Automatic gain control circuit

An automatic gain control (AGC) circuit based on a cascade of a step automatic gain control unit and a second automatic gain control unit connected to adjust their gain in a synchronized manner to result in a combined impulsive gain variation. Hereby it is possible, e.g. to use a step automatic gain control unit instead of a continuous automatic gain control unit in an RF part of a TV tuner without resulting in visual artifacts—this applies both for digital and analog TV tuners. According to a preferred embodiment, the synchronization between the two automatic gain control units may be achieved by the first automatic gain control unit generating a control signal upon gain adjustment, this control signal serving to speed up a loop bandwidth controlling gain change of the second automatic gain control unit. In another embodiment, the control signal generated by the first automatic gain control unit is used to control a gain step of the second automatic gain control unit. In addition, the invention provides a tuner with an automatic gain control unit as mentioned, and a device, e.g. a TV or a hard disc recorder with such tuner.

FIELD OF THE INVENTION

The present invention relates to the field of automatic gain controls.

BACKGROUND OF THE INVENTION

In wide band receivers for analog or digital TV, usually continuous Automatic Gain Controls (AGCs) are used in the RF receiver circuit to adapt to the level of the input signal. Continuous AGCs are used since use of step AGCs with a step size of e.g. 1 dB or larger will cause severe impairments to the image. In an analog TV an annoying glitch in the image will occur in case gain in the RF receiver circuit is adjusted in steps, whereas in a digital TV such stepwise adjustment of gain can in some cases result in a complete loss of the image when a gain step higher than 0.5 dB is used. Moreover, for multi-tuner applications a step of gain in one tuner can cause impairment in the image received by the second tuner. Thus, in multi-tuner applications a low noise amplifier with a fixed gain is normally used to provide a splitter function before the signal is delivered to the two or more tuners.

From a design point of view it is preferred for a number of reasons to implement a step AGC, since it is easier to meet the requirements in terms of low power consumption, low noise, together with a high linearity and gain with a step AGC than with a continuous AGC. In general, a step AGC allows a simpler design and a receiver with improved performance with respect to a number of parameters can be obtained.

WO 2004/086757 describes a TV tuner for receiving analog TV signals. The TV tuner includes a step AGC amplifier1that is adjusted in steps of 0.1, 0.5 or 1 dB only during a vertical synchronization interval in order not to generate severe visual artifacts in the image. In order to obtain this, the step AGC is adjusted based on a vertical synchronization signal v-sync provided by a separate circuit, namely an Intermediate Frequency (IF) demodulation circuit.

SUMMARY OF THE INVENTION

It is an object of the invention to provide an automatic gain control circuit, such as for use in a TV tuner, where it is possible to use a step AGC while still providing images without severe visual artifacts. The invention is defined by the independent claims. The dependent claims define advantageous embodiments.

A first aspect of the invention provides an automatic gain control (AGC) circuit comprising a cascade of a first AGC arranged to adjust its gain in steps, and a second AGC, wherein the first and second AGCs are connected so as to adjust their gains in a synchronized manner.

It is appreciated that by “cascade” is understood that the first and second AGCs can either be connected to directly follow each other, i.e. an output of the first AGC is directly connected to an input of the second AGC, or the first and second AGC can be connected in cascaded by indirectly following each other, i.e. another circuit may be interconnected between an output of the first AGC and an input of the second AGC.

The AGC circuit according to the first aspect is advantageous for example for incorporating into an RF part of a tuner, such as a TV tuner. By providing an AGC with a step AGC in cascade with a second AGC and synchronizing their gain adjustments, it is possible to obtain a combined gain variation that complies with the Digital Video Broadcasting-Terrestrial (DVB-T) requirements to acceptable input noise pulses. Thus, with the AGC circuit of the first aspect it is possible to implement a TV tuner using a step AGC with a gain step of more than 1 dB that still allows the tuner to receive digital TV signals without image artifacts. The AGC circuit can also be used for analog TV tuners where gain adjustment will only result in minor artifacts in the image.

