MXPA97008288A - Automatic gain control circuit disposition for a tune - Google Patents

Abstract

The present invention relates to a tuner wherein each plurality of RF AGC gain controllable amplifiers are individually controlled by individual AGC control signals generated by an AGC controller, such that the level of the output signal of each of the RF controllable AGC gain amplifiers are individually optimized for tuning operation

Description

DISPOSITION OF AUTOMATIC GAIN CONTROL CIRCUIT FOR A TUNER FIELD OF THE INVENTION The present invention relates to an automatic gain control (AGC) arrangement useful for a tuner.

BACKGROUND OF THE INVENTION At present, tuners do not work well enough to process digital signals received using various transmission systems. The RF and IF characteristics of the receivers need an improvement, since a digital signal occupies the entire spectrum of a channel, and the delay and frequency response errors have a more severe effect on digital signals than on analog signals. . In addition, since the energy transmitted from the digital signals is considerably less than the energy transmitted for analogue transmissions, the acceptable reception of the desired signal may be more difficult in the presence of strong, adjacent channel signals. In addition, the conditions of the signal in the marginal zone of transmissions over air are also a problem. For marginal areas, some tenths of a loss in dB of a signal-to-noise ratio (SNR) or an increase in distortion between modulation can make signal recovery difficult, if not difficult. Conventional automatic gain control AGC systems in TV receivers are typically responsible for the level of the demodulated video signal. After comparing the demodulated video signal with a reference, error voltages are generated to control the gain of the IF amplifier and the RF state of the tuner. To obtain a good SNR over a wide variety of input levels, it is common to practice delaying the application of AGC to the tuner until s find relatively high signal levels. This works well in the absence of strong adjacent signals. However, if the signal level is low in the presence of strong adjacent signals, cross modulation with strong adjacent signals in the mixer may occur, and the bit error rate (BER) of the demodulated television information may be increased. Typically, there is an individual AGC control signal for IF and an individual AGC control signal for RF, which may or may not be delayed. In addition, the AGC control signal for RF is typically derived from the AGC control signal for IF. Although the relative amplitudes of the AGC control signals can be made different for the various stages through the use of dividers, the total ratios between the various stages remain constant for several signal levels. More particularly, the AGC control signal for RF is not individually adjustable according to the input signal characteristics, especially for optimizing the characteristics of the tuner with respect to the adjacent channel and SNR signals. Finally, the last criterion for digital reception quality is the BER, which is effected by the SNR and the distortion of the intermodulation. Currently, much research has been conducted to develop improved devices to minimize such difficulties in the tuner. However, even if such new devices are successful, the limitations of the mixer will remain problematic. In this way, it is desirable that the quality of the signal coupled to several stages of a tuner be optimized independently of the devices used in the various stages of the tuner.

COMPENDIUM OF THE INVENTION A tuner is presented, in which there are two separate AGC RF controllable gain amplifier stages, desirably separated by a controllable gain stage without AGC. Each of the AGC RF stages are individually controlled by respective individual AGC control signals generated by an AGC controller, so that the level of the output signal from each of the AGC RF controllable gain amplifiers is individually optimized for operation of the tuner.

BRIEF DESCRIPTION OF THE DRAWINGS Figure 1 is a block diagram showing an RF section, an IF section and an AGC section of a receiver according to the prior art. Figure 2 is a block diagram showing RF sections, an IF section and an AGC section of a receiver according to aspects of the present invention. Figure 3 shows a modification of the AGC arrangement of Figure 2 according to aspects of the present invention.

DETAILED DESCRIPTION OF THE PREFERRED MODALITY HARDWARE (MACHINERY AND EQUIPMENT) Figure 1 shows an RF, IF and AGC circuit arrangement according to the prior art. A signal source is coupled to an input 10 and filtered through the input filter 12. The signal of the input filter 12 is coupled to the amplifier 14, the gain of which is controllable by AGC. The amplified signal of the amplifier 14 is coupled to an interstage filter 16 and to a mixer 18, where it is mixed with the signal generated by the local oscillator 20 to produce the IF signal on the cable 21. The IF signal is processed and amplified by section IF 22, which includes a gain controllable AGC amplifier and a video detector. A video output signal detected on the cable 23 is coupled to the AGC generator 24 to provide a responsible AGC control signal. A version of the AGC control signal is coupled to the IF section on the cable 25 to adjust the gain of the IF section to maintain the video signal on the cables 23 at a reasonably constant level for variations of the source signal level in the RF input terminal 10. If the signal level at terminal 10 is too high, a delayed AGC control signal is coupled via cable 26 to the RF AGC gain controllable amplifier 14. This delayed AGC control signal is derived from the IF AGC control signal, except that it is applied when a higher threshold signal level is reached. The delay is provided so that the maximum gain in the amplifier 14 is maintained whenever it is possible to obtain a good SNR. Figure 2 shows a current arrangement, according to the aspects of the present invention, wherein common members to Figure 1 are given similar designations. A second AGC controllable amplifier 28 is included in the RF section before the mixer. The amplifier 28 is separated from the first controllable amplifier AGC 14 by, in the present embodiment, a interstage filter controllable without AGC 16, or by a controllable amplifier without AGC (not shown). In this arrangement, the output signal of the amplifier 14 is optimized for the filter 16 and the output signal of the second controllable gain amplifier AGC 28 is optimized for the mixer 18. An AGC controller 30 has input signals from the cable 21, which it couples the output signal IF converted directly from the mixer 18, the video signal detected in the cable 23, and an indication of the BER of the digital decoder (not shown). In response to these input signals, the AGC controller 30 provides individually adjustable control signals 1, 2 and 3 AGC, which are coupled to respective gain control amplifiers 1, 2, and 3 AGC 14, 28 and 22. The controller AGC 30 may include a microprocessor, which analyzes the characteristics of its various input signals, as discussed above, and adjusts each of the AGC control signals to provide optimum tuning characteristics according to the conditions programmed in EPROM (not shown) ).

