ZERO IF TRANSCEIVER

Abstract: A zero Intermediate Frequency (IF) transmitter and receiver are implemented within a transceiver to eliminate interference in the receive band. An output of a tunable high oscillator to generate a frequency source that is an integer multiple of the desired transmit LO frequency. The frequency source is coupled to an amplitude limiter and frequency divider. The output of the frequency divider is used as the transmit LO to directly upconvert baseband signals to the desired output frequency without the need for an IF stage. Direct upconversion of the baseband transmit signals without an IF stage eliminates spurious frequency products that are produced in the receive band.

Description: ZERO IF TRANSCEIVER BACKGROUND OF THE INVENTION I. Field of the Invention The present invention relates to wireless communications. More particularly, the present invention relates to a novel and improved zero IF transceiver. II. Description of the Related Art Communications have experienced a tremendous growth due in part to the improved capabilities of wireless devices. Wireless devices utilize radio waves to provide distant communications without the physical constraints of a wire-based system. Information is provided to devices using radio waves transmitted over predetermined frequency bands. Allocation of available frequency spectrum is regulated to enable numerous users access to communications without undue interference. Information that is to be transmitted from a source is seldom acquired in a format that is ready for radio transmission. Typically, the transmitter takes an input signal and formats it for transmission in the predetermined frequency band. The input signal is used to modulate a carrier in the desired frequency band. The input signal is also referred to as a baseband signal. As an example, a radio transmitter that receives an audio input signal modulates a carrier frequency with the input signal. A corresponding remote receiver tuned to the same carrier frequency as the transmitter is required to receive and demodulate the transmitted signal. The remote receiver recovers the baseband signal from the modulated carrier. The baseband signal may be directly presented to a user or may be further processed prior to being presented to the user. A majority of wireless devices designed for consumer use are solely receivers. Examples of receivers include radios, televisions, and pagers. Some wireless devices integrate a transmitter and receiver in a single package. These integrated devices are commonly known as transceivers and are used to allow nearly instantaneous two way communications. Examples of transceivers designed for consumer use are two way radios, walkie- talkies, two way pagers, and wireless phones. A problem associated with transceivers is interference due to the proximity between the transmitter and the receiver. The physical proximity between the transmitter and the receiver improves the possibility of signals generated in one of the sections to be coupled to the other section. Additionally, the proximity of the operating frequency bands of the transmitter and receiver make the receiver susceptible to interference generated by the transmitter in the receive band. The proximity of the receive band to the transmit band makes it more difficult to filter the transmitter signals such that they do not couple any energy into the receive band. The inability to filter the transmitter signals is further exacerbated by the use of tunable oscillators that are required for multiple channel operation. An example of a wireless device that integrates a transceiver that potentially suffers performance degradation due to interference is a wireless phone. Wireless phones may form a part of a wireless communication system such as those defined in Telecommunications Industry Association (TIA)/ Electronics Industries Association (EIA) IS-95-B, MOBILE STATION- BASE STATION COMPATIBILITY STANDARD FOR DUAL-MODE SPREAD SPECTRUM SYSTEMS and American National Standards Institute (ANSI) J-STD-008, PERSONAL STATION-BASE STATION COMPATIBILITY REQUIREMENTS FOR 1.8 TO 2.0 GHZ CODE DIVISION MULTIPLE ACCESS (CDMA) PERSONAL COMMUNICATIONS SYSTEMS. Wireless phones used in the two aforementioned systems must conform, respectively, to the standards TIA/EIA IS-98-B, RECOMMENDED MINIMUM PERFORMANCE STANDARDS FOR DUAL-MODE SPREAD SPECTRUM CELLULAR MOBILE STATIONS and ANSI J-STD-018, RECOMMENDED MINIMUM PERFORMANCE REQUIREMENTS FOR 1.8 TO 2.0 GHZ CODE DIVISION MULTIPLE ACCESS (CDMA) PERSONAL STATIONS. Wireless phones that are capable of operating in the above mentioned communication systems are often implemented in physical designs that are smaller than nine cubic inches. The transmitters in these wireless phones are often spaced much less than one inch away from the receivers. Thus, physical proximity between the transmitter and receiver tends to contribute to the coupling of interfering signals from one to the other. Also, the transmit and receive bands are within close proximity to one another in these communication systems. Wireless phones operating in accordance with TIA/EIA IS-95-B transmit on the frequency band 824 - 849 MHz and receive on the frequency band 869 - 894 MHz. Additionally, the transmit and receive frequencies for a wireless phone operating on any particular channel within the band are separated by only 45 MHz. This spacing between the transmit frequency and the receive frequency is known as the duplex frequency. Thus, the transmit and receive frequencies are separated by only approximately five per cent of the carrier frequency for that channel. Similarly, for PCS phones the transmit frequency band is 1850 - 1910 MHz and the receive frequency band is 1930 - 1990 MHz. The transmit frequency is