Circuit Note

CN-0283

Rev. 0 | Page 3 of 6

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

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COMPOSITE OUTPUT POWER (dBm)

OIP3 ADL5375 AND ADL5320

OIP3 ADL5375

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.

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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.

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dBFS Level (dB)

I AND Q INPUTS TERMINATED WITH 100Ω

I AND Q INPUTS UNTERMINATED

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