Television – Receiver circuitry – Tuning
Reexamination Certificate
2000-04-20
2004-03-30
Kostak, Victor R. (Department: 2611)
Television
Receiver circuitry
Tuning
C725S111000, C725S131000
Reexamination Certificate
active
06714261
ABSTRACT:
BACKGROUND OF THE INVENTION
1. Field of the Invention
The present invention relates to a CATV tuner. More specifically, the present invention relates to a cable modem tuner incorporated in a cable modem used for enabling high speed data communication at home, utilizing an unused channel of cable television (hereinafter referred to as CATV), and a CATV tuner used for a digital set box (hereinafter referred to as STB) for high speed data communication at home utilizing a different frequency band.
2. Description of the Background Art
In a CATV system, introduction of HFC (Hybrid Fiber/Coax) has been in progress, in which a coaxial cable is kept as a subscriber's drop wire and the main network is implemented by optical fibers. This system attempts to provide broad-band data communication service of several Mbits/sec at home. Utilizing this system, it is possible to realize high speed data line having the transmission rate of 30 Mbits/sec with the band width of 6 MHz using 64 QAM (Quadrature Amplitude Modulation), which may not be called the state of the art any more. The cable modem is used in this system, and realizes high speed data communication of 4 Mbits/sec to 27 Mbits/sec, utilizing an unused channel of CATV.
FIG. 11
is a block diagram of a conventional cable modem tuner. An up signal transmitted from the cable modem tuner to a CATV station, not shown, has the frequency of 5 MHz to 42 MHz, and a down signal transmitted from the CATV station to the cable modem tuner has the frequency of 54 MHz to 860 MHz, and transmitted to a cable network through a CATV input terminal
11
of the tuner. The up signal transmitted from the cable modem is received by a data receiver of the CATV station (system operator), and enters a computer of a center. In the cable modem, a data signal subjected to quadrature phase shift keying from a QPSK transmitter, not shown, is input to a data terminal
10
, as the up signal. The data signal is transmitted through an upstream circuit
9
and an input terminal
11
, to the CATV station.
The down signal is passed through an HPF (High Pass Filter)
1
as an IF (Intermediate Frequency) filter having an attenuation range of 5 to 42 MHz and a passband of not lower than 54 MHz and to a buffer amplifier
35
to be supplied to various circuits of the succeeding stages.
The circuits of the succeeding stages provide receiving circuits for UHF band (B
3
band) having the frequency of 470 to 860 MHz, VHF High band (
132
band) of 170 to 470 MHz and VHF Low band (B
1
band) of 54 to 170 MHz, respectively. Band division is not limited thereto.
Further, the cable modem tuner includes, in addition to the receiving circuits described above, IF amplifying circuits
19
and
21
, an SAW filter
20
, an IF output terminal
12
and a PLL channel selection circuit
27
.
The receiving circuits for the B
1
to B
3
bands described above respectively include input switching circuits
200
,
140
and
220
; UHF high frequency amplification input tuning circuits
300
, VHF HIGH BAND high frequency amplification input tuning circuit
150
and VHF LOW BAND high frequency amplification input tuning circuit
230
; a UHF high frequency amplifier
4
, a VHF HIGH BAND high frequency amplifier
16
and a VHF LOW BAND high frequency amplifier
24
; a UHF high frequency amplification output tuning circuit
50
, VHF HIGH BAND high frequency amplification output tuning circuit
170
and VHF LOW BAND high frequency amplification output tuning circuit
250
; a UHF mixing circuit
6
, a VHF HIGH BAND mixing circuit
18
and a VHF LOW BAND mixing circuit
26
; and a UHF oscillating circuit
7
, a VHF HIGH BAND oscillating circuit
13
and a VHF LOW BAND oscillating circuit
8
, corresponding to the mixing circuits, respectively.
Switching method using a switching diode, or a method using a filter for band splitting is applied to the input switching circuits
200
,
140
and
220
.
Generally, a dual gate type MOSFET device is used for the high frequency amplifiers
4
,
16
and
24
. An AGC (Automatic Gain Control) voltage from an AGC terminal
36
is input to the gate electrode of the device, and therefore the gain in these amplifiers is controlled by the AGC voltage.
Input switching circuits
200
,
140
and
220
receive as inputs the signals of B
1
to B
3
bands, and selectively outputs the received signals of prescribed frequency bands only.
High frequency amplification input tuning circuits
300
,
150
and
230
tune the received signals selectively output from input switching circuits
200
,
140
and
220
to respective desired frequencies (frequencies of the desired channels) using a tuning coil or the like, in respective bands.
High frequency amplifiers
4
,
16
and
24
receive the output signals from high frequency amplification input tuning circuits
300
,
150
and
230
, amplify these signals so as to prevent degradation of SN ratio such as signal distortion, using the voltage level of AGC terminal
36
receiving the AGC voltage as a reference, and output the resulting signals. The RF (high frequency) AGC voltage supplied to AGC terminal
36
is supplied to the gate electrode of the dual gate MOSFET in each of the high frequency amplifiers
4
,
16
and
24
, and therefore the dual gate MOSFET operates such that the power gain of the high frequency amplifier attains the full gain when the input signal level is higher than 60 dB&mgr;, and operates so that the output level of the tuner is always kept at a constant level when the input signal level is not higher than 60 dB&mgr;, so that degradation of SN ratio such as distortion, of the signal can be prevented.
High frequency amplification output tuning circuits
50
,
170
and
250
tune the output signals from high frequency amplifiers
4
,
16
and
24
to desired frequencies by using a tuning coil or like in respective bands, and provide the resulting signals.
Local oscillating circuits
7
,
13
and
8
oscillate to provide prescribed intermediate frequencies corresponding to respective bands, and mixing circuits
6
,
16
and
26
convert the signals output from high frequency amplification output tuning circuits
50
,
170
and
250
to desired intermediate frequency signals by using the oscillation signals from the corresponding local oscillating circuits, and therefore, local oscillating circuits
7
,
13
and
18
together with the mixing circuits
6
,
18
and
26
form frequency converting circuits for respect bands.
Thereafter, the output signals of the receiving circuits are amplified to prescribed levels by an IF amplifying circuit
19
, frequency-converted to a prescribed level by SAW filter and IF amplifying circuit
21
, and output through IF output terminal
12
.
In operation, the down signal passes through HPF
1
and applied to input switching circuits
200
,
140
and
220
. Therefore, among the three receiving circuits, only that receiving circuit of which operational frequency corresponds to the frequency of the down signal operates, and other receiving circuits do not operate. The operations of the receiving circuits are common.
The receiving circuit of each band will be described in the following.
CATV signal is passed through input switching circuits
200
,
140
and
220
as well as high frequency amplification input tuning circuits
300
,
150
and
230
, amplified by high frequency amplifiers
4
,
16
and
24
, and provided as received signals through high frequency amplification output tuning circuits
50
,
170
and
250
.
Thereafter, the received signals are passed through mixing circuits
6
,
18
and
26
as well as local oscillating circuits
7
,
13
and
8
whereby the signals are converted to desired intermediate frequency signals, and subjected to LOW IF conversion by IF amplifying circuits
19
and
21
and SAW filter
20
, and provided at output terminal
12
.
The above described series of operations are implemented as a channel selection data is transmitted from a CPU, not shown, to PLL channel selection circuit
27
so that the channel is selected accordingly and, at the same time,
Birch & Stewart Kolasch & Birch, LLP
Kostak Victor R.
Sharp Kabushiki Kaisha
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