FIELD OF THE INVENTION
The present invention concerns an automatic gain control (AGC) arrangement
useful in a tuner.
BACKGROUND OF THE INVENTION
Present day tuners do not work sufficiently well for processing digital
signals received using various transmission systems. Both the RF and IF characteristics
of the receivers need improvement since a digital signal occupies the entire spectrum
of a channel, and delay and frequency response errors have a more severe effect
on digital signals than on analog signals. Moreover, since the transmitted power
of digital signals is considerably less than the transmitted power for analog
transmissions, acceptable reception of the desired signal can be more difficult
in the presence of strong adjacent channel signals. Further, the signal conditions
in the fringe area of over-the-air transmissions are also a problem. For fringe
areas, a few tenths of a dB loss of signal-to-noise ratio (SNR) or an increase
of intermodulation distortion, can make the signal recovery difficult, if not impossible.
Conventional automatic gain control AGC systems in TV receivers typically
are responsive to the level of the demodulated video signal. After comparison of
the demodulated video signal with a reference, error voltages are generated to
control the gain of the IF amplifier and RF stage of the tuner. To obtain a good
SNR over a wide range of input levels, it is a common practice to delay the application
of AGC to the tuner until relatively high signal levels are encountered. This
works well in the absence of strong adjacent signals. However, if the signal level
is low in the presence of strong adjacent signals, cross modulation with the strong
adjacent signals could occur in the mixer and the bit error rate (BER) of the demodulated
television information would increase.
Typically, there is a single AGC control signal for the IF and a
single AGC control signal for the RF, which may or may not be delayed. Additionally,
the AGC control signal for the RF typically is derived from the AGC control signal
for the IF. Although the relative amplitudes of the AGC control signals can be
made different for the various stages by the use of dividers, the overall ratios
between the various stages remains constant for various signal levels. More particularly,
the AGC control signal for the RF is not individually adjustable according to the
characteristics of the input signal, especially for optimising the tuner characteristics
with respect to adjacent channel signals and SNR.
Finally, the ultimate criterion for the quality of digital reception
id the BER, which is effected by SNR and intermodulation distortion. Currently,
there is much research being conducted to develop improved devices to minimize
such difficulties in the tuner. However, even if such new devices are successful,
the limitations of the mixer will still be troublesome. Thus, it is desirable that
the quality of the signal coupled to various stages of a tuner be optimized independently
of the devices used in the various tuner stages.
An example of apparatus providing control of RF circuits prior to
mixing is described by Erat in EPO application EP-A-0 455 974 A2 entitled CONTROL
CIRCUIT FOR SUPER-HETERODYNE RECEIVER which was published 13 November 1991. The
receiver includes a preselector stage coupled to apply RF signals to a mixer. The
preselector stage comprises an attenuator to which an antenna is connected. The
attenuator is coupled to the mixer via a series connection of three tuned circuits
which may be tuned by voltage-variable diodes. An amplifier (of fixed gain) is
coupled between the first and second tuned circuits.
The amplitude of signals before and after mixing (in the IF path)
are measured and applied to a control signal generator which provides an indication
of the presence or absence of noise in the received signal. If noise is absent,
the attenuator is made inactive. If noise is present, the attenuator is turned
on. In the case where the input signal is noisy and of high level, the generator
also sends correction signals to the filters to correct for detuning.
An example of a receiver in which signal carrier to noise ratio is
used for filter control is described by Nonaka et al. in Patent Abstracts of Japan,
vol. 017, no. 371 (E-1396), 13.07 1993 and JP-A-05056434 (Sony Corp.), 05.03.93
entitled SATELLITE BROADCAST RECEIVING DEVICE. This receiver includes switching
circuitry which inserts a noise reduction filter at the output of a video processor
under conditions of low carrier to noise ratio (C/N) and by-passes the filter otherwise.
