SC5313A Operating & Programming Manual
Rev 1.0.2
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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 SC5313A is a single-stage, direct coversion Inphase-Quadrature (IQ) demodulating mixer, or simply
an IQ demodulator. The SC5313A can operate as a single-stage downconverter or as an IQ demodulator.
The SC5313A demodulator operates in the 400 MHz to 6 GHz RF range with a typical 3 dB IF bandwidth
of 160 MHz in single-stage converter mode, or 320 MHz in IQ mode. The RF input stage has adjustable
gain to allow the user to adjust the incoming RF signal prior to the demodulation process for the
purpose of optimizing RF dynamic range. The IF stage has adjustable gain to ensure that linearity and
noise of the IF output are optimized. The SC5313A has the necessary RF amplifiers, attenuators, IF
amplifiers, and IF control via DACs to allow the user to optimally operate the device over the entire
frequency range as well as for both small and large RF input levels. Figure 2 shows a simplified block
diagram of the SC5313A, showing only the signal conditioning components critical for the following
discussion. The following sections below 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.
RF Input Section
In the design of the RF input section, care was taken to ensure that the dynamic range of the IQ
demodulator is preserved as seen at the input port of the device. This requires that the demodulator is
not driven too hard (high signal amplitude) nor too soft (low signal amplitude). When the device is
driven hard, nonlinear effects dominate the system. When driven too softly, signal-to-noise dynamic
range suffers. A general rule is to apply more attenuation earlier in the RF signal path to improve
linearity, and more gain to improve signal-to-noise performance. As an example, for a given input signal
level and while maintaining a relatively constant output IF level, the user would switch in RF AMP#1 and
apply attenuation on ATTEN#3 to improve signal-to-noise dynamic range. The factory default state sets
all the RF amplifiers off, all attenuators set to 0 dB attenuation, and the IF gain set to 8 dB (DAC code of
32). In this default state, the device is optimized for a -10 dBm RF signal in the 1.0 GHz to 2.4 GHz range.
The IF output is typically 0.5 V – 1.0 V peak-to-peak differential at these settings.
The RF amplifiers are used to improve the gain of the device if the input signal is too low or when the
losses at higher frequencies are large. RF AMP#1 is usually selected when the RF signals are lower than
-25 dBm at the input port. With RF AMP#1 enabled, the device sensitivity is improved and the detection
of low level signals is better resolved. RF AMP#2 should be selected and switched into the signal path at
RF frequencies greater than 5 GHz, where the signal power loss through the front end prior to the
demodulator can be as high as 15 dB due to filter and switch insertion losses. At these high RF
frequencies, if the IF gain is at its maximum of 15.75 dB (DAC code = 63) and the IF output level falls
below -10 dBm or outside the digitizers optimal levels, RF AMP#2 should be enabled.