separated from the receive frequency by 80 MHz. Therefore, the duplex frequency is approximately four per cent of the carrier frequency. The physical and spectral proximity of the transmitter and receiver in a portable wireless communication device make it difficult to minimize transmitter interference in the receive band. The transmitter in a wireless phone is typically capable of providing +27 dBm output power while the receiver is able to simultaneously detect signals at or below a power level of -104 dBm. Yet, interference must be minimized in order to optimize the receiver performance. SUMMARY OF THE INVENTION The disclosed embodiments show a novel and improved zero IF transceiver. The transmitter upconverts a baseband signal to a desired RF output frequency without first converting the baseband signal to an Intermediate Frequency (IF). In a first embodiment the zero IF transmitter incorporates a first Local Oscillator (LO) that outputs a frequency that is at least twice the frequency of the desired receive frequency. The output of the first LO is coupled to a divider that provides frequency division. The divider may incorporate both a limiter and frequency divider. The transmitter uses a mixer whose first input is coupled to a baseband signal and whose second input is coupled to the divider output. The mixer upconverts the baseband signal to the desired RF output signal without the need for an Intermediate Frequency (IF) stage. When the transmitter is implemented with a receiver, it may use the first LO to generate the receiver LO. The receiver uses the first LO output to directly downconvert the receive channel. In another embodiment of the transceiver, the transmitter LO is generated using the first LO in conjunction with a second LO that produces an offset frequency and an LO mixer. The second LO is coupled to a first input of the LO mixer and the first LO is coupled to a second input of the LO mixer. The LO mixer may be a SSB mixer configured as a high side band SSB mixer or a low sideband SSB mixer. The resultant mixer output is coupled to the divider. An offset divider can also be used between the second LO and the LO mixer. When an offset divider is used the frequency of the second LO is, in one embodiment, greater than two times the transmit frequency band. Alternatively, the second LO frequency is, in another embodiment, greater than two times the duplex frequency. In another embodiment, the output frequency of the second LO is greater than eight times the transmit frequency band. In still another embodiment, the output frequency of the second LO is greater than eight times the duplex frequency. In this embodiment the offset divider is configured to divide by eight and the second LO output frequency may be configured to 1280 MHz or 1440 MHz. Additionally, the first LO may be configured to operate in the frequency bands 3476 MHz - 3576 MHz and 3860 MHz - 3980 MHz respectively. In another embodiment a wireless communication device capable of communicating in a wireless communication system is composed of a transmitter and a receiver. The transmitter may be configured as a zero IF transmitter as previously described. In an alternative embodiment, the transmitter and receiver may operate over multiple communication systems. The transmitter is capable of operating in multiple transmit frequency bands when the transmitter is configured to operate in multiple communication systems. Similarly, the receiver is capable of operating in multiple frequency bands when the receiver is configured to operate in multiple communication systems. The transmitter may be implemented as a zero IF transmitter and may also incorporate a first amplifier chain configured to operate in the transmit frequency band of a first communications system and a second amplifier chain configured to operate in the transmit frequency band of a second communication system. When the two amplifier chains are used the embodiment may incorporate a diplexer having a first input coupled to the output of the first amplifier chain and a second diplexer input coupled to the output of the second amplifier chain. BRIEF DESCRIPTION OF THE DRAWINGS The features, objects, and advantages of the disclosed embodiments will become more apparent from the detailed description set forth below when taken in conjunction with the drawings in which like reference characters identify correspondingly throughout and wherein: FIG. 1 is a block diagram of a transceiver; FIG. 2 is a block diagram of a first transceiver embodiment; FIG. 3 is a block diagram of a zero IF transceiver embodiment; FIG. 4 is a block diagram of a second zero IF transceiver embodiment; FIG. 5 is a block diagram of a zero IF receiver embodiment; FIG. 6 is a block diagram of a zero IF transmitter embodiment; and FIG. 7 is a spectrum diagram of narrow band modulated signals. DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS Many wireless devices utilize a dual conversion architecture design for the receiver and transmitter. FIG. 1 shows a block diagram of a wireless transceiver such as may be used in a wireless phone capable of working in one of the communication systems mentioned above. An antenna 10 is used to interface the wireless device 100 to incoming radio waves. The antenna 10 is also used to broadcast the signal from the transmitter. Incoming radio waves coupled to the wireless device 100 at the antenna 10 are coupled to a duplexer 20. The duplexer 20 filters the incoming receive band signal but, more importantly, is used to electrically isolate the transmit power from the receive path while allowing the transmitter and receiver to use the same antenna. The duplexer 20 couples the signals in the receive path to a Low Noise Amplifier (LNA) 22 while simultaneously rejecting signals