SC5305A Operating & Programming Manual
Rev 2.1.0
12
Figure 4. Graphical representation of IF inversion.
Inverted spectral conversion is convenient for digitizers that sample the IF in the even Nyquist zones
because it eliminates the need to perform digital inversion of the acquired spectrum.
All local oscillators are phase-locked to an internal 100 MHz voltage controlled crystal oscillator (VCXO),
which sets their close-in phase noise performance. The 100 MHz VCXO is in turn phase-locked to the
internal 10 MHz TCXO for frequency accuracy and stability. For better frequency accuracy and stability
than the TCXO onboard the SC5305A, or for frequency synchronization, the user can programmatically
set the device to phase lock the TCXO to an external 10 MHz reference source by programming the
REFERENCE_SETTING register. It is important to note that the TCXO will only attempt to lock to an
external source if one is detected. A typical external reference source minimum level of -10 dBm is
required for detection to be successful. A reference source level of 0 dBm to +3 dBm is recommended
for normal operation. The reference source is fed into the device through the
“ref in”
port. The device
can also export a copy of its internal reference through the
“ref out”
port. The output reference
frequency is selectable for either 10 MHz or 100 MHz output. By default, routing of the reference signal
to the “
ref out
” port is disabled. It can be enabled by programming the REFERENCE_SETTING register.
This reference frequency is sourced from the internal 100 MHz OCXO, and the default output selection is
10 MHz, which is divided down from the 100 MHz VCXO. The output reference level is typ3 dBm.
Frequency Tuning Modes
Tuning of SC5305A superheterodyne downconverter is accomplished through the tuning of LO1. LO1 has
two sets of control parameters that can be explored to optimize the device for any particular
application. The first set of parameters, TUNE SPEED, set the tuning and phase lock time as the
frequency is changed. TUNE SPEED consists of two modes - Fast Tune mode and the Normal mode; both
of these modes directly affect the way the YIG oscillator is configured. The Fast Tune mode deactivates a
noise suppression capacitor across the tuning coil of the YIG oscillator, and doing so increases the rate of
current flow through the coil, correspondingly increasing the rate of frequency change. In Normal mode
the capacitor is activated, slowing down the rate of frequency change. The advantage of activating the
capacitor is that it shunts the noise developed across the coil, decreasing close-in phase noise. Refer to
Appendix A for specifications regarding tuning speed.
RF
A
f
c
IF
A