Data communications by wireless LAN (local area network) according to IEEE802.11 are now widely used. They are used as signal-transmitting means in place of personal computers (PCs); PC peripherals such as printers, hard disk drives, broadband routers, etc.; electronic appliances such as facsimiles, refrigerators, standard-definition televisions (SDTVs), high-definition televisions (HDTVs), digital cameras, digital video recorders, cell phones, etc.; and wired communications of automobiles and aircrafts, and wireless data transmission is conducted among these electronic appliances.
There are IEEE802.11a, IEEE802.11b and IEEE802.11g as the standards of wireless LAN. IEEE802.11a is adapted to high-speed data communications of 54 Mbps at maximum in a frequency band of 5 GHz, using an orthogonal frequency division multiplex (OFDM) system. IEEE802.11b is adapted to high-speed communications of 5.5 Mbps and 11 Mbps in an industrial, scientific and medical (ISM) band of 2.4 GHz that can be freely used without wireless license, using a direct sequence spread spectrum (DSSS) system. IEEE802.11g is adapted to high-speed data communications of 54 Mbps at maximum in a 2.4-GHz band like IEEE802.11b, using the OFDM system.
WO 03/092997 describes a multi-band communications apparatus for such wireless LAN. As shown in FIG. 15, a high-frequency circuit used in this multi-band communications apparatus comprises two dual-band antennas capable of transmitting and receiving signals in two communications systems (IEEE802.11a, IEEE802.11b) with different frequency bands, a high-frequency switch SW1 having four ports for switching the connections of transmission circuits and receiving circuits, a diplexer 3 disposed between one port of the high-frequency switch SW1 and the transmission circuits, and a diplexer 5 disposed between another port of the high-frequency switch SW1 and the receiving circuits, thereby being able to conduct diversity reception.
There is IEEE802.11h, a communications system making wireless LAN usable in Europe. This requires a transmission power control (TPC) function to reduce a transmission power, when good transmission can be obtained even though the transmission power is suppressed, for instance, because terminals are close to base stations. For this purpose, the output power should be able to be controlled more precisely than the conventional wireless LAN.
As shown in FIG. 15 of WO 03/092997, the conventional power control circuit for wireless LAN comprises a coupler between a power amplifier PA1 and a diplexer 3 for transmission signals, a detection signal from the coupler being rectified by a detector comprising a detection diode D2 and a smoothing circuit C2, R2, to control an output signal of RFIC based on the resultant detection voltage. This power control circuit requires a coupler, and a detector comprising a detection diode and a smoothing circuit for both of PA1 in a 2.4-GHz band and PA2 in a 5-GHz band, as well as an analog switch 6 for selecting detection voltage terminals for the 2.4-GHz band and the 5-GHz band. Accordingly, it needs many parts, resulting in difficulty in reducing the size of an communications apparatus.
To solve this problem, Japanese Patent 3,371,887 proposes a circuit comprising a coupler 7 connected to a common end of a diplexer 3, as shown in FIG. 16. This circuit has parts whose number is reduced to ½ or less of that in the detection circuit shown in FIG. 15, and can monitor an output power at a position near an antenna, resulting in improved detection precision. However, because both transmission powers in a 2.4-GHz band and a 5-GHz band should be detected in the circuit shown in FIG. 16, it is extremely difficult to keep the relation between output power and detection voltage constant, when the coupling degree of the coupler largely depends on a frequency.
A detection diode (Schottky diode) should be disposed to detect transmission powers in both 2.4-GHz and 5-GHz bands, but it generates harmonic signals due to its detection distortion. The harmonic signals mainly having frequencies 2 or 3 times those of the transmission signals are reflected toward the coupler, so that most of them are radiated from the antenna via the coupler. With respect to the frequency dependency of the coupling degree of the coupler, for instance, small, low-loss, laminated couplers generally have coupling degrees of −15 dB and −10 dB, respectively, in a 2.4-GHz band and a 5-GHz band, this difference directly affecting their detection voltages. In this case, the detection voltage is +0.5 V at 2.45 GHz and +1 V at 5.4 GHz, for instance, when there is an output of +20 dBm at an antenna end as shown in FIG. 14. Such variations of the detection voltage depending on the frequency should be reduced to enable the TPC function.