outside of the receive band. Ideally, the duplexer 20 rejects all signals in the transmit band such that they do not interfere with the receive band signals. However, practical implementations of duplexers 20 provide only limited rejection of signals in the transmit band. The LNA 22 following the duplexer 20 is used to amplify the receive signal. The LNA 22 is also the major contributor to the receiver's noise figure. The noise figure of the LNA 22 adds directly to the noise figure of the receiver while the noise figure of subsequent stages is reduced in proportion to the LNA 22 gain. Thus, the LNA 22 is chosen to provide a minimal noise figure in the receive band while amplifying the receive signal with sufficient gain to minimize noise figure contributions from subsequent stages. There are competing design requirements, such as DC power requirements and device third order intercept point, that make the choice of LNA 22 gain difficult. The signal amplified in the LNA 22 is coupled to an RF filter 24. The RF filter 24 is used to provide further rejection to signals outside of the receive band. The duplexer 20 is not capable of supplying sufficient rejection of signals outside of the receive band so the RF filter 24 supplements the prior filtering. The RF filter 24 is used after the first LNA 22 stage in order to reduce its contribution to the receiver noise figure. The output of the RF filter 24 is coupled to a second LNA 26. The second LNA 26 is used to further amplify the received RF signal. A second LNA 26 stage is used where sufficient gain cannot be achieved in a single LNA stage while also satisfying third order intercept constraints. The output signal from the second LNA 26 is coupled to an input of a RF mixer 30. The RF mixer 30 mixes the amplified receive signal with a locally generated frequency signal to downconvert the signal to an Intermediate Frequency (IF). The IF output of the RF mixer 30 is coupled to an IF amplifier 32 that is used to increase the signal level. The IF amplifier 32 has limited frequency response and does not amplify the upconverted signal that is output from the RF mixer 30. The output of the IF amplifier 32 is coupled to an IF filter 34. The IF filter 34 is used to filter only the IF from a single receive channel. The IF filter 34 has a much narrower frequency response than does the RF filter 24. The IF filter 32 can have a much narrower bandwidth since the RF mixer 30 downconverts the desired RF channel to the same IF regardless of the frequency of the RF channel. In contrast, the RF filter 24 must pass the entire receive band since any channel in the receive band can be allocated to the communication link. The output of the IF filter 34 is coupled to a receive Automatic Gain Control (AGC) amplifier 36. The AGC amplifier 36 is used to maintain a constant amplitude in the receive signal for the subsequent stages. The gain of the AGC amplifier 36 is varied using a control loop (not shown) that detects the amplitude of the amplifier's output. The output from the AGC amplifier 36 is coupled to an IF mixer 40. The IF mixer 40 downconverts the IF signal to a baseband signal. The Local Oscillator (LO) used in conjunction with the IF mixer 40 is separate and distinct from the first LO 150. The baseband output of the IF mixer 40 is coupled to a baseband processor 102. The baseband processor 102 block represents all subsequent processing that is performed on the baseband signal. Examples of subsequent processing include, but are not limited to, despreading, deinterleaving, error correction, filtering, and amplification. The received information is then routed to the appropriate destination. The information may be used within the wireless device or may be routed to a user interface such as a display, loudspeaker, or data port. The same baseband processor 102 may also be used in the complementary transmitter. Information to be transmitted is input to the baseband processor 102 where it may be, for example, interleaved, spread, and encoded. The processed signal is coupled to a transmit IF mixer 110 where the baseband signal is upconverted to a transmit IF. The transmit LO 112 used in conjunction with the transmit IF mixer 110 is generated separately from the first LO 150 and the receive IF LO 42. The upconverted transmit IF signal output from the IF mixer 110 is coupled to a transmit AGC amplifier 114. The transmit AGC amplifier 114 is used to control the amplitude of the transmit IF signal. Amplitude control of the IF signal may be required to ensure the signal is maintained within the linear regions of all subsequent amplifier stages, or may be used for transmit power control as is required for wireless handsets used in the IS-95 and J-STD-008 communication systems. The output of the AGC amplifier 114 is coupled to an transmit IF filter 116 that is used to reject unwanted mixer and amplifier products. The filtered output is coupled to a transmit RF mixer 120. The transmit RF mixer 120 is used to upconvert the transmit IF to the proper transmit RF frequency. The upconverted RF output from the transmit RF mixer 120 is coupled to a first transmit RF filter 122. The first transmit RF filter 122 is used to reject undesired mixer products. The output of the first transmit RF filter 122 is coupled to a driver amplifier 124. The driver amp 124 amplifies the signal to a level required by the subsequent power amplifier 128. Before the signal is applied to the power amplifier 128 the signal is filtered in a second transmit RF filter 126. The second transmit RF filter 126 is used to further reject mixer products and is also used to reject out of band products that are generated by the driver amplifier 124. The out of band products generated by the driver amplifier 126 may be harmonic products generated