Television – Receiver circuitry – Tuning
Reexamination Certificate
1998-05-22
2003-09-30
Harvey, David E. (Department: 2714)
Television
Receiver circuitry
Tuning
C348S726000, C455S304000
Reexamination Certificate
active
06628343
ABSTRACT:
BACKGROUND OF THE INVENTION
This invention relates generally to a circuit which involves phase shifting and specifically to a television reception circuit suitable to receive a broadcasting channel selected from among a plurality of channels and output a signal of the received broadcasting channel as an intermediate frequency signal, and more particularly to an integrated television reception circuit which can efficiently remove an image frequency. The preasent invention relates also to a phase shifting circuit for shifting the phase of a signal inputted thereto, and more particularly to an automatic phase shift control circuit which can set a phase shift amount accurately within a broad frequency range. The present invention further relates to an equal amplitude addition circuit which adds signals inputted thereto with amplitudes equal to each other, and more particularly to an equal amplitude addition circuit suitable to add two balanced input signals with amplitudes equal to each other.
A reception circuit for selecting and receiving a desired channel from among a large number of television broadcasting channels includes a radio frequency amplifier for amplifying the selected broadcasting signal and a frequency mixing circuit for converting the radio frequency output of the radio frequency amplifier into an intermediate frequency, and has such a circuit construction as shown, for example, in FIG.
17
.
Referring to
FIG. 17
, the reception circuit shown includes a filter
1
of the single-tuned type for selecting a desired frequency from among channel frequencies of broadcasting radio waves received by an antenna, and an amplifier
2
of the automatic gain control type for controlling the level of the received signal selected by the filter
1
. The amplifier
2
is controlled based on a demodulation output (AGC) not shown so that the output level thereof may remain within a predetermined level range.
The reception circuit further includes a double-tuned filter
3
for limiting the pass band of the television signal. The double-tuned filter
3
cooperates with the single-tuned filter to vary the pass frequency of the broadcasting radio wave of the selected channel in accordance with a channel selection signal not shown.
The reception circuit further includes a trap circuit
4
for removing from within the received broadcasting frequency an image frequency which is mixed into the received broadcasting frequency when the broadcasting frequency is converted into an intermediate frequency. As hereinafter described, the frequency to be removed is set by the trap circuit
4
so that an image intermediate frequency which serves as a disturbing wave may not be produced. The reception circuit further includes a frequency conversion circuit (mixing circuit)
5
for converting the inputted radio frequency signal into an intermediate frequency (58.75 MHz) signal. The frequency conversion circuit
5
is formed from an integrated circuit which includes a local signal oscillator Lo, a frequency mixer MIX, and an intermediate frequency amplifier IFA for selecting an intermediate frequency.
In the television reception circuit described above, a local oscillation frequency (which may be hereinafter referred to as local frequency) fLo is set corresponding to a frequency (which may be hereinafter referred as selected frequency) fD of a desired selected channel as seen from
FIG. 18
, and the local frequency fLo and the selected frequency fD are mixed by the frequency mixer MIX so that a frequency of a difference between them is outputted as an intermediate frequency fIF from the frequency mixer MIX.
By the way, if an image frequency fIM is mixed at a point spaced by the intermediate frequency fIF from the local frequency fLo in the frequency mixer MIX, then the frequency mixer MIX outputs an undesired intermediate frequency fIF(IM) which includes the same frequency component also with regard to the image frequency fIM. Therefore, in order to prevent the image frequency component fIM, which makes an image diturbance, from being mixed in the frequency mixer MIX, the television reception circuit of
FIG. 17
includes the trap circuit
4
for removing the image frequency.
Consequently, the television reception circuit described above is disadvantageous in that the channel selection control circuit thereof is complicated because the three circuits of the single-tuned circuit, double-tuned circuit and image trap circuit must be adjusted each time the selected channel is changed and also in that it is difficult to adjust the channel selection control circuit so that an image disturbance may be reduced in an entire reception band.
Thus, another reception circuit has been proposed wherein a mixing circuit formed as an integrated circuit includes two frequency mixers to cancel an image frequency.
FIG. 19
shows a construction in principle of the mixing circuit just mentioned. Referring to
FIG. 19
, a frequency fD of a desired selected channel is supplied to a first frequency mixer MIX(Q) and a second frequency mixer MIX(I). Meanwhile, local frequencies fLo(Q) and fLo(I) produced by a &pgr;/2 phase shifter PH and having a phase difference of 90 degrees from each other are supplied to the frequency mixers MIX(Q) and MIX(I), respectively, and intermediate frequencies corresponding to frequency differences between the selected channel frequency and the local frequencies are outputted from the first and second frequency mixers MIX(Q) and MIX(I), respectively. The intermediate frequencies are outputted through a phase shifter PSN by which they are shifted by &pgr;/2 relative to each other, and then combined by an adder ADD, thereby cancelling an inputted intermediate frequency fIM (refer to
IEEE Transactions on Consumer Electronics
, Vol. 38, No. 3, August, 1992).
However, while the image cancelling mixing circuit described above has been put into practical use in reception circuits for radio frequencies included in an ordinary comparatively low frequency band, it is difficult to use it for a television reception circuit.
The reason is that, with regard to a broadcasting signal having a frequency band of several MHz as in the case of television broadcasting waves, it is difficult to form the &pgr;/2 phase shifter PH of the image cancelling mixing circuit so that it may shift the phases of local oscillation frequencies accurately by &pgr;/2 whichever broadcasting channel is received because of an influence of a dispersion of resistance, a dispersion of capacitance and parasitic floating capacitance when an integrated circuit is formed. Further, a high cost is required because a large number of elements are involved. From those reasons, it is considered difficult to put the image cancelling mixing circuit described above into practical use.
A phase shifting circuit which is employed in such an image cancelling mixing circuit as described above and causes the phase of an ac signal to lead or lag has been realized using various circuits. Where a phase shifting circuit is formed, for example, from an analog circuit in which a circuit device formed from resistors and capacitors is used, it is difficult to form the phase shifting circuit so that it may exhibit a fixed phase shift amount over a broad frequency range.
This arises from the fact that, with a circuit wherein capacitors having impedance and resistors are integrated, it is difficult to set a phase shift amount having a high degree of accuracy because of an absolute dispersion in the impedance or resistance and the fact that the phase shift amount is varied by an influence of a floating capacitance and so forth of the circuit by the frequency of an ac signal inputted to the phase shifting circuit. It is difficult to form a phase shifting circuit of the type described so that it shifts the phase of an input signal by a fixed angle over a broad frequency range.
Particularly where a phase shifting circuit is formed as an integrated circuit, dispersions in resistance of the circuit and floating capacitance derived in the circuit make it difficult to provide a
Kitazono Shinichi
Ohya Koichi
Yamaguchi Toshihiro
Harvey David E.
Kananen Ronald P.
Rader & Fishman & Grauer, PLLC
Sony Corporation
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