Invention
The present invention relates generally to amplifier circuits, and more particularly to an apparatus and method for attenuating an undesired frequency in a signal output from an RF amplifier circuit in a wireless communications system, for example.
2. State of the Art
Wireless communications systems may typically include a chain of amplifier circuits in stages, each stage comprising an amplifier an filter circuit through which a received or modulated signal is passed in series. At each stage the filter circuit filters out unwanted (out of band) frequencies while the amplifiers amplify the remaining signals. Typically, each stage in the chain of amplifier circuits is a Radio Frequency (“RF”) amplifier circuit.
RF amplifier circuits are well known and widely used in, for example, receivers, transmitters and transceivers including devices such as cellular telephone handsets, base stations, pagers and wireless modems.
An example of an RF amplifier circuit suitable for use in a cellular telephone handset is shown in FIG.
1
. Referring to
FIG. 1
, a conventional RF amplifier circuit
10
typically includes an amplifier
12
having at least one active element or device
14
for amplifying a desired frequency or frequencies in a signal received on an input
16
thereto, and a network
18
performing the dual role of impedance transformation and suppressing or attenuating an undesired frequency or frequencies in the signal from an output
20
thereof. In the example shown in
FIG. 1
, the filter
18
is a particular type of low-pass filter (LPF), known as a three-section LPF, having a shunt capacitor
22
, a series inductor
26
and another shunt capacitor
24
, connected in that order. Values of the capacitors
22
,
24
, and the inductor
26
are selected to pass substantially unimpeded all frequencies below a predetermined first frequency (f
0
) while attenuating all frequencies above f
0
. For simplicity the amplifier circuit
10
is shown as including a single amplifier
12
with a single active element
14
and a single filter
18
, however it will be appreciated that the amplifier circuit can include additional active elements and filters.
A graph of the output versus frequency of the amplifier circuit
10
of
FIG. 1
is illustrated in FIG.
2
.
FIG. 2
is a graph of the gain, that is the change in strength of the signal between the input
18
and the output
20
, versus frequency. As shown by line
28
in
FIG. 2
, the amplifier
12
is biased and the filter designed such that all frequencies below f
0
have a generally constant gain and are passed through the filter
18
substantially unimpeded, while all frequencies greater than f
0
are attenuated by an amount or factor that increases in proportion to the frequency. Generally, it is desirable to suppress the undesired frequencies above f
0
to avoid distortion of the desired output waveform.
A particular problem with convention amplifier circuits is the suppression of harmonics of the desired frequency, and more particularly the suppression of a second harmonic of a desired or fundamental frequency. Because of the proximity of the second harmonic, 2f
0
, to the fundamental frequency, f
0
, conventional amplifier circuits using simply a low-pass filter have generally been unable to sufficiently suppress the second harmonic to avoid signal distortion. For example, as shown in
FIG. 2
, for an amplifier circuit using a conventional LPF
18
as shown in
FIG. 1
, the signal out will include in addition to the fundamental frequency a second harmonic that is attenuated by a factor of less than about 30 dB relative to the fundamental.
Several approaches have attempted to provide an amplifier circuit having a filter or apparatus for sufficiently suppressing the second harmonic while pass the fundamental frequency substantially unattenuated. One approach, also shown in
FIG. 1
, is the addition of a series-resonant trap
30
in shunt with the output
20
of the amplifier circuit
10
. The series-resonant trap
30
is designed to have a low impedance to any frequencies occurring at the second harmonic, thereby shunting a portion of this component of the signal to ground. The result, as shown by line
32
in
FIG. 2
, is a dip, or notch, in the output from the amplifier circuit
10
at the second harmonic. However, while this approach is effective to a degree, it is not wholly satisfactory. For example, to generate 1 W of power from a 3 V battery requires the active device to have load impedance below 4 ohms. For the shunt trap to successfully remove signals at the second harmonic, its impedance must be significantly lower, such as 0.5 ohms. Such low impedances are difficult to attain.
Another problem with the use of the series-resonant trap of
FIG. 1
is the impact of the trap on other characteristics of the circuit.


REFERENCES:
patent: 3461372 (1969-08-01), Pickup et al.
patent: 5072200 (1991-12-01), Ranky
patent: 5095285 (1992-03-01), Khatibzadeh
patent: 5202651 (1993-04-01), Yoshimasu
patent: 5880649 (1999-03-01), Tai et al.
patent: 5939939 (1999-08-01), Gaynor et al.
patent: 5969582 (1999-10-01), Boesch et al.
patent: 6100776 (2000-08-01), Furutani et al.
patent: 6104259 (2000-08-01), Miyaura
patent: 0213009 (1991-09-01), None
patent: 0114505 (1992-04-01), None

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