by driving the amplifier into a non-linear operating range. The output from the second transmit RF filter 128 is coupled to a high powered amplifier 128. The high powered amplifier 128 is used to amplify the transmit signal to a power level sufficient to ensure a communication link to a recipient. The output of the high powered amplifier 128 is coupled to an isolator 130. The isolator 130 is used to protect the output of the high power amplifier 128. Signals from the high power amplifier 128 are able to pass through the isolator 130 with minimal loss but signals that are incident at the output of the isolator 130 are greatly attenuated at the input to the isolator 130. Thus, the isolator 130 provides a good impedance match to the output of the high power amplifier 128 and protects the amplifier from reflected signals due to impedance mismatches in subsequent stages. The output of the isolator 130 is coupled to the duplexer 20 that is used to couple the transmit signal to the single antenna 10 while simultaneously rejecting the transmit signal from the receive path. FIG. 2 is a block diagram of a transceiver 200 implementing the receiver as a zero IF receiver. Implementing the receiver as a zero IF receiver reduces complexity of the receiver by reducing the number of parts in the receiver. This in turn reduces the cost of the receiver. The RF, high frequency, paths in both the receiver and the transmitter are the same as for the transceiver shown in FIG. 1. Thus, the reference numbers for the RF elements in FIG. 2 match those of FIG. 1. The frequency conversion stage in the receiver of FIG. 2 differs from that of FIG. 1 since the receiver in FIG. 2 is a zero IF receiver. The receive RF mixer 230 converts the RF signal directly to a baseband signal. In order to accomplish this direct conversion to baseband, the RF LO 250 operates at exactly the desired receive frequency. The output of the receive RF mixer 230 is coupled to a baseband filter 234 before being coupled to the baseband processor 202. The baseband filter 234 may be external to the baseband processor 202 or may be incorporated within the baseband processor 202. The same RF LO 250 may be used in the transmitter with minor changes to the transmitter frequency plan. The transmitter IF must be equal to the duplex frequency. The baseband processor 202 couples the baseband transmit signal to an IF mixer 210. The IF LO 212 is configured to operate at the duplex frequency. The resultant output of the IF mixer 210 is a replica of the baseband signal upconverted to the duplex frequency. The transmit IF signal is then coupled to an AGC amplifier 214 and IF filter 216. The output of the IF filter 216 is coupled to the input of the transmit RF mixer 220. The transmit RF mixer 220 uses the same RF LO 250 as was used for the receiver. The transmit IF is thus upconverted to the desired RF transmit frequency. A disadvantage of the transceiver embodiment of FIG. 2 is the presence of the RF LO frequency in the upconverted transmit signal. A frequency component at the LO frequency occurs at the output of the transmit RF mixer 220 due to the inability of the mixer to reject the LO frequency. The LO frequency component is amplified in the transmit RF path but is also rejected in the transmit RF filter 126 as well as the duplexer 20. However, the LO frequency component remains greater than the receiver threshold. This creates an interference problem because the LO frequency is tuned to the desired receive frequency. The result is desensitization of the receiver at the desired receive frequency. The transceiver architecture shown in FIG. 2 may also have problems associated with spurious products generated by the LO coupling directly into the receiver. The close physical proximity of the LO to the receiver allows the LO signal to couple into the receive RF path. The effect of spurious products in the receiver is the desensitization of the receiver. The receiver is not able to accurately receive signals that are near the receive threshold because of the presence of the spurious product. A block diagram of an alternative transceiver 300 embodiment is shown in FIG. 3. The receiver is still implemented as a zero IF receiver and the receive signal path remains unchanged from that shown in FIG. 2. The only change to the receiver is in the method of generating the LO. The transmitter is now implemented as a zero IF transmitter, eliminating the IF stage at the duplex frequency. The transmit RF section remains the same as shown in FIG. 2 but the baseband and upconversion elements change. Additionally, the method of generating the LO is changed. The transmitter is implemented as a zero IF transmitter. The signal from the baseband processor 302 is coupled to a baseband filter 304 prior to being coupled to a transmit mixer 320. The baseband filter 304 is typically a low pass filter used to reject any high frequency components outside of the signal bandwidth. The baseband filter 304 may be implemented external to the baseband processor 302 or may be eliminated in favor of a filter implemented within the baseband processor 304. The output of the baseband filter 304 is coupled to a first input of the transmit mixer 320. A Local Oscillator signal at the desired transmit RF frequency is coupled to a second input of the transmit mixer 320. The upconverted output from the transmit mixer 320 is coupled to an RF filter 122 to reject out of band mixer products. The output of the RF filter 122 is coupled to an RF amplifier 324. The RF amplifier 324 is implemented as an AGC amplifier, such that the transmit path RF gain may be varied. The output of the RF amplifier 324 is coupled to a second RF filter 126. The remainder of the transmit path is identical to the previously discussed transmitters. The