BACKGROUND OF THE INVENTION 
     I. Field of the Invention 
     The present invention relates to radio communications. More particularly, the present invention relates to radios having the capability of communicating over more than one frequency simultaneously. 
     II. Description of the Related Art 
     There are presently numerous different radiotelephone systems. The cellular analog advanced mobile phone system (AMPS), the two digital cellular systems: code division multiple access (CDMA) and time division multiple access (TDMA), or the new personal communication systems (PCS) that can use both TDMA and CDMA technologies. The CDMA cellular system is described in greater detail in Telecommunications Industry Association/Electronic Industries Association (TIA/EIA) Interim Standard IS-95. 
     The CDMA cellular system and the CDMA PCS share some common attributes. They are typically composed of numerous fixed base stations, each base station transmitting over a forward channel in a cellular area to one or more mobile radios. 
     The cell&#39;s base station is connected to the public switched telephone network (PSTN). This enables a mobile radio transmitting within the cell, over a reverse channel, to communicate with a land line telephone through the base station. Additionally, a mobile radio can communicate through the base stations and the PSTN to another mobile radio in the same cell or another cell. 
     In a CDMA cellular telephone system or a CDMA PCS, a common frequency band is used for communication with all base stations in a system. The common frequency band allows simultaneous communication between a mobile radio and more than one base station. The transmitters operate at a low power allowing the frequencies to be reused in nearby systems without substantial interference. 
     Signals occupying the common frequency band are discriminated at the receiving terminal (either within the mobile radio or base station) through the spread spectrum CDMA waveform properties based on the use of high speed pseudo noise (PN) codes and orthogonal Walsh codes. The high speed PN codes and orthogonal Walsh codes are used to modulate signals transmitted from the base stations and the mobile radios. Transmitting terminals (either within a mobile radio or within a base station), using different PN codes or PN codes that are offset in time, produce signals that can be separately received at the receiving terminal. 
     In a typical CDMA system, each base station transmits a pilot signal having a common PN spreading code that is offset in code phase from the pilot signal of other base stations. During system operation, the mobile radio is provided with a list of code phase offsets corresponding to neighboring base stations surrounding the base station through which communication is established. The mobile radio is equipped with a searching element that allows the mobile radio to acquire and track the signal strength of the pilot signal from a group of base stations including the neighboring base stations. 
     CDMA technology provides for soft hand-off between cells across one frequency by the changing of code phase offsets. When there is a need to use more than one frequency so that a hand-off between two frequencies is required, a hard hand-off is performed. Hand-off between sectors of one cell across one frequency is referred to in the art as a softer hand-off. 
     A method and system for providing a communication with the mobile radio through more than one base station during the hand-off process are disclosed in U.S. Pat. No. 5,267,261 issued Nov. 30, 1993, titled Mobile Assisted Soft Hand-Off In a CDMA Cellular Telephone System and assigned to the assignee of the present invention. Using this system, communication between the mobile radio and the end user is uninterrupted by the eventual hand-off from an original base station to a subsequent base station. This type of hand-off may be considered as a &#34;soft&#34; hand-off in that communication with the subsequent base station is established before communication with the original base station is terminated. When the mobile radio is in communication with two base stations, a single signal for the end user is created from the signals from each base station by a cellular or personal communication system controller. 
     Mobile radio assisted soft hand-off operates based on the pilot signal strength of several sets of base stations as measured by the mobile radio. The Active Set is the set of base stations through which active communication is established. The Neighbor Set is a set of base stations surrounding an active base station comprising base stations that have a high probability having a pilot signal strength of sufficient level to establish communication. The Candidate Set is a set of base stations having a pilot signal strength of sufficient level to establish communication. 
     When communications are initially established, a mobile radio communicates through a first base station and the Active Set contains only the first base station. The mobile radio monitors the pilot signal strength of the base stations of the Active Set, the Candidate Set, and the Neighbor Set. When a pilot signal of a base station in the Neighbor Set exceeds a predetermined threshold level, the base station is added to the Candidate Set and removed from the Neighbor Set at the mobile radio. 
     The mobile radio communicates a message to the first base station identifying the new base station. A cellular or PCS controller decides whether to establish communication between the new base station and the mobile radio. Should the cellular or PCS controller decide to do so, the controller sends a message to the new base station with identifying information about the mobile radio and a command to establish communications with the mobile radio. 
     A message is also transmitted to the mobile radio through the first base station. The message identifies a new Active Set that includes the first and the new base stations. The mobile radio searches for the new base station&#39;s transmitted information signal and communication is established with the new base station without termination of communication through the first base station. This process can continue with additional base stations. 