The receiver comprises an "operating" circuit which operates the noise level of
each channel from the receiver IF AGC voltage at the time of an empty channel selection,
a memory that memorizes the result of the operation, a comparator which compares
the noise level of the currently selected channel as memorized and the AGC voltage
SUMMARY OF THE INVENTION
that shows the current carrier level and a discriminating circuit
that discriminates whether noise reduction processing is to be carried out or not
in the noise reduction circuit from the C/N value that is outputted from the comparator
A tuner is presented in which there are two separate RF AGC gain controllable
amplifier stages desirably separated by a non-AGC gain controllable stage. Each
of the RF AGC stages are individually controlled by respective individual AGC control
signals generated by an AGC controller so that the level of the output signal from
each of the RF AGC gain controllable amplifiers is individually optimised for tuner
The present invention has application to tuning apparatus of a type
comprising a source of an input signal; a local oscillator ; a mixer coupled to
the local oscillator for converting the input signal into an intermediate frequency
(IF) signal; and an IF processing means including a detector coupled to the mixer
for detecting information on the IF signal.
The invention is characterized by a plurality of gain-controlled amplifiers
coupled in series between the source and the mixer for amplifying the input signal;
and control means for generating control signals for respective ones of the plurality
of gain-controlled amplifiers in response to an output of the mixer and an output
of the detector.
In accordance with a feature of the invention, the apparatus is further
characterized in that the control means further generates a modification signal
for modifying the operation of one of the plurality of gain-controlled amplifiers.
In accordance with another feature of the invention, the apparatus
is further characterized in that the IF processing means further includes an IF
gain-controlled amplifier; and the control means generates another control signal
for the IF gain-controlled amplifier in response to an output of the mixer and
an output of the detector.
BRIEF DESCRIPTION OF THE DRAWINGS
DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENT HARDWARE
- Fig. 1 is a block diagram showing an RF section, an IF section and an AGC section
of a receiver according to the prior art.
- Fig. 2 is a block diagram showing an RF sections an IF section and AGC section
of a receiver according to aspects of the present invention.
- Fig. 3 shows a modification of the AGC arrangement of Fig.2 according to aspects
of the present invention.
Fig. 1 shows an RF, IF and AGC circuit arrangement according to the
prior art. A signal source is coupled to input 10 and filtered by input filter
12. The signal from input filter 12 is coupled to amplifier 14, the gain of which
is AGC controllable. The amplified signal from amplifier 14 is coupled to an interstage
filter 16 and to mixer 18 where it is mixed with the signal generated by local
oscillator 20 for producing the IF signal at lead 21. The IF signal is processed
and amplified by IF section 22 which includes a gain controllable AGC amplifier
and a video detector. A detected video output signal at lead 23 is coupled to AGC
generator 24 to provide a responsive AGC control signal.
A version of the AGC control signal is coupled to the IF section
at lead 25 to adjust the gain of the IF section to keep the video signal at lead
23 at a reasonably constant level for variations of source signal level at RF input
terminal 10. If the level of signal at terminal 10 is very high, a delayed AGC
control signal is coupled via lead 26 to RF AGC gain controllable amplifier 14.
This delayed AGC control signal is derived from the IF AGC control signal except
that it is applied when a higher threshold signal level is reached. The delay is
provided so that the maximum gain at the amplifier 14 is maintained for as long
as possible to maintain a good SNR.
Fig. 2 shows a circuit arrangement, according to aspects of the present
invention, wherein members common with Fig. 1 are given like designations. A second
AGC gain controllable amplifier 28 is included in the RF section prior to the mixer.
Amplifier 28 is separated from the first AGC controllable amplifier 14 by, in the
present embodiment, a non-AGC controllable interstage filter 16, or by a non-AGC
controllable amplifier (not shown). In this arrangement, the output signal from
amplifier 14 is optimized for the interstage filter 16 and the output signal from
the second AGC gain controllable amplifier 28 is optimized for mixer 18. An AGC
controller 30 has input signals from lead 21 which couples the converted IF output
signal directly from mixer 18, the detected video signal at lead 23, and an indication
of the BER from the digital decoder (not shown). In response to these input signals,
AGC controller 30 provides individually adjustable AGC 1, 2 and 3 control signals
which are coupled to respective AGC 1, 2 and 3 gain controllable amplifiers 14,
28 and 22. AGC controller 30 can include a microprocessor which analyzes the characteristics
of its various input signals, as discussed above, and adjusts each of the AGC control
signals for providing optimum tuner characteristics according to a conditions
programmed into an EPROM (not shown).
