If Output Section - SIGNALCORE SC5312A User Manual

14 SC5312A & SC5313A Hardware Manual

IF Output Section

The IF outputs are differentially driven. Each of the in-phase and quadrature components of the
demodulator are conditioned prior to leaving the IF ports. The user can programmatically adjust the
parameters of the differential signal such as the common output voltage, DC offset between the (-) and (+)
terminals, and its amplitude. The differential output impedance of each component is 100  and DC
coupled. However, all ports can be operated as AC coupled single-ended 50  ports. All unused ports
should be terminated into AC coupled 50  loads.
There are voltage DACs within the device to control the signal parameters of each of the IQ components.
For each component, the Vcom (common voltage) DAC controls the common output voltage of the
differential outputs. The Vcom DAC values range from 0 to 16383 (14 bits) and change the voltage between
1 - 3.5 V. For a wider output voltage swing range, this voltage should be set to about 2.4 - 2.5 V. Having a
wider swing range improves the output compression point of the device. This is not a hard requirement
and the user will need to adjust the voltage levels to suit their specific requirements. As an example, setting
to some other voltage may be required to optimize the dynamic range of the receiving digitizer and as a
result better optimize the entire system.
DC offsets may limit the dynamic range of the receiving digitizer and where it is critical the user can "tune
out" to minimize these offsets using the DC Offset DAC. This 14 bit DAC can correct offsets up to +/-0.050
V with less than 0.010 mV resolution.
The IF amplifiers have adjustable gain ranges from 0 to 15.75 dB, with a tuning resolution of 0.25 dB. The
gain is controlled by programming a 6-bit DAC whose codes range from 0 to 63. Writing 63 to the DAC
provides the highest gain. Increasing the IF gain instead of the RF gain to achieve a required IF level will
improve the linearity of the system but with the chance of a slight increase in output noise. For a common
output voltage of 2.4 V, the output compression/saturation point of the amplifier is around 10 dBm. It is
recommended to operate the output at least 6 dBm below this value to avoid running into saturation from
signals with high crest factors. When deciding the operating point of the digitizer, it is recommended that
the user not operate the output voltage too close to the saturation point of the digitizer input.
The linearity DAC controls the current flow throw the demodulator core and thus affects the linearity of
the device. Generally, increasing the voltage results in higher current consumption, and as a result the
linearity improves. However slight adjustments to the voltage may improve the linearity further; this is
dependent on the frequency and input power.
Rev 2.1 | SC5312A & SC5313A User Manual SignalCore, Inc.