Abstract:
A local oscillator circuit for generating a local frequency signal is provided. The local oscillator circuit may cooperate with a radio circuit for providing wireless reception or transmission. The radio circuit performs modulation or demodulation processes with reference to a defined carrier signal frequency. The local oscillator circuit has a voltage controlled oscillator that generates a VCO signal at frequency different than the carrier frequency. A frequency scaling circuit applies a scaling factor to the VCO signal, with the scaled signal generated at the frequency of the defined carrier frequency.

Description:
BACKGROUND  
       [0001]     The field of the present invention is electronic circuits for generating a frequency signal. More particularly, the invention relates to an electronic circuit and process for generating a local oscillator signal for a radio.  
         [0002]     Wireless communication systems generally transmit a modulated radio frequency (RF) signal that is converted to a baseband signal in a receiver. A conventional receiver does this conversion in a two-stage process. In a first stage, the RF signal is down converted to an intermediate frequency (IF) signal, and then in a second stage, the IF signal is further down converted to the baseband frequency. In a similar manner, a conventional radio transmitter generates the modulated radio frequency (RF) signal in a two-stage process. In a first stage, the baseband signal is up converted to an intermediate frequency (IF), and then in a second stage, the IF signal is further up converted on to the carrier signal. This two stage process enables simplified filtering and processing, but the two-stage architecture consumes valuable space and power in wireless devices. Accordingly, a newer single-stage architecture is being deployed. This single-stage architecture converts directly between the RF signal directly and the baseband signal, and is typically, referred to as a direct conversion radio. The direct conversions process may be applied to the receiver section, the transmitter section, or both the receiver and the transmitter.  
         [0003]     As an alternative, some of the benefits of the direct conversion structure may be realized using a low IF architecture, while retaining some of the simplified filtering and processing of the IF structure. A low IF radio uses an intermediate frequency that is much lower than the IF of a conventional radio. In this way, some of the difficulties of implementing the direct conversion radio are avoided, but the low IF also does not enable the full benefit of direct conversion. To simplify discussion, it will be understood that direct conversion also includes such low-IF systems.  
         [0004]     In operation, a direct or low IF radio uses a voltage controlled oscillator to generate a signal operating at the desired carrier frequency. For example, if a radio is operating on a CDMA standard, then a carrier frequency of 824 MHz may be needed. In such a case, the voltage controlled oscillator is set to output a 824 MHz signal to the radio circuit. The radio circuit receives the 824 MHz signal, and uses it as the reference carrier signal. There are numerous telecommunications standards, with each standard defining specific transmitter and receiver carrier frequencies. If the radio is operating as a transmitter, then a baseband signal is modulated on to the carrier signal, and the modulated signal is transmitted via an antenna. If the radio is operating as a receiver, then the carrier signal is removed, and the demodulated baseband signal processed in the baseband circuit of the radio.  
         [0005]     When implementing a low IF or direct conversion transmitter, a voltage controlled oscillator generates a local oscillator signal. Typically, the local oscillator signal operates between about 400 MHz and 2.2 GHz, depending on the particular telecommunications standard being used. This local oscillator signal is then used as the carrier frequency for the radio. A baseband portion of the radio provides a baseband signal, which operates at a much lower frequency than the carrier signal, generally in the range of a few hundred kilohertz. This baseband signal is then modulated on to the carrier signal. Since the carrier frequency is so much faster than the baseband signal, the frequency of the modulated signal is very close to the frequency of the frequency of the carrier signal itself. The modulated signal is amplified and transmitted from the radio via an antenna or other radiating device.  
         [0006]     However, the transmitted signal is radiated at a relatively high power, and, as discussed above, is operating at a frequency close to the frequency of the carrier signal in the radio circuitry. Even though the radio may be well shielded, it is likely that the transmitted signal still couples to and interferes with the radio circuitry. For example, the transmitted signal may affect the voltage controlled oscillator (VCO). If the transmitted signal couples back to the VCO, then the VCO may become unstable, resulting in frequency shifts and phase noise. These effects, commonly referred to as “VCO pulling” cause an undesirable frequency jitter and a distortion in the output signal. The effects of VCO pulling may be reduced by positioning the VCO farther from the antenna, or by increasing the amount of shielding around the VCO. Unfortunately, as wireless devices become smaller, and radios are offered as single-chip devices, it becomes more difficult to adequately decouple the VCO from the transmitted signal.  