Local Oscillator (LO) used to upconvert the baseband signal is generated using two independent oscillators and a series of dividers. A first LO 350 generates a first frequency that is at least twice the desired receive RF frequency. A UHF oscillator is used as the first oscillator 350 in an exemplary transceiver used in a wireless phone designed to operate in the frequency bands specified in IS-95 or J-STD-008. The output of the first oscillator 350 is coupled to a receive LO limiter 352 as well as to a transmit LO limiter 356. The single output of the first oscillator 350 can be directly coupled to the inputs of the two limiters, 352 and 356, or may be coupled using a signal splitter, hybrid, or any other means as is known to one of ordinary skill in RF design. The receive LO signal is generated by coupling the output from the receive LO limiter 352 to a receive LO divider 354. The receive LO divider 354 scales the amplitude limited frequency output of the first oscillator 350 by the appropriate factor. In the exemplary embodiment shown in FIG. 3 the first oscillator 350 is tuned to operate at twice the desired receive frequency and the receive LO divider 354 is implemented to scale the frequency by a factor of two. The scaled output of the receive LO divider 354 is at the desired receive frequency and is used as the receive LO signal. The transmit LO is generated in a similar manner except an offset frequency oscillator 360 is required to compensate for the duplex frequency. The output of the first oscillator 350 is coupled to the transmit LO limiter 356 where the signal is amplitude limited. The output of the transmit LO limiter 356 is coupled to a transmit LO divider 358. The transmit LO divider 358 scales the frequency by the same factor used in the receive LO divider 354. The output of the transmit LO divider 358 is coupled to a first input of an LO offset mixer 370. The output of an offset frequency oscillator 360 is coupled to a second input of the LO offset mixer 370. It can be seen from the block diagram that the frequency of the offset frequency oscillator 360 is equal to the duplex frequency. The LO offset mixer 370 is a Single Side Band (SSB) mixer that outputs only one primary mixer product. A SSB mixer provides a frequency output that is either the sum of the two input frequencies or is the difference of the two input frequencies. The SSB mixer is used to minimize the unwanted mixer products at the output of the LO offset mixer 370. AN embodiment uses a SSB mixer that outputs the difference of the two input frequencies. The output of the LO offset mixer 370 is at the desired transmit frequency and is used as an input to the transmit mixer 320 to directly upconvert the transmit baseband signal to the RF frequency. The embodiment shown in FIG. 3 eliminates a majority of the receive frequency component from coupling into the transmit path. The physical proximity of the receive LO divider 354 to the receive mixer 230 minimizes any coupling of the signal into the transmit path. The use of an LO offset mixer 370 in the generation of the transmit LO minimizes the amplitude of the receive frequency component in the transmit LO signal. However, the receive frequency is not entirely eliminated. Some of the receive frequency component will leak through the LO offset mixer 370. The amount of rejection provided by the LO offset mixer 370 is specified as the RF rejection and is not infinite. The result of having some receive frequency component in the transmit LO is that undesired signals at the transmit output will appear at the receive frequency. FIG. 4 is a block diagram of an embodiment of a transceiver 400 using a zero IF transmitter and associated zero IF receiver. The transmitter and receiver elements are the same as those detailed in FIG. 3 as reflected in the reference numbers. The elements of the transmit and receive signal path are arranged in the same structure as the block diagram shown in FIG. 3. The only difference is in the manner that the transmit LO is generated, A first oscillator 350 is still used to generate a signal that is an integer multiple of the receive frequency. The output of the first oscillator 350 is coupled to a buffer amplifier 452. The buffer amplifier 452 serves several purposes. The amplifier 452 provides a stable impedance for the first LO 350 output and buffers the LO's output to minimize interference and load variations on the LO output. The buffer amplifier 452 need not be included in applications where the first LO 350 has sufficient output power and insensitivity to output load variations and interference. Indeed, for applications where the first LO 350 output power is greater than required for the subsequent stage an attenuator may be used in place of the buffer amplifier 452. However, in embodiments using the buffer amplifier 452 the output of the amplifier is coupled to a first input of an LO mixer 470. A second input of the LO mixer 470 receives a signal generated by a second LO 460. The second LO 460 is configured to oscillate at a frequency that is at least twice the frequency of the transmitter bandwidth. Additionally, the operating frequency of the second LO 460 is chosen such that integer multiples of the output frequency do not produce products in the receive band. The output of the second LO 460 is coupled to an offset LO divider 462. The offset LO divider 462 generates an output frequency that is an integer division of the input frequency. In a first embodiment the offset LO divider 462 performs a division by eight. The offset LO divider 462 can be implemented as a single IC, multiple IC's, discrete components, or a combination of discrete components and IC's. The offset LO divider 462 can be implemented in any means available to one of ordinary skill