PC radio card capable of operating at a nonstandard power output level by limiting the current being drawn from a power amplifier

The present invention relates to personal computer (PC) card radio modems. Small hand-held personal computing devices can use Personal Computer Memory Card Interface Association (PCMCIA) radio modems to connect to a wireless network. PC card radio modems typically use battery power from the hand-held PC or from a battery pack extension.

A problem that exists in prior-art systems in which the wireless modem operates on a network such as the North American AMPS or CDPD network, is a battery pack extension is used to supplement the power supplied to the wireless modem. Typically, hand-held PCs use two AA batteries, which suffer from low capacity and a low peak current capability.

Six different standard RF power levels are required by the CDPD standard (CDPD R1.1 Part 409) and the AMPS standard (IS-19B) for a class III cellular device. With two AA batteries supplying the hand-held PC and the PCMCIA wireless card, it is difficult to achieve the highest RF power level; which, for a class III cellular device, is an output power of 28 dBm where 0 dBm is 1 mW output power. Typically, prior art uses a battery pack extension on the PCMCIA card to achieve the highest RF power output. This is an undesirable solution as the esthetics of the complete product are poor.

It is desirable to have an improved system, which avoids some of the problems of the prior art.

In one embodiment, the present invention relates to a PC card radio transceiver which uses a non-standard power output level. The non-standard power output level is limited by the peak sustainable current available from the battery inside the hand-held PC. In one embodiment, this current is limited to 600 mA. The non-standard RF power obtainable, given a 600 mA maximum limit, is typically between 24 dBm and 28 dBm (in one implementation, 26.5 dBm). The non-standard power output level has a nominal value less than the nominal value required by the standard, yet is still within the acceptable range (+2, −4 dB) specified in the standard.

The advantage of the present invention is that the radio transceiver in the PC card can operate while attached to a hand-held PC without requiring a battery pack extension. The system allows the two AA batteries of the hand-held PC to produce an acceptable, high-level RF output power.

Implementation of a non-standard RF power level, which is based on a current restriction, requires the automatic leveling circuit (ALC) of the power amplifier to be a current leveling circuit as opposed to a power leveling circuit as in prior art systems. A current leveling circuit achieves the same effect as a power leveling circuit, given the load on the power amplifier remains constant. In practice, this is a reasonable assumption.

FIG. 1 is a block diagram illustrating a PCMCIA cellular modem attached to a hand-held PC.

FIG. 2 is a diagram illustrating the power amplifier, current leveling circuit, duplexer and antenna used in the PC card cellular radio transceiver of one embodiment of present invention.

FIG. 3 is a diagram illustrating both the implemented and non-implemented standard power output levels as well as the non-standard power output level of one embodiment of present invention.

FIG. 4 is a block diagram illustrating one embodiment of the current leveling system sent invention.

FIG. 5 is a schematic illustrating one embodiment of the current leveling circuit for one embodiment of the present invention.

FIG. 1 is a block diagram that illustrates a hand-held personal computer 20 and PC card 22 . In one embodiment, the PC card is a cellular modem with a built-in radio transceiver.

The hand-held PC 20 sends signals to be transmitted to the PC card cellular modem 22 . Additionally, the hand-held PC supplies power for the operation of the PC card cellular modem 22 . Typically, two AA batteries in the hand-held PC power both the hand-held PC and the PC card cellular modem 22 . Since relatively small batteries are used, the maximum current which can be drawn by the PC card cellular modem 22 is limited to less than what is needed to meet the nominal output power requirements of IS-19B and CDPD Part 409 standards given present power amplifier and duplexer technologies. In one embodiment, the PC card cellular modem 22 should not draw more than 600 mA of current.

FIG. 2 is a simplified diagram of the elements in the PC card cellular modem of one embodiment of the present invention. A current leveling circuit 32 controls the current supplied to power amplifier 30 and hence indirectly controls the output power of amplifier 30 . In a PC card cellular modem, the current supplied to the power amplifier makes up a large percentage of the total current consumed by the device. The output of the power amplifier 30 is sent to a duplexer 34 , then out the antenna 36 .

In the present invention, the current leveling circuit 32 limits the current used by the power amplifier 30 at the highest power output level. This highest power output level is the non-standard power output level described below. In one embodiment, the current leveling circuit 32 limits the current drawn by the power amplifier 30 to 400 mA when it is transmitting at the highest output power level. The current consumption of the entire card is therefore limited to 600 mA.

FIG. 3 illustrates the RF output levels for the standard and non-standard power outputs for CDPD standard R.1.1 Part 409 and AMPS standard IS-19B. There are six standard output power levels for a class III cellular device. The standard, nominal power output levels range from 8 to 28 dBm in 4 dB steps. In one embodiment, the non-standard power output level is between the highest standard power level, which is not implemented (28 dBm), and the highest implemented power level (24 dBm). In one embodiment, by limiting the current draw by the power amplifier to 400 mA or less (the total PC card current draw to 600 mA or less), the non-standard power output level is about 26.5 dBm. This level is below the nominal 28 dBm power level specified in the standard, yet still falls within the acceptable range of +2 dB to −4 dB from nominal specified in the standard.

FIG. 4 is a block diagram of a circuit to implement one embodiment of the present invention. A set point value 48 is sent to adder 40 and combined with a feedback value on line 44 from current-sensor 42 to produce a gate bias (power control) signal for power amplifier 46 . The set point values are controlled by the micro-processor. Each set point value corresponds to a power output level for the power amplifier 46 . The current leveling circuit limits the power output by the power amplifier. In a preferred embodiment, the maximum set point value is such that the current drawn by the power amplifier is less than 400 mA.

FIG. 5 illustrates details of one embodiment of the current leveling circuitry for operation with the PC card radio transceiver of the present invention.

Other modifications and implementations will occur to those skilled in the art, without departing from the spirit and scope of the present invention. Accordingly, the above description is not intended to limit the invention, which is to be limited only by the following claims.