Preferably, the first AGC is arranged to generate a control signal to the second AGC upon adjusting its gain. The second AGC is preferably arranged to adjust its gain in response to the control signal. In one embodiment the second AGC is arranged to adjust its gain in response to the control signal to counter act the adjustment of the first AGC. In another embodiment, the second AGC is arranged to speed up a rate of gain adjustment in response to the control signal.

Preferably, the first and second AGCs are synchronized such that, upon gain adjustment, a combined gain of the first and second AGCs exhibits an impulse variation. Preferably, the combined gain impulse variation complies with the DVB-T requirements with respect to allowable impulsive noise without picture failure. Thus, in such embodiment, the AGC circuit can be used for digital TV tuners without artifacts of the image.

In preferred embodiments, both the first and second AGCs are step type AGCs since the step AGCs have a number of advantages. However, the second AGC may be alternatively be arranged to adjust its gain continuously which can still be advantageous since the use in connection with the first AGC of a step type relaxes the requirements to the second AGC even though the second AGC is a continuous type AGC.

The second AGC can be implemented using a dual gate MOSFET, especially a dual gate MOSFET can be used to implement the second AGC of a continuous type.

In a second aspect, the invention provides a Radio Frequency (RF) tuner comprising at its input an automatic gain control circuit according to the first aspect. In some embodiments, at least a tracking filter is connected between the first and second AGCs. Even more parts of the RF tuner circuit, such as a mixer, may be connected between the first and second AGCs. In case the second AGC is connected after a mixer, it is preferred that it is a continuous gain AGC type.

For a multi-tuner application, the first step AGC may be connected at an input of a first tuner, whereas the second AGC is connected in relation to a second tuner. Thus, by splitting the two AGCs between the two tuners, it is possible to allow use of step AGCs in multi-tuner arrangements. Since the second tuner reacts upon gain change of the AGC in the first tuner, the first tuner can be seen as a master tuner for the second tuner. In general, this principle can be extended for multi tuners where on tuner acts as master for two or more slave tuners.

In a third aspect the invention provides a device comprising an RF tuner according to the second aspect. Especially, the device may be a TV set, either a digital TV set or an analog TV set. In case of an analog TV set, the automatic gain control circuit may be connected to synchronize its gain adjustment according to a vertical scan synchronization signal of the TV set, e.g. as disclosed in WO 2004/086757. Hereby it is possible to use the AGC circuit for receipt of analog TV signal without any visual artifacts in connection with gain adjustment. The device may in principle be any kind of device with an RF tuner, e.g. a hard disc recorder, a media center etc.

In a fourth aspect, the invention provides an automatic gain control method including adjusting gain of a first AGC in steps, and adjusting gain of a second AGC connected in cascade with the first AGC, wherein the adjustments of gain of the first and second AGC are performed in a synchronized manner.

It is appreciated that advantages and embodiments mentioned for the first aspect also apply for the second, third and fourth aspects of the invention. Thus, any one aspect of the present invention may each be combined with any of the other aspects.

DETAILED DESCRIPTION OF EMBODIMENTS

FIG. 1shows a first AGC circuit embodiment according to the invention for use e.g. in an RF input part of a TV tuner. A first step AGC1is connected in cascade with a second AGC2. The second AGC2may be either a continuous or a step type AGC with a small step size, e.g. of 0.5 dB or smaller. In the shown embodiment, the first and second AGCs1,2are directly connected in cascade via connection3. In addition, the first AGC1delivers a control signal4to the second AGC2when the first AGC1changes its gain, thus the control signal4allows the two AGCs1,2to synchronize their gain adjustments. The second AGC changes its gain slowly (i.e. it has a gain loop bandwidth in the range from a few Hertz to a few hundred Hertz) to cope with the field condition like disturbance of amplitude modulated unwanted signals that can disturb the wanted signal. However, by receiving the control signal4, the loop bandwidth of the second AGC is increased, and thus the rate of gain change of the second AGC is speeded up in order to generate a fast counter gain step. The combined action of the first and second AGC will provide the desired gain response as shown by means of an example in the graphs ofFIGS. 2a-2d.