         [0007]     The VCO pulling problem results from the transmitted signal coupling back to the VCO circuit. In a similar manner, another problem exists when the VCO signal couples to the radio circuit. This problem, often referred to as “carrier feedthrough” exists when the VCO signal couples to the transmitter circuitry. In such a case, the stray VCO signal is amplified and transmitted from the wireless device. Accordingly, even when no baseband signal is being transmitted, the wireless device is still transmitting the VCO signal, which wastes device power and may substantially reduce capacity in some telecommunication architectures such as CDMA. For these reasons, some telecommunications standards have strict limits on the level of allowable carrier feedthrough.  
         [0008]     Just as with the direct conversion transmitter, the direct conversion receiver also suffers from implementation difficulties. When implementing a low IF or a direct conversion receiver, there is typically some amount of offset (referred to as “DC offset”) that appears on the downconverted baseband signal. The DC offset may occur due to due to self-mixing that can occur between the local oscillator (LO) signal from the VCO and the received radio frequency (RF) signal. Correction for DC offset is typically performed on the baseband amplifier located in the receiver. Many techniques have been proposed to minimize DC-offset. For example, it is possible to minimize DC offset using digital calibration techniques in the analog-to-digital converter (A/D) located in the receiver. Alternately, sampling techniques and Sample-and-Hold (S/H) circuits have been used to subtract the estimated offset of the variable gain amplifier from the received signal.  
         [0009]     Unfortunately, one or all of these techniques can only be applied to a system in which the receiver does not continuously operate, such as in a TDMA communication system, and even then add an undesirable level of complexity. In a CDMA system, these techniques will not be effective because the receiver works continuously with no interruption. Furthermore, DC-offset correction using so called “auto-zeroing” techniques during start-up is not practical in a CDMA system because of dynamic offsets. In a CDMA system the only option that shows promise is the implementation of a so called “servo-loop” like architecture around the variable gain amplifier.  
         [0010]     In a servo-loop architecture, the high pass cut-off frequency is dependent upon the gain characteristics of the variable gain amplifier and the amplifiers in the servo-loop. Because the transconductance of the variable gain amplifier varies significantly with the applied gain control signal (usually above 50 dB of range), the cut-off frequency varies by more than 50 dB, which places the cut-off frequency at a point at which data carried in the received signal will likely be lost. It is possible to adjust the high pass cut-off frequency by varying the gain of the amplifiers in the servo-loop inversely proportional to the transconductance amplification of the VGA. Since the transconductance amplification of the VGA varies proportionally to the exponential of the control voltage, the amplification of the amplifiers in the servo-loop must vary with the inverse of the exponential of the control voltage. Unfortunately, such a servo-loop increases significantly the complexity, power consumption and the area on the device occupied by the architecture.  
         [0011]     Therefore, it would be desirable to reduce the effects VCO pulling and carrier feedthrough in a direct conversion transmitter. Further, it would be desirable to reduce the effects of DC offset in a direct conversion receiver.  
       SUMMARY  
       [0012]     Briefly, the present invention provides a local oscillator circuit for generating a local frequency signal. The local oscillator circuit may cooperate with a radio circuit for providing wireless reception or transmission. The radio circuit performs modulation or demodulation processes with reference to a defined or determined carrier signal frequency. The local oscillator circuit has a voltage controlled oscillator that generates a VCO signal at frequency different than the carrier frequency. A frequency scaling circuit applies a scaling factor to the VCO signal, with the scaled signal generated at the frequency of the defined carrier frequency.  
         [0013]     Advantageously, the local oscillator circuit operates the VCO at a frequency different from the carrier frequency. By operating at different frequencies, the local oscillator circuit substantially reduces VCO pulling or carrier feedthrough effects when the radio is operating as a transmitter, and reduces the effects of LO mixing and DC offset when the radio is operating as a receiver. 
     
    
     BRIEF DESCRIPTION OF THE DRAWINGS  
       [0014]     The invention can be better understood with reference to the following figures. The components within the figures are not necessarily to scale, emphasis instead being placed upon clearly illustrating the principles of the invention. Moreover, in the figures, like reference numerals designate corresponding parts throughout the different views. It will also be understood that certain components and details may not appear in the figures to assist in more clearly describing the invention.  
         [0015]      FIG. 1  is a block diagram of a direct conversion radio in accordance with the present invention;  
         [0016]      FIG. 2  is a block diagram of a direct conversion transmitter in accordance with the present invention;  
         [0017]      FIG. 3  is a block diagram of a local oscillator circuit in accordance with the present invention;  
         [0018]      FIG. 4  is a is flow diagram of a method of providing a carrier frequency in accordance with the present invention;  
         [0019]      FIG. 5  is a block diagram of a local oscillator circuit in accordance with the present invention; and  
         [0020]      FIG. 6  is a block diagram of a direct conversion receiver in accordance with the present invention.  
