GB1568513A - Automatic signal level control system - Google Patents
Automatic signal level control system Download PDFInfo
- Publication number
- GB1568513A GB1568513A GB4297376A GB4297376A GB1568513A GB 1568513 A GB1568513 A GB 1568513A GB 4297376 A GB4297376 A GB 4297376A GB 4297376 A GB4297376 A GB 4297376A GB 1568513 A GB1568513 A GB 1568513A
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- GB
- United Kingdom
- Prior art keywords
- level
- signal
- voltage
- minimum
- control
- Prior art date
- Legal status (The legal status is an assumption and is not a legal conclusion. Google has not performed a legal analysis and makes no representation as to the accuracy of the status listed.)
- Expired
Links
- 239000003990 capacitor Substances 0.000 claims description 19
- 230000000694 effects Effects 0.000 claims description 4
- 230000032683 aging Effects 0.000 claims description 3
- 230000003321 amplification Effects 0.000 claims description 2
- 238000003199 nucleic acid amplification method Methods 0.000 claims description 2
- 230000001934 delay Effects 0.000 description 2
- 238000010586 diagram Methods 0.000 description 2
- 238000000034 method Methods 0.000 description 1
- 230000001932 seasonal effect Effects 0.000 description 1
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
- H03G3/3073—Circuits generating control signals when no carrier is present, or in SSB, CW or pulse receivers
-
- 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
- H03G3/3005—Automatic control in amplifiers having semiconductor devices in amplifiers suitable for low-frequencies, e.g. audio amplifiers
Landscapes
- Engineering & Computer Science (AREA)
- Multimedia (AREA)
- Control Of Amplification And Gain Control (AREA)
Description
(54) IMPROVEMENTS IN OR RELATING TO AUTOMATIC
SIGNAL LEVEL CONTROL SYSTEMS
(71) 1, SECRETARY OF STATE FOR
DEFENCE, LONDON, do hereby declare the invention, for which I pray that a patent may be granted to me, and the method by which it is to be performed, to be particularly described in and by the following statement:
This invention relates to automatic signal level control systems.
Automatic signal level control systems are commonly employed to stabilise the amplitude level of a signal at an optimum value by compensating for variations in the strength of the signal resulting from, for example, component ageing and temperature drift in communication equipment, and diurnal or seasonal variations in cables and radio propagation paths.
Such systems essentially comprise a variable gain amplifying element connected in the path of the signal, and gain control means for automatically controlling the gain of the amplifying element in dependence upon variations in the amplitude level of the input signal in such a manner as to produce an output signal having a substantially constant optimum amplitude level under widely varying input signal conditions.
In conventional automatic signal level control systems, the automatic gain control means includes a peak level detector arranged to detect and follow variations in the peak amplitude level of the incoming signal waveform envelope, and to vary the gain of the amplifier element in inverse proportion to this peak level so as to maintain the peak amplitude of the output signal at a substantially constant optimum level.
However, conventional automatic level control systems of this kind are unsuitable for use with information signals which do not have a continuous carrier waveform, or in which the information is transmitted in intermittent form, such as radar pulses and human speech, owing to the long time delays which frequently occur between successive peaks in the signal waveform envelope. In attempting to maintain a constant output amplitude level during such delays, the system increases the gain of the amplifying element, thus considerably amplifying the background noise of the signal and often introducing distortion on the next succeeding signal peak until the gain of the amplifying element can be restored to an appropriate level.
It is an object of the present invention to provide an alternative form of automatic signal level control system which can usefully be applied to stabilise the amplitude level of a signal in which the information is transmitted in non-continuous or intermittent form.
According to the present invention, an automatic signal level control system comprises:
variable-gain amplifying means connected in the path of the signal;
minimum signal level detecting means including a storage capacitance and electronic switching means responsive to variations in the amplitude level of the signal to cause the magnitude of the voltage stored by the storage capacitance to follow rises in the amplitude level of the signal with a first time constant, and to follow drops in the amplitude level of the signal with a second time constant substantially shorter than the first time constant whereby the stored voltage represents the minimum level of the signal;;
and level control means for applying the stored voltage to control the gain of the amplifying means whereby to tend to maintain the minimum amplitude level of the signal at the output of the amplifying means at a substantially constant value.
With this arrangement, the storage capacitance will tend to hold a charge representative of the minimum amplitude level of the signal, even during periods between successive minima in the signal waveform envelope.
