Abstract:
In a dynamic automatic loop for control of the overall gain of an input circuit of a superheterodyne receiver, the response time of the HF-AGC circuit of the TUNER, in response to the action of the TUNER DELAY circuit activated by the IF-AGC in the case of autonomously uncontrollable abrupt increases in the level of the antenna signal from the same HF-AGC of the TUNER, is markedly reduced using an additional TUNER DELAY PLUS circuit able to absorb for a determined interval of time, a discharge current from the storage capacitor the control voltage of the HF-AGC in addition to the discharge current absorbed by the existing TUNER DELAY circuit. The relevant intensity of this additional discharge current and its duration are optimized by way of suitable circuital arrangements in the design of said TUNER DELAY PLUS circuit. The response time is reduced without modifying the time constant of the HF-AGC, which cannot be freely reduced because of inter- and cross-modulation problems.

Description:
BACKGROUND OF THE INVENTION 
     1. Field of the Invention 
     The present invention relates to superheterodyne receivers and more particularly to systems for automatic gain control (AGC) used, in the amplifying and tuning (TUNER) stages and in the intermediate frequency (IF) stages of receivers to compensate for the effects of the variation in intensity of the signal picked up. 
     2. Description of the Prior Art 
     A conventional system for superheterodyne TV reception can be represented by the block diagram of FIG. 1. The system comprises a wideband amplification stage provided with a first automatic gain control circuit, a tuner circuit for preselecting the signal, a mixer stage for deriving an intermediate frequency signal from the amplified signal provided by said wideband amplification circuit and by said tuner circuit (indicated as a whole in the figure by the (TUNER+HF-AGC) block), a band filter for said intermediate frequency (indicated in the figure by the IF BAND FILTER block) and by an intermediate frequency circuit comprising at least one amplifier provided with a second automatic gain control circuit (indicated in the figure by the IF+IF-AGC STAGE block). The system is such as to maintain constant the amplitude of the output signal V OUT  on variation of the amplitude of the input signal V IN  and for this purpose is provided with a first circuit for automatic control of the gain of the tuner, namely with a HF-AGC, with a second circuit for automatic control of the gain of the intermediate frequency stages, namely with a IF-AGC, and with a dynamic automatic control loop for the overall gain of the input circuit of the receiver using a particular circuit commonly known as TUNER DELAY, as indicated in the diagram of FIG. 1. The voltage V 1  and V 3  regulate respectively the gain of the automatic circuit for control of the gain of the TUNER (HF-AGC), and of the circuit for automatic control of the gain of the intermediate frequency stage (IF-AGC), and these two DC control voltages are respectively stored on two storage capacitors, C 1  and C 2 , indicated in the diagram of FIG. 1. The TUNER DELAY circuit constitutes in practice an &#34;open collector&#34; able to absorb a fixed maximum current (e.g. of the order of 2 mA) I 1  in order to discharge the capacitor C 1  (decrease V 1 ) and reduce the gain of the TUNER which, for low levels of antenna signal, is maximum, with the purpose of improving the noise pattern of the entire reception system. By way of the variable resistor R T  the voltage level V 3  =V REF  (i.e. V REF  depending on R T ) can be chosen, on the attainment of which is activated the TUNER DELAY circuit which absorbs a current I 1   which discharges the capacitor C 1  decreasing the voltage V 1  of the node 1 by an appropriate value V 1  asymptotically towards an asymptotic discharge level determined by the product 
     
         R.sub.(1-2)eq ·I.sub.1 
    
     where ##EQU1## depending on a time constant τ 1  which is given by: ##EQU2## 
     This is graphically illustrated in the graph of FIG. 5 while the graphs of FIGS. 2, 3 and 4 show respectively the development of the gain of the TUNER as a function of the voltage V 1 , the development of the gain of the IF STAGE as a function of the voltage V 3 , and the development of the discharge current I 1  of the capacitor C 1  as a function of the voltage V 3  for various parameters V REF  which can be selected by way of the variation of the parameter R T . 
