Second generation (2G) phones use TDMA or CDMA technology, using digital channels modulated directly onto the transmission carrier. The result—greater spectral efficiency—improves the value of signal quality, security, actual data service volume, and international roaming.
The goal of the third generation (3G) terminal is to provide global seamless mobility and achieve global compatibility with some access technologies, such as wireless local loop, cellular, cordless and satellite systems. A technical challenge and difficulty in achieving global seamless mobility of terminals is to achieve a globally unified frequency plan. In every region of the world, at least part of the necessary spectrum has been allocated to other wireless services.
A technical entity called the 3rd Generation Partnership Project (3GPP) was assigned to analyze these proposed technologies. As a result of this work, WCDMA became the technology that was most favored for 3G systems. 3GPP once wrote a technical specification, in which Chapter 25.101 included the core performance requirements of the RF hardware part of WCDMA mobile terminals. 3GPP also defined two optional working modes for WCDMA terminals:
3GPP also specifies that FDD terminals use only 60MHz bandwidth with a duplex spacing of 190MHz: 2110MHz-2170MHz for mobile RX, and 1920MHz-1980MHz for mobile TX.
CDMA Principles
Before discussing WCDMA transmitters, this section briefly summarizes the principles of CDMA. The signal spreading method used by the CDMA system is the "direct sequence" spreading method. To spread the signal, the CDMA system multiplies the unmodulated baseband data with a unique code called a spreading code, which contains a certain number of chips.
The generated spread data is modulated onto a carrier for transmission, and the modulated carrier bandwidth is directly affected by the spread spectrum coding chip rate. WCDMA uses a 3.84MHz chip rate to generate a very wide bandwidth transmission spectrum, so the term "wideband" is used.
To extract the original information, the CDMA receiver demodulates the information carrier and uses a correlator (with the original transmitter spreading code) to regenerate (despread) the desired signal. The extracted data is passed through a narrowband bandpass filter and further processed as needed.
3G WCDMA transmitter specification requirements
Chapter 25.101 of the 3GPP specification (mentioned above) includes the electrical specification requirements for FDD 3G mobile terminal Rx/Tx. Before discussing the requirements for WCDMA transmitters, this section will describe several key transmitter parameters and their importance in transmitter design.
Adjacent Channel Power Ratio [ACPR]: ACPR measures the amount of interference or adjacent frequency channel power. Usually defined as the ratio of the average power in the adjacent channel (or offset) to the average power in the transmitted signal channel, ACPR describes the amount of distortion caused by transmitter hardware nonlinearity.
ACPR is critical for WCDMA transmitters because CDMA modulation produces closely adjacent spectral components in the modulated carrier. The intermodulation of these components causes spectral regeneration on both sides of the center carrier, and the nonlinearity of the transmitter will cause these spectral regeneration components to enter adjacent channels.
Error Vector Magnitude [EVM]: The error vector (both magnitude and phase) is the vector difference between the ideal error-free reference signal and the actual transmitted signal at a given moment. Because it is constantly changing with each symbol change, this new parameter (EVM) is defined as the RMS value of the error vector over a period of time.
EVM is also very important for WCDMA transmitter performance because it indicates the modulation quality of the transmitted signal. Large EVM values will lead to poor detection accuracy, thereby reducing the performance of the transceiver.
Frequency Error: The difference between the specified carrier frequency and the actual carrier frequency. Large frequency errors degrade transceiver performance by causing adjacent channel interference and poor quality detection accuracy.
Spurious and harmonics: Spurious are signals generated by the combination of different signals in the transmitter, and harmonics are distortion products produced by the nonlinear characteristics of the transmitter. Harmonics are generated at frequencies that are integer multiples of the transmitted signal frequency.
