Amplifiers – With semiconductor amplifying device – Including frequency-responsive means in the signal...
Reexamination Certificate
2000-07-07
2002-07-23
Mottola, Steven J. (Department: 2817)
Amplifiers
With semiconductor amplifying device
Including frequency-responsive means in the signal...
C330S294000, C330S305000
Reexamination Certificate
active
06424226
ABSTRACT:
BACKGROUND OF THE INVENTION
1. Field of the Invention
The invention relates to a signal processing system comprising an output circuit that is tuned to a first frequency, an output of which circuit being connected to an input of a two-port having non-linear amplification, and a frequency-selective external network, an output of the two-port being connected to an input of an input circuit that is tuned to a second frequency.
The invention further relates to a communications system arranged for processing signals in different frequency channels with a predetermined channel spacing, comprising, in succession, an output circuit that is tuned to a channel frequency an output of which being connected to an input of a two-port having non-linear amplification, and a frequency-selective external network, an output of the two-port being connected to an input of an input circuit that is tuned to a second frequency.
Systems of this type are known from the AMPS standard for mobile telecommunication, the two-port having the form of an output amplifier. With the known arrangements it is not very well possible to satisfy the requirements as to reception sensitivity made by the network managers when there is an external interference signal.
If a first and a second signal are available on a two-port that has non-linear amplification, intermodulation signals will arise due to this non-linear amplification. These intermodulation signals appear on the output of the two-port. If the spacing between the first frequency and the second frequency is equal to the spacing between the transmitting frequency of the system and the receiving frequency of the system, an intermodulation component will arise that is exactly equal to the receiving frequency. As a result, the signal-to-noise ratio of the received signal is reduced.
If either of the two signals available on the input of the two-port arrives via an internal feedback from the output to the input, a similar effect will arise. This effect is called Backward Intermodulation.
2. Description of the Related Art
A method of reducing specific components of intermodulation distortion is known from WO 96/25791 Moazzan et al. When two input signals available on the input of a two-port and, having a frequency F
1
and F
2
respectively, are amplified by a non-linear amplifier, an unwanted third-order IM product having frequency 2*F
1
−F
2
will arise. The signals and IM products will further be referenced by their frequencies. By mixing the second-order distortion component 2*F
1
of either of the input signals with the other input signal F
2
, a new mixed product will arise having a frequency equal to 2*F
1
−F
2
. This mixed product 2*F
1
−F
2
is added to the originally unwanted third-order IM product and, with suitable amplitude and phase, compensates for the unwanted third-order IM product. A disadvantage of this method is that both signals must be present on the input and that the second-order distortion component has a frequency that is twice as high as the system frequency, so that the correct amplitude and phase of the correction signal are hard to control. Furthermore, the transistor and associated adaptation networks are also to work at a frequency that is twice as high as the system frequency.
With an input signal F
1
having a frequency of, for example, 1800 MHz, and an input signal F
2
having a frequency of, for example, 1755 MHz, a third-order IM product will arise at 2*1800−1755=1845 MHz. The second harmonic distortion component lies at 2*1800=3600 MHz. If the frequency of input signal F
1
of 1755 MHz is mixed with this, a mixed product will arise having a frequency of 3600−1755=1845 MHz. This frequency is equal to the frequency of the third-order IM product. With anti-phase and equal amplitude relative to the unwanted third-order IM product, a reduction of this third-order IM product is possible.
SUMMARY OF THE INVENTION
It is an object of the invention to minimize the influence of the external interference signal on the receiving sensitivity by compensating a third-order IM product developed in the two-port in a manner that does not have the above identified disadvantages.
For this purpose, the system according to the invention is characterized in that the external network is tuned to a difference frequency between the first frequency and the second frequency and is connected at least to the input of the two-port and to the output of the two-port.
The invention is based on the following recognition:
Besides the unwanted third-order IM product 2*F
1
−F
2
, from a first input signal having frequency F
1
and a second input signal having frequency F
2
, with intermodulation as a result of the non-linear amplification of the two-port, also a second-order IM product F
1
−F
2
rises having a relatively low frequency. After this second-order IM product has been fed back and mixed with the input signal F
1
, inter alia the signal F
1
+(F
1
−F
2
) arises which, provided that a suitable choice of phase and amplitude is made, is capable of reducing the unwanted third-order IM product 2*F
1
−F
2
. As a result of the relatively low frequency of F
1
−F
2
, the phase and amplitude control becomes considerably simpler.
With a frequency of the first input signal F
1
of, for example, 1800 MHz and of the second input signal F
2
of, for example, 1755 MHz, a third-order IM product arises at 2*1800−1755=1845 MHz. If the receiving frequency is also 1845 MHz, the receiving sensitivity of the signal processing system is reduced. A second-order IM product F
1
−F
2
lies at 1800−1755=45 MHz. If this 45 MHz signal is mixed with the first transmit signal F
1
of 1800 MHz, there will also be 1800+45=1845 MHz, which is the frequency of the third-order IM product. If this signal is in anti-phase and has equal amplitude to the unwanted third-order IM product, a reduction of the unwanted signal is possible.
In the case of a communications system, the difference between the transmitting frequency and the receiving frequency is the channel spacing, while a similar effect may be obtained if the second-order IM product used for compensation lies at a frequency equal to the channel spacing. In the above example the transmitting frequency F
1
is, for example, 1800 MHz, the second input signal F
2
, for example, 1755 MHz and the receiving frequency of a nearby receiver, for example, 1845 MHz. The amplified third-order IM product, also at 1845 MHz, is radiated by the antenna and may reduce the receiving sensitivity of a nearby receiver. With the aid of the same measures as with the signal processing system, the third-order IM product may be reduced, so that a reduction of the receiving sensitivity of a nearby receiver is avoided.
In a first embodiment of the signal processing system according to the invention, two signals F
1
and F
2
are present on the input of the two-port. The resulting second-order IM product F
1
−F
2
on the output of the two-port is frequency-selectively applied with the correct phase and amplitude from the two-port output to the two-port input through the external network, at which input the product is mixed with the transmit signal F
1
by means of the non-linear amplification of the two-port. The resulting new mixed product F
1
+(F
1
−F
2
)=2*F
1
−F
2
has, when the external network is suitably selected, exactly the same amplitude, but an opposite phase to the unwanted third-order IM product 2*F
1
−F
2
. When the two signals are added together, the unwanted third-order IM product is compensated for. Since the external network is frequency-selective, mainly the second-order IM product is present on the input of the two-port, whereas the original input signals are fed back the least possible from the output to the input.
In a further embodiment of the signal processing system according to the invention, in which the two-port comprises an internal feedback, the input signal F
2
is not present
Koninklijke Philips Electronics , N.V.
Mak Theodorus N.
Mottola Steven J.
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