     
    
     DETAILED DESCRIPTION  
       [0021]     Referring now to  FIG. 1 , a direct conversion radio  10  is illustrated. The direct conversion radio  10  may be constructed to comply with a wireless standard, such as CDMA, WCDMA, UMTS, CDMA 2000, GSM, or other wireless standard. It will be appreciated that other wireless standards exist, and that existing standards may be revised and modified over time. Also, the general construction of a direct conversion radio is well-known, so will not be discussed in detail herein.  
         [0022]     The direct conversion radio  10  comprises baseband circuitry  12  for operating on an informational signal. This informational signal may be, for example, a voice signal, a video signal, a text signal, or other informational or data signal. The baseband circuitry  12  couples to radio frequency circuit  14 . The radio circuitry  14  may include transmitter circuitry, receive circuitry, or both. In one example, the radio circuitry  14  is included as part of a wireless mobile device. In this way, the radio circuitry  14  includes both transmitter circuitry and receiver circuitry. The radio circuitry  14  couples to an RF (radio frequency) radiator in the form of antenna  16 . The antenna  16  is used to receive or transmit modulated radio frequency signals. These modulated signals have a baseband informational signal modulated onto an RF carrier. The frequency of the RF carrier and the frequency content of the baseband signal are generally defined in the relevant communication standard. For example, a direct conversion radio compliant with a CDMA standard may have a carrier signal in the range of 824 MHz to 849 MHz, while the baseband signal may be provided at around 600 KHz. In another example, a wideband CDMA signal may transmit at 1920-1980 MHz, and receive at 2110-2170 MHz. It will be understood that other frequency ranges are used in compliance with other telecommunication standards.  
         [0023]     The direct conversion radio  10  has a frequency source, generally in the form of a voltage controlled oscillator  21 , for providing a stable and accurate frequency signal. The voltage controlled oscillator  21  provides its frequency signal at a frequency different than the carrier frequency required under the relevant communication standard. The signal generated by the voltage controlled oscillator  21  is received into frequency scaler  19 . The frequency scaler  19  has scaling circuitry for scaling the frequency of the received signal to the desired carrier frequency. For example, if the direct conversion radio  10  requires a carrier frequency of 1850 MHz, the VCO  21  may generate a signal having a frequency of 1233 MHz. The frequency scaler  19  may then apply a scaling factor of 3/2. In this way, the 1233 MHz signal is first multiplied by 3 and then divided by 2 to generate a signal at 1849.5 MHz. It will be appreciated that other VCO frequencies may be used, provided the scaling factor is adjusted accordingly.  
         [0024]     The frequency scaler  19  is a relatively simple circuit, generally comprising multiplication and division circuitry, and may be readily incorporated into the radio circuitry  14 . In this way, fewer components and traces are operating at or near the carrier frequency, thereby reducing VCO pulling and carrier feed-through effects. Advantageously, the voltage controlled oscillator  21  is operating at a frequency different than the desired carrier frequency. In this way, the amplified and transmitted modulated signal may be readily restricted from distorting or otherwise affecting the voltage controlled oscillator  21 . In a similar manner, stray VCO signals that are received by the radio circuitry  14  may be more easily filtered or removed as these stray signals have a frequency different than the carrier frequency.  
         [0025]     Referring now to  FIG. 2 , a direct conversion transmitter  50  is illustrated. The direct conversion transmitter  50  has baseband circuitry  52  that converts an informational signal to a baseband signal. The information signal may be, for example, a voice signal, a video signal, a text signal, or an audio signal. The baseband signal is received into transmitter circuitry  54 , where the baseband signal is modulated onto an RF carrier signal. The modulated RF signal is then transmitted using antenna  56 . The RF carrier signal is derived from a frequency signal generated by the voltage controlled oscillator  61 . The voltage controlled oscillator  61  provides a stable and accurate frequency signal at a frequency different than the desired RF carrier frequency. The signal from the voltage controlled oscillator is received into a frequency scaler  59 , where the frequency of the signal is scaled to the desired carrier frequency. In one example, the frequency scaler implements a scaling factor of 3/2. In this way, the carrier frequency is generated by multiplying the VCO signal by 3, and dividing the resulting signal by 2. Since the RF carrier operates at a frequency that is 3/2 different than the VCO signal, the VCO may be operated without significant interference or pulling due to the transmitted signal. In a similar manner, any VCO signal that leaks through to the transmitter circuit is readily filtered, reducing any effects from carrier feedthrough. It will be appreciated that other VCO frequencies and scaling factors may be used.  