in the art and its implementation is not a limitation on the zero IF transmitter implementation. The output of the LO divider 462 is at a multiple of the duplex frequency. The output of the offset LO divider 462 is coupled to the second input of the LO mixer 470. The output of the LO mixer 470 is coupled to a limiter 456. The limiter 456 amplitude limits the signal such that the amplitude of the signal remains relatively stable. A limiter 456 may be an IC, amplifier, or discrete components, or the limiter 456 may be incorporated into an adjacent stage in the signal path. The output of the limiter 456 is coupled to an LO divider 458. The LO divider 458 generates the desired LO signal by dividing the input signal by an integer value. In one embodiment the division value is two. In another embodiment the division value is four. The LO divider 458 and the limiter 456 may be incorporated into a single device. An LO divider 458 that inherently performs limiting of the input signal will allow the elimination of a separate limiter 456. The divided output is used as the desired LO signal for upconverting the baseband signal to the desired transmit frequency. If the transceiver of FIG. 4 is configured to communicate in a system defined by IS-95 the transmit band is 824-849 MHz and the receive band is 869-894 MHz. The first oscillator 350 is configured to operate from 3476-3576 MHz. The second oscillator 460 is configured to operate at the fixed frequency of 1440 MHz. The offset LO divider 462 is configured to divide the second LO 460 by a factor of eight. The output of the offset LO divider 462 is a signal at 180MHz. The offset LO signal is then mixed with the first oscillator in a SSB mixer 470. The SSB mixer 470 in this embodiment only retains the lower sideband. If desired, the oscillator frequencies could be adjusted such that the upper sideband is retained. In that case, an upper sideband SSB mixer is required. The output of the SSB mixer 470 is coupled to the limiter 456 and divider 458. The divider 458 is configured to divide the resultant transmit LO signal by a factor of four. The output of the divider is within the frequency band of 824-849 MHz and thus allows for direct upconversion of the transmit baseband signal to the RF frequency band. FIG. 5 shows a block diagram of an embodiment of a zero IF receiver 500 for use in a transceiver. An antenna 510 is coupled to a LNA 522. The block diagram shows a direct coupling but it should be understood that in a transceiver the antenna 510 would couple to the LNA 522 through a duplexer. The output of the LNA 522 is then coupled to a filter 524 and amplifier 526. The output of the amplifier 526 is coupled to two receive mixers 530 and 532. Each of the mixers 530 and 532 is fed an LO signal. The LO signals to the two mixers 530 and 532 differ in phase by ninety degrees. Each of the mixers 530 and 532 outputs a baseband signal. The baseband signals are in quadrature due to the phase relationship of the LO signals. The baseband signals are then filtered and coupled to a baseband processor 502. The LO signal is generated as in the transceiver of FIG. 4 except a quadrature splitter 556 is used to generate the two LO signals that are in quadrature. FIG. 6 is a block diagram of an embodiment of a zero IF transmitter 600 for use in a wireless phone capable of communicating in the systems specified in both IS-95 and J-STD-008. The transmitter 600 of FIG. 6 uses a baseband unit 602 to provide formatted signals for transmission. However, unlike the baseband processors in the previous embodiments, the baseband processor 602 of FIG. 6 provides a pair of output signals with each output signal implemented as a balanced or differential pair. The two outputs from the baseband processor 602 are identical and are provided as separate signal paths to allow for quadrature modulation of the signals in the following stages. A first baseband output is coupled to a first RF mixer 612 that upconverts the baseband signal directly to the desired RF frequency. A second baseband output is coupled to a second RF mixer 610 that upconverts the second baseband output directly to the same RF frequency as at the output of the first RF mixer 612. The difference between the mixer outputs is due to the relative phase difference of the LO signals used to upconvert the baseband signals. The phases of the LO signals driving the first and second RF mixers, 612 and 610, differ by ninety degrees. The ninety degree phase difference in the LO signals results in upconverted signals having a phase difference of ninety degrees. The signals having a ninety degree phase difference are said to be in quadrature. The quadrature RF signals are then coupled to a signal summer 620 that combines the two quadrature signals into a single signal by summing the two RF signals together. The inputs of the signal summer 620 are balanced to correspond with the balanced outputs from each of the RF mixers, 610 and 612. The output of the signal summer 620 is also a balanced signal to minimize signal interference from common mode noise sources. The output of the signal summer 620 is simultaneously coupled to two amplifier chains. A first amplifier chain 660 is configured to operate in the PCS transmit band, such as that defined in J-STD-008. A second amplifier chain 670 is configured to operate in the cellular transmit band, such as that defined in IS-95. Only one amplifier chain is operational at any time. When the transmitter 600 is configured to transmit in a particular frequency band, only the amplifier chain supporting that frequency band is operational. The idle amplifier chain is powered down by control circuits (not shown) in order to conserve power. An upconverted quadrature signal that is configured to be transmitted in the PCS