EP1632027A1

EP1632027A1 - Channel selector with automatic gain control

Info

Publication number: EP1632027A1
Application number: EP04734584A
Authority: EP
Inventors: Kam C. c/o Philips I.P. & StandardsGmbh Kwong
Original assignee: Koninklijke Philips Electronics NV
Current assignee: NXP BV
Priority date: 2003-05-27
Filing date: 2004-05-24
Publication date: 2006-03-08
Also published as: CN100546184C; JP2007500989A; WO2004107574A1; KR20060013554A; CN1795612A

Abstract

In a channel selector with and RF preamplifier and a mixer for converting the RF input signals to intermediate frequency output signals, an automatic gain control (AGC) voltage is derived from the IF output signals for reducing the gain of the RF preamplifier after a predetermined “take over point” (delayed AGC). In the IF output path of the mixer a switched attenuator is provided that is switched “on” and “off” under the control of the automatic gain voltage. The attenuator is “on” at levels above the “take over point”.

Description

CHANNEL SELECTOR WITH AUTOMATIC GAIN CONTROL

The invention relates to a channel selector comprising in cascade a gain- controlled RF preamplifier for amplifying received RF channel signals, a mixing stage for converting the amplified RF channel signals to IF channel signals, an IF filter connected to the output of the mixing stage for selecting a desired channel signal from said IF channel signals and means connected to the output of the IF filter for deriving from the selected IF channel signal an AGC voltage that reduces the gain of the RF preamplifier in a high level mode, when the level of said selected IF channel signal is above a predetermined level, and that keeps the gain of the RF preamplifier substantially constant in a low level mode, when the level of said selected IF channel signal is below said predetermined level.

Such channel selector is well known in the art and serves to derive from a plurality of received RF channel signals one desired IF channel signal with the best possible signal to noise (S/N) ratio and without substantial non-linear distortion. Present day television tuners may for instance be designed to operate optimally with an input signal of 60 dBμV (= 1 mV) at the antenna input and a gain of 45 dB, so that the IF output is 105 dBμV. The antenna input signal may have a S/N-ratio of 51 ,3 dB and the tuning arrangement itself may have a noise figure of 4 dB, so that the S/N-ratio at IF is 47,3 dB.

The automatic gain control (AGC) is a so-called delayed AGC with a low level mode in which the input signal level and consequently also the IF output level is so low that a gain reduction of the RF preamplifier is not appropriate. When the IF output level surpasses a "take over point" (TOP) the automatic gain control enters the high level mode wherein the gain of the RF preamplifier is reduced so as to maintain the IF output level substantially constant. Usually a channel selector has to be used for the reception of off-the-air signals as well as for the reception of signals from cable systems. Especially when the cable is the main signal input, the input signal may be substantially stronger than 60 dBμV, for instance 66 dBμV. To protect later stages from strong signals with non linear distortion and radiation effects, the automatic gain control (AGC) of the RF preamplifier reduces the gain of this stage as soon as the level of the IF output signal is stronger than e.g.105 dBμV, the "take over point" (TOP). A drawback however is that every dB gain reduction of the RF preamplifier increases the noise factor of the tuning arrangement by one dB, so that the S/N-ratio at stronger input signals remains at the S/N-ratio at the "take over point". In the above given example: when the input signal is increased to 66 dBμV, the gain is reduced to 39 dB and the noise figure of the tuning arrangement is increased to 10 dB, so that the S/N-ratio at IF remains 47,3 dB.

Nowadays there is a strong demand in the market of television tuners to have a higher S/N-ratio at stronger antenna input signals. It is the primary object of the present invention to provide a tuning arrangement with such higher S/N-ratio at stronger antenna input signals and the tuning arrangement according to the present invention is therefore characterized by a switched attenuator between the output of the mixer and the input of the IF filter, said switched attenuator being controlled by the AGC voltage so that it is operative in at least part of said high level mode and non operative in at least part of said low level mode. By switching the IF attenuator into operation at or above the "take over point", the automatic gain control seeks to compensate for the loss of gain in the IF path by an increase in gain of the RF preamplifier. The end-result is that the IF output signal is at the original level but with improved signal to noise ratio, because the addition of noise by the preamplifier is reduced while the attenuator itself does not substantially add noise. For instance, at 66 dBμV input level and a 6 dB attenuator the AGC loop will react as if the input level would be 60 dBμV, with the result that the noise figure of the preamplifier is 6 dB less. It will be apparent that the extent of the added attenuation is limited by the capability of the mixer and of the IF-path in front of the attenuator to handle larger signals without distortion problems. The channel selector according to the present invention may further be characterized by a switched amplifier connected in parallel with said switched attenuator, said switched amplifier being controlled by the AGC voltage so that it is operative in a part of said low level mode in which the switched attenuator is not operative. While the above-described switched attenuator improves the dynamic range of the channel selector at the higher signal levels, the switched amplifier improves the dynamic range at the low signal levels. The switched amplifier does not improve the signal to noise ratio because the AGC is not active in this range and thus the IF signal and the accompanying noise are equally amplified by the switched amplifier. However, the switched amplifier does increase the lower IF signal levels and therewith the sensitivity of the channel selector, i.e. the smallest signal which gives a useful picture. The amplifier should be switched off before the high end of the low level mode is reached, so as to prevent that the AGC becomes operative with the extra noise then added. For instance, when the input level of the "take over point" lies at 60 dBμV and the switched amplifier has a gain of 10 dB, the amplifier should be switched off at an input level at or lower than 50 dBμV. It is remarked that a continuous IF AGC in combination with RF AGC is known in the art, for instance from U.S. Patent