     Assuming that the voltage at the storage node 3 of the circuit for automatic control of the gain of the IF block is equal to a value V 3 , i.e. to the insertion limit of the TUNER DELAY block, if the antenna signal undergoes a large sudden increase, for example larger than 10 dB, the gain of the TUNER would not be able to undergo a corresponding instantaneous diminution as the time constant which regulates the operation of the automatic gain control loop is relatively high being, as already seen, given by: ##EQU3## where R 1  and R 2  are the two resistors which form a voltage divider for charging the capacitor C 1  to a predefined voltage for supplying the pilot transistor of the HF-AGC. 
     Conversely the intermediate frequency stage will respond to this situation of sudden increment in the level of the signal at its input, i.e. at node 5 of FIG. 1, so as nevertheless to maintain constant the output level V OUT  and this will occasion loading of an IF-AGC output storage capacitor C 2  (raising the voltage V 3 ) and simultaneously reducing the gain of the stage so as to compensate for the increase in level of the signal at its input. The response time of the IF-AGC is greatly less than the response time of the HF-AGC, but the presence of a signal with too high a level at the input node 5 of the IF STAGE creates inter-modulation and cross-modulation problems. 
     Naturally, when the increase in the level of the antenna signal is relatively slow, the problems do not exist since the HF-AGC is in itself able to contain the increase in the level of the signal at the output node 4 of the TUNER and consequently also at the node 5. 
     On the other hand, unexpected increases in the level of the antenna signal can occur suddenly and also temporarily, for example through the so-called &#34;fast fading&#34; phenomenon caused by a reflection of the arrival signal for example on the wings of an aircraft in transit. 
     It is a practical impossibility to reduce the time constant τ 1  for spurious frequency filtering reasons, which would modulate the output of the TUNER which necessitate the use of a capacitor C 1  of large capacitance. 
     OBJECT AND SUMMARY OF THE INVENTION 
     The need therefore exists to reduce the response time of the HF-AGC so as to avoid an excessive increase in the level of the signal at the input of the intermediate frequency stage consequent upon autonomously uncontrollable sudden broad increases in the level of the antenna signal of the HF-AGC and which involve the activation of the so-called TUNER DELAY circuit of the dynamic automatic gain control loop without reducing the intrinsic time constant characteristics of the HF-AGC. 
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS 
     FIG. 1 is a block diagram of a typical input circuit of a superheterodyne TV receiver according to the known art, as already described previously; 
     FIGS. 2, 3, 4 and 5 are graphs which illustrate the operation of the systems for automatic control of the gain of a receiver of FIG. 1, as already described previously; 
     FIG. 6 is a graph which illustrates the mode of operation of the dynamic automatic gain control loop which is the subject of the present invention; 
     FIG. 7 is a block diagram of an input circuit of a superheterodyne receiver &#34;equivalent&#34; to the circuit of FIG. 1 but provided with an improved dynamic automatic gain control loop produced according to the present invention; 
     FIG. 8 is a graph which shows the operation of the dynamic automatic gain control loop in two different conditions one of which determines the activation of the special circuit which is the subject of the present invention; 
     FIG. 9 is an explanatory graph of the operation of the circuit of the invention according to a determined embodiment of the latter; 
     FIG. 10 is a graph which illustrates a particular aspect of the operation of the circuit of the invention, according to a determined embodiment of the latter; 
     FIG. 11 is a circuit diagram of the special circuit of the invention according to a first embodiment; 
     FIG. 12 is a graph which shows the development of the discharge current absorbed by the special circuit of the invention; 
     FIG. 13 is a graph which shows the development of the discharge current absorbed by the normal TUNER DELAY circuit; 
     FIG. 14 is a graph which shows the development of the total discharge current absorbed jointly by the normal TUNER DELAY circuit and by the auxiliary discharge circuit of the invention, when the latter is triggered; 
     FIG. 15 is the circuit diagram of the special circuit of the invention according to a second embodiment. 
