Wave transmission lines and networks – Coupling networks – Wave filters including long line elements
Reexamination Certificate
2001-11-30
2003-09-16
Pascal, Robert (Department: 2817)
Wave transmission lines and networks
Coupling networks
Wave filters including long line elements
C333S012000
Reexamination Certificate
active
06621382
ABSTRACT:
BACKGROUND OF THE INVENTION
1. Field of the Invention
The present invention relates to a noise filter and a high frequency transmitter using the same. More specifically, the present invention relates to a noise filter formed by a microstrip line provided on a substrate and a high frequency transmitter provided with such a noise filter on the output side of a transmission power ampflifier.
2. Description of the Background Art
In recent years, rapid progress is made in the market for the radio communication using high frequencies in many systems such as broadcasting satellites and communications satellites. At the same time, the demand is increasing day by day for two-way communication according to the development of the Internet. In the two-way communication in the satellite communication, reception is implemented by LNB (Low Noise Block Down Converter) as is conventionally done, while transmission is implemented by newly using a high frequency transmitter.
FIG. 8
is a block diagram representing an arrangement of a conventional high frequency transmitter,
FIG. 9
is a diagram illustrating the shape of a reception band noise filter used in the conventional high frequency transmitter, and
FIG. 10
is a simulation result for a conventional reception band noise filter.
Now, a high frequency transmitter of a conventional example will be described with reference to
FIGS. 8
to
10
. An IF (intermediate frequency) signal input to the high frequency transmitter shown in
FIG. 8
is input to a mixer circuit
2
after having its gain ensured by an IF amplifier
1
. In mixer circuit
2
, a local oscillation signal from a local oscillation circuit
3
and the IF signal are mixed, and the IF signal is frequency-converted into a high frequency signal. The high frequency signal output from mixer circuit
2
, after passing through a band-pass filter
4
that attenuates the spurious that is generated in mixer circuit
2
, obtains a large gain from a circuit configured by three high frequency amplifiers
5
,
6
, and
7
.
The output from high frequency amplifier
7
is input via a band-pass filter
8
that attenuates the amplified spurious to a high frequency amplifier
9
, and together with a succeeding driver amplifier
10
, more gain is earned. The output of driver amplifier
10
is input via a reception band noise filter
11
that limits the noise level of the reception frequency band down to a negligible level to a power amplifier
12
, and becomes a high power signal required for transmission to a satellite. The high frequency signal output from power amplifier
12
passes via a reception band noise filter
13
that once again attenuates the noise level of the reception frequency band that has risen from the thermal noise level due to the gain of power amplifier
12
and an isolator
14
for ensuring isolation between an RF output and reception band noise filter
13
and is output from the high frequency transmitter (not shown).
Now, as shown in
FIG. 9
, for reception band noise filters
11
and
13
, a microstrip filter is generally employed which is formed by a main microstrip line
15
, one end of which has an input signal supplied thereto and the other end of which outputs a signal, and three sub-microstrip lines
16
,
17
, and
18
that are disposed together one by one such that they run orthogonal to main microstrip line
15
.
The reason for employing a filter of such a shape lies in that it allows large attenuation to be obtained in relation to the reception frequency band, while at the same time, the loss in the transmission frequency band can be limited to as low as 1 dB. When the loss is great in the transmission frequency band of reception band noise filter
13
disposed downstream to power amplifier
12
, there is a need to select a power amplifier of the type having large output power (the type having large saturation power) for power amplifier
12
. The power amplifier with large output power also involves high power consumption and greater heat generation so that the shape of the overall high frequency transmitter must be enlarged for the purpose of heat radiation, which, as a result, goes against the conditions such as compactness and low power consumption for its widespread use. Therefore, a filter of the shape as shown in
FIG. 9
that has small loss in the transmission frequency band is employed.
The signal pass characteristic of the filter shown in
FIG. 9
is indicated by the simulation result shown in FIG.
10
. As shown in
FIG. 10
, the filter is optimized such that a signal can pass through at a transmission frequency of 14 to 14.5 GHz and attenuates at a reception frequency of 10.95 to 12.75 GHz, and the loss of the transmission frequency is about 1 dB and the attenuation of the reception frequency obtained is at least 25 dB.
When the noise level of the reception frequency band that is input to power amplifier
12
is lowered to the thermal noise level (−173.5 dBm/Hz (25° C.)) due to the attenuation of band-pass filters
4
and
8
and reception band noise filter
11
, and when the small signal gain of power amplifier
12
is 20 dB and the noise figure is 7 dB, the noise level of the reception frequency band output from power amplifier
12
rises as high as −173.5+20+7=−146.5 dBm/Hz. This level, however, would be limited to −146.5−25=−171.5 dBm/Hz by being input into reception band noise filter
13
having the shape and characteristic of
FIGS. 9 and 10
. The specifications of the reception band noise level of a common high frequency transmitter is about −165 dBm/Hz or below, and it can be recognized that the specifications are satisfied by the effect of reception band noise filter
13
.
The recent development trends involve movements toward widely spreading high frequency transmitters among ordinary households as well as achieving lower cost and compactness, and a high gain type power amplifier with a small signal gain of about 35 dB is increasingly being adopted. By employing a high gain type power amplifier, components such as a driver amplifier and a high frequency amplifier become unnecessary, which contributes to cost and size reduction.
The increase in the small signal gain of the power amplifier, however, leads to greater increase in the noise level of the reception frequency band, which leads to the problem of the specifications of the reception band noise level not being satisfied.
Let us assume a case where a power amplifier
12
shown in
FIG. 8
is replaced by a power amplifier
19
having a small signal gain of 35 dB. When the noise level of the reception frequency band input to power amplifier
19
is lowered to the thermal noise level (−173.5 dBm/Hz (25°)), with the small signal gain of power amplifier
19
being 35 dB and the noise figure being 7 dB, the noise level of the reception frequency band output from power amplifier
19
rises as high as −173.5+35+7=−131.5 dBm/Hz. By inputting a signal to reception band noise filter
13
having the shape and characteristic of
FIGS. 9 and 10
, the level can be limited to −131.5−25=−156.5 dBm/Hz; however, this level does not satisfy the specifications of the reception band noise level of a general high frequency transmitter of about −165 dBm/Hz.
SUMMARY OF THE INVENTION
Thus, the principal object of the present invention is to provide a noise filter having large attenuation in the reception frequency band and a high frequency transmitter using the same.
In short, according to the present invention, a noise filter formed by a microstrip line disposed on a substrate includes a main microstrip line, one end of which has an input signal supplied thereto and other end of which outputs a signal, and at least first to fifth sub-microstrip lines disposed together one by one such that they intersect with the main microstrip line and their lengths from the intersections to their respective ends vary.
Thus, according to the present invention, the attenuation can be made large in t
Jones Stephen E.
Nixon & Vanderhye P.C.
Pascal Robert
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