Rainbow Electronics MAX5888 - Manual (Page 13)

MAX5888
3.3V, 16-Bit, 500Msps High Dynamic
Performance DAC with Differential LVDS Inputs
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carriers spread their IM products over a bandwidth of
20MHz on either side of the 20MHz total carrier band-
width. In this four-carrier scenario, only the energy in
the first adjacent 3.84MHz side band is considered for
ACLR 1. To measure ACLR, drive the converter with a
WCDMA pattern. Make sure that the signal is backed
off by the peak-to-average ratio, such that the DAC is
not clipping the signal. ACLR can then be measured
with the ACLR measurement function built into your
spectrum analyzer.
Figure 9 shows the ACLR performance for a single
WCDMA carrier (f
CLK = 184.32MHz, f OUT = 61.44MHz)
applied to the MAX5888 (including measurement sys-
tem limitations*).
Figure 10 illustrates the ACLR test results for the
MAX5888 with a four-carrier WCDMA signal at an out-
put frequency of 61.44MHz and sampling frequency of
184.32MHz. Again, the noise floor of the instrument
restricts the signal’s real dynamic range of the signal,
and the measured ACLR 1 understates the actual by
more than 2.5dB. Considerable care must be taken to
ensure accurate measurement of this parameter.
Multitone Testing for GSM/EDGE
Applications
The transmitter sections of multicarrier base station
transceiver systems for GSM/EDGE usually present
communication DAC manufacturers with the difficult
task of providing devices with higher resolution, while
simultaneously reducing noise and spurious emissions
over a desired bandwidth.
To specify noise and spurious emissions from base sta-
tions, a GSM/EDGE Tx mask is used to identify the DAC
requirements for these parameters. This mask shows
that the allowable levels for noise and spurious emis-
sions are dependent on the offset frequency from the
transmitted carrier frequency. The GSM/EDGE mask
and its specifications are based on a single active car-
rier with any other carriers in the transmitter being dis-
abled. Specifications displayed in Figure 11 support
per-carrier output power levels of 20W or greater.
Lower output power levels yield less stringent emission
requirements. For GSM/EDGE applications, the DAC
demands spurious emission levels of less than -80dBc
for offset frequencies
≥6MHz. Spurious products from
the DAC can combine with both random noise and spu-
rious products from other circuit elements. The spuri-
ous products from the DAC should therefore be backed
off by 6dB more to allow for these other sources and
still avoid signal clipping.
*Note that due to their own IM effects and noise limitations, spectrum analyzers introduce ACLR errors, which can falsify the measure-
ment. For a single-carrier ACLR measurement greater than 70dB, these measurement limitations are significant, becoming even more
restricting for multicarrier measurement. Before attempting an ACLR measurement, it is recommended consulting application notes pro-
vided by major spectrum analyzer manufacturers that provide useful tips on how to use their instruments for such tests.
-125
-100
-110
-120
-90
-80
-70
-60
-50
-30
-40
OUTPUT POWER (dBm)
-25
3.5MHz/div
f CENTER = 61.44MHz
f CLK = 184.32Mbps
ACLR = 73dB
Figure 9. ACLR for WCDMA Modulation, Single Carrier
-130
-100
-110
-120
-90
-80
-70
-60
-50
-30
-40
3.5MHz/div
f CENTER = 61.44MHz
f CLK = 184.32Mbps
ACLR = 65dB
OUTPUT POWER (dBm)
Figure 10. ACLR for WCDMA Modulation, Four Carriers