Fig. 3 shows a modification of Fig. 2 where an AGC modified control
signal is coupled from AGC controller 30 to AGC 1 amplifier 14 in addition to the
AGC 1 control signal. An AGC modified control signal is generated by the AGC controller
and coupled to the AGC 1 amplifier. This AGC modified control signal: 1) can either
modify the operation of the AGC 1 amplifier's response to the AGC 1 control signal,
or 2) change the gain of the AGC 1 amplifier in a manner different from the AGC
1 control signal. Further, if warranted, as a third alternative, the AGC 1 stage
can be bypassed if the input signal is very strong, e.g., by actuation of a switch,
e.g., a diode, in response to the AGC modified signal. Modifications (1) and (2)
of the AGC operation on the first AGC amplifier stage would be actuated by the
microcontroller only when a weak digital signal is received and a further improvement
of the SNR is desirable. The third modification can be initiated when the input
signal is so strong that the first AGC amplifier may be overloaded.
Suitable digital decoders for providing the BER signal or the equivalent,
i.e., a signal which is indicative of the BER, are a STEL-2030B decoder available
from Stanford Telecommunication Company of Sunnyvale, California, U.S.A. or an
AHA 4210 decoder available from Advanced Hardware Architecture of Pullman, Washington,
U.S.A. Such a signal indicative of the BER could be a signal indicative of the
amount of error correction being performed or a symbol error signal derived from
a Viterbi decoder.
It has been found, according to aspects of the present invention,
that the providing of individually adjustable AGC control signals to each of a
plurality of AGC adjustable amplifier stages prior to the mixer, improves the performance
of the tuner. For example, a 3dB signal reduction after a tuner first amplifier
reduces second order distortions in the output signal from the mixer by 6dB and
third order distortions by 9dB, with little degradation of the SNR. By distributing
the AGC responsive gain reduction over two AGC stages in the tuner before the
mixer and one AGC section in the IF section after the mixer, distortions and the
BER of the system can be optimized.
The peak signal level of the mixer provides a reasonable criteria
for mixer distortions, and such a level can be measured at the unfiltered IF output
signal at the mixer. Additionally, as in conventional systems, the level of the
output signal at the video detector is also important. The magnitudes of these
two signals, i.e., the unfiltered IF output signal at the mixer and the output
signal at the video detector, provide information about the nature of the received
signal. For example, selecting a channel with a low level signal but with strong
adjacent channel signals produces a low level output signal at the video detector
and a high level output signal at the mixer. On the other hand, a weak selected
signal without adjacent channel signals causes both levels to be low.
From the measured signal levels, an intelligent controller can calculate
and adjust the three AGC control signals to substantially reduce distortion. For
example, if the attenuation in the second AGC in the tuner is reduced, but the
attenuation of the third AGC in the IF is increased to maintain the same signal
level, the SNR would improve, but more intermodulation distortion is produced.
Once such operations are accomplished, either the BER or a similar error indicative
signal, both of which are often readily available from a digital decoder (not shown),
could be used as the final criteria for the fine adjustment of the AGC stages,
i.e., to verify whether or not the operation was beneficial. For example, if the
BER or other error indicative signal is above a threshold, the microprocessor controller
modifies the three AGC control signal voltages until the BER becomes minimum or
falls below a threshold value.
It should be noted that this arrangement is also applicable to standard
analog received signals. The effect on the change of SNR for standard analog reception
may be little importance. However, even a slight decrease in SNR can have a detrimental
effect for digital signals received at or close to a threshold level. For such
a situation, the value of the AGC control signal coupled to the first AGC stage
can be chosen so that the gain of the AGC stage is not reduced.