CN-0283 Datasheet, PDF (3/6 Pages) Analog Devices – Providing Fixed Power Gain at the Output of an IQ Modulator

Circuit Note
Figure 3 shows a plot of OIP3 vs. output power (P_OUT) measured at
the IQ modulator output and at the output of the composite circuit.
The shape of the two OIP3 profiles are quite similar, just shifted in
terms of output power and OIP3. This reinforces the idea that the
IP3 is only slightly degraded as the signal passes through the RF
amplifier
50
45
40
35
30
25
20
15
10
5
OIP3 ADL5375 AND ADL5320
OIP3 ADL5375
0
–10
–5
0
5
10
15
20
COMPOSITE OUTPUT POWER (dBm)
Figure 3. OIP3 vs. P_OUT at 2100 MHz for ADL5375 IQ Modulator and for the
Composite Circuit (ADL5375 and ADL5320 Driver Amplifier)
Choosing an Output Power Level
While the circuit achieves OIP3 levels in the 35 dBm to 40 dBm
range for output power levels up to 15 dBm, operation is not
practical up to these levels, particularly with nonconstant envelope
modulation schemes that tend to have relatively high peak-to-
average ratios. To understand why, look at the volts-in to power-out
transfer function of the circuit and consider the typical drive
levels that are available at the input to the IQ modulator.
Figure 4 shows the transfer function of the circuit in terms of
output power (in dBm) and input voltage (in V p-p) with a CW
sine wave, drive signal. An IQ modulator, such as the ADL5375, is
driven typically by a dual, current-out, digital-to-analog converter
(DAC). Normally, the two current outputs (0 mA to 20 mA
nominal) of the DAC are terminated to ground with two 50 Ω
resistors and two 100 Ω shunt resistors are placed across each of the
IQ inputs (for more information on this interface, see Circuit Note
CN-0205). With the DAC running at 0 dBFS, this corresponds
to a drive level at the IQ modulator of 1 V p-p or 0.353 V rms
(this is neglecting the insertion loss of the low-pass filter that is
generally placed between the DAC and the IQ modulator). This
results in an output power of approximately 13 dBm.
CN-0283
25
P_OUT ADL5375 AND ADL5320
20
15
10
5
0
–5
–10
0.10
1
10
V_IN (V p-p DIFFERENTIAL)
Figure 4. Transfer Function of Circuit in Terms of Output Power in dBm and
Input Level in V p-p Differential
If it is assumed that the I and Q inputs of the IQ modulator are
terminated with 100 Ω as previously discussed, the output power
relative to the dBFS drive level of a typical Analog Devices, Inc.,
DAC can be plotted (see Figure 5). Therefore, a drive level of
0 dBFS corresponds to 1 V p-p, resulting in the same 13 dBm
output power previously discussed.
20
I AND Q INPUTS UNTERMINATED
I AND Q INPUTS TERMINATED WITH 100Ω
15
10
5
0
–5
–10
–20
–15
–10
–5
0
dBFS Level (dB)
Figure 5. Transfer Function of Circuit in Terms of Output Power vs. DAC Drive Level
with IQ Modulator I and Q Inputs Terminated with 100 Ω and with I and Q
Inputs Unterminated
Figure 5 also shows the transfer function of the circuit when the
I and Q inputs are not terminated with 100 Ω resistors. Because the
resulting DAC voltage drive level is doubled (2 V p-p maximum),
the resulting output power is higher by 6 dB for the same DAC
drive level.
While operation of the circuit without I and Q termination resistors
is possible, it does pose some problems for the filter that is usually
placed between the DAC and IQ modulator. Because this filter
is generally terminated at both ends, it is desirable to have some
resistance across the I and Q inputs of the IQ modulator (the
unterminated input resistance of these inputs is approximately
60 kΩ). A value that is in the 100 Ω to 1000 Ω range can be
used to increase the resulting DAC voltage drive level and
corresponding output power. However, take care to design
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