The invention relies to some extent on the realisation that in most circumstances the minimum amplitude level of an intermittent or non-continuous signal, usually the background noise level, will vary in the same manner as the overall signal strength, so that the overall signal strength will be maintained substantially constant at an optimum value. However, systems in accordance with the invention may also be used to ensure that the background noise level of an information signal does not exceed a predetermined optimum value.
Preferably the level control means is arranged to compare said voltage representative of the minimum amplitude of the signal with a reference voltage, and to control the gain of the amplifying means in dependence upon the difference between them.
This reference voltage will therefore represent the desired optimum value of the minimum amplitude level of the output signal, and may conveniently be adjustable to permit manual control of the desired minimum signal amplitude level.
The minimum level detecting means may be arranged to detect the minimum amplitude level of the signal at the input side of the amplifying means to effect feed forward control, although preferably the level detecting means is arranged to detect the minimum signal level at the output side of the amplifying means whereby to effect feedback control of the gain of the amplifying means.
The invention will now be further described, by way of example only, with reference to the drawing accompanying the provisional specification of which:
Figure 1 is a block schematic diagram of a first automatic signal level control system in accordance with the invention;
Figure 2 is a block schematic diagram of a second automatic signal level control system in accordance with the invention, and
Figure 3 represents typical waveforms associated with the operation of the systems shown in Figures 1 and 2.
Referring to the drawings, the automatic signal level control system shown in Figure 1 comprises a variable gain amplifier 1 connected in the path 2 of a signal of which the amplitude level is to be controlled, and an automatic gain control (AGC) loop 3 connected between the signal input, and a gain control terminal 4 of the amplifier 1.
The AGC loop 3 includes a signal rectifier 5, an integrator 6, a minimum level detector 7, and a level control circuit 8 whose output is connected to the gain control terminal 4 of the amplifier 1.
The rectifier 5, integrator 6 and level control means 8 may be of any suitable known form, and are not shown in detail.
The minimum level detector 7 comprises a detector diode D, connected in series in the gain control loop 3, a resistor R connected in shunt across the diode D, and a storage capacitor C connected between the output side of the diode D and earth.
The operation of the system will now be described with reference to the waveforms shown in Figure 3 of the drawings.
On application of a signal of the form shown in Figure 3 (a) (which represents a speech signal having a background or threshold noise component arising, for example, from intermodulation, cross-talk or thermal energy) along the signal path 2, a portion of the signal at the input side of the amplifier 1 is fed through the AGC loop 3.
This portion of the signal is first applied to the rectifier 5 to produce a signal of the form shown in Figure 3 (b), and this signal is then applied to the integrator 6 which acts as a low pass filter to produce a smooth unidirectional DC voltage of the form shown in solid lines in Figure 3 (c), representing variations in the amplitude of the input signal waveform envelope.
The DC output voltage from the integrator 6 is then applied to the detector diode D of the minimum level detector 7. Disregarding the voltage drop across the diode D, when the value of the DC output voltage from the integrator 6 exceeds the value of the voltage stored by the storage capacitor
C, the diode D will be reverse biassed, ie non-conducting, and the capacitor will charge very slowly with a long time constant, typically a few seconds, established by the resistor R. However, when the value of the DC voltage from the integrator 8 falls below the stored voltage on the capacitor C, diode D will conduct, causing C to discharge rapidly, with a very short time constant, to the minimum value of the DC voltage.
Thus the storage capacitor C will tend to store and hold the output voltage of the minimum level detector 7 at a value corresponding to the minimum amplitude level of the input signal, which in this case is the level of the threshold noise component of the signal.
The output voltage of the level detector, represented by the dotted lines in Figure 3 (c), is then applied to the level control circuit 8 in which it is compared with a reference voltage of opposite polarity to produce an output voltage representing the difference between them. This output voltage from the level control circuit 8 is then applied to the gain control terminal 4 of the amplifier 1 to increase or decrease the gain of the amplifier in dependence upon whether the magnitude of the level control circuit output voltage is smaller or greater than the reference voltage respectively. It will be seen that the value of this reference voltage determines the level at which the minimum amplitude of the signal will be stabilised, and adjustment of the reference voltage may be used to control the minimum amplitude, or noise level of the signal.
An alternative form of automatic level control system in accordance with the invention is shown in Figure 2, and where appropriate, identical reference numerals will be used to denote parts of this system corresponding to those in Figure 1.
