LOCAL OSCILLATOR FOR A DIRECT CONVERSION TRANSCEIVER

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

"" ^•J f,- f ;qucncy (RF) signal, that is converted to a baseband signal in a m-eiv^t

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 Io w-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 2.2GHz, 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 proviαes 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.

[Out)?] ihe v CO 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.

, ^{» !} - *

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

;rn _± >. ;•» ,vβ ^r 'ι of the VGA varies proportionally to the exponential of ιh. < < ^~, >-...:•!

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

irequencies, the local oscillator circuit substantially reduces VCO puiiing 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

HI 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 iis 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 sealer 19. The frequency sealer

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 sealer 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 iactor is adjusted accordingly.

[0024] The frequency sealer 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

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 sealer 59, where the

frequency of the signal is scaled to the desired carrier frequency. In one example,

the frequency sealer 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.

_l ϋu2ϋj 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

.Ta ¹ IVj; factor has <ι sufficient frequency difference between tVe VCC slgivu n.pιά

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 v ^" CO frequency 87 is identified, along with an associated scaling ractor

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

io iIluoii.ciC£vi. The local oscillator circuit 125 is intended io 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

v CvJ 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 sealer 159, where the frequency of the signal is scaled

to the received carrier frequency. In one example, the frequency sealer

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

' ^~ y 1. / oca if si- the signal generated by the VCO is different ^ >λ- <". 'h- e frequency ot

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.