frequency band is coupled to the first amplifier chain 660. The initial element in the first amplifier chain 660 is a first transmit AGC amplifier 662. In addition to providing gain control over the transmit signal, the first transmit AGC amplifier 662 also converts a balanced input signal to a single ended output signal. The output of the first transmit AGC amplifier 662 is coupled to a first transmit filter 664. The first transmit filter 664 rejects any signals outside of the desired RF frequency band. The output of the first transmit filter 664 is coupled to a first power amplifier 666 that coupled the amplified RF signal to a first isolator 668. The output of the first isolator 668 is the output of the first amplifier chain 660. The output of the first amplifier chain 660 is coupled to a first input of a diplexer 680. The diplexer 680 is used to combine the signals from two distinct frequency bands into a single signal while providing signal isolation from one input signal path to the other. The diplexer 680 is distinct from the duplexer previously shown coupling a single antenna to a transmitter and receiver. Here, the diplexer 680 couples two distinct transmit signal paths to a single antenna 610. A duplexer is used to couple the transmit and receive signals to the antenna where the transmitter 600 is implemented with a receiver in a transceiver with a common antenna. Similarly, an upconverted quadrature signal that is configured to be transmitted in the cellular frequency band is coupled to a second amplifier chain 670. The input stage of the second amplifier chain 670 is a second transmit AGC amplifier 672. The second transmit AGC amplifier 672 utilizes a balanced input and provides a single ended output. The output of the second transmit AGC amplifier is coupled to a second transmit filter 674. The second transmit filter 674 rejects signals outside of the cellular frequency band. The output of the second transmit filter 674 is coupled to a second power amplifier 676. The output of the second power amplifier 676 is coupled to a second isolator 678 that is the final stage in the second amplifier chain 670. The output of the second amplifier chain 670 is coupled to a second input of the diplexer 680. As mentioned earlier, the diplexer 680 couples the second input to the antenna 610. The LO's used to drive the mixers are generated from two different oscillators. A first LO 550 operates at a frequency greater than a minimum of twice the desired RF receive frequency. When the transmitter 600 is configured to operate in the PCS band the first LO operates in the frequency range of 3860-3980 MHz. The first LO 550 operates in the frequency band of 3476-3576 MHz when the transmitter 600 is configured to operate in the cellular band. The output of the first LO 550 is coupled to a buffer amplifier 632 that increases the amplitude of the first LO 550 and provides a stable terminating impedance for the first LO 550 output. The buffer amplifier 632 provides a single ended input and a balanced output configuration to minimize the effects of common mode noise. The output of the buffer amplifier 632 is coupled to a SSB mixer 650. The SSB mixer 650 outputs the lower sideband and attenuates the upper sideband. The second LO 640 operates at a frequency of 1440 MHz when the transmitter is configured to operate in the cellular band and operates at a frequency of 1280 MHz when the transmitter is configured to operate in the PCS band. The output of the second LO 640 is a balanced signal. The balanced output is coupled to an offset frequency divider 642. The offset frequency divider 642 is configured to divide the second LO 640 signal by a factor of eight. The divided output is a frequency source at 180 MHz when the transmitter 600 is configured to operate in the cellular band and 160 MHz when the transmitter 600 is configured to operate in the PCS band. The divided output is couple to a second input on the SSB mixer 650. The output of the SSB mixer 650 is then coupled to a limiter 652 and LO divider 656. The LO divider 656 is configured to divide the input signal by a factor of two when the transmitter 600 is configured to operate in the PCS band and divides the inputό signal by a factor of four when the transmitter 600 is configured to operate in the cellular band. The output of the LO divider 656 is coupled to a ninety degree splitter 658. The 90 degree splitter 658 splits the LO signal into two equal amplitude signals with one output having a 90 degree phase shift relative to the other splitter output. The two outputs are said to be in quadrature and are used to generate the quadrature upconverted signals in the transmit path. The transmitter 600 shown in FIG. 6 is able to operate in both the cellular and PCS frequency bands. The transmitter 600 can be integrated with a corresponding receiver to produce a transceiver capable of operating in the cellular and PCS bands without transmitter spurious products interfering with receiver operation. FIG. 7 is a frequency spectrum view showing the effect of a limiter and divider on an RF signal. FIG. 7A shows a frequency spectrum of a dominant signal with a much smaller adjoining frequency component. FIG. 7B shows the frequency spectrum of a narrow band AM signal where the frequency of modulation corresponds to the frequency offset of the adjoining signal in FIG. 7A. FIG. 7C shows the frequency spectrum of a narrow band PM signal where the frequency of modulation corresponds to the frequency offset of the adjoining signal in FIG. 7A. The inverted frequency component to the left of the center frequency in FIG 7C is used to show an inverted phase of the lower side band. It can be seen by examining FIG.'s 7A-7C that a dominant signal with a single side band can be generated by summing an