FIG. 2ashows a gain of the first AGC1as a function of time. As seen, the gain is adjusted one step upwards at a certain time.FIG. 2bshows the control or “speed up” signal4, generated by the first AGC1, as a function of time. As seen, this control signal4is “high” for a short period in connection with the gain adjustment of the first AGC.FIG. 2cindicates a gain variation of the second AGC as a function of time. As seen, the second AGC responds to the control signal4being “high” by decreasing the its gain. The result of the combined gain of the two AGCs1,2is shown versus time inFIG. 2d. As seen, the increased gain of the first AGC1is counter acted by the decreased gain of the second AGC, and the result is an impulse variation in combined gain of the two AGCs and thereby by the total AGC circuit.

FIG. 3illustrates another AGC circuit embodiment. Again, a step AGC11is connected in cascade with a second AGC12that can be a continuous AGC or a step AGC with a small gain step (smaller than 0.5 dB steps). In this embodiment the two AGCs11,12are synchronized by a control signal13generated by the step AGC11and received by the second AGC12along with a change of gain of the step AGC11. The second AGC12generates in response to the control signal13a counter gain step so as to counter act the gain step of the step AGC11.

FIGS. 4a-4dillustrate an example of the gain variations and control signal13of the embodiment ofFIG. 3.FIG. 4aillustrates a one step gain increase at a certain time for the step AGC11. The control signal13generated by the step AGC11along with its gain variation is shown inFIG. 4b. The gain variation of the second AGC12in response to the control signal13is seen inFIG. 4c. As seen, the gain of the second AGC12is adjusted in one step in order to counter act the step of the step AGC11.FIG. 4dillustrates the combined gain variation of the AGC circuit ofFIG. 3. As seen, due to the second AGC12responding with its counter gain step delayed a short time compared to the initial gain step of the step AGC11, the combined gain variation of the two AGCs11,12is an impulse.

Considering the behavior of a Digital Video Broadcasting-Terrestrial (DVB-T) channel decoder, such channel decoder has to satisfy some requirements about impulsive noise (specified in D-book). It is specified that no picture failure is allowed to occur in case the channel decoder received impulsive noise where the duration and level of the impulse is below a specified range. For example a level of 15 dB is allowed for an impulse duration of 0.3 micro seconds, a level of 10 dB is allowed for an impulse duration of 1 micro second, and level of 5 dB is allowed in case the impulse duration is 2 micro second. The AGC circuit embodiments of the present invention exploit this behavior of the DVB-T decoders, since an AGC circuit with an impulse gain variation behavior as illustrated inFIGS. 2dand4dsatisfies the conditions of no picture failure in case it is used in an RF chain of a tuner for the reception of the digital TV, as long as the mentioned DVB-T conditions for the impulsive gain variation are met. These conditions can easily be achieved in practical embodiments. Thus, step AGCs can be used for an RF input part of a TV tuner for digital TV reception.

The AGC circuit embodiments can also be used for analog TV tuners. Here, the impulsive gain behavior will cause a luminance variation that is far less annoying than a glitch in the image, which will be the result of using a step AGC with a large step size in an RF part of a TV tuner for analog TV reception.

FIG. 5illustrates a possible implementation of a continuous AGC element that can be used as the second AGC for any of the AGC circuit embodiments ofFIGS. 1 and 3. A current source25is connected to feed a MOSFET21with a dual gate22,23. The NMOST connected to the first gate22is used to change the drain voltage of the NMOS connected to23. Then changing the drain voltage the current and the gm of the NMOST connected to the second gate23is changed. The resistance R of the variable resistor24determines together with the gm of the NMOST the voltage gain A of the entire circuit as A=R·gm. Thus, the voltage gain A can be adjusted by either changing the resistance R of the variable resistor24or by changing gm of the NMOST. Therefore, gate22is used as control signal input whereas gate23is used as AGC input.