Application Publication US 2001/0016950 Al. Such double AGC is useful in receivers with a digital demodulator to obtain a high and constant IF level over a large range of input levels.

Preferably the channel selector according to the invention is further characterized in that the switched attenuator and the switched amplifier are each switched by a comparator that compares the said AGC voltage with a predetermined DC voltage.

The invention will now be described with reference to the accompanying figures. Herein shows: Figure 1 a block diagram of a channel selector according to the invention,

Figure 2 a graph for explaining the operation of the channel selector of figure 1 and

Figure 3 a detailed schematic diagram of a switched attenuator and switched amplifier for use in the channel selector of figure 1.

The channel selector of figure 1 comprises a first tuned filter 2 whose input may be connected to an antenna 1 and whose output is connected to the input of an RF preamplifier 3. The RF channel signals amplified by the preamplifier 3 are fed through a second tuned filter 4 to a mixer 5 where the RF signals are multiplied with an oscillation from a tuned local oscillator 6. The mixer converts the RF signals to intermediate frequency (IF) signals that are subsequently amplified in an IF amplifier 7 and supplied to a surface acoustic wave filter 8 where the channel filtering takes place.

The arrangement described so far is well known in the art. The tuned filters 1 and 4 do substantially not attribute to the channel filtering but serve primarily to suppress the image frequencies that would otherwise fold into the desired IF channel in the mixer 5. The desired IF channel signal, with sufficiently suppressed neighbouring channel signals in the SAW filter 8, is applied to an IF unit 9 where further IF amplification and IF demodulation takes place. Moreover, the IF unit 9 generates an automatic gain control (AGC) voltage V_agc for controlling the gain of the preamplifier 3. This is a delayed automatic gain control: at low level input signals (low level mode) the AGC voltage does substantially not reduce the gain of the preamplifier 3 whereas at higher input levels (high level mode) the AGC voltage reduces the gain of the preamplifier so as to maintain the IF output signal of the SAW-filter substantially constant. The point where the AGC voltage starts reducing the gain of the preamplifier is the "take over point" (TOP). In such delayed AGC a delay device (e.g. a single diode) prevents transfer of AGC voltage to the preamplifier in the low level mode and enables such transfer in the high level mode. It is also possible to construct the preamplifier 3 so that its gain is substantially not reduced by the AGC-voltages generated in the low level mode.

In accordance with the invention, the channel selector of figure 1 comprises a switched attenuator 10 and a switched amplifier 1 1 between the output A of the IF amplifier 7 and the input B of the SAW filter 8. The attenuator 10 can be switched "on" and "off by means of a comparator Ci that compares the AGC-voltage V_a c with a predetermined reference voltage Vι and the amplifier 11 can be switched "on" and "off by means of a comparator C₂ that compares the AGC-voltage with a predetermined reference voltage V₂. When both the attenuator 10 and the amplifier 11 are "off, the combination provides a reference gain that is not necessarily equal to 0 dB. When the switched amplifier is "on" its gain is increased by e.g. 10 dB and when the switched attenuator is "on" its gain is decreased by e.g. 6 dB. Of course the amplifier and the attenuator should not be "on" simultaneously.

The operation of the switched attenuator and the switched amplifier is illustrated in the diagram of figure 2 that shows the IF level V₀ at the output of the SAW filter 8 in dBμV as a function of the antenna input level V, also in dBμV. The dashed line in figure 2 represents this function without the attenuator 10 and the amplifier 11. At low input levels the amplifier 11 is "on" and the sensitivity of the arrangement is increased by e.g. 10 dB. When the level increases the amplifier switches off at an input level L₂ of e.g. 50 dBμV. The level then passes the "take over point" that is situated at an input level T of e.g. 63 dBμV. At the input level Li of e.g. 65 dBμV the attenuator 10 is switched on so that from this level on the signal to noise ratio of the arrangement is increased.

