New DAC Generation significantly simplifies WCDMA TX Designs
By Heinz-Peter Beckemeyer - Texas Instruments
Direct Up-Conversion has become the popular architecture for WCDMA transmit architectures. While in earlier days the I/Q modulators have achieved required performance nodes, the latest DAC technology even now allows simplified architectures because of an improved noise floor for avoiding noise filtering in or close to the TX band. The following article gives an in-depth overview of this major achievement with the lately introduced DAC5687. The high speed D/A-converter play an important role in WCDMA base-station (Node B) technology. The architecture can be optimized significantly by choosing the right D/A-converter with its main features like sampling speed, linearity and especially the noise floor. The lately introduced DAC5687, which can be used for various TX architectures, helps on the optimization with its improved linearity and noise performance compared to previous DAC generations.
Figure 1 shows the block diagram of the DAC5687, which consists of two D/A-converter with a maximum sampling rate of up to 500 MSPS. It has in addition several digital features integrated like interpolation, which allows for low data input rate and therefore simplified system partitioning. The digital Quadrature modulator and the NCO on chip allow for an operation in real high IF or complex IF mode.
Direct Up-Conversion in WCDMA As already indicated earlier, the direct up-conversion is an attractive architecture for lowest cost and as well for high bandwidths needed in today's WCDMA systems. The bandwidth requirements come from the desire of multi-carrier designs and the digital pre-distortion technology, which needs a bandwidth of around 5-7 times the signal bandwidth. The D/A-converter is in such architecture followed by an I/Q-modulator, which output is directly the RF frequency without any intermediate frequency stage. The advantage in this architecture is that no image reject or channel filtering is required, resulting into reduced complexity and cost.
Figure 2 shows the block diagram of the direct up-conversion architecture with the DAC5687 and the I/Q modulator TRF3702. The DAC5687 was in-depth studied for this architecture in WCDMA transmitter. A common example is a sampling rate of 491.52 MSPS for the DAC5687 while the input data rate is at 122.88 MHz, resulting into an interpolation factor of 4. The improved noise performance of the DAC5687 helps specifically on meeting the spurious emission limits without a filtering in or close to the TX band.
Spurious Emission Limits
The spurious emission limits in 3GPP define the maximum limits at a certain offset from the carrier. The most relevant specification is the spurious emission at an offset of 50 MHz (-25 dBm) and at an offset of 60 MHz (-30 dBm). The measurement bandwidth for these is 1 MHz. An example for the 60 MHz case is shown in Figure 3. The 60 MHz case is for the single carrier operation the worst case scenario, where the carrier is either located at the most upper frequency of 2167.5 MHz or the lowest frequency of 2112.5 MHz. The 60 MHz offset (2172.5 MHz and 2107.5 MHz) is in this case just outside of the TX frequency band where the duplexer filter cannot yet provide any attenuation. The direct up-conversion architecture also does not allow for a channel filter to reduce the noise floor away from the carrier, because of the 60 MHz bandwidth (2110 MHz to 2170 MHz) to operate in. The I/Q modulator itself needs to serve with the sufficient noise performance.
Figure 3: Spurious emission at an offset of 60 MHz The -30 dBm represents an absolute noise level, which results into a carrier to noise performance of the I/Q modulator. The calculation in Table 1 shows the required performance for the carrier to noise ratio.
An output power of 43 dBm was considered in this analysis. This is the rms power of a single WCDMA carrier, measured in a 3.84 MHz bandwidth. The spurious emission of -30 dBm is however measured in a 1 MHz bandwidth. This corresponds to a power of -24.16 dBm in a bandwidth of 3.84 MHz. With a system margin of 6 dB for the noise floor, the resulting noise floor results into -30.16 dBm at the output of the PA. This corresponds to a relative carrier to noise floor of -73.16 dBc/3.84 MHz or -139 dBc/Hz. This means that the noise floor at the output of the I/Q modulator with an output power of -10 dBm shall not exceed -149 dBm/Hz. Previous designs had at the output of the D/A-converter a noise filter between the DAC and the I/Q-modulator for meeting the noise performance at 50- and 60 MHz offset with a certain margin. The DAC5687 with its improved noise floor provides such margin that this noise filtering at the output is not anymore necessary.
DAC5687 ACPR and Noise Floor Performance
The DAC5687 was specifically tested for this WCDMA TX system test in terms of noise and ACPR performance. Figure 4 shows at first the ACPR1 and ACPR2 behavior, where a 2-carrier signal at +5 MHz and -5 MHz was fed into the DAC5686. The ACPR1 shows here a performance on the range of 76 dBc (-6.42 dBm –(-82.28 dBm)), while the ACPR2 is in the range of 80 dBc, well above the 70 dBc typically demanded from a DAC in WCDMA TX architectures. The linearity and the noise performance of the DAC play an important role for those excellent results.
Figure 4: WCDMA ACPR Performance of the DAC5687 For the investigation of the noise performance related to the spurious emissions, the DAC output noise needs to be analyzed at the offset frequencies described earlier in the chapter Spurious Emissions. Figure 5 shows the noise measurement according to the 3GPP standard. The resolution bandwidth and vide bandwidth is set to 1 MHz. In parallel, the FSQ spectrum analyzer from Rohde and Schwarz allows for measuring the noise floor in a certain band, indicated in this figure at an offset of 40-, 50- and 60 MHz. The noise floor is in the range of -99 dBm. This is an improvement in the order of 5 dB compared to earlier DAC generations. The two measurements show with a carrier output power in the range of -6 dBm in a bandwidth of 3.84 MHz and the noise in the order of -99 dBm in a 1 MHz bandwidth, that the carrier to noise is in the order of -152 dBc/Hz. This results into a margin of around 13 dB compared to the calculation shown earlier for the noise floor calculation of the I/Q modulator. This results into an I/Q modulator noise degradation of less than 0.25 dB.
This margin allows therefore avoiding the filtering of the noise at an offset of 50- and 60 MHz, resulting into less complexity. The resulting available bandwidth is therefore also not limited because of such filtering making the DAC5687 ideally suited for wider band systems with digital pre-distortion of higher order.
Literature:
[1] DAC5687 datasheet http://focus.ti.com/docs/prod/folders/print/dac5687.html [2] TRF3702 datasheet http://focus.ti.com/docs/prod/folders/print/trf3702.html
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