    
    
     DESCRIPTION OF THE PREFERRED EMBODIMENT 
     Basically, as illustrated graphically in the graph of FIG. 6, the system of the present invention allows the response time of the dynamic automatic gain control loop to be reduced without altering the intrinsic time constant characteristics thereof operating in such a way that the capacitor C 1  for storing the DC voltage for control of the gain of the HF-AGC of the TUNER is discharged, for a limited time, by a discharge current I&#39; 1  suitably increased with respect to the predefined normal discharge current I 1  drawn by the normal TUNER DELAY circuit of the dynamic automatic gain control loop. In other words, the voltage V 1  for control of the HF-AGC, which under conditions of maximum gain of the HF-AGC of the TUNER has the value V 1  (given by dividing the supply voltage by means of R 1  and R 2 ), diminishes in response to a front of variation of the level of the antenna signal of determined characteristics detected by the second system for automatic control of the IF-AGC gain of the intermediate frequency section of the receiver through the activation of the TUNER DELAY circuit which absorbs a discharge current I 1  from the capacitor C 1  as a function of a time constant τ 1=R .sub.(1-2)eq ·C 1  tending to reduce the value of the voltage V 1  to a first asymptotic level given by the product R.sub.(1-2)eq ·I 1  and for a first initial period of time t 2  -t 1 , this diminution in the voltage V 1  follows a law which although being determined by the same time constant τ 1  characteristic of the circuit, tends to a second asymptotic level given by the product R.sub.(1-2)eq ·I&#39; 1 , where the discharge current of the capacitor C 1 , I&#39; 1 , is equivalent to the sum of the current I 1  drawn by the TUNER DELAY circuit plus an additional discharge current I tp  which, in the period t 2  -t 1 , is drawn by a special discharge circuit operating essentially in parallel with the existing TUNER DELAY circuit. 
     The circuit of the invention is shown in the block diagram of FIG. 7. Relative to the block circuit of a normal receiver of the prior art (FIG. 1) the dynamic automatic gain control loop comprises a second discharge circuit, called TUNER DELAY PLUS, to which is applied the same reference voltage V REF  which establishes the intervention voltage level of the TUNER DELAY acting on the variable resistor R T , as already described in relation to the circuit of the prior art of FIG. 1, as well as the voltage V 3  of the node for storing the DC voltage for control of the IF-AGC circuit (across the storage capacitor C 2 ) and a certain constant current I A  which can be provided by the same automatic control circuit IF-AGC when the output of the IF-AGC circuit itself exceeds a determined threshold, this latter being a condition which, given the very high response speed of the IF-AGC system, can occur only in the presence of a sudden large increase in the antenna signal (e.g.≧3 dB) and which initiates an excursion of V 3  towards and possibly beyond the value V REF . 
     In practice the activation of the TUNER DELAY PLUS circuit which is determined by the generation of said (enabling) current I A  on the part of the IF-AGC and by a possible successive exceeding of the specified value V REF  on the part of V 3 , depends exclusively on the occurrence of a rising front of the output signal V 3  of the IF-AGC circuit and not on the value of a &#34;DC component&#34; thereof. Moreover, the TUNER DELAY PLUS circuit does not intervene if such a rapid variation in the value of the voltage V 3  (which induces the same IF-AGC circuit to liberate said enabling current I A ) occurs when the same voltage V 3  has already exceeded the preset value V REF , insertion limit of the TUNER DELAY circuit. 
     Therefore, as graphically shown in FIG. 8, the dynamic automatic gain control loop can have two operating modes with the occurrence of respective conditions of increase in the level of the antenna signal V IN . A first operating mode is that represented by the characteristic a) which is equivalent to the insertion of the TUNER DELAY circuit alone operating in entirely analogous mode to that of a circuit of the prior art whereas, on the occurrence of particular conditions of increase in the antenna signal, the operating mode of the loop becomes that represented by the characteristic b) relevant to the additional activation of the TUNER DELAY PLUS circuit of the invention besides the TUNER DELAY circuit with the evident effect of reducing the intervention delay time of the limiting action fulfilled by the HF-AGC circuit upstream of node 5 of the input circuitry of the superheterodyne receiver. 
