Pulse or digital communications – Receivers – Automatic gain control
Reexamination Certificate
1999-06-30
2003-09-30
Pham, Chi (Department: 2631)
Pulse or digital communications
Receivers
Automatic gain control
C455S224000, C455S232100
Reexamination Certificate
active
06628731
ABSTRACT:
TECHNICAL FIELD
The invention relates to a method for controlling the signal level in a digital receiver. The invention also relates to an apparatus for controlling the signal level in a digital receiver.
BACKGROUND OF THE INVENTION
With various digital receivers, such as digital TV receivers, it is desirable that the digital input signal level be at all times optimal. Due to losses in transmission networks and other non-idealities, real signal levels received from a transmission network may vary a great deal. To level out such variations digital receivers typically are equipped with various signal level control devices which aim at keeping the digital signal level constant regardless of the signal level of the signal received from the transmission network. These systems are generally called automatic gain control (AGC) systems. For example, many demodulator circuit manufacturers are offering circuits with an AGC output that provides a voltage dependent of the signal level of the input signal.
FIG. 1
shows a prior-art arrangement
10
for realizing signal level control in a digital receiver, based on a circuit
14
with an AGC output. The arrangement
10
is based on the use of two successive control loops
11
and
12
. A first control circuit
11
controls the signal level of a received analog RF signal and filters by means of a bandpass filter
18
the desired frequency band to be sent forward. A second control loop
12
controls the level of the filtered analog IF signal produced by the first control loop
11
. For simplicity,
FIG. 1
does not show components, such as mixer elements, which are essential to the operation of the receiver but inessential as regards the operation of the control circuits.
From the second control circuit
12
the analog signal is taken to an A/D converter
13
which converts the analog signal into a digital one. The digital signal thus produced is fed to an AGC circuit
14
which may be a demodulator, for example. The AGC circuit
14
estimates the level of the digital signal coming from the A/D converter
13
and produces an AGC signal
19
proportional to the signal level. Since variable gain amplifiers usually are controlled using an analog signal, the AGC signal has to be D/A-converted. This can be advantageously realized internally in the AGC circuit
14
so that the AGC circuit
14
directly produces an analog AGC signal
19
or, if the circuit
14
has no D/A converter, in a separate D/A converter circuit (not shown). The AGC signal
19
is taken direct to a variable-gain amplifier
16
in the second control circuit
12
and via a threshold circuit
17
to a variable-gain amplifier
15
in the first control circuit
11
.
FIG. 2
is a graph illustrating how the level of the analog signal, which is produced by the second control circuit
12
in the arrangement
10
according to FIG.
1
and fed to an A/D converter
13
, affects the second control signal (IF Gain) controlling an amplifier
16
in the second control circuit and on the first control signal (RF Gain) following the threshold circuit
17
and controlling an amplifier
15
in the first control circuit
11
. Stepwise dependence of amplification factors on the value of the analog signal is the result of quantization noise caused by the limited conversion accuracy of the D/A converter in the AGC circuit
14
or separate D/A converter circuit. Quantization noise is caused by the fact that digital apparatus only have a limited number of bits at their disposal so that the smallest possible adjustment step that can be achieved with digital systems equals the operating range divided by the number of bit combinations available. For example, a one-volt control range in an 8-bit system (256 bit combinations) produces a smallest possible adjustment step of about 4 mV (1 V/256).
In the example depicted in
FIG. 2
the system is specified such that when the level of the analog signal coming from the second control circuit
12
is at its minimum, both amplification factors are at their maximum. The behavior of the amplification factors may be realized in other ways, too. When signal levels start to rise in the example depicted in
FIG. 2
, the amplification factor (IF Gain) of only the second control circuit
12
is reduced at first. The reduction of the amplification factor (RF Gain) of the first control circuit
11
is begun only when the signal level exceeds a certain threshold level
21
defined by means of a threshold circuit
17
. This arrangement produces a noise performance which is optimal for the system whole.
Since the values of the amplification factors may vary greatly in an arrangement such as the one described above, the control range also needs to be wide. As the D/A conversion is realized using a limited number of bits, it subsequently follows that the control system always includes quantization noise which degrades the quality of the signal available by preventing accurate signal level control.
A solution obvious to a person skilled in the art for the prior-art quantization noise problem described above is to increase the number of bits available. In the simplest case this would require more accurate circuits, i.e. circuits with more bits, to perform the D/A conversion. AGC circuits with more accurate D/A converters are considerably more expensive as well as more limited in variety than the most common circuits used currently. In the ideal case the user could design and build a circuit with the necessary characteristics but then the unit price of the circuit would be significantly higher.
Another disadvantage of the prior art described above is the system's sensitivity to disturbances. As the AGC signal
19
is used for directly controlling the amplifier
16
in the second control circuit
12
, it is possible that a small spurious signal may cause the digital control system to oscillate between two or more amplification values such that the oscillation continues even after the spurious signal has disappeared.
A solution obvious to a person skilled in the art for the disturbance-sensitivity of the arrangement according to the prior art would be to attempt to shield the apparatus as effectively as possible against electrical interference by placing it in a grounded metal casing, for example. Such protective systems add to the size and manufacturing costs of the apparatus but will not, however, provide adequate protection e.g. against spurious signals accompanying the received signal.
SUMMARY OF THE INVENTION
An object of this invention is to provide a new method and apparatus for controlling the signal level in a digital receiver, eliminating the above-mentioned disadvantages of the prior art. These objects are achieved by realizing coarse control of signal level by means of an analog control circuit and fine-controlling the resulting signal by means of a digital control circuit which is functionally separate from the analog control circuit.
More specifically, the present invention is a method for controlling signal level in a digital receiver, comprising at least the following steps: an analog signal is received from a transmission network or the like, the level of the received analog signal is controlled in an analog control circuit, whereafter the analog signal is converted to a digital signal in an A/D converter, wherein the level of the analog signal coming from the analog control circuit is controlled in a digital control circuit which is functionally substantially separate from the analog control circuit, and the amplification factor of the digital control circuit is adjusted on the basis of the level of the digital signal coming from the A/D converter.
The present invention is also an apparatus for controlling signal levels in a digital receiver, comprising at least an analog control circuit for controlling the level of an analog signal, wherein the apparatus includes a digital control circuit, which is functionally substantially separate from the analog control circuit, for controlling the level of the analog signal coming from the analog control circuit.
Compare
Auranen Tommi
Vasenkari Petri
Kumar Pankaj
Nokia Technology GmbH
Pham Chi
Ware Fressola Van Der Sluys & Adolphson LLP
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