Figure 3 shows a modification of Figure 2, wherein a modified control signal AGC is coupled from the AGC controller 30 to the AGC 1 amplifier 14 in addition to the AGC control signal 1. A modified AGC control signal is generated by the AGC controller and coupled to the AGC 1 amplifier. This AGC modified control signal: 1) can either modify the operation of the AGC 1 amplifier response to the AGC control signal 1, or 2) change the gain of the AGC 1 amplifier in a different way than the AGC 1 control signal. Furthermore, if it is guaranteed, as a third alternative, the AGC 1 stage can be diverted if the input signal is very strong, for example, by actuating a switch, for example, a diode, in response to the modified AGC signal. . The modifications (1) and (2) of the AGC operation in the first AGC amplifier stage could be driven by the microcontroller only when a weak signal is received and a further improvement of the S N R is desired. The third modification can be initiated when the output signal is so strong that the first AGC amplifier can be overloaded. The digital decoders suitable for providing the BER signal or the equivalent, that is, a signal, which is indicative of the BER, are a STEL-2030B decoder available from the Stanford Telecommunication Company of Sunnyvale, California, E. U .A. , or an A HA 4210 decoder available from Advanced Hardware Architecture of Pullman, Washington, E. U .A. Said signal indicative of the BER may be a signal indicative of the amount of error correction being performed or a symbol error signal derived from a Viterbi decoder.

OPERATION It has been found that, in accordance with the aspects of the present invention, the provision of individually adjustable AGC control signals for each of a plurality of AGC adjustable amplifier stages before the mixer, improves the operation of the tuner. For example, a signal reduction of 3 dB after a first tuner amplifier reduces second order distortions in the output signal of the mixer by 6 dB and third order distortions by 9 dB, with a small degradation of the SNR. By distributing the AGC responsible gain reduction over two stages AGC in the tuner before the mixer and an AGC section in the IF section after the mixer, the distortions and the BER of the system can be optimized. The peak signal level of the mixer provides a reasonable criterion for distortions of the mixer, and that level can be measured in the IF output signal not filtered in the mixer. In addition, as in conventional systems, the level of the output signal in the video detector is also important. The magnitudes of these two signals, ie the unfiltered IF output signal in the mixer and the output signal in the video detector, provide information regarding the nature of the received signal. For example, the selection of a channel with a low level signal, but with strong adjacent channel signals, produces a low level output signal in the video detector and a high level output signal in the mixer. On the one hand, a weak selected signal without adjacent channel signals causes both levels to be low. From the measured signal levels, an intelligent controller can control and adjust the three AGC control signals to substantially reduce the distortion. For example, if attenuation is reduced at the second AGC in the tuner, but the attenuation of the third AGC in the IF is increased to maintain the same signal level, the SNR could be improved, but more distortion occurs between modulations. Once such operations are achieved, either the BER or a signal indicative of similar error, both of which are generally readily available from the digital decoder (not shown), can be used as the final criterion for fine tuning the stages of AGC, that is, to verify if the operation is beneficial or not. For example, if the BER or other signal indicative of error is above the threshold, the microprocessor controller modifies the three AGC control signal voltages until the BER becomes minimal or falls below a threshold value. It should be noted that this provision is also applicable to normal analogue received signals. The effect on the change of SNR for normal analog reception may be of little importance. However, even a slight reduction in the SNR can have a damaging effect for digital signals received at or near a threshold level. For said situation, the value of the control signal AGC coupled to the first stage AGC can be chosen so that the gain of the AGC stage is not reduced.

Claims (8)

1 .- A signal processing system, comprising: first AGC means for adjusting the amplitude of a first signal in response to a first AGC control signal and providing a first adjusted output signal; first signal processing means for receiving the first adjusted output signal of the first AGC means and providing a first output signal; second AGC means for adjusting the amplitude of the first output signal in response to a second AGC control signal and providing a second adjusted signal; mixing means for changing the frequency of the second adjusted output signal and providing a signal I F; signal processing means I F and means AGC I F for respectively processing the IF signal and adjusting the amplitude of the signal I F in response to a third AGC control signal, and providing a third output signal; means for generating said first, second and third AGC control signals in response to an output of the mixer.

2. The signal processing system according to claim 1, wherein the means for generating include a microprocessor, which also responds to a second characteristic of the third adjusted output signal.

3. The signal processing system according to claim 2, wherein the second feature is a bit error regime.

4. A receiver having input signal processing means, a mixer for providing an IF signal, and IF signal processing means, wherein the input signal processing means includes two AGC means separated by a processor of IF. signal, each of the AGC means adjusting the gain of the input signal processing means in response to an output characteristic of the IF signal processing means.

5. The receiver according to claim 4, wherein the signal processor comprises one of one of the amplifying means and filter means.

6. A receiver having input RF signal processing means, a mixer for providing an IF signal of an RF signal, and an IF signal processor, wherein the RF signal processor includes at least two AGC means, each of the AGC means being controlled by an individual control signal provided by a microcontroller that responds to a characteristic of an I F signal processor output.

7. - A receiver having input RF signal processing media. , a mixer for providing an IF signal, and an IF signal processor, wherein the RF signal processor includes a plurality of AGC means separated from each other, by one of the amplifying means and filter means, each of the AGC means being controlled by an individual control signal provided by a microcontroller.

8. A receiver having means of RF signal processing, a mixer, and RF signal processing means, wherein the RF signal processing means includes more than one AGC means, which are separated from each other, but which are controlled by an individual control signal provided through common control means.