AM signal FIG. 7B with a corresponding PM signal FIG. 7C. The effect of having both AM and PM components in a signal is used in the generation of the LO signals in the transmitter. FIG. 7D shows a frequency spectrum of a dominant frequency located at fl and a much smaller frequency component located at fl+f2. FIG. 7E shows the resulting frequency spectrum after the passing the signal through a limiter 702. The effect of the limiter 702 is to reject all of the AM component in the signal. The remaining frequency component is the PM signal. The phase inversion of the lower sideband is not illustrated in FIG. 7E. FIG. 7F shows the frequency spectrum of the signal after passing through a divider 704. The original frequency component at fl is divided down by a factor of two in this example to result in a frequency component at (fl)/2. However, note that the sidebands from PM modulation of the signal retain their spacing relative to the center frequency. The upper sideband is at (fl)/2 + f2 and the lower sideband is at (fl)/2 - f2. It can be seen that this retention of frequency spacing is used to an advantage in the generation of the transmit LO signal in the block diagrams of FIG. 3 and FIG. 6. The previous description of the preferred embodiments is provided to enable any person skilled in the art to make or use the present invention. The various modifications to these embodiments will be readily apparent to those skilled in the art, and the generic principles defined herein may be applied to other embodiments without the use of the inventive faculty. Thus, the present invention is not intended to be limited to the embodiments shown herein but is to be accorded the widest scope consistent with the principles and novel features disclosed herein. WE CLAIM:

Claims: CLAIMS 1. A wireless communication device comprising: a transmitter configured to transmit RF signals from the wireless communication device in at least one predetermined transmit frequency band of a wireless communication system; and a receiver configured to receive RF signals at the wireless communication device in at least one predetermined receive frequency band of the wireless communication system; wherein the transmitter directly converts baseband signals to a desired RF output frequency within the predetermined transmit frequency band without the use of an Intermediate Frequency (IF) stage. 2. The wireless communication device of Claim 1 wherein the transmitter comprises: a first Local Oscillator (LO) configured to output a frequency at least twice the frequency of the desired RF receive frequency; a divider having an input and an output produced by dividing an input signal; a mixer having a first mixer input operatively coupled to a baseband signal and a second mixer input operatively coupled to the divider output; a second LO configured to output an offset frequency; and an LO mixer having a first input operatively coupled to the second LO, a second input operatively coupled to the first LO and an output operatively coupled to the divider; wherein the LO mixer produces an output frequency that is offset from the first LO output frequency and the LO mixer operatively couples the first LO output to the divider and wherein an output of the mixer is an upconverted baseband signal at the desired RF output. 3. The wireless communication device of Claim 2 wherein the transmitter further comprises an offset divider that operatively couples the second LO output to the LO mixer first input. 4. The wireless communication device of Claim 3 wherein the offset frequency output of the second LO is at least two times greater than an operating frequency band of the zero IF transmitter. 5. The wireless communication device of Claim 3 wherein the offset frequency output of the second LO is at least eight times greater than an operating frequency band of the zero IF transmitter. 6. The wireless communication device of Claim 5 wherein the offset divider is configured to divide by eight. 7. The wireless communication device of Claim 6 wherein the offset frequency output of the second LO is 1280 MHz. 8. The wireless communication device of Claim 6 wherein the first LO operates in the frequency range of 3476 MHz - 3576 MHz and the divider is configured to divide by four. 9. The wireless communication device of Claim 6 wherein the offset frequency output of the second LO is 1440 MHz. 10. The wireless communication device of Claim 9 wherein the first LO operates in the frequency range of 3860 MHz - 3980 MHz and the divider is configured to divide by two. 11. The wireless communication device of Claim 2 wherein the LO mixer is a Single Side Band (SSB) mixer. 12. The wireless communication device of Claim 11 wherein the SSB mixer is a low side SSB mixer. 13. The wireless communication device of Claim 11 wherein the SSB mixer is a high side SSB mixer. 14. The wireless communication device of Claim 2 wherein the divider comprises: a limiter; and a frequency divider operatively coupled to a limiter output. 15. The wireless communication device of Claim 2 further comprising: a first amplifier chain configured to operate in a first transmit frequency band operatively coupled to the mixer output; and a second amplifier chain configured to operate in a second frequency band operatively coupled to the mixer output. 16. The wireless communication device of Claim 15 further comprising a diplexer having a first input operatively coupled to an output of the first amplifier chain and a second input operatively coupled to an output of the second amplifier chain.

Inventors: PETERZELL PAUL E, SEE PUAY HOE, WALKER BRETT C, PETERZELL, PAUL, E., SEE, PUAY, HOE, WALKER, BRETT, C.
Applicants: QUALCOMM INC, QUALCOMM INCORPORATED
Application year: 2001
Main classification: H04B1
IPC codes: H04B1