     The time graph of FIG. 8 represents in practice a partial illustration (zone delimited by the dashed-line frame) of the time graph of FIG. 9. A scale representation of the various magnitudes in the case of an exemplary embodiment of the invention is expressly maintained in this latter time graph. The effect of the temporary drawing of an additional discharge current from the capacitor C 1  by the special TUNER DELAY PLUS discharge circuit with the purpose of rendering more rapid the fall in the control voltage V 1  (t) of the HF-AGC of the receiver, determines the development of a discharge characteristic which is characterized, for a first tract, by a &#34;pseudo time constant&#34; T 2  smaller than the actual time constant of the circuit given by τ 1  =R.sub.(1-2)eq ·C 1 . In the example considered T 2  =τ 1  /15. When the action of drawing the additional current I tp  by the TUNER DELAY PLUS circuit ceases, the tension V 1  (t) reverts to following the trend dictated by the time constant τ 1  as regards the first asymptotic discharge level of C 1  determined by the product R.sub.(1-2)eq ·I 1 . This TUNER DELAY PLUS intervention time interval (t 2  -t 1 ) should nevertheless never exceed the value T 2   given by: ##EQU4## otherwise the capacitance C 1  would discharge excessively and the voltage V 1  (t) would drop below the asymptotic value given by: V1-R.sub.(1-2)eq ·I 1 . 
     An unacceptable situation is in fact that in which the voltage V 1  (t) drops excessively below the abovementioned asymptotic value, for example by an amount greater than 10% of R.sub.(1-2)eq ·I 1 . This condition of excessive diminution of the voltage V 1  (t) is graphically represented by the characteristic 1 of FIG. 10 where, for the purpose of comparison, the characteristic curve 5 relevant to the operation of the TUNER DELAY circuit alone without superposition of the action exercised by the TUNER DELAY PLUS circuit of the invention, is likewise indicated. 
     On the other hand if the TUNER DELAY PLUS circuit becomes disconnected before V 1  (t) has reached a value close to the asymptotic value, for example a value V 1  -0.9 R.sub.(1-2)eq ·I 1  (i.e. greatly above the first asymptotic discharge level of the capacitor C 1 ), the effect of the special TUNER DELAY PLUS circuit would not be fully exploited. This condition is represented by the characteristic curve 3 of FIG. 10. 
     A preferable situation is that according to which the voltage V 1  (t) drops slightly below the asymptotic value, as represented by the characteristic 2 of FIG. 10. 
     In practice an optimal condition is that according to which the voltage V 1  (t) attains the asymptotic level in the shortest possible time as represented by the characteristic 4 of FIG. 10. This is attainable when operating in such a way that the TUNER DELAY PLUS circuit becomes disactivated at the instant at which the voltage V 1  (t) attains the asymptotic value, i.e. at a time t 2  =T 2 , i.e. with the minimum time for attainment of the steady state equivalent to the value of said &#34;pseudo time constant&#34; T 2 . 
     Naturally, an interval of ±10% relative to the value of the first asymptotic level, as indicated in the graph of FIG. 10, is an indicative design choice and represents a substantially satisfactory operating condition for the system. 
     According to one embodiment, the TUNER DELAY PLUS circuit of the invention can have the circuit diagram shown in FIG. 11 in which are indicated the connections to the respective circuit nodes 1 and 3 of the block diagram of FIG. 7, as well as the two input terminals to which are applied the voltage V REF  and the enabling current I A  respectively, which current, in the circuit diagram, is schematically shown generated by a generic generator of constant current I A . 