     When the mobile radio is communicating through multiple base stations, it continues to monitor the signal strength of the base stations of the Active Set, the Candidate Set, and the Neighbor Set. Should the signal strength corresponding to a base station of the Active Set drop below a predetermined threshold for a predetermined period of time, the mobile radio generates and transmits a message to report the event. The cellular or PCS controller receives this message through at least one of the base stations with which the mobile radio is communicating. The controller may decide to terminate communications through the base station having a weak pilot signal strength. 
     The controller, upon deciding to terminate communications through a base station, generates a message identifying a new Active Set of base stations. The new Active Set does not contain the base station through which communication is to be terminated. The base stations through which communication is established send a message to the mobile radio. The controller also communicates information to the base station to terminate communications with the mobile radio. The mobile radio communications are thus routed only through base stations identified in the new Active Set. 
     Because the mobile radio is communicating with the end user through at least one base station at all times throughout the soft hand-off process, no interruption in communications occurs between the mobile radio and the end user. A soft hand-off provides significant benefits in its inherent &#34;make before break&#34; communication over conventional hard hand-off or &#34;break before make&#34; techniques employed in other cellular communication systems. 
     When a mobile radio moves from one cell to another, the radio may need to change frequencies, i.e., execute a hard hand-off. This frequency change in PCS may be due to the use of Operational Fixed Services (OFS) that share the PCS spectrum. Near these OFS&#39;s, the PCS mobile cannot use the OFS frequency in order to avoid interference. The PCS mobile, therefore, has to change frequencies in these areas. 
     In handing-off from one frequency to another, the mobile radio searches the Neighbor Set for another pilot channel, synchronizing channel, paging channel, and traffic channel. If only the pilot and/or synchronizing channels are present, the mobile radio moves on to the next frequency. 
     The problem with a soft hand-off from one frequency to another is that as the mobile radio searches the Neighbor Set for other pilot channels, the synthesizer must change frequencies rapidly while allowing a settling time of 2 milliseconds on the frequency to enable the frequency to stabilize. This is difficult to accomplish and requires a more complex design to do so. Additionally, the mobile must leave the frequency being used, causing an interruption in communications. There is a resulting need for an economical radio that can rapidly communicate over multiple frequencies, thus allowing the mobile radio to efficiently perform a soft hand-off between frequencies. 
     SUMMARY OF THE INVENTION 
     The present invention encompasses a multiple band radio that can transmit and receive multiple frequency signals simultaneously. The radio has a transmit path and a receive path. The transmit path is comprised of a plurality of mixing paths. Each mixing path has an amplifier whose input is coupled to the signal to be transmitted. The output of each amplifier is coupled to an input of a mixer. Another input of the mixers is coupled to an output of a frequency synthesizer. The resulting signals from the mixers are summed by a summer. The sum signal is input to a power amplifier which inputs the signal to an antenna to be radiated. 
     The receive path is comprised of an amplifier coupled to the antenna for amplifying received signals. The output of the amplifier is input to a plurality of down converting paths. Each down converting path has a mixer coupled to the amplified signal. Another input of each mixer is coupled to the frequency synthesizers. The resulting down converted signals are input to filters. The output of each filters is input to a variable gain amplifier. The amplified signals from the down converting paths are input to a summer. The sum signal is then input to a common filter that generates a signal for use by the rest of the radio. 
     The multiple frequency synthesizers in combination with the multiple transmit and receive paths enables the apparatus of the present invention to transmit and receive on multiple frequencies simultaneously, communicate on one frequency while searching others, and to perform a soft hand-off between frequencies. This alleviates the problem of the prior art of settling time of a frequency synthesizer since the frequency synthesizers do not have to change frequencies as rapidly and as frequently as previously required. It also avoids an interruption in communications, as normally results from monitoring one frequency at a time. 
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS 
     FIG. 1 shows a block diagram of the multiple band radio of the present invention. 
     FIG. 2 shows an alternate embodiment of the multiple band radio of the present invention. 
     FIG. 3 shows another alternate embodiment of the multiple band radio of the present invention. 
    
    
     DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENT 
     The apparatus and method of the present invention enables a mobile radio to operate on multiple frequencies. By increasing the number of intermediate frequency paths in the radio and separately mixing each signal to be transmitted to different frequencies, the number of frequencies that the radio can communicate over is increased. The use of CDMA technology then allows these signals to be separated later. 
