FR2500235A1 - CIRCUIT FOR ADJUSTING THE AMPLIFICATION OF THE HIGH FREQUENCY AND INTERMEDIATE FREQUENCY STAGES IN RADIO AND TELEVISION RECEIVERS - Google Patents

Abstract

DUE TO STRONGER ADMINISTRATIVE REQUIREMENTS, PROBLEMS OCCUR WITH THE PROCESSING OF SIGNIFICANT SIGNALS IN RECEIVING SYSTEMS. TO AVOID NON-LINEAR SATURATIONS AND DISTORTIONS, WITHOUT DETERIORING THE SIGNAL-NOISE GAP (OR RATIO), A CIRCUIT IS PROVIDED IN WHICH IS CONNECTED, AFTER MIXER STAGE 4 OF THE RECEIVER, A REGULATION ELEMENT 7 THAT RECEIVES ITS INFORMATION CONTROL FROM AN AUTOMATIC GAIN CONTROL CIRCUIT 9 ALSO CONNECTED TO THE MIXER STAGE, THE ADJUSTMENT ELEMENT 7 BEING FIRST ADJUSTED BY THE AUTOMATIC GAIN CONTROL CIRCUIT 9 AT APPROXIMATELY 10DB, THE ADJUSTMENT OF THE HIGH FREQUENCY PRE-AMPLIFICATION STAGE 3 BEING INTRODUCED THROUGH A THRESHOLD 10 CIRCUIT.

Description

Circuit for adjusting the amplification of the high frequency stalls and

  at intermediate frequency in radio and television receivers In general, in radio and television receivers, the amplification stages are adjusted

  so that saturation is avoided. Thus, the dis-

  nonlinear twists of stage output signals

  amplification are also avoided.

  It is known to take control voltage (ACC or CAG)

  for example from an inter- frequency amplifier

  medium, which voltage makes it possible to adjust one after the other, downwards, the various stages of intermediate frequency as well as the stages of the input amplifier, in the tuning circuit, with antenna signals croissants. It is probably now possible to adjust the floors with

  smaller input voltages but it is also pos-

  likely to shift the intervention point of the setting to higher values of the input voltage (setting

  delayed). In the first case, there are few pro-

  problems with non-linear distortions, intermodulation, etc. The signal-to-noise difference (or ratio) is, however, very small. This difference, for a determined input voltage, is, in a known manner, inversely proportional to the point of introduction of the adjustment. The maximum possible signal-to-noise difference is reached, in this case, only for

the highest input voltages.

  Due to new administrative prescriptions, the problems relating to the solution of the treatment of

  important signals appear again in the construction

  tion of receiving systems such as, for example,

  television and radio receivers. The new pres-

  crictions make it necessary to introduce the adjustment of am-

  essentially earlier planning to prevent saturation of separate floors. However, this entails a cost in terms of the signal-noise difference which is all the worse when the point of introduction of the adjustment is located earlier. Due to the non-linear operation according to the frequencies of the amplification stages, the amplification for the reception frequency band is not constant, also the amplification system

  is set for an entry point of the deter-

  mined for the frequency f For a frequency f 2 'for which the amplification is for example smaller, the adjustment is introduced later, again with the risk of saturations, while for another frequency f3 requiring a stronger amplification, the adjustment will be introduced essentially earlier. But this has the consequence that the signal-to-noise difference is not large enough until very late, that is to say for the strongest

antenna input tensiors.

  The object of the invention is to satisfy the new administrative requirements according to which the adjustment

  is introduced very early, without resulting in deterioration

ration of the signal-to-noise difference.

  This object is achieved according to the invention, by the fact that after the mixer stage of the receiver, is connected an adjustment element which receives its control information from an automatic gain control circuit also connected to the mixer stage, the adjusting element being first adjusted by the automatic gain control circuit at around 10 db, the adjustment of the high frequency preamplification stage

  being introduced via a threshold circuit.

  Preferably, the control voltage delivered to

  from the control circuit is obtained, selectively

  tive, via a filter, from the mixer stage. The invention has the advantage of reducing the whole

  system dispersions (amplification dispersions).

  This type of adjustment works precisely, which is very important, so that the maximum possible signal-to-noise difference is obtained as soon as possible. The knowledge which is the basis of the invention resides in the fact that the part of the circuit threatened with saturation, in this case for example the intermediate frequency amplifier, must first be protected from saturation, before introducing the adjustment. of the tuning circuit, the ampli

  for the highest input voltages, is still linear

  re. Thus, the signal-to-noise difference increases very quickly, so that measured for the whole amplifier (high frequency and intermediate frequency) the maximum possible signal-to-noise difference is already reached, according to the noise rate of The amplifier for half up to a third of the input voltage. Thanks to the invention, the point of introduction of the adjustment can be established so

  io much more precise because the input transistor of the circuit

  cooked okay does not shift its working point

  than for the highest antenna voltages.

  An exemplary embodiment of the invention will be

  now described with reference to the accompanying drawing.

  The single figure in this drawing is a diagram of a control circuit for adjusting the amplification

  a receiver, according to the invention.

  The input signals arrive, via the antenna 1, at the input of the tuning circuit 2. In the preamplification stage 3, these signals are amplified and, in the mixer stage 4, these signals are mixed using a signal taken from an oscillator 5 to

  obtaining an intermediate frequency ZF. According to the-

  vention, between the output of the tuning circuit 2 and the input of the amplifier 6, an adjustment element 7 is connected,

  element which regulates the input signal for the amplified stage

  cateur 6 of intermediate frequency. In addition a circuit

  9 for the production of a signal for automatic control

  that gain (ACC or CAG), is connected to the output of the tuning circuit 2, if necessary via a filter 8. When the signal for the intermediate frequency amplifier ZF has reached its value of maximum possible setting, setting for tuning circuit 2

  is delayed or slowed down using a threshold circuit 10.

  The AGC circuit can be constructed in the usual way.

  As an adjustment element 7, an attenuator made up of a PIN diode or a stage provided with a

field effect transistor.

  For antenna input voltages of 1.5 mV,

  the signal-to-noise difference (or ratio) has already improved by 4 db.

  In the circuit which has just been described, the adjustment criterion is taken at the output of the mixer from the tuning circuit, as required, selectively or along a band 1 a r g e. This setting mode allows

  to limit the amplification differences to the tuning circuit

  cation which appear in a complete branch of ampli-

  from the antenna to the receiver, so that

  meet the current linearity requirements with a simultaneous improvement in the signal-to-noise difference (or ratio).

Claims (2)

Claims   1. Circuit for adjusting the amplification of the high-frequency and intermediate-frequency stages in radio and television receivers, characterized in that after the mixer stage (4) of the receiver is connected an adjusting element (7) which receives control information from a control circuit (9)   automatic g a i n also connected to the rack   mixer, the adjusting element (7) being first adjusted by the automatic gain control circuit (9)   around 10 db, the pre-stage adjustment (3)   high frequency amplification being introduced by the   through a threshold circuit (10).   2. Circuit according to claim 1 characterized in that the control voltage delivered from the control circuit is obtained selectively by means of a filter (8) from the stage mixer (4).