The Figure 2 system differs from the system shown in Figure 1 in several respects.
The automatic gain control voltage is derived from the output side of the variable gain amplifying means, which in this embodiment is provided by an analogue multiplier 1, to provide a feed-back control of the gain in the signal path 2. Any variable gain amplifier may equally be used, however, in place of the analogue multiplier.
As in the Figure 1 embodiment, the gain control loop 3 includes a rectifier 5, an integrator 6, a minimum level detector 7 and a level control circuit 8 all of which perform the same function as the corresponding elements in the Figure 1 embodiment.
However the level detector 7 is of more sophisticated form and includes, in addition to a detector diode D, resistor R and storage capacitor C, an operational amplifier A.
The output of the integrator 6 is connected directly to the non-inverting input 11 of the amplifier A, and via the resistor R to the inverting input 10 of the amplifer A. The output of the amplifier is connected, via diode D, to the input of the level control circuit 8, which input is in turn connected to the inverting input 10 of the amplifer A by means of a negative feedback path 13.
Storage capacitor C is connected between the output side of diode D and earth.
The operation at the level detector 7 of
Figure 2 is similar in principle to that of the
Figure 1 system. When the output voltage at the integrator 6 exceeds the stored voltage on capacitor C, diode D is reverse biassed and capacitor C charges slowly through resistor R with a long time constant. When the integrator output voltage falls below the stored voltage on capacitor C, diode D conducts causing the voltage on capacitor C to follow the input voltage from the integrator 6 with a short time constant.
The voltage thus established by the capacitor C at the output of the detector 7 is compared, in the level control circuit 8, with a reference voltage as before to produce a difference voltage which in this case is then multiplied with the input signal by the analogue multiplier 1, to produce the desired gain control in the signal path 2.
It will be appreciated that the automatic level control systems described above with reference to Figures 1 and 2 may be modified in many ways without departing from the scope of the present invention. In particular the form of minimum level detector 7 in Figure 1 may readily be replaced by the level detector of the Figure 2 embodiment and vice versa. Furthermore, these two forms of level detector are described by way of example only, and may readily be replaced by any other suitable form of minimum amplitude level detector circuit.
In some cases improved performance may be achieved by including an amplifier stage in the AGC loop 3.
Although the invention has been described in its application to the control of background noise level in speech signals, it may equally be applied to stabilise the minimum amplitude level of any type of signal in which information is transmitted in intermittent form. Typical examples of such applications of the invention are as follows:
a) compensation for such phenomena as component ageing and temperature drift in electronic circuits in which amplification or attenuation is important;
b) control of noise levels applied to signal energy recognition and electronic decision cjircuits to achieve optimum conditions for these to perform effectively;
c) adjustment of threshold noise levels between different media of communication, such as line signals to radio or magnetic tape; and
d) balance of noise levels for optimum working between parallel paths in such systems as HF or space diversity radio communication systems.
WHAT I CLAIM IS:
1. An automatic signal level control system comprising:
variable-gain amplifying means connected in the path of the signal;
minimum signal level detecting means including a a storage capacitance and electronic switching means responsive to variations in the amplitude level of the signal to cause the magnitude of the voltage stored by the storage capacitance to follow rises in the amplitude level of the signal with a first time constant. and to follow drops in the amplitude level of the signal with a second time constant substantially shorter than the first time constant whereby the stored voltage represents the minimum level of the signal; ;
and level control means for applying the stored voltage to control the gain of the amplifying means whereby to tend to maintain the minimum amplitude level of the signal at the output of the amplifying means at a substantially constant value.
