Cost-Effective
Direct Up-Conversion for WCDMA NodeB
Heinz-Peter Beckemeyer -
Direct up-conversion is the
most cost effective solution for transmitter architectures, the most important
criterion in base station designs. The carrier to noise ratio of the I/Q
modulator is the most critical parameter in the WCDMA technology. The TRF3702
I/Q-modulator provides a carrier to noise performance with margin while
maintaining an excellent ACPR performance. This article details the technical
requirements related to the carrier to noise floor of an analog quadrature
modulator in a single- and multi carrier WCDMA BTS.
The
direct up-conversion architecture in transmitter systems is often considered
for a low system cost since it can reduce the complexity and component count. Figure 1 shows the block diagram of a WCDMA direct
up-conversion architecture. Opposed to a dual conversion transmitter, the
direct up-conversion
Figure 1: TX block diagram for WCDMA direct up-conversion
transmitter
The
main requirements for the performance of an I/Q modulator in WCDMA technology
is the ACPR (Adjacent Channel Power Ratio) and the noise performance at an
offset of 50 MHz and 60 MHz offset from the carrier, with the noise requirement
being the most stringent. This noise performance requirement is determined by
the spurious emissions limits, which shall not be exceeded (section 6.6.3.1.2
[1]).
Spurious Emission Limits
The
spurious emission limits 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 2. 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 2: 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.
Table 1: Link budget, noise floor due to spurious emissions
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.
The
noise performance is in a multi-carrier design not necessarily more difficult
to meet since the 60 MHz offset pushes to frequency to measure at into the
region where the duplexer filter already attenuates the noise by a certain
amount. Here, the spurious emission at an offset of 50 MHz might be more
stringent, with the maximum value at -25 dBm (in 1 MHz bandwidth).
This is 5 dB more relaxed to the 60 MHz offset spurious emission
specification. The 50 MHz case becomes especially important in a
15 MHz bandwidth system, where the 50 MHz offset falls exactly to the
same frequency of 2107.5 MHz or 2172.5 MHz like in the single carrier
case, with no help from the duplexer.
ACPR requirement
The
ACPR requirement is next to the tough carrier to noise specification an
important criterion for the I/Q modulator in WCDMA. The ACPR (Adjacent Channel
Power Ratio), or in the UMTS standard used expression ACLR (Adjacent Channel
Leakage Ratio) describes the power ratio between the main carrier and the lower
and upper adjacent channel (5 MHz offset) and the alternate channel
(10 MHz offset). The UMTS standard asks for a value 45 dBc for the
alternate and 50 dB for the adjacent channel. These values are referred to
the output of the power amplifier, better values are asked for in the
individual components in the signal chain before to not degrade the PA
performance. A performance typically in the range of 60 dBc is the
requirement for the I/Q modulator.
TRF3702 performance with the
DAC5686
The TRF3702
[2] has been tested in combination with the DAC5686 [3], a 16-bit, 500 MSPS
dual DAC, in a passive interface to demonstrate the performance in terms of the
ACPR and noise behavior. The passive interface is shown in Figure 3. The filtering of the DAC image is left out for
simplicity reason.
Figure 3: Passive interface between the DAC5686 and TRF3702
The
DAC5686 output compliance (AVDD – 0.5V to AVDD + 0.5V)
range can easily be interfaced to the common mode input voltage (3.7V) of the
TRF3702. The achieved noise performance at an offset of 50 MHz and
60 MHz is in the range of -144 dBc/Hz, which is well within the
calculated requirement including the 6 dB margin (see Table 1). The output power of the TRF3702 I/Q modulator was
in this configuration -8 dBm. The obtained ACPR performance under this
condition was in the range of 64 dBc.
Figure 4: Noise at 60 MHz offset versus WCDMA channel
power
Figure 4 shows the noise at 60 MHz offset versus the
channel power at the output of the TRF3702.
The calculated carrier to noise specification can be maintained down to
an output power in the range of -15 dBm, resulting into a potentially
better ACPR
The
ACPR of the TRF3702 can go beyond 71 dBc with a lower output power. Figure 1 shows the ACPR performance chart with a single
carrier WCDMA signal at an output power, where the ACPR is 71.2 dBc and
the carrier to noise floor still within the limits.
Figure 5: Best in class ACPR with -14 dBm output power
.
Conclusion
The
analysis on the maximum noise floor for an analog modulator for direct
up-conversion in WCDMA base station shows that the noise requirement can be met
with sufficient margin on the TRF3702. The noise floor shall not exceed the
-139 dBc/Hz, including a 6 dB margin. Measurment results with the
TRF3702 connected with a passive interface to the DAC5686 show a noise
performance in the range of -143 dBc/Hz. The ACPR requirement can be met
at a high output power of -8 dBm with 64 dBc. The ACPR can go as high
as 71 dBc for a lower output power of -14 dBm, while still meeting
the carrier to noise requirements.
[1] 3GPP TS 25.104
version 6.3.0 Release 6
[2] TRF3702 datasheet, http://focus.ti.com/docs/prod/folders/print/trf3702.html
[3] DAC5686 datasheet, http://focus.ti.com/docs/prod/folders/print/dac5686.html