Classifications

• • H—ELECTRICITY
  • H03—ELECTRONIC CIRCUITRY
  • H03G—CONTROL OF AMPLIFICATION
  • H03G3/00—Gain control in amplifiers or frequency changers
  • H03G3/20—Automatic control
  • H03G3/30—Automatic control in amplifiers having semiconductor devices
  • H03G3/3052—Automatic control in amplifiers having semiconductor devices in bandpass amplifiers (H.F. or I.F.) or in frequency-changers used in a (super)heterodyne receiver
  • H03G3/3068—Circuits generating control signals for both R.F. and I.F. stages
• • H—ELECTRICITY
  • H04—ELECTRIC COMMUNICATION TECHNIQUE
  • H04N—PICTORIAL COMMUNICATION, e.g. TELEVISION
  • H04N5/00—Details of television systems
  • H04N5/44—Receiver circuitry for the reception of television signals according to analogue transmission standards
  • H04N5/52—Automatic gain control
• • H—ELECTRICITY
  • H03—ELECTRONIC CIRCUITRY
  • H03G—CONTROL OF AMPLIFICATION
  • H03G1/00—Details of arrangements for controlling amplification
  • H03G1/0005—Circuits characterised by the type of controlling devices operated by a controlling current or voltage signal
  • H03G1/0088—Circuits characterised by the type of controlling devices operated by a controlling current or voltage signal using discontinuously variable devices, e.g. switch-operated

Definitions

• 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.
• 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 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.
• TOP take over point
• 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.
• 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.
• the input signal may be substantially stronger than 60 dB ⁇ V, for instance 66 dB ⁇ V.
• 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 dB ⁇ V, 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".
• 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.
• 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 dB ⁇ V input level and a 6 dB attenuator the AGC loop will react as if the input level would be 60 dB ⁇ V, 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.
• 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.
• 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 dB ⁇ V 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 dB ⁇ V. It is remarked that a continuous IF AGC in combination with RF AGC is known in the art, for instance from U.S. Patent
• 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.
• 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.
• 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.
• IF intermediate frequency
• 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 is applied to an IF unit 9 where further IF amplification and IF demodulation takes place.
• the IF unit 9 generates an automatic gain control (AGC) voltage V agc for controlling the gain of the preamplifier 3.
• AGC automatic gain control
• the point where the AGC voltage starts reducing the gain of the preamplifier is the "take over point" (TOP).
• TOP take over point
• a delay device e.g. a single diode
• 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 V ⁇ and the amplifier 11 can be switched “on” and “off by means of a comparator C 2 that compares the AGC-voltage with a predetermined reference voltage V 2 .
• the combination provides a reference gain that is not necessarily equal to 0 dB.
• 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.
• 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 V 0 at the output of the SAW filter 8 in dB ⁇ V as a function of the antenna input level V, also in dB ⁇ V.
• the dashed line in figure 2 represents this function without the attenuator 10 and the amplifier 11.
• the amplifier 11 is "on” and the sensitivity of the arrangement is increased by e.g. 10 dB.
• the level increases switches off at an input level L 2 of e.g. 50 dB ⁇ V.
• the level passes the "take over point" that is situated at an input level T of e.g. 63 dB ⁇ V.
• the attenuator 10 is switched on so that from this level on the signal to noise ratio of the arrangement is increased.
• FIG 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, R 2 and bypass capacitor C ⁇ , an inductance Li in the collector lead and a feedback resistor R in series with a capacitor C 3 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 C 2 to the base electrode of the transistor Ti and the amplified signals from the collector electrode are applied through a capacitor C 4 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 T 2 that is controlled by the comparator C 2 .
• the collector electrode of this switching transistor is connected through a collector resistor R 4 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 C 5 , a resistor R ⁇ , a diode Di, a second diode D 2 , a resistor R 7 and a capacitor C O .
• a resistor Rs couples the interconnected anodes of the diodes Di and D 2 to the positive supply voltage.
• the collector electrode of a switching transistor T 3 is connected through a resistor R 9 to the interconnection of C 5 and R « and through a resistor Rio to the interconnection of C O and R 7 .
• the comparator Ci controls the switching transistor T 3 .
• the values of the passive components of the arrangement of figure 3 are as follows:
• capacitors C 2 to C ⁇ have only a DC-separation function and that the resistors R 4 , R 9 and Rio have only a DC-supplying function.
• 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 T 2 and T 3 are cut off.
• the diodes Di and D 2 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.
• the voltage V agc passes the predetermined voltage V 2 so that the output of the comparator C becomes high, the switching transistor T 2 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 C 2 , the feedback path C 3 , R 3 and the capacitor C to the output terminal B.
• resistor R 3 determines the reference gain, i.e. the gain between the terminals A and B when both the amplifier and the attenuator are off.
• the input signal passes the level Li of figure 2
• the voltage Va g c passes the predetermined voltage Vi and the output of the comparator becomes high, with the result that the switch transistor T 3 and the attenuator diodes Di and D become conducting.
• 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.

Landscapes

• Engineering & Computer Science (AREA)
• Multimedia (AREA)
• Signal Processing (AREA)
• Control Of Amplification And Gain Control (AREA)
• Circuits Of Receivers In General (AREA)

Priority Applications (2)

Applications Claiming Priority (2)

Publications (1)

Family

ID=33485268

Family Applications (1)

Country Status (5)

Cited By (1)

Families Citing this family (1)

Citations (3)

Family Cites Families (4)

• 2004
  • 2004-05-24 JP JP2006530891A patent/JP2007500989A/ja not_active Withdrawn
  • 2004-05-24 WO PCT/IB2004/050758 patent/WO2004107574A1/en not_active Application Discontinuation
  • 2004-05-24 EP EP04734584A patent/EP1632027A1/en not_active Withdrawn
  • 2004-05-24 CN CNB2004800147532A patent/CN100546184C/zh not_active Expired - Fee Related
  • 2004-05-24 KR KR1020057022395A patent/KR20060013554A/ko not_active Application Discontinuation

Patent Citations (3)

Non-Patent Citations (1)

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