After defining some key transmitter parameters, we now list some important requirements for specification and design of 3G WCDMA transmitter terminals. (Table 1)
Table 1. 3GPP transmitter specification requirements
| Parameter | 3GPP Specification | Reference |
| RF frequency range | 1920 - 1980MHz | 25.101 [5.2] |
| Channel spacing | Nominally 5MHz | |
| Chip rate | 3.84Mcps | |
| Maximum output power | 24dBm +1/- 3dB [power class 3] | 25.101 [6.2] |
| Minimum output power | -50dBm | 25.101 [6.4.3.1] |
| Transmit off power | < -56dBm | 25.101 [6.5.1.1] |
| Adjacent channel leakage power | > -33dBc [if adjacent channel power is > -50dBm] | 25.101 [6.6.2.2.1] |
| Alternate channel leakage power | >-43dBc | 25.101 [6.6.2.2.1] |
| Frequency error | Within +/- 0.1ppm | 25.101 [6.3] |
| Transmit intermodulation | > -31dBc [@5MHz offset]
> -41dBc [@10MHz offset] | 25.101 [6.7.1] |
| Error Vector magnitude | <17.5% | 25.101 [6.8.2.1] |
| Spurious emissions | 100kHz RBW | -67dBm ; 925 < f < 935MHz | 25.101 [6.6.3.1] |
| -79dBm ; 935 < f < 960MHz |
| -71dBm;1805 < f < 1880MHz |
| -36dBm ; 30 < f < 1000MHz |
| 300 KHz RBW | -41dBm; 1893.5 = f = 1919.6MHz |
| 1MHz RBW | -30dBm ; 1GHz = f = 12.75GHz |
| 10KHz RBW | -36dBm ; 150KHz = f = 30MHz |
| 1KHz RBW | -36dBm ; 9KHz = f = 150KHz |
WCDMA Transmitters
Maxim offers a variety of WCDMA transmitter ICs covering most common frequency ranges. For example, the superheterodyne system devices feature the industry's highest integrated transmitter chip (MAX236X), providing a typical 380MHz Tx intermediate frequency (IF). Another example of a superheterodyne system chip is the MAX2383 upconverter driver, which uses a high Tx IF frequency of 570MHz. To show that the hardware meets the 3GPP specification (with margin), this section provides some system-level and discrete component test results based on the first generation of Maxim WCDMA transmitter ICs that are part of the v1.0 WCDMA reference design. Contact the manufacturer for information on the newer zero-IF WCDMA reference design.
Figure 1. WCDMA transceiver block diagram
WCDMA Superheterodyne Transmitter
This transmitter is part of a complete WCDMA transceiver reference design, which includes 4 main ICs:
MAX2388 Receiver Front-EndMAX2309 IF Quadrature DemodulatorThe MAX2363 Quadrature Modulator/Upconverter Transmitter ICThe MAX2291 RF Power AmplifierThe transmitter hardware uses an IF of 380MHz and a Tx frequency of 1920MHz to 1980MHz. The duplexer enables full-duplex operation by connecting the Tx channel (and the Rx channel) to the antenna.
At the back end of the Tx circuit, the MAX2363 receives the I, Q differential signals transmitted from the baseband as input, performs quadrature modulation, IF and RF LO frequency synthesis, and RF up-conversion. The IF LO is synthesized by the internal VCO and PLL at a frequency of 760MHz. The -7dBm signal provided by the external RF VCO module is input to the MAX2363 up-converter in high-side injection mode. The on-chip RF driver enables the chip to directly drive an external PA.
In the front end of the Tx circuit, the chip-scale packaged linear PA (MAX2291) provides 28dB of gain in this application and an output power of +28dBm. Since the insertion loss after the PA is about 4dB, the maximum antenna output achieved by the system is 24dBm.
After fully entering the working state, the WCDMA system works at medium power instead of full power most of the time. The MAX2291 provides two output power optimization modes to meet this requirement, extending the talk time while having the following expected performance:
Vcc is 3.5V DC, measured in high power mode:
Pout = 28dBmFrequency = 1.95GHzACP1 = -39dBc (measured at 5MHz offset, 3.84MHz bandwidth)Power added efficiency = 37%Standby current Icc = 97mAVcc is 3.5V DC, measured in low power mode:
Pout = 16dBmFrequency = 1.95GHzACP1 = -38dBc (measured at 5MHz offset, 3.84MHz bandwidth)Power added efficiency = 14%Standby current Icc = 30mAThe 3GPP specification given above stipulates that the output power of the WCDMA transmitter must be between +24dBm and -50dBm to meet the required 74dB dynamic range. The v1.0 reference design board is designed for 80dB dynamic range, leaving some margin.
