SC5412A Operating & Programming Manual
Rev 1.1.0
8
S C 5 4 1 2 A
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H E O R Y O F
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P E R A T I O N
Overview
The SC5412A is a single-stage, direct conversion Inphase-Quadrature (IQ) modulator. The modulator
operates in the 400 MHz to 6 GHz RF range with a typical 3 dB IQ IF bandwidth of 320 MHz. IF signals are
conditioned by input differential drivers prior to the IQ mixers. These differential drivers adjust the DC
level and minimize the DC offset effects on the modulated RF signal. The RF output stage has adjustable
gain to allow the user to adjust the signal levels, and to also to optimize for signal dynamic range. The
SC5412A has the necessary RF amplifiers, attenuators and IF amplifiers to give the user optimal control
of the device over the entire frequency range. Figure 3 shows a simplified block diagram of the SC5412A,
showing only the signal conditioning components critical for the following discussion. The following
sections provide more in-depth discussion on how to optimize the converter for linearity and signal-
noise dynamic range. Power supply generation and regulation, and digital control functions are not
covered. Should the user require more information than what is provided in this manual, please contact
SignalCore.
IF Input Section
The IF inputs are typically driven differentially, however they may also be driven single-ended. The IF
input is DC coupled with a differential impedance of 100
. Singled-ended operation is recommended
for AC-coupled operation due to the fact that DC-coupled single-ended operation may cause large DC
offsets at the input driver amplifier that the DC offset compensation may not be about to overcome. A
large DC offset will lead to high LO leakage, so unless the DC points of the differential inputs are bias the
same, AC coupling is recommended for single-end operation. For differential DC-coupled input it is
recommended to drive the common mode input voltage between 1.5 V to 2.5 V for best linearity
performance. See Figure 2 for details of the input section.
From Figure 3, the IF signals are first filtered by a 160 MHz filter, and then passed to the input
differential drivers before being driven into the IQ mixers. The DC characteristics of the output of each
differential driver are controlled by two 14 bit DACs. One DAC controls the driver common output
voltage, while a separate DAC controls the DC offset of the differential output pair. The common output
voltage has a range from 0V to 2.5V (DAC code 0 to 16383), and a differential voltage range of -50 mV to
+50 mV (DAC code 0 to 16383, mid code is 0V). The common output voltage controls the bias to the
mixer inputs, which affects the mixer linearity. The user may need to adjust this voltage to optimize
linearity (IP3 and harmonics), which is dependent on IF input power level and the operating frequency. A
typical bias voltage may range between 1 V and 3 V.
DC offset to the differential modulating mixer is controlled using the DC offset DAC. This DAC provides
approximately -50 mV to +50 mV of differential voltage adjustment. Adjusting the DAC offset on both
the I and Q channels can minimize the LO leakage at the RF output. This is commonly known as “nulling”
out the LO. The process typically involves a few adjustment steps alternating between the I-Channel DAC
and the Q-Channel DAC until an optimal level of LO suppression is achieved. However, nulling of the LO
is typically only effective over a small amplitude, frequency, and temperature range. Once the operating