     The apparatus of the present invention is illustrated in FIG. 1. The apparatus is comprised of a transmit path (103) and a receive path (104). Both the transmit (103) and receive paths (104) have a common automatic gain control (AGC) amplifier (101 and 102) for amplifying a signal at the intermediate frequency. In the preferred embodiment, the receive intermediate frequency is 85 MHz and transmit intermediate frequency is 130 MHz. Alternate embodiments use other intermediate frequencies. 
     The common AGC amplifiers (101 and 102) are used for both open loop power control and closed loop power control of the radio. Open loop power control is explained in greater detail in U.S. Pat. No. 5,056,109 to Gilhousen et al. and assigned to Qualcomm, Incorporated. Open loop power control is accomplished by the radio estimating the path loss of the forward link based on the total power received by the radio. The total power is the sum of the power from all base stations operating on the same frequency assignment as perceived by the radio. From the estimate of the average forward channel loss, the radio sets the transmit level of the reverse channel signal to compensate for the channel loss. Closed loop power control is accomplished through commands from the base station. 
     The apparatus of the present invention performs this power control using the common AGC amplifiers (101 and 102). When a signal is received by the radio, the gain of the receive common AGC amplifier (102) is adjusted so that the gain of the receiver is substantially equal to the gain of the transmitter minus 73 dB. The difference is the estimated path loss. 
     The transmit path (103) of the apparatus of the present invention is further comprised of multiple mixing paths (110 and 115). In the preferred embodiment, there are two mixing paths (110 and 115) enabling the radio to communicate on two different frequencies simultaneously. Alternate embodiments could use more than two mixing paths to enable the radio to communicate with a larger number of base stations. 
     Each mixing path (110 and 115) contains a differential AGC amplifier (120 and 125) each feeding the input of a mixer (130 and 135). These amplifiers (120 and 125) have a variable gain that is adjustable over a 20 dB range, in the preferred embodiment. Alternate embodiments have different ranges for the amplifier gain. 
     The inputs to the differential AGC amplifiers (120 and 125) are coupled to the output of the transmit common AGC amplifier (101). The differential AGC amplifiers (120 and 125) amplify the signal to be transmitted. During normal operation of the radio, the gain of one of the amplifiers is set to zero. When the radio is handing-off or searching another frequency, the gains are approximately equal. If it is desired to change the hand-off region of the system, one gain can be increased over the other. This increases the transmit power of one signal over the other and therefore the distance the radio can operate from the base station using the frequency of the higher power signal. 
     Frequency synthesizers (140 and 145) are coupled to the other inputs of the mixers (130 and 135). These synthesizers (140 and 145), in the preferred embodiment, are variable frequency synthesizers that cover the frequency spectrum set aside for either the cellular radiotelephone systems or the personal communication systems. The frequency output by the synthesizers (140 and 145) is controlled by the radio&#39;s microcontroller. The radio receives instructions from the base stations on what frequency to operate and the microcontroller varies the frequency of the synthesizers (140 and 145) so that the radio transmits and receives at these frequencies. 
     Each mixer (130 and 135) in the mixing paths (110 and 115) multiplies the signal from its respective differential AGC amplifier (120 or 125) with the signal from the respective frequency synthesizer (140 or 145). The outputs of both mixers (130 and 135) are combined by a summer (160). The sum signal is amplified by a power amplifier (165). In the preferred embodiment, the amplifier (165) is set at a gain of approximately 30 dB. Alternate embodiments use other gains depending on the noise levels of the components. 
     The amplified signal is input to a duplexer (170) that is connected to an antenna (175). The duplexer (170) enables the antenna (175) to be connected to both the transmit (103) and receive paths (104) by separating the transmitted signals from the received signals. 
     The receive path (104) is comprised of a low noise amplifier (180) feeding multiple down converting paths (116 and 117), each path down converting a received signal to the same IF frequency. The low noise amplifier amplifies the received signal by a gain of 20 dB in the preferred embodiment. 
     In the preferred embodiment, the amplified signal is input to the two down converting paths (116 and 117). Alternate embodiments use more down converting paths if it is desired to communicate with more than two base stations simultaneously. 
     Each down converting path (116 and 117) is comprised of a mixer (185 and 190) that combines the frequency from one of the frequency synthesizers (140 or 145) with the received, amplified signal. Therefore, if a mixing path (110 or 115) operates at a frequency of 850 MHz, there is a corresponding down converting path (116 or 117) that also operates at that frequency offset by the duplexor offset. Band pass filters (122 and 132) are used to filter the signals from the mixers (185 and 190). 