The dynamic range of the transmitter chip is limited. The DD is usually limited by the ACPR at high power and by the noise floor at low power. In order to obtain a carrier-to-noise ratio (C/N) of more than 15dB at low power, an additional 20dB of variable attenuation (introduced by the gain control attenuator of the PA) was designed into the v1.0 reference design board. The key performance parameters extracted from the comprehensive test results (Table 2) prove that the Maxim v1.0 WCDMA transmitter meets the specification requirements.
Table 2. Tx output characteristics at full power
| Parameter | Specification | Data @1980MHz | Data @ 1920MHz |
| O/p power @ antenna port | 24dBm | 24.8dBm | 25.5dBm |
| +/- 3.8MHz ACP * | -50dBc | -52dBc | -52dBc |
| +/- ACPR1 * | -33dBc | -37dBc | -37dBc |
| +/- ACPR2 * | -42dBc | -54dBc | -54dBc |
| Icc @ 3.3V (TX only) | - | 620mA | 615mA |
| Noise @ Rx. band | | -137dBm/Hz | -137dBm/Hz |
| Noise @ 1880MHz | | | -135dBm/Hz |
*For specific minimum/maximum ACP diagrams, see Figures 2-5 below.EVM and ACPR of the Tx Circuit
The EVM measured from the v1.0 WCDMA reference design board is about 5.7% (3.5% from the MAX2291 PA and 4.6% from the MAX2363 Tx chip) when the Tx output power is +24dBm. The overall EVM value fully meets the requirements of 3GPP (<17.5%). The EVM and ACP test results of the Tx circuit are shown below:
Figure 2. EVM of Tx circuit, output -20dBm
Figure 3. EVM of Tx circuit, output +24dBm.
Figure 4. ACP of Tx circuit, output +24dBm.
Figure 5. ACP of Tx circuit, output -20dBm. According to the suburban voice output power distribution function (a statistical performance parameter that describes the power variation in different situations such as urban and rural areas, data and voice), the measured Tx circuit current is 550mA at maximum output power and 365mA at 22dBm output power.
When the Tx power is at maximum, the Tx noise measured in the Rx band is -137.0dBm/Hz. If the isolation between Tx and Rx is -50 dB, the Tx noise in the Rx channel is -187.0dBm/Hz, which is much lower than the thermal noise. In other words, the contribution of Tx to the total Rx noise is almost zero. (This calculation has been confirmed by the test results at maximum power and lower power.)
The two graphs show the typical spectral shapes expected at the antenna port of the V1.0 Tx circuit. When the antenna outputs 24dBm power (Figure 6), the situation is:
Icc = 490mA (TX only), and 535mA (TX + RX)MAX2363 IF DAC Setting = 110VGC = 2.4V.
Figure 6. Typical spectrum shape at the antenna port of the V1.0 Tx circuit when the antenna outputs 24dBm power. When the antenna outputs -53dBm power, i.e., low Tx output power with VGC = 1.35V and absolute output power = -38dBm (Figure 7), the situation is:
Icc = 166mA (TX only)VGC = 1.35VIF DAC = 000PA Bias Setting = 1Pout decay = MAX.
Figure 7. Typical spectrum shape of the antenna port of the V1.0 Tx circuit when the antenna outputs -53dBm low power Summary
The purpose of this article is to provide the reader with a reasonable overview of the theory, design, and specification requirements of a WCDMA transmitter, using the first generation Maxim WCDMA superheterodyne reference design as an example. Maxim's current product portfolio also includes direct-conversion (zero-IF) WCDMA transceivers.
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