Telecommunications – Transmitter and receiver at same station – Radiotelephone equipment detail
Reexamination Certificate
2000-12-13
2004-12-28
Gelin, Jean (Department: 2681)
Telecommunications
Transmitter and receiver at same station
Radiotelephone equipment detail
C455S114200, C455S114300
Reexamination Certificate
active
06836671
ABSTRACT:
FIELD OF THE INVENTION
The present invention relates to feed-forward amplification circuitry and a method of providing feed-forward amplification. The invention finds application in cellular radio networks and, in particular, but not exclusively, those operating in accordance with the GSM standard.
BACKGROUND OF THE RELATED ART
In a cellular radio network each of a plurality of base transceiver stations serves a particular geographic area (a cell). When a transceiver terminal such as a mobile phone is in one of these areas it is served by the base station associated with that area. The base stations are designed to serve one or more transceiver terminals simultaneously. The base stations typically communicate with the transceiver terminals by transmitting signals within a predetermined transmission radio frequency band. Each of the base stations has amplification circuitry for amplifying signals before transmission to a transceiver terminal.
All amplifier circuits suffer from distortion. The level of distortion is dependent upon the design of the circuit and the conditions under which the circuit operates. In the case of a high power amplifier such as those typically used in base stations, intermodulation distortion (IMD) is the most significant form of distortion. Intermodulation distortion generates intermodulation product signals (INTERMOD) at frequencies which are a mixing function of the signals supplied to the amplifier. The most significant INTERMODs are the third order products. As an example, a signal for transmission may comprise two component signals at frequencies F
1
and F
2
which are to be transmitted. The amplifier will have an operational range which includes both F
1
and F
2
. The third order INTERMODs are generated by the mixing of the two frequencies by the amplifier and will be generated at frequencies 2F
1
-F
2
and 2F
2
-F
1
. These third order products will typically fall within the operational range of the amplifier and also within the transmission frequency band of the base station. These INTERMODs consequently form a source of noise in the transmitted signal.
The effect of INTERMODs can be reduced by making the amplifier operate in a more linear fashion. Amplifier linearity can be enhanced by using feed-forward compensation.
FIG. 1
illustrates a feed-forward amplifier circuit
1
. The feed-forward amplifier circuit
1
receives an input signal
11
at a radio frequency and-produces a compensated amplified signal
37
at a radio frequency. The feed-forward amplifier circuitry
1
contains a first hybrid
12
, a first phase trimmer
14
, delay circuitry
18
, a radio frequency power amplifier
20
, a directional coupler
22
functioning as a detector, a variable attenuator
24
, a second hybrid
26
, a second variable attenuator
28
, a second phase trimmer
30
, an amplifier
32
, second delay circuitry
34
and a second directional coupler
36
functioning as a combiner.
The input signal
11
is supplied to the first hybrid
12
. This 3 dB hybrid splits signal
11
into two paths. The first path supplies part of the input signal
11
as an input to the power amplifier
20
. The second path supplies a hybrid output signal
13
which is a proportion of the input signals
11
as an input to the first phase trimmer
14
. The hybrid output signal
13
has a phase difference with respect to the input signal
11
equivalent to +90°. The first phase trimmer
14
introduces a phase shift to its input signal, the hybrid output signal
13
, to produce a phase compensated signal
17
. The phase shift introduced by the first phase trimmer
14
may vary between 0-360°. The first phase trimmer
14
receives as a controlling input a first phase control signal
15
. The first phase control signal
15
controls the value of the phase shift introduced to the hybrid output signal
13
. The phase compensated signal
17
passes through delay circuitry
18
to produce the delayed phase compensated signal
19
which is supplied as a first input to the second-hybrid
26
.
The power amplifier
20
which receives input signal
11
from the first hybrid
12
produces amplified signal
21
. The amplified signal
21
passes through the first directional coupler
22
, and second delay circuitry
34
to produce a delayed amplified signal
35
. The directional coupler
22
detects the amplified signal
21
and produces a detected amplified signal
23
which is supplied as an input to the first variable attenuator
24
. The variable attenuator
24
reduces the power of the detected amplified signal
23
to produce the attenuated detected signal
39
which is supplied as a second input to the second hybrid
26
. The value of attenuation effected by the variable attenuator
24
is controlled by a first attenuation control signal
25
supplied to the variable attenuator
24
. The attenuator
24
ensures that the two signals
19
and
39
input to the second hybrid
26
are of a similar magnitude. The second hybrid
26
introduces a phase shift equivalent to +90 ° into the delayed phase compensated signal
19
and combines this signal with the attenuated detected signal
39
to produce their vector sum, the error signal
27
.
Referring to
FIG. 2
a
an illustrated example of a frequency spectrum for one type of exemplary input signal
11
is illustrated. According to this example the input signal
11
has two frequency components having frequencies F
1
and F
2
.
FIG. 2
b
illustrates the amplified signal
21
which may be formed when the input signal
11
illustrated in
FIG. 2
a
is passed through the power amplifier
20
. It can be seen that the amplified signal
21
has third order INTERMODs
21
a
and
21
b
at respectively frequencies 2F
1
-F
2
and 2F
2
-F
1
. These INTERMODs will typically lie within the band of frequencies at which the transmitter containing the amplifying circuit is designed to transmit. The circuitry described in
FIG. 1
isolates the additional frequency component which has been introduced to the input signal
11
by the amplifier
20
as the error signal
27
. The error signal
27
therefore has a frequency spectrum which is essentially the subtraction of the frequency spectrum of the input signal
11
illustrated in
FIG. 2
a
from the frequency spectrum of the amplified signal
21
illustrated in
FIG. 2
b
. The frequency spectrum of the error signal
27
therefore has a form similar to that illustrated in
FIG. 2
c
. It will therefore be appreciated that the error signal is created by taking samples of the input signal
11
and the amplified signal
21
and adjusting their relative phase and amplitude relationships to obtain cancellation of the undistorted components in the amplified signal
21
to leave remaining the distorted components of the amplified signal
21
as the error signal
27
. The first and second hybrids
12
and
26
introduce a phase shift equivalent to 180°. The phase trimmer
14
and delay circuitry
18
introduce a further, variable, phase shift which compensates for the different delays experienced between the signal being input at the first hybrid and received at the first input of the second hybrid and a signal being input at the input of the first hybrid and being received at the second input of the second hybrid.
Referring back to
FIG. 1
, the error signal
27
has its power modified by the second variable attenuator
28
. The phase of the error signal
27
is then varied relative to the amplified signal
21
by the second phase trimmer
30
. The error signal is then buffered by the amplifier
32
to produce a compensated error signal
33
. The second variable attenuator
28
receives at a control input a second attenuation control signal
29
which controls the power level of the compensated error signal
33
. The second phase trimmer
30
receives at a control input a second phase control signal
31
which controls the phase of the compensated error signal
33
relative to the delayed amplified signal
35
produced by the second delay circuitry
34
. The compensated error signal
33
is supplied as an input to
Haigh John Anthony
Small John
Soghomonian Manook
Gelin Jean
Squire Sanders & Dempsey L.L.P.
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