FIG. 6illustrates an RF tuner embodiment30according to the invention where an AGC circuit32,34according to the invention is used. An RF signal from an antenna31is received by a step AGC32. This step AGC is connected in cascade with a second AGC34, not directly since another tuner circuit part33, including e.g. a tracking filter and a mixer, is connected between the two AGCs32,34. A control signal36serves to synchronize the two AGCs32,34as described in connection with the embodiments ofFIGS. 1 and 3. Possible further tuner circuits35, e.g. an Intermediate Frequency (IF) filter, follow the second AGC34. The tuner30can serve as a TV tuner, e.g. for integration into a TV set, and a hard disc recorder etc.

FIG. 7illustrates a two-tuner embodiment40. An RF signal from an antenna41is received by a step AGC42. Output from the step AGC42is applied to further circuit43of a first tuner. In addition, the output of the step AGC42is applied to an input of a second AGC44whose output is connected to further circuit45of a second tuner. A control signal46serves to synchronize the two AGCs42,44as described above. Thus, in order to avoid impairment of the image of the two-tuner embodiment40, the AGC circuit is split between the two separate tuners so that a first tuner, the one with the step AGC42, serves as a master while a second tuner, the one with the second AGC44, serves a slave, i.e. the AGC44in the second tuner reacts to a gain step made by the step AGC42in the first tuner. As explained for one-tuner arrangements, the AGC44in the second tuner can in principle be connected anywhere in the second tuner circuit, i.e. it needs not to be inserted at an input of the second tuner.

It is appreciated that the same principle applies in general for multi-tuner arrangements, i.e. for three, four, five, six or more tuners coupled in a master-slave configuration. The speed up signal4or gain control signal13described for the embodiments ofFIG. 1or3must be provided to the slave tuners to avoid impairments of the resulting image.

FIG. 8illustrates an analog TV set50embodiment. An RF signal from antenna51is received by an RF tuner52, e.g. the one illustrated inFIG. 6. Output of the tuner is applied to a display unit53of the TV set50. A vertical synchronization signal54from the display unit53is applied to the tuner52in order to allow the AGC system of the tuner52to adjust gain only during a vertical synchronization interval, thereby eliminating a possible luminance variation of the produced image upon gain adjustment.

In conclusion, the invention provides an AGC circuit based on a cascade of step AGC and a second AGC connected to adjust their gain in a synchronized manner to result in a combined impulsive gain variation. Hereby it is possible, e.g. to use a step AGC instead of a continuous AGC in an RF part of a TV tuner without resulting in visual artifacts—this applies both for digital and analog TV tuners. According to a preferred embodiment, the synchronization between the two AGCs may be achieved by the first AGC generating a control signal upon gain adjustment, this control signal serving to speed up a loop bandwidth controlling gain change of the second AGC. In another embodiment, the control signal generated by the first AGC is used to control a gain step of the second AGC. In addition, the invention provides a tuner with an AGC circuit as mentioned, and a device, e.g. a TV or a hard disc recorder with such tuner.

Although the present invention has been described in connection with the specified embodiments, it is not intended to be limited to the specific form set forth herein. Rather, the scope of the present invention is limited only by the accompanying claims. In the claims, the term “comprising” does not exclude the presence of other elements or steps. Additionally, although individual features may be included in different claims, these may possibly be advantageously combined, and the inclusion in different claims does not imply that a combination of features is not feasible and/or advantageous. In addition, singular references do not exclude a plurality. Thus, references to “a”, “an”, “first”, “second” etc. do not preclude a plurality. Furthermore, reference signs in the claims shall not be construed as limiting the scope.