Wave transmission lines and networks – Coupling networks – Wave filters including long line elements
Reexamination Certificate
2002-08-13
2004-03-02
Wamsley, Patrick (Department: 2817)
Wave transmission lines and networks
Coupling networks
Wave filters including long line elements
C333S246000
Reexamination Certificate
active
06700462
ABSTRACT:
BACKGROUND OF THE INVENTION
1. Field of the Invention
The present invention relates to a microstrip line filter and a high-frequency transmitter using the microstrip line filter. In particular, the present invention relates to a microstrip line filter constituting a low-pass filter which eliminates any unwanted radiation and relates to a high-frequency transmitter using the microstrip line filter.
2. Description of the Background Art
In recent years, the radio (high-frequency) communication has undergone remarkable developments in numerous systems like the broadcast and communication satellites for example. On the other hand, the widespread use of the Internet has caused increasing demands for the two-way communication.
FIG. 11
schematically shows a system for two-way communication by means of a communication satellite. Referring to
FIG. 11
, an IDU (indoor unit)
1
is contained within a television receiver or housed in a board in a personal computer, and processes a signal for two-way communication with a broadcast station via a communication satellite
2
. IDU
1
is connected to a high-frequency transmitter
4
via a transmission-adapted coaxial cable
3
and IDU
1
is also connected to an LNB (low noise block down converter)
6
via a reception-adapted coaxial cable
5
.
High-frequency transmitter
4
and LNB
6
are coupled to a feed horn
8
via an orthogonal polarization isolator
7
. A transmission signal from high-frequency transmitter
4
is radiated as the microwave from feed horn
8
, reflected by a parabolic antenna
9
and transmitted toward communication satellite
2
. The microwave from communication satellite
2
is reflected by parabolic antenna
9
and then received by LNB
6
via feed horn
8
.
FIG. 12
is a block diagram of the high-frequency transmitter employed in the system shown in FIG.
11
. Referring to
FIG. 12
, high-frequency transmitter
4
receives, from IDU
1
shown in
FIG. 11
, a transmission signal of an intermediate frequency ranging from 950 to 1450 MHz superimposed on a direct-current voltage. The intermediate-frequency signal is supplied via a high-pass filter (HPF)
401
to an IF amplifier
402
to obtain a gain, adjusted to a proper level by an attenuator
403
, further amplified by an IF amplifier
404
, and then supplied to a mixer
406
via a bandpass filter (BPF)
405
.
A local oscillator
407
generates a local oscillator signal of 13.05 GHz which is provided via a buffer amplifier
408
to mixer
406
. Mixer
406
combines the local oscillator signal of 13.05 GHz with the intermediate-frequency signal of 950-1450 MHz in order to convert the intermediate-frequency signal into a high-frequency signal of 14.0-14.5 GHz. The high-frequency signal supplied from mixer
406
is input to a half-wave bandpass filter
409
where any unwanted radiation component (spurious radiation component) of the high-frequency signal that is generated in mixer
406
is attenuated, and then amplified by two high-frequency amplifiers
410
and
411
to obtain a great gain.
The output from high-frequency amplifier
411
is supplied to a bandpass filter
412
where the amplified spurious component is attenuated, and then supplied to a driver amplifier
413
to obtain a further gain. The output from driver amplifier
413
is supplied to a reception-bandwidth noise filter
414
where any noise level in a reception frequency range is substantially reduced to a thermal noise level. Then, the high-frequency signal is converted by a power amplifier
415
to a signal of high power required for transmission to the satellite. The high-frequency signal from power amplifier
415
is provided to a reception-bandwidth noise filter
416
where the noise level in the reception frequency range that is increased from the thermal noise level due to the gain of power amplifier
415
is attenuated, and then the signal supplied via noise filter
416
from high-frequency transmitter
4
is radiated as the microwave from feed horn
8
, reflected by parabola antenna
9
and transmitted toward communication satellite
2
that are shown in FIG.
11
.
The DC voltage with the intermediate-frequency signal superimposed thereon is supplied via an inductor L to a power supply circuit
421
. Inductor L prevents the intermediate-frequency signal from being input to power supply circuit
421
. Power supply circuit
421
converts the supplied DC voltage into a predetermined voltage which is provided to a power supply sequence circuit
422
. Then, the converted DC voltage is supplied to IF amplifiers
402
and
404
, mixer
406
, local oscillator
407
, buffer amplifier
408
, high-frequency amplifiers
410
and
411
, driver amplifier
413
and power amplifier
415
.
In high-frequency transmitter
4
shown in
FIG. 12
, the gain of IF amplifiers
402
and
404
and the degree or amount of attenuation by attenuator
403
are adjusted to prevent the output level from varying when the level of the input intermediate-frequency signal varies in the range from −5 dBm to −25 dBm. Even if a high-level signal of approximately −5 dBm is input, IF amplifiers
402
and
404
operate in a saturation region to distort the signal component in order to output the signal at a predetermined level. However, the distorted signal component generates harmonic components resulting in increase of spurious components.
Any spurious of 14.95-15.95 GHz generated in mixer
406
resultant from mixing of the input signal of twice the frequency of 950 MHz-1450 MHz and the local oscillator signal of 13.05 GHz differs from the output frequency range 14 GHz-14.5 GHz of high-frequency transmitter
4
merely by 450 MHz. Then, in order to reduce such a spurious, a microstrip filter as shown in
FIG. 13
is used as the half-wave bandpass filter
409
shown in FIG.
12
.
The microstrip filter shown in
FIG. 13
includes a plurality of (e.g.
8
) rectangular elements shifted so that respective halves of the longitudinal sides of respective elements are opposite to and in parallel with each other. This bandpass filter
409
has a passband of 14 GHz-14.5 GHz so as to attenuate an image-frequency signal of 11.6-12.1 GHz and a signal above 14.5 GHz. However, proper attenuation of the spurious of 14.95 GHz which is close to 14.5 GHz could be impossible.
FIG. 14
shows cutoff characteristics of a combination of half-wave bandpass filter
409
and high-frequency amplifiers
410
and
411
. It is seen from
FIG. 14
that the attenuation achieved by the cutoff characteristics is merely 11.9 dB, which means that an attenuation of 20 dB or more by half-wave bandpass filter
409
with its elements arranged as shown in
FIG. 13
is extremely difficult. Even if attenuation of at least 20 dB is possible, it is impossible to make the cutoff characteristics more steeper.
SUMMARY OF THE INVENTION
One object of the present invention is to provide a microstrip line filter constituting a low-pass filter with a large out-of-band attenuation and a small in-band deviation, and to provide a high-frequency transmitter employing the microstrip line filter.
In summary, according to one aspect of the present invention, a microstrip line filter formed on a substrate includes a plurality of composite elements arranged in parallel with each other. The composite elements each include a rectangular microstrip line element, an input microstrip line and an output microstrip line that are formed on the substrate. The composite elements are connected to constitute a low-pass filter.
The rectangular microstrip line element has one longer side, the other longer side, one end and the other end. The input microstrip line is connected at the one end to the one longer side, and the output microstrip line is connected at the other end to the other longer side.
The composite elements adjacent to each other have respective input microstrip line and output microstrip line connected to each other and, the adjacent composite elements are symmetrical with respect to a center line between the input microstrip line and the output microstrip line connected to ea
Nagano Atsushi
Nakamura Makio
Okami Mitsutoshi
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