CN-0283 Datasheet, PDF (4/6 Pages) Analog Devices – Providing Fixed Power Gain at the Output of an IQ Modulator

CN-0283
the filter between the DAC and IQ modulator so that it can
support different source and load impedances.
As already noted, from Figure 4 and Figure 5, it can be seen that
a 1 V p-p sine wave (0 dBFS) is provided an output power of
approximately 13 dBm (the I and Q inputs terminated with 100 Ω).
In practice, the DAC drive level must be reduced slightly from
0 dBFS to reduce distortion (typically 1 dB to 2 dB). In addition to
this, the rms drive level should be lower again by an amount
equal to the peak-to-average ratio of the modulation of the carrier.
The ratio of peak envelope power (PEP) to rms power is typically
in a range from 5 dB for QPSK-like modulation schemes (0 dB in
the special case where the modulation is constant envelope) to
around 10 dB for higher order QAM-based modulation. Referring
to Figure 6, this suggests that output power levels in the 0 dBm
to 10 dBm range are feasible.
The adjacent channel power ratio (ACPR) of a single carrier,
wideband code division multiple access (WCDMA) signal has
become a popular metric for assessing the system level distortion
of a circuit (that is, as opposed to an assessment that is solely based
on IP3 and IMD levels). Figure 6 shows the measured ACPR of
the circuit vs. the output power level. In the case of a WCDMA
signal, ACPR is defined as the ratio of the power in the carrier
(in a bandwidth of 3.84 MHz) to the power in an adjacent channel
(channel spacing = 5 MHz), also measured in a 3.84 MHz
bandwidth. The plot also shows an alternate channel power ratio
that is the same type of measurement; however, at a carrier
offset of 10 MHz.
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ADJACENT CHANNEL POWER RATIO (dB)
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ALTERNATE CHANNEL POWER RATIO (dB)
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OUTPUT POWER (dBm)
Figure 6. Plot of OIP3 and WDCMA ACPR vs. Output Power
In this case, the signal has a PEP-to-rms ratio of approximately
10 dB (the peak-to-average ratio of a WCDMA signal can vary
based on how the carrier is configured and loaded). Based on
this plot and the desired level of ACPR, select an output power
level in the 0 dBm to 10 dBm range. At power levels less than
0 dBm, the ACPR becomes dominated by the degrading signal-
to-noise ratio of the circuit.
Circuit Note
COMMON VARIATIONS
The ADL5320 driver amplifier is specified to operate from
400 MHz to 2.7 GHz. This conveniently covers the lower end
of the specified frequency range of the ADL5375 IQ modulator.
For operation at frequencies in the 2.3 GHz to 4 GHz range, the
ADL5321 driver amplifier is recommended. Both the ADL5320
and ADL5321 must be tuned to the frequency at which they will
be operating. The data sheets of both devices contain tables that
provide recommended values for tuning components at popular
operating frequencies.
A broadband internally matched gain block, such as the ADL5601
or the ADL5602, can also be used to provide gain at the output
of the IQ modulator. However, because these devices have lower
OIP3 (than ADL5320 and ADL5321), they tend to dominate
and reduce the overall IP3 of the circuit.
A number of narrow-band IQ modulators are available that
provide higher performance over their operating frequency ranges.
Examples are ADL5370/ADL5371/ADL5372/ADL5373/ADL5374.
These narrow-band devices provide higher gain and OIP3
compared to ADL5375. When paired with the ADL5320 and
ADL5321 driver amplifiers, the net result is overall higher
output power with similar composite OIP3.
The ADRF6701/ADRF6702/ADRF6703/ADRF6704 families of
narrow-band IQ modulators include an integrated phase-locked
loop (PLL) and voltage controlled oscillator (VCO). These devices
provide similar performance to the ADL5370/ADL5371/ADL5372/
ADL5373/ADL5374 family; however, with a higher level of
integration.
A number of options exist to drive the I and Q inputs of the IQ
modulator. The AD9125 and AD9122 are 16-bit dual DACs that
operate at 1 GSPS or 1.2 GSPS, respectively. These devices can be
used to generate either a baseband spectrum (centered at 0 Hz) or a
complex IF spectrum typically in the 100 MHz to 200 MHz range.
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