     The circuit comprises a pair of transistors Q1 and Q2 which form a first current mirror for the input current I A  which powers a differential circuit constituted by the transistors Q3 and Q4 of which the former is a transistor formed of two distinct fractional collectors, and a second differential-output current mirror formed by the transistors Q5 and Q6-Q7, these last two transistors being powered respectively through said two respective fractional collectors of the multiple-collector transistor Q3 of the differential, which constitute two distinct output nodes of the mirror. By means of one of these two output nodes there is directly driven (t&gt;t 1 ) an output circuit composed of a Darlington transistor pair, Q9 and Q10, which represents an &#34;open collector&#34; capable of drawing or not a current I tp  from the node 1, namely from storage node of the DC control voltage of the HF-AGC (across the capacitor C 1  ; FIG. 7) whereas a transistor Q8, connected between said node for driving the output circuit and ground, is driven by means of the other output node of the mirror so as to cut off the transistor pair of the output circuit after a certain period of time (t=t 2 ). 
     The operation of the circuit is as follows. 
     In a starting situation: V 3  =V 3  &lt;V REF , with the arrival of a climbing front in the level of the signal at time t=t 1  such as to determine the delivery of an enabling current I A , the latter is mirrored by Q1 and Q2 and passes through the transistor Q4 which renders it available to the transistor Q5 so as to then be mirrored by the transistors Q6 and Q7. Since Q3 is cut off (I B  =0), no current is available at the collectors of the transistors Q6 and Q7 which then saturate almost dragging to ground the voltage present on the respective collectors. Under these conditions the transistors Q8, Q9 and Q10 remain cut off and no current is absorbed by the node 1. 
     At the instant at which the voltage V 3  begins to rise, the transistor Q3 starts to conduct (I B  ≠0) providing current to the collectors of the transistors Q6 and Q7. It should be noted that the current I B  divides up by 1/4 on the collector of the transistor Q6 and by the remainders 3/4 on the collector of the transistor Q7 as a function of the respective areas of the multiple collector zones of the transistor Q3. 
     Because of this fact and moreover bearing in mind the disparate ratio between the emitter areas of the transistors Q6 and Q7, where Q6 has an emitter area five times (5×) greater than the emitter area of the transistor Q7, the voltage on the base of the transistor Q9 will begin to increase. 
     More precisely, the transistor Q7 starts to desaturate at the instant t 1  at which: ##EQU5## 
     From this instant onwards (t&gt;t 1  and with the voltage V 3  exceeding V REF ) a current i x  is available for the base of Q9, which will produce a current β·i x  for the base of Q10 and hence the total current I tp  absorbed by the node 1 will be given by: 
     
         β(β÷1)i.sub.x ≈β.sup.2 i.sub.x 
    
     Only when the voltage at node 3 of the circuit of FIG. 7 has further increased (and hence after a certain delay), will the transistor Q6 also begin to leave the saturated condition. 
     The transistor Q6 starts to desaturate at the instant t 2  at which: ##EQU6## i.e. practically when the differential constituted by the two transistors Q3 and Q4 becomes completely unbalanced and hence the transistor Q4 is off. 
     From the instant t 2  onwards, 1/4 of the current I B  enters the base of Q8, for which the collector current of Q8 is given by β I B  /4 which is certainly larger than 3 I B  /4. As a result the saturation of Q8 is obtained and hence the cutting off of the pair of transistors Q9 and Q10. 
     As has been stated previously, the TUNER DELAY PLUS circuit must not intervene despite the existence of an enabling current I A  in the case in which the voltage V 3  has already exceeded the value V REF . 
     Under this last condition the differential is in fact already unbalanced, and the collector current of Q8 being equal to β I B  /4 and hence larger than 3 I B  /4, no base current would be available for Q9; this however if the response time of Q8 were instantaneous. Since this is not the case, this instantaneous current available at the base of Q9 has to be discharged and this is obtained through the capacitance C, avoiding a multiplication by β 2 . 
     Referring again to the functional block diagram of FIG. 7 and summarizing, the development of the current I tp  can be represented graphically as in FIG. 12. The maximum value obtainable by the current peak I tp  is equivalent to about β 2  3 I B  /4. 
     Hence it is possible to attain relatively high values of current, typically about 30 mA, and with the purpose of limiting excessive values a resistor R L  can be introduced into the circuit diagram of the TUNER DELAY PLUS circuit of FIG. 11. 