2. An automatic signal level control system as claimed in claim 1 wherein the
**WARNING** end of DESC field may overlap start of CLMS **.
Claims (7)
- **WARNING** start of CLMS field may overlap end of DESC **.circuit output voltage is smaller or greater than the reference voltage respectively. It will be seen that the value of this reference voltage determines the level at which the minimum amplitude of the signal will be stabilised, and adjustment of the reference voltage may be used to control the minimum amplitude, or noise level of the signal.An alternative form of automatic level control system in accordance with the invention is shown in Figure 2, and where appropriate, identical reference numerals will be used to denote parts of this system corresponding to those in Figure 1.The Figure 2 system differs from the system shown in Figure 1 in several respects.The automatic gain control voltage is derived from the output side of the variable gain amplifying means, which in this embodiment is provided by an analogue multiplier 1, to provide a feed-back control of the gain in the signal path 2. Any variable gain amplifier may equally be used, however, in place of the analogue multiplier.As in the Figure 1 embodiment, the gain control loop 3 includes a rectifier 5, an integrator 6, a minimum level detector 7 and a level control circuit 8 all of which perform the same function as the corresponding elements in the Figure 1 embodiment.However the level detector 7 is of more sophisticated form and includes, in addition to a detector diode D, resistor R and storage capacitor C, an operational amplifier A.The output of the integrator 6 is connected directly to the non-inverting input 11 of the amplifier A, and via the resistor R to the inverting input 10 of the amplifer A. The output of the amplifier is connected, via diode D, to the input of the level control circuit 8, which input is in turn connected to the inverting input 10 of the amplifer A by means of a negative feedback path 13.Storage capacitor C is connected between the output side of diode D and earth.The operation at the level detector 7 of Figure 2 is similar in principle to that of the Figure 1 system. When the output voltage at the integrator 6 exceeds the stored voltage on capacitor C, diode D is reverse biassed and capacitor C charges slowly through resistor R with a long time constant. When the integrator output voltage falls below the stored voltage on capacitor C, diode D conducts causing the voltage on capacitor C to follow the input voltage from the integrator 6 with a short time constant.The voltage thus established by the capacitor C at the output of the detector 7 is compared, in the level control circuit 8, with a reference voltage as before to produce a difference voltage which in this case is then multiplied with the input signal by the analogue multiplier 1, to produce the desired gain control in the signal path 2.It will be appreciated that the automatic level control systems described above with reference to Figures 1 and 2 may be modified in many ways without departing from the scope of the present invention. In particular the form of minimum level detector 7 in Figure 1 may readily be replaced by the level detector of the Figure 2 embodiment and vice versa. Furthermore, these two forms of level detector are described by way of example only, and may readily be replaced by any other suitable form of minimum amplitude level detector circuit.In some cases improved performance may be achieved by including an amplifier stage in the AGC loop 3.Although the invention has been described in its application to the control of background noise level in speech signals, it may equally be applied to stabilise the minimum amplitude level of any type of signal in which information is transmitted in intermittent form. Typical examples of such applications of the invention are as follows: a) compensation for such phenomena as component ageing and temperature drift in electronic circuits in which amplification or attenuation is important; b) control of noise levels applied to signal energy recognition and electronic decision cjircuits to achieve optimum conditions for these to perform effectively;c) adjustment of threshold noise levels between different media of communication, such as line signals to radio or magnetic tape; and d) balance of noise levels for optimum working between parallel paths in such systems as HF or space diversity radio communication systems.WHAT I CLAIM IS: 1. An automatic signal level control system comprising: variable-gain amplifying means connected in the path of the signal; minimum signal level detecting means including a a storage capacitance and electronic switching means responsive to variations in the amplitude level of the signal to cause the magnitude of the voltage stored by the storage capacitance to follow rises in the amplitude level of the signal with a first time constant. and to follow drops in the amplitude level of the signal with a second time constant substantially shorter than the first time constant whereby the stored voltage represents the minimum level of the signal;; and level control means for applying the stored voltage to control the gain of the amplifying means whereby to tend to maintain the minimum amplitude level of the signal at the output of the amplifying means at a substantially constant value.
- 2. An automatic signal level control system as claimed in claim 1 wherein thelevel control means is arranged to compare the stored voltage with a reference voltage, and to control the gain of the amplifying means in dependence upon the difference between them.
- 3. An automatic signal level control system as claimed in claim 2 wherein the reference voltage is adjustable to permit manual control of the desired minimum signal level.
- 4. An automatic signal level control system as claimed in any one of Claims 1 to 3, wherein variations in the amplitude level of the signal are converted into a unidirectional voltage varying in amplitude therewith, before application to the minimum level detecting means.
- 5. An automatic signal level control system as claimed in any one of Claims to 4, wherein the minimum level detecting means is arranged to detect the minimum amplitude level of the signal at the output side of the amplifying means, whereby to effect feedback control of the gain of the amplifying means.
- 6. An automatic signal level control system as claimed in any one of Claims 1 to 4, wherein the minimum level detecting means is arranged to detect the minimum amplitude level of the signal at the input side of the amplifying means, whereby to effect feedforward control of the gain of the amplifying means.