If there is a possibility that under specific conditions the loop comprising the elements 8, 9, CI , C2, 10 and 11 has a tendency to oscillate, this can be simply avoided by providing the comparators CI and C2 with hysteresis or a latch.

Figure 3 shows a detailed implementation of the switched attenuator 10 and the switched amplifier 11 of figure 1. The switched amplifier comprises a bipolar npn transistor Ti with an emitter impedance of resistors Ri, R₂ and bypass capacitor Cι, an inductance Li in the collector lead and a feedback resistor R in series with a capacitor C₃ between the collector- and base-electrodes of the transistor. IF input signals from IF-amplifier 7 of figure 1 are applied, through input terminal A and a capacitor C₂ to the base electrode of the transistor Ti and the amplified signals from the collector electrode are applied through a capacitor C₄ and via output terminal B to the input of the SAW filter 8 of figure 1. The transistor Ti is switched on and off by means of a switching transistor T₂ that is controlled by the comparator C₂. The collector electrode of this switching transistor is connected through a collector resistor R₄ to the supply voltage and through a series resistor R5 to the base electrode of the amplifier transistor T,. The switched attenuator comprises, between the IF input terminal A and the IF output terminal B the series arrangement of a capacitor C₅, a resistor R^, a diode Di, a second diode D₂, a resistor R₇ and a capacitor C_O. A resistor Rs couples the interconnected anodes of the diodes Di and D₂ to the positive supply voltage. The collector electrode of a switching transistor T₃ is connected through a resistor R₉ to the interconnection of C₅ and R« and through a resistor Rio to the interconnection of C_O and R₇. The comparator Ci controls the switching transistor T₃. The values of the passive components of the arrangement of figure 3 are as follows:

It may be noted that the capacitors C₂ to Cβ have only a DC-separation function and that the resistors R₄, R₉ and Rio have only a DC-supplying function.

In operation, at low RF input level, the automatic gain control voltage V_agc is high with the result that both the comparators Ci and C deliver low output voltages and that consequently the switching transistors T₂ and T₃ are cut off. The diodes Di and D₂ are non-conducting so that the switched attenuator is off. The amplifier transistor Ti is conducting and the IF signal from the input A is amplified by the transistor Ti to the output B. When the input level passes the level l of figure 2, the voltage V_agc passes the predetermined voltage V₂ so that the output of the comparator C becomes high, the switching transistor T₂ becomes conducting and the transistor Ti is cut off. The IF signals from the input terminal are not any more amplified, but pass unamplified through the capacitor C₂, the feedback path C₃, R₃ and the capacitor C to the output terminal B. Therewith the value of resistor R₃ determines the reference gain, i.e. the gain between the terminals A and B when both the amplifier and the attenuator are off. Finally, when the input signal passes the level Li of figure 2, the voltage Va_gc passes the predetermined voltage Vi and the output of the comparator becomes high, with the result that the switch transistor T₃ and the attenuator diodes Di and D become conducting. Now part of the IF signal current from the input terminal A flows through resistor R₈ so that the signal transfer to the output terminal O is attenuated. The attenuator is in the "on"-state. The attenuator which is formed by the three resistors Re, R and Rg is configured so that its output impedance is maintained as close as possible to the output impedance of the switched amplifier in the "on"-state.

Claims

1. Channel selector comprising in cascade a gain-controlled RF preamplifier (3) for amplifying received RF channel signals, a mixing stage (5) for converting the amplified RF channel signals to IF channel signals, an IF filter (8) connected to the output of the mixing stage for selecting a desired channel signal from said IF channel signals and means (9) connected to the output of the IF filter for deriving from the selected IF channel signal an AGC voltage (V_agc) that reduces the gain of the RF preamplifier in a high level mode, when the level of said selected IF channel signal is above a predetermined level (T), and that keeps the gain of the RF preamplifier substantially constant in a low level mode, when the level of said selected IF channel signal is below said predetermined level (T), characterized by a switched attenuator (10) between the output (A) of the mixer and the input (B) of the IF filter, said switched attenuator being controlled by the AGC voltage so that it is operative in at least part of said high level mode and non operative in at least part of said low level mode.

2. Channel selector as claimed in claim 1, characterized by a switched amplifier (11) connected in parallel with said switched attenuator (10), said switched amplifier being controlled by the AGC voltage (V_agC) so that it is operative in a part of said low level mode in which the switched attenuator is not operative.

3. Channel selector as claimed in claim 2, characterized in that the switched attenuator (10) and the switched amplifier (11) are each switched by a comparator (Ci, C₂) that compares said AGC voltage (V_agC) with a predetermined DC voltage (Vj, V₂).