Telecommunications – Transmitter – With feedback of modulated output signal
Reexamination Certificate
1997-12-30
2001-09-11
Hunter, Daniel (Department: 2684)
Telecommunications
Transmitter
With feedback of modulated output signal
C455S127500, C455S116000, C330S129000, C330S279000
Reexamination Certificate
active
06289205
ABSTRACT:
BACKGROUND OF THE INVENTION
1. Field of the Invention
This invention relates to a transmitter for communication devices as presented in the preamble of claim
1
and to a mobile station as presented in the preamble of claim
9
.
2. Description of the Prior Art
The transmitters of communication devices have a high frequency power amplifier, in which the signal to be transmitted is amplified. The output of the high frequency power amplifier is connected to the adapter circuit of the antenna, in which circuit the impedance of the antenna is adapted to the output impedance of the high frequency power amplifier. The purpose of the adaption is, among other things, to prevent the formation of reflection waves from the antenna towards the high frequency power amplifier. However, high frequency power amplifiers are sensitive to load variations. Load variations cause distortion in the signal to be amplified, among other things. It is also possible that the high frequency power amplifier is damaged in difficult load conditions. In portable communication devices, in particular, the load variations of high frequency power amplifiers are due to the interaction between the antenna and the operation environment and changes in operating conditions. Metal objects in the vicinity of the antenna, for example, can remarkably change the antenna impedance of the portable communication device. This, in turn, has an effect on the operation point of the last stage of the high frequency power amplifier, whereby the transistor is exposed to large voltage and current variations. In time, these voltage and current variations can impair the performance of the output stage transistor of the high frequency power amplifier, and possibly also shorten its lifetime.
There are prior art solutions, in which the power signal formed by the high frequency power amplifier is measured by means of a directional coupler and a detector diode. For example,
FIG. 1
shows a prior art coupling, in which the directional coupler DIR
1
samples the power fed to the output. The samples are detected by a detector diode D
1
. A method like this, based on a directional coupler, operates well when the load impedance Z is constant. However, the method provides incorrect information in situations in which the load impedance varies, as usually happens when portable communication devices are used. In order to indicate this, the operation of the coupling in
FIG. 1
has been simulated. The simulation results can be seen in
FIGS. 2
a
-
2
e
. In this simulation, a bipolar transistor biased into the class AB was used as the power transistor T
1
of the output stage, and a harmonic trap was used to form the harmonics. Samples of the output power were taken by a directional coupler DIR
1
, and the samples were detected with a detector diode D
1
. The detector diode D
1
was biased to the linear region of operation, whereby the output power is proportional to the square of the voltage V
meas
formed by the detector diode D
1
.
Load variations are common in portable communication devices, such as mobile stations, because the interaction between the environment and the antenna cause load variations in the high frequency power amplifier. Table 1 shows various impedance values used in the simulation. In the first simulation, the value of the load impedance Z was such that it resulted in an optimum resistive load for the simulated amplifier. Different values of the load impedance Z were used in other simulations, resulting in an incorrect adaptation. The values used correspond to a reflection loss of −6 dB to a 6 ohmn load. Simulation results with different values of load impedance are shown in
FIGS. 2
a
-
2
e
. The power measurement was calibrated to produce the correct power reading with an output power of two watts.
FIGS. 2
a
-
2
e
show both the square of the voltage V
meas
formed by the detector diode D
1
and the output power P
out
of the amplifier in different load situations.
TABLE 1
Simulation no.
Value of the load impedance Z
1
6 &OHgr;
2
2 &OHgr;
3
18 &OHgr;
4
3.6 + j4.8 &OHgr;
5
3.6 − j4.8 &OHgr;
It can be seen from
FIG. 2
a
that the power measurement gives an accurate result in optimum load conditions. It can further be seen from
FIGS. 2
b
-
2
e
that when the load of the amplifier varies, the square of the detected voltage is no more the same as the power conveyed to the load, whereby the measurement does not give the correct idea of the load situation of the amplifier.
Still another disadvantage of using a directional coupler is the fact that a directional coupler causes power loss in the signal to be transmitted. In practical applications, the directional coupler is typically implemented by means of conductor traces incorporated directly on the printed circuit board (PCB), whereby the power loss of the directional coupler is typically approx. 0.5 dB. In addition, a directional coupler formed directly on the circuit board takes an unnecessarily large amount of space.
OBJECTS OF THE INVENTION
Measurement of the signal power is used to adjust the output power of high frequency transmitters. It is an object of the present invention to reduce the disadvantages presented above and to achieve a device for adjusting the power in the power amplifier of the transmitter of a communication device, and a mobile station in which the invention can be advantageously applied. The invention is based on the idea that voltage and current are measured at the output stage of the high frequency power amplifier, whereby the load impedance at the output stage can be calculated and the transmission power adjusted accordingly. The transmitter according to the invention is characterized in what is put forth in the characterizing part of claim
1
. A mobile station according to the invention is characterized in what is put forth in the characterizing part of claim
9
.
SUMMARY OF THE INVENTION
The present invention has many advantages compared to prior art transmitters and mobile stations. Preferably, the high frequency current running through the amplifier and the high frequency voltage at the output of the amplifier are measured, whereby the load impedance can be calculated very accurately and the output stage can be adjusted on the basis of this to the optimum point of operation. The high-frequency current and the high-frequency voltage are measured as close to the output of the last stage as possible, whereby potential transmission line losses and other losses having an effect on the measurement results can be eliminated and the reliability of the measurements improved compared to the measurements of prior art technique. In addition, it is possible to find out the real load during the transmission, which has an effect on the transistor of the output stage, and thereby to improve the adjustment of the optimum point of operation of the transistor in varying operating conditions. The efficiency of the transmitter according to the present invention is improved as compared to the prior art transmitters, because the measurement of high-frequency current and voltage does not cause a substantial power loss in the output signal. Due to better efficiency, the output power of the transmitter can be somewhat reduced. The measurement coupling can also be implemented in a small space by integrating it to the same semiconductor chip as the power transistor of the output stage of the power amplifier. Space is also saved on the circuit board, and the size of communication devices can be reduced. Potential variations of the load impedance are also taken into account in the measurements according to the invention, which also increases the reliability of the measurement.
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patent: 4602218 (1986-07-01), Vilmur et al.
patent: 4631491 (1986-12-01), Smithers
patent: 4994757 (1991-02-01), Bickley et al.
patent: 5101175 (1992-03-01), Vaisanen
patent: 5109538 (1992-04-01), Ikonen et al.
patent: 5118965 (1992-06-01), Vaisanen et al.
patent: 5119506 (1992-06-01), McGann
patent: 5128629 (1992-07-01), Trinh
patent:
Jarvinen Esko
Pollanen Ossi
Hunter Daniel
Nokia Mobile Phones Limited
Perman & Green LLP
Tran Pablo
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