- 7. An automatic signal level control system substantially as shown in, and as hereinbefore described with reference to, Figure 1 or Figure 2 of the drawing accompanying the Provisional Specification.
Priority Applications (1)
| Application Number | Priority Date | Filing Date | Title |
|---|---|---|---|
| GB4297376A GB1568513A (en) | 1977-10-07 | 1977-10-07 | Automatic signal level control system |
Applications Claiming Priority (1)
| Application Number | Priority Date | Filing Date | Title |
|---|---|---|---|
| GB4297376A GB1568513A (en) | 1977-10-07 | 1977-10-07 | Automatic signal level control system |
Publications (1)
| Publication Number | Publication Date |
|---|---|
| GB1568513A true GB1568513A (en) | 1980-05-29 |
Family
ID=10426788
Family Applications (1)
| Application Number | Title | Priority Date | Filing Date |
|---|---|---|---|
| GB4297376A Expired GB1568513A (en) | 1977-10-07 | 1977-10-07 | Automatic signal level control system |
Country Status (1)
| Country | Link |
|---|---|
| GB (1) | GB1568513A (en) |
Cited By (7)
| Publication number | Priority date | Publication date | Assignee | Title |
|---|---|---|---|---|
| WO1991011854A1 (en) * | 1990-01-23 | 1991-08-08 | Ampsys Limited | Amplitude locked loop circuits |
| US5087893A (en) * | 1989-12-18 | 1992-02-11 | Motorola, Inc. | RF driven gate bias |
| EP0498003A1 (en) * | 1991-02-05 | 1992-08-12 | Siemens Aktiengesellschaft | Integrated circuit for detecting signals freed of background noise |
| EP0638997A1 (en) * | 1990-08-24 | 1995-02-15 | Sony Corporation | Servo circuit |
| US6107880A (en) * | 1998-08-06 | 2000-08-22 | Motorola, Inc. | Method and apparatus for increasing the linearity of the phase and gain of a power amplifier circuit |
| EP1355436A1 (en) * | 2002-04-16 | 2003-10-22 | Samsung Electronics Co., Ltd. | Burst mode optical receiver generating a reset signal for an automatic threshold control device during inter-burst periods |
| EP1355437A1 (en) * | 2002-04-19 | 2003-10-22 | Samsung Electronics Co., Ltd. | Bottom level detection device for burst mode optical receiver |
-
1977
- 1977-10-07 GB GB4297376A patent/GB1568513A/en not_active Expired
Cited By (10)
| Publication number | Priority date | Publication date | Assignee | Title |
|---|---|---|---|---|
| US5087893A (en) * | 1989-12-18 | 1992-02-11 | Motorola, Inc. | RF driven gate bias |
| WO1991011854A1 (en) * | 1990-01-23 | 1991-08-08 | Ampsys Limited | Amplitude locked loop circuits |
| US5341106A (en) * | 1990-01-23 | 1994-08-23 | The Governors Of Paisley College Of Technology | Amplitude locked loop circuits |
| EP0638997A1 (en) * | 1990-08-24 | 1995-02-15 | Sony Corporation | Servo circuit |
| EP0498003A1 (en) * | 1991-02-05 | 1992-08-12 | Siemens Aktiengesellschaft | Integrated circuit for detecting signals freed of background noise |
| US6107880A (en) * | 1998-08-06 | 2000-08-22 | Motorola, Inc. | Method and apparatus for increasing the linearity of the phase and gain of a power amplifier circuit |
| EP1355436A1 (en) * | 2002-04-16 | 2003-10-22 | Samsung Electronics Co., Ltd. | Burst mode optical receiver generating a reset signal for an automatic threshold control device during inter-burst periods |
| US7266312B2 (en) | 2002-04-16 | 2007-09-04 | Samsung Electronics Co., Ltd. | Burst mode optical receiver |
| EP1355437A1 (en) * | 2002-04-19 | 2003-10-22 | Samsung Electronics Co., Ltd. | Bottom level detection device for burst mode optical receiver |
| US7330670B2 (en) | 2002-04-19 | 2008-02-12 | Samsung Electronics Co., Ltd. | Bottom level detection device for burst mode optical receiver |
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Legal Events
| Date | Code | Title | Description |
|---|---|---|---|
| PS | Patent sealed | ||
| PCNP | Patent ceased through non-payment of renewal fee |