     The outputs of the bandpass filters (122 and 132) are each amplified by a differential amplifier (142 and 152). The amplifiers (142 and 152) operate in a similar fashion to the differential amplifiers (120 and 125) in the transmit path. The receive differential amplifiers (142 and 152) normally have a gain that is approximately equal. The gain of one can be offset from the other, however, to emphasize one signal frequency over the other. This enables the mobile to monitor either frequency channel or both at once. 
     The outputs of the receive differential amplifiers (142 and 152) are input to a summer (162) that adds them together. The sum signal from the summer (162) is input to a bandpass filter (172) for filtering. In the preferred embodiment, this bandpass filter (172) is a surface acoustic wave (SAW) filter. The filtered signal is input to the common AGC amplifier (102) that was explained in greater detail above. The amplified signal from this amplifier (102) is then input to the radio&#39;s circuitry for further processing as is already known in the art. TIA/EIA IS-95 describes this processing in greater detail. 
     If the differential AGC amplifiers (120, 125, 142, and 152) are always set equal, in other words the hand-off region is always at the equal power point, the amplifiers (120, 125, 142, and 152) can be replaced by switches (220, 225, 242, and 252). Such an embodiment is illustrated in FIG. 2. The switches can take the form of diodes, transistors, relays, or other switch devices to allow the circuit to be simplified. 
     This alternate embodiment operates in a similar fashion to the preferred embodiment, the difference being the switches. The switch position is controlled by the radio&#39;s microcontroller, depending on the number of frequencies required by the radio. If the radio is not operating near the hand-off region of the cellular system, only one frequency is required and, therefore, only one switch in each path is closed. As the radio approaches the hand-off region, the second switch in each path is closed to enable the radio to communicate over multiple frequencies. 
     Yet another alternate embodiment is illustrated in FIG. 3. The structure and operation of the receive path of this embodiment is the same as the preferred embodiment. The transmit path (301) of this embodiment, however, is comprised of the common AGC amplifier (302), performing the same closed loop power control function as in the preferred embodiment, a mixer (310), a bandpass filter (330), and a power amplifier (315). 
     Two frequency synthesizers (320 and 325) each generate a signal having a different frequency. A switch or multiplexer (330) connects both of the frequency synthesizers to the mixer (310). The switch is controlled by the radio&#39;s microcontroller as are the frequency synthesizers (320 and 325). The radio can now rapidly switch between the first frequency synthesizer (320) and the second frequency synthesizer (325) as required by the frequency of each base station with which the radio is communicating. These frequencies are determined by the received signals since this alternate embodiment can still receive on multiple frequencies. 
     The output of the amplifier (302) is input to the mixer (310). The other input of the mixer (310) is connected to the switch (330). When synthesizer 1 (320) is needed, the switch connects it to the mixer (310). When synthesizer 2 (325) is needed, the switch (330) disconnects synthesizer 1 (320) and connects synthesizer 2 (325) to the mixer (310). If in other alternate embodiments additional synthesizers are used, the operation of the switch would be the same. 
     The bandpass filter (330) filters the output of the mixer (310). As in the preferred embodiment, the pass band of the filter (330) is adjusted depending on the signal desired from the mixer (310). 
     The output of the bandpass filter (330) is input to a power amplifier (315). As in the preferred embodiment, this amplifier is adjusted to the desired transmit power required for the cellular radio system in which the apparatus of the present invention operates. 
     The alternate embodiment of FIG. 3 cannot transmit on multiple frequencies simultaneously. However, it can receive and down convert multiple frequencies simultaneously. This embodiment requires fewer components and therefore is less expensive and needs less real estate on a printed circuit board than the preferred embodiment since it does not require additional amplifiers, mixers, and bandpass filters. Yet another advantage is that the power amplifier only has to transmit one frequency at a time. This is critical to maintain linearity and efficiency of the power amplifier.
The dual band code division multiple access radio of the present invention operates on multiple frequencies simultaneously on either transmit or receive. The transmit path operates by splitting the transmit intermediate frequency (IF) path (103) and mixing the IF to two different transmit frequencies using two frequency synthesizers (140 and 145). The receive path (104) has two RF channels (116 and 117) that are mixed with the frequencies generated by the synthesizers (140 and 145) and summed into one IF strip. Thus the radio of the present invention can perform a soft hand-off between frequencies while communicating with both base stations simultaneously. Additional mixing paths and synthesizers can be added if it is desired to communicate with more than two base stations simultaneously.