Pulse or digital communications – Receivers – Automatic gain control
Reexamination Certificate
2000-02-22
2004-08-03
Bocure, Tesfaldet (Department: 2631)
Pulse or digital communications
Receivers
Automatic gain control
C455S234100, C455S240100, C455S241100
Reexamination Certificate
active
06771719
ABSTRACT:
BACKGROUND
(1) Field of the Invention
The present invention relates to automatic gain control systems which automatically control the gain of digital modulating signal, more particularly to dual AGC control system which has double gain control loop of RF AGC and IF AGC.
(2) Description of the Related Art
As digital modulation techniques used in order that signal level may generally transmit digital signal data (namely, symbol) expressed with binary methods, digital modulation method such as Phase shift Keying (PSK), Quadrature Amplitude Modulation (QAM), and Orthogonal Frequency Division Multiplexing (OFDM), are known.
In modulating equipment which adopted these digital modulation techniques, orthogonal modulation by analog carrier is performed using two symbol of I and Q which are sampled and quantized at predetermined time interval.
Further, thus the modulated signal is converted to radio frequency (RF), such as VHF band and UHF band, and is sent out to them.
In digital demodulating equipment, after tuning in signal of desire wave and carrying out frequency conversion to Intermediate Frequency (IF), carrier component is eliminated, compare is carried out to predetermined symbol level, and the nearest symbol level is judged a playback symbol value.
Since signal level of received signal is fluctuated according to field streng that the time, signal level of recovery signal is always fixed using Automatic Gain Control (AGC).
Hereafter, conventional automatic gain control system is explained with reference to FIG.
13
.
FIG. 13
shows a block diagram of conventional automatic gain control system.
After amplifying RF signal provided from the terminal
601
in the RF AGC amplifier
602
, it is provided to the 1st frequency converter
604
.
The 1st frequency converter
604
transforms output of the RF AGC amplifier
602
into predetermined 1st intermediate frequency based on frequency control signal provided to the terminal
603
.
After carrying out band limitation of the output of the 1st frequency converter
604
by the 1st filter
605
, it is provided to the 2nd frequency converter
606
.
The 2nd frequency converter
606
transforms output of the 1st filter
605
into predetermined 2nd intermediate frequency based on frequency control signal provided to the terminal
603
. After carrying out band limitation of the output of the 2nd frequency converter
606
by the 2nd filter
607
, it is provided to the IF AGC amplifier
608
. The IF signal
609
amplified by the IF AGC amplifier
608
is provided to the orthogonal wave detector
610
and the signal level detector
614
.
The orthogonal wave detector
610
provides I and Q component signal which eliminated the carrier component to the demodulator
611
. The demodulator
611
recovers the transmitted digital signal data from two component signals, I and Q, and provides it to the error corrector
612
.
The error corrector
612
corrects the error in digital signal data, and provides it to the output terminal
613
.
After detecting signal level of IF signal
609
provided from the IF AGC amplifier
608
, the signal level detector
614
provides the error between the signal level and predetermined level, i.e., signal level error signal agcerr, to the loop filter
615
. The loop filter
615
integrates signal level error signal agcerr, and provides the AGC signal
616
which eliminated the noise component to the comparator
617
.
The comparator
617
compares the AGC signal
616
with predetermined level, i.e., the delay point level. When the AGC signal
616
is smaller than delay point level, the control signal is provided to the IF AGC amplifier
608
. When the AGC signal
616
is larger than delay point level, the control signal is provided to the RF AGC amplifier
602
.
The operation of the conventional automatic gain control system shown in
FIG. 13
is explained.
The RF AGC amplifier
602
and the IF AGC amplifier
608
have the gain property that the gain becomes small, when the control signal provided from the comparator
617
becomes large, respectively. Moreover, the signal level detector
614
has the characteristics that the output becomes large, when the signal level of IF signal
609
becomes larger than predetermined signal level.
For this reason, since the output of the signal level detector
614
becomes small when the signal level of IF signal
609
is small, it is controlled so that the gain of the IFAGC amplifier
608
or the RF AGC amplifier
602
becomes large and the signal level of IF signal
609
becomes large.
Moreover, since the output of the signal level detector
614
becomes large when the level of the input signal is large, it is controlled so that the gain of the IF AGC amplifier
608
or the RF AGC amplifier
602
becomes small and the signal level of IF signal
609
becomes small.
Thus, the signal level of IF signal
609
is controlled to desired level by changing the gain of the IF AGC amplifier
608
or the RF AGC amplifier
602
. This control is called automatic gain control.
It is decided on delay point level using which of the IF AGC amplifier
608
and the RF AGC amplifier
602
the automatic gain control is carried out.
When the AGC signal
616
is smaller than delay point level, the gain of the RF AGC amplifier
602
is fixed and the automatic gain control is performed by changing the gain of the IF AGC amplifier
608
.
When the automatic gain control signal is larger than delay point level, the gain of the IF AGC amplifier
608
is fixed and the automatic gain control is performed by changing the gain of the RF AGC amplifier
602
.
Thus, changing IF AGC and RF AGC according to the signal level of the input signal is performed in order to take the large control range of AGC, maintaining low noise characteristics. Namely, on the premise that the total gain of the IF AGC amplifier
608
and the RF AGC amplifier
602
is fixed, in the case of low gain of the RF AGC amplifier
602
and high gain of the IF AGC amplifier
608
, the noise factor (NF) becomes higher than the reverse case.
For this reason, when signal level is low, it is more advantageous to fix RF AGC on the maximum gain in respect of noise characteristics.
On the other hand, if the automatic gain control is applied only by the IF AGC amplifier
608
, since the control range of AGC is limited by the dynamic range of the IF AGC amplifier
608
, the range becomes narrow. Then, when signal level is high, the automatic gain control range can be made wide by applying AGC using the RF AGC amplifier
602
.
As explained above, the conventional automatic gain control system has the characteristics of having the wide control range by the low noise.
However, the conventional automatic gain control apparatus causes the following faults, when the high disturbance wave of signal level is in the adjacent channel.
Since there is no filter which limits the band of the signal before RF AGC, with the RF AGC amplifier
602
, the signal of the adjacent channel is also amplified with the signal of the desire wave. On the one hand, with the latter filter, band limitation of the automatic gain control signal is carried out, and it is generated according to the level of the signal of the desire wave.
For this reason, if the disturbance wave of high signal level is in the adjacent channel, the gain of the RF AGC amplifier
602
is too high, and may become the saturation state. The saturation state of the RF AGC amplifier
602
causes the distortion of the signal, and produces the remarkable degradation for the performance of the demodulator. Since the RF AGC amplifier
602
is saturated when the worst, the demodulation of the signal may not be made.
Thus, in the conventional automatic gain control system, when the adjacent disturbance wave of high signal level existed, there was a problem that the performance of the demodulator degraded remarkably.
SUMMARY
The inventions provide in part an automatic gain control method and its system which are the few circuit scale, and have low noise characteristics and high adjacent disturbance oppression characteristics, a
Koyama Takeshi
Seki Takashi
Taga Noboru
Bocure Tesfaldet
Kabushiki Kaisha Toshiba
Pillsbury & Winthrop LLP
Tran Khanh Cong
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