         [0026]     Referring now to  FIG. 3 a  local oscillator circuit  75  is illustrated. The local oscillator circuit  75  may be advantageously used in association with a wireless radio system. For example, the local oscillator circuit  75  may provide a local oscillator signal for modulating or demodulating in an associated radio circuit. The local oscillator circuit  75  includes a voltage controlled oscillator  76 . The voltage controlled oscillator  76  provides a stable and accurate frequency signal to an input line  77 . The design and construction of a voltage controlled oscillator is well known so will not be discussed in detail. The output from the voltage controlled oscillator  76  is received into a frequency scaling circuit  79 . The frequency scaling circuit applies a scaling factor to the signal received from the voltage controlled oscillator  76 .  
         [0027]     The scaling factor is selected such that the frequency of the voltage controlled oscillator signal multiplied by the scaling factor equals the frequency of the desired carrier frequency. The scaling factor is selected so that the frequency of the voltage controlled oscillator is sufficiently different from the carrier frequency so that VCO pulling and carrier feed through effects may be substantially reduced through filtering or other processes. Also, the scaling factor is selected to avoid significant harmonics near the carrier frequency. However, the scaling factor should also be selected such that the signal from the VCO has sufficient resolution and accuracy as required by the relevant communication standard. In one example, the scaling factor is set to 3/2. A 3/2 scaling factor has a sufficient frequency difference between the VCO signal and the carrier frequency such that the effects of VCO pulling and carrier feed through may be easily reduced. Also, no substantial harmonics are produced near the frequency of the carrier. Further, the VCO signal is generated at a frequency that provides sufficient resolution and accuracy to support most communication standards. For example, a CDMA system may require a carrier in the range of 1850 to 1910 MHz. Using a 3/2 scaling factor, the VCO would operate from 1233 to 1273 MHz. Since the VCO is still operating in excess of 1.2 GHz, it provides a stable and accurate frequency signal with sufficient resolution to support the required carrier signals and channel separations.  
         [0028]     In one example, the frequency scaling circuit  79  is implemented as a multiplier  82  placed in series with a divider  83 . Such multiplier  82  and divider  83  circuits may be efficiently and easily constructed. In the example of applying a 3/2 scaling factor, the frequency of the VCO signal at input  77  is first multiplied by 3 by multiplier  82 , and then divided 2 by divider  83 . The signal is then output on output line  81  for use as a carrier signal. It will be appreciated that the division may be performed before the multiplication, and that other scaling algorithms may be used.  
         [0029]     Table  85  illustrates five common telecommunication standards in current use. For each standard, the common name of the band  86  is shown, with the frequency range  89  defined for the carrier frequency. For each band, a possible VCO frequency  87  is identified, along with an associated scaling factor  88 . The scaling factor  88  is applied to the VCO frequency  87  to generate an output carrier signal  89  in the identified ranges. For example, the US PCS band requires an output carrier signal  89  in the range from 1850 to 1910 MHz. If a scaling factor  88  is selected to be 3/2, then the VCO  87  is set in the range of 1233 to 1273 MHz. Other bands, such as cellular CDMA, J-CMDA, K-PCS, and NMT450 are also illustrated. It will be appreciated that other bands may be used, and that other scaling factors and VCO frequencies may be substituted.  
         [0030]     Referring now to  FIG. 4 , a method of providing a carrier frequency is illustrated. Method  100  has a frequency signal provided by a VCO as shown in block  102 . The VCO frequency is scaled by a scaling ratio as shown in block  104 , with the output sent to the radio as illustrated in block  106 . The output signal  108  may be provided as a carrier signal to a transmitter  115  or receiver  117  operation within the radio. The VCO frequency and the scaling ratios may be set by a control system  110 . The control system  110  may be part of the radio system  106  and in one example may be included on a single integrated circuit with the radio system. The scaling ratio  104  may be implemented by a multiplication  111  and a division  113 . It will be appreciated that other scaling algorithms may be used.  