     A typical current peak I tp  absorbed by the circuit in the time interval between t 1  and t 2  is shown graphically in FIG. 12. 
     Furthermore, the development of the discharge current I 1  absorbed by the TUNER DELAY CIRCUIT is shown graphically in the graph of FIG. 13. 
     The overall effect of the two circuits (TUNER DELAY and TUNER DELAY PLUS) acting substantially in parallel with one another in case both are activated, is that represented graphically in the graph of FIG. 14 which represents the development of the discharge current I&#39; 1  of the capacitor C 1  for storing the DC voltage for controlling the HF-AGC circuit. 
     Returning to consider an exemplary embodiment described in relation to the graph of FIG. 9 and wishing the additional circuit of the invention referred to as TUNER DELAY PLUS to remain switched on until the voltage V 1  (t) becomes equal to the value of the first asymptotic level (characteristic 4 of the graph of FIG. 10), i.e. for a time T 2  =τ 1  /15, the equation: ##EQU7## must be satisfied, where I IF  is the discharge current of the capacitor C 2  of the IF-AGC circuit and δV equals 75 mV-7 mV=68 mV. 
     This value of δV might not be sufficient for the equality t 2  -t 1  =T 2 , to be satisfied, in fact I IF  is fixed, C 2  is a specified design value and T 2  =τ 1  /n, where τ 1  is specified in the design phase, R1, R2 and C 1  usually being discrete external components and n is the number of times which it is wished to reduce the effective time constant of the system by way of activation of the additional circuit of the invention, TUNER DELAY PLUS. 
     Two different possibilities exist for increasing the value of δV, having determined the value n (equal to 15 in the example proposed) and the fractionalizing of Q3: 
     a) further increasing the emitter area of the transistor Q6 relative to Q7, for example from 5× to 10×; or 
     b) introducing resistors between the emitter of Q7 and ground and between the emitter of Q5 and ground. 
     In the event of adopting the first solution and the emitter area of the transistor Q6 becoming 10 times the area of Q7 (10×) there will be obtained: ##EQU8## and hence δV=92 mV-7 mV=85 mV. 
     In the event of adopting the second solution according to the modified circuit diagram of FIG. 15, the two resistors R3 and R4 do not modify the mirror ratio between Q5 and Q7 which still remains unitary and hence the additional discharge current I tp  becomes available from the moment at which V 3  -V REF  =7 mV. Assuming that the two resistors R3 and R4 both have the value R, the following transcendental equation will hold: ##EQU9## (again assuming that the transistor Q6 has an emitter area 5 times greater than the area of the transistor Q7). 
     Setting I A  =10 μA and R=20 kΩ, I B  ≈9.64 μA and hence ##EQU10## and hence δV=82 mV-7 mV=75 mV. 
     If simultaneously the transistor Q6 is made with an area 10 times greater than Q7, the transcendental equation will be modified in the following way: ##EQU11## and has as solution I B  ≈9.79 μA from which it results that V 3  -V REF  =96 mV and δV=89 mV. 
     Modifying in this way the limits of δV, namely the respective values which when attained by the voltage V 3  (t) determine firstly the desaturating of the transistor Q7 (t 1 ) modifying the fractionalizing of Q3 and then of the transistor Q6 (t 2 ), the start and the duration of the peak discharge current absorbed by the circuit (FIGS. 12 and 14) can be modified. 
     Naturally the ways in which said current I A  can be generated or liberated when the level of the output signal V OUT  of the IF STAGE exceeds a determined threshold, which, given the extreme rapidity of response of the IF-AGC, can happen only in the presence of a sudden climb in the level of the signal at the input of the same IF STAGE (node 5), can be different and all easily recognizable by a person skilled in the art. The use of a differential stage, to one input of which is applied a certain reference voltage and to the other input of which is applied a signal replica of the level of the signal from the IF-AGC representing one of the circuital arrangements suitable for generating said current I A  for activating the TUNER DELAY PLUS circuit of the invention.