Telecommunications – Receiver or analog modulated signal frequency converter – Local control of receiver operation
Patent
1996-06-21
1998-03-24
Eisenzopf, Reinhard J.
Telecommunications
Receiver or analog modulated signal frequency converter
Local control of receiver operation
4552411, 4552451, 4552501, 348678, H04B 116
Patent
active
057323429
DESCRIPTION:
BRIEF SUMMARY
BACKGROUND
The invention is based on a radio-frequency receiver. Such radio-frequency receivers are used in particular in television receivers, videorecorders and radio receivers.
In practice, such receivers have an unsatisfactory signal-to-noise ratio in certain circumstances or at specific frequencies of the received RF signal. The invention is based on the object of improving the signal-to-noise ratio in such a radio-frequency receiver. According to a development of the invention, the AFT circuit for automatic sharp tuning by adjusting the mixer oscillator is also improved.
SUMMARY OF THE INVENTION
Conventional circuits operate with a fixed voltage of a desired value for the AGC automatic fading compensation circuit such that the amplitude of the RF voltage at the input of the mixer stage is adjusted to a constant value. The invention is based on the realization that the mixer stage does not have the same properties at all frequencies of the supplied RF signal. Instead, the mixer stage can be modulated to a greater extent at specific frequencies and to a lesser extent at other frequencies. This fact is allowed for by the invention in that the amplitude at the input of the mixer stage is no longer adjusted to a constant value. Instead, the amplitude is adjusted to different values which are dependent on the respective frequency of the RF signal. This takes place in such a way that the mixer stage is always modulated up to the limit, that is to say to a point just before distortions occur in the mixer stage. As a result of this adjustment as far as the modulation limit at all frequencies of the received RF signal, the signal-to-noise ratio is thus increased at least in specific frequency ranges and the image presented is improved with respect to the noise ratio and noise.
The aforesaid development of the invention is based on the following consideration. Circuits are known in which a PLL circuit is connected to the output of the IF amplifier. The PLL circuit produces the unmodulated image carrier from the amplitude-modulated image carrier. The said unmodulated carrier is used in the demodulator for synchronous demodulation or demodulation with addition of the received amplitude-modulated image carrier. The control voltage which is effective within the PLL circuit is dependent here on the frequency of the IF carrier. This control voltage is therefore additionally used for the adjustment of the mixer oscillator with the effect that the IF carrier is at its desired value at the output of the IF amplifier and thus the IF band filter lying between the mixer stage and the IF amplifier is driven to an optimum degree.
BRIEF DESCRIPTION OF THE DRAWINGS
Exemplary embodiments of the invention are explained below with reference to the drawing, in which:
FIG. 1 shows a radio-frequency receiver with the circuit according to the invention and
FIG. 2 shows a radio-frequency receiver according to FIG. 1 with an AFT circuit according to the aforesaid development of the invention.
DESCRIPTION OF THE PREFERRED EMBODIMENT(S)
FIG. 1 shows a radio-frequency receiver with the antenna 1, the RF amplifier 2, the mixer stage 3, the IF filter 4, the IF amplifier 5 and the demodulator 6 which supplies the composite colour video signal to the terminal 7. The output of the mixer oscillator 8 is connected to the mixer stage 3. The said mixer oscillator 8 is driven by the microprocessor 9 in order to be tuned to the respectively desired transmitter.
The AGC circuit 10 for automatic fading compensation is connected to the output of the demodulator 6. The said AGC circuit 10 supplies a first control voltage AGC1 for the RF amplifier 2 and a second control voltage AGC2 for the IF amplifier 5. In known circuits, a fixed voltage Us of a desired value is applied by the potentiometer 11 to the AGC circuit 10. This voltage determines at which amplitude of the RF voltage at the input of the mixer stage 3 the control takes place. The RF voltage at the input of the mixer stage 3 is adjusted here to a constant value dependent on the voltag
REFERENCES:
patent: 4355414 (1982-10-01), Inoue
patent: 4709408 (1987-11-01), Itakura
patent: 4827511 (1989-05-01), Masuko
patent: 5125106 (1992-06-01), Kato
IEEE Transactions on Consumer Electronics, vol. 36, No. 3, 30 Aug. 1990, New York US, McGinn M. et al, "An Advanced I.F. Amplifier & AFT System Suitable for HDTV", pp. 407-413.
Rieger Martin
Roth Sabine
Deutsche Thomson Brandt GmbH
Eisenzopf Reinhard J.
Emanuel Peter M.
Le Thanh
Tripoli Joseph S.
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