Wave transmission lines and networks – Coupling networks – With impedance matching
Reexamination Certificate
2002-10-01
2004-11-09
Jones, Stephen E. (Department: 2817)
Wave transmission lines and networks
Coupling networks
With impedance matching
C333S204000
Reexamination Certificate
active
06816030
ABSTRACT:
BACKGROUND OF INVENTION
1. Field of the Invention
The present invention relates in general to an impedance matching circuit, and more particularly, to an impedance matching circuit having a plurality of microstrip structures providing a lossless target signal transmission and rejecting image signals of heterodyne noise and super-heterodyne noise.
2. Description of the Prior Art
Although a wired communication system is taken for granted as a means of signal transmission for most local area networks (LAN), wireless network applications have gained popularity over the past few years. With continuing development of related technology, the changes brought about by wireless communication are gradually penetrating into almost every field of traditional communication systems. Additionally, the cost of switching from a wired signal transmission to a wireless signal transmission has been slashed due to the maturity of the related technology. Today, in order to avoid troublesome wiring problems, a wireless communication system becomes more and more crucial and is demanded by some local area signal transmissions, such as a wireless application protocol (WAP) browser of a mobile phone, which lets users connect to the Internet and access special designed services and WAP pages.
With the aim of quickly spreading wireless communication technology into every phase of office life and enhancing technological improvement, a standard must be created to ensure compatibility and reliability of signal transmission among all the related devices and systems. Therefore, wireless transmission standards were created by the Institute of Electrical and Electronics Engineers (IEEE), such as the IEEE 802.11 standard in 1997, and the newer standards of IEEE 802.11a and IEEE 802.11b created in 1999. The early standards define the specification of RF-band usages and regulate signal transmission rate. The new version standards of IEEE 802.11a and IEEE802.11b are based on the band signals of 5.8 GHz and 2.4 GHz to specify the physical layer transmission rate. All these specifications can be applied to general transmission signals in the Industrial-Scientific-Medical (ISM) bands, such as bands of 902-928 MHz, 2.4-2.4835 GHz, 5.150-5.350 GHz, and 5.725-5.850 GHz.
Please refer to
FIG. 1
, which shows a function block diagram of a prior art transceiver
10
. The transceiver
10
is a front-end circuit to receive a low-power radio-frequency (RF) signal. Traditionally, there are various modes of processing the RF signal received by the transceiver
10
, such as a heterodyne, a super-heterodyne, or a zero intermediate frequency (IF) topology. Because of a DC voltage offset, a transceiver with zero-IF topology is known to have a narrower dynamic range. Moreover, because of circuit design considerations, although a transceiver with either a heterodyne or a super-heterodyne topology is known to have a broader dynamic range, extra filters are required to get rid of unwanted image signals.
The prior art transceiver
10
in
FIG. 1
comprises an antenna
11
, an input circuit
13
, a RF amplifier
14
, a mixer
16
, a local oscillator
18
, an IF amplifier
20
, a demodulator
22
, and an output device
23
. After receiving an RF signal
12
through the antenna
11
, the input circuit
13
is utilized to pick up desirable signals and match the impedance between the RF amplifier
14
and the antenna
11
. In addition, the input circuit
13
is able to avoid secondary radiation coming from the RF signal
12
received by antenna
11
. The RF amplifier
14
is not a necessity in circuits of the transceiver
10
, however, with the aid of the RF amplifier
14
, the receiving performance of the transceiver
14
is improved. For instance, if the RF amplifier
14
increases the gain of the RF signal
12
, the back-end circuits, such as mid-band or base-band circuits, can be easily driven by the amplified signal. Nevertheless, unwanted noises are amplified at the same time. In order to improve the signal-to-noise ratio of the circuit and avoid unwanted radiation coming from the local oscillator
18
through the antenna
11
, devices having features of low noise, high forward gain and high reverse isolation are required, with GaAs hetero-junction field effect transistors (FETs) among the candidates.
The mixer
16
functions to convert the RF signal
12
into an IF signal
17
for later amplification. The operation of the mixer
16
is based on the received RF signal
12
and the oscillating signal
17
generated by the local oscillator
18
. Taking advantage of a nonlinear circuit, the mixer
16
is capable of generating various kinds of signals, such as an RF signal having the same frequency as the RF signal
12
, a signal having a frequency equal to the sum of the frequencies of the RF signal
12
and the oscillating signal
17
, a signal having a frequency equal to the difference of the frequencies of the RF signal
12
and the oscillating signal
17
, and the other high frequency harmonic signals. Using a filter, a signal having a frequency equal to the difference of the frequencies of the RF signal
12
and the oscillating signal
17
is extracted from all the harmonic signals by the mixer
16
. Because it is more difficult and costs more to design a high frequency amplifier for the RF signal
12
, than to design a IF frequency amplifier for the IF signal
19
, the mixer
16
converts the RF signal
12
into the IF signal
19
and sends the IF signal
19
to the IF amplifier
20
. Consequently, the major gain, signal selectivity of the transceiver
10
is determined by the IF circuits, which typically comprises of a channel selection filter and the IF amplifier
20
. In the end, a demodulator
22
, such as an envelope detector, or a frequency discriminator, is utilized to retrieve the RF signal
12
having larger power from the amplified IF signal
19
and provides the amplified RF signal
12
to drive the output device
23
, such as a loudspeaker.
As described hereinbefore, the transceiver
10
generates the IF signal
19
through a signal mixing process of the oscillating signal
17
and the low-power RF signal
12
received by the antenna
11
, and an amplified RF signal
12
having enough power to drive the output device
23
can be retrieved from the amplified IF signal
19
by a demodulating process. The transceiver
10
has been used in almost every kind of signal modulation systems, such as amplitude modulation (AM) systems, frequency modulation (FM) systems, single side band (SSB) modulation systems, television systems, radar systems, mobile communication systems, and wireless communication systems. The major reason for such an extensive application comes from the high-selective bandpass effect of the IF amplifier
20
, which removes the undesirable band signals other than the IF signal
19
.
If the transceiver
10
is operated with a heterodyne or super-heterodyne topology, there are unwanted signals having two optional frequencies produced based on the oscillating signal
17
and the RF signal
12
. One unwanted signal has a frequency higher than the frequency of the RF signal
12
received by the antenna
11
and the related phenomenon is called LO high-side injection. The other unwanted signal has a frequency lower than the frequency of the RF signal
12
and the related phenomenon is called LO low-side injection.
Taking LO high-side injection for an example, if the frequency of the oscillating signal
17
is F
O
, the frequency of the RF signal
12
is F
RF
, and the frequency of the IF signal
19
is F
IF
, the relationship of the three frequencies can be expressed as F
O
=F
RF
+F
IF
. Ideally, after the signal mixing process, the IF signal
19
having a frequency equal to the difference of the frequencies of the oscillating signal
17
and the RF signal
12
, that is F
IF
=F
O
−F
RF
, is the only signal passing through the IF amplifier
20
. However, there is a noise signal having a frequency F
I
, where F
I
=F
RF
+2F
IF
, only partially attenuated by the input circuit
13
. Aft
Chang Sheng-Fuh
Chen Jia-Liang
Liu Cheng-Cheng
Accton Technology Corporation
Hsu Winston
Jones Stephen E.
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