         [0031]     In determining the scaling ratio  104 , three factors are generally considered. First, the scaling factor should provide a sufficient difference in frequency between the VCO frequency and the carrier frequency such that the effects from VCO pulling and carrier feedthrough may be readily reduced. Second, the scaling factor should be selected so that substantial harmonics of the VCO frequency are not generated near the carrier frequency. And third, the scaling factor should be selected so that the VCO frequency has sufficient resolution and accuracy to support the relevant communication standard. Also, scaling factors closer to 1 require less power to implement. For example, a scaling factor of 3 requires more power to implement than a scaling factor of 3/2, and in a similar manner, a scaling factor of 0.3 requires more power to implement than a scaling factor of 3/4. Therefore, in a wireless environment, such as a mobile wireless environment, where power considerations are important, scaling factors should be selected as close to 1 as appropriate in light of the factors identified above. In one specific example, a scaling factor of 3/2 has been found effective for the US PCS CDMA band. The selection of 3/2 enables sufficient difference in frequency to allow undesirable effects to be easily removed, avoids substantial harmonics at the carrier frequency, provides sufficient resolution to provide required carrier and channel frequencies, and may be implemented using relatively low powered circuitry. It will be appreciated, however, that other application requirements may dictate or allow the use of other scaling factors.  
         [0032]     Referring now to  FIG. 5 , a local oscillator circuit for a CDMA system is illustrated. The local oscillator circuit  125  is intended to create a carrier frequency according to present CDMA telecommunications standards. It will be appreciated that future versions of the CMDA standard may require other carrier frequency ranges, and that other VCO frequencies and scaling factors may be applied to achieve those new frequencies. Local oscillator circuit  127  has a voltage controlled oscillator generating a frequency onto an input line  127 . The input frequency is received into a frequency scaling circuit  129 . The frequency scaling circuit applies a scaling factor to generate a carrier frequency on output line  131 . As illustrated in table  140 , the scaling factor  142  may be selected to generate carriers in different CDMA bands. A first scaling factor  142  of 3/2 is implemented by first multiplying by three  132  and then dividing by two  133 . In this way, when the VCO frequency  141  is set at 1233 MHz, the output frequency  143  is 1849.5 MHz for implementing the carrier frequency at 1850 MHz. In a similar manner, when the VCO frequency  141  is set to 1273 MHz, the output carrier frequency is at 1909.5 MHz, which implements the 1910 MHz carrier frequency. The 3/2 scaling factor thereby enables the VCO to generate carrier frequencies in the range of 1850 MHz to 1910 MHz to implement a first CDMA band.  
         [0033]     To implement a second CDMA band, which extends from 824 MHz to 849 MHz, the scaling factor  142  is selectively set to 3/4. Accordingly, the VCO signal is first multiplied by three  132  and then divided by four  134 . When the VCO frequency  141  is set to 1098 MHz then the carrier frequency is output at 823.5 MHz, which implements the 824 MHz carrier frequency requirement. In a similar manner, when the VCO frequency  141  is set to 1132 MHz, then the frequency carrier output  143  is at 849 MHz. A controller (not shown) may be used to select between a scaling factor of 3/2 and 3/4. This enables a single local oscillator circuit  125  to implement a dual band CDMA radio circuit.  
         [0034]     Referring now to  FIG. 6 , a direct conversion receiver  150  is illustrated. The direct conversion receiver  150  has an antenna  156  for receiving a modulated RF signal. The modulated RF signal is received into receiver circuitry  154 , where a baseband signal is demodulated from a carrier signal. The baseband signal is received into baseband circuitry  152 , where the signal is further processed for use by the wireless device. In the demodulation process, the receiver circuitry  154  uses a locally generated signal at the same frequency as the carrier signal. This local signal is derived from a frequency signal generated by the voltage controlled oscillator  161 . The voltage controlled oscillator  161  provides a stable and accurate frequency signal at a frequency different than the received RF carrier frequency. The signal from the voltage controlled oscillator is received into a frequency scaler  159 , where the frequency of the signal is scaled to the received carrier frequency. In one example, the frequency scaler implements a scaling factor of 3/2. In this way, the local signal frequency is generated by multiplying the VCO signal by 3, and dividing the resulting signal by 2. Because the signal generated by the VCO is different than the frequency of the carrier, any undesirable mixing effect between the voltage controlled oscillator signal and the carrier signal is substantially reduced. In this way, undesirable DC offset effects are reduced. It will be appreciated that other VCO frequencies and scaling factors may be used.  
         [0035]     While particular preferred and alternative embodiments of the present intention have been disclosed, it will be appreciated that many various modifications and extensions of the above described technology may be implemented using the teaching of this invention. All such modifications and extensions are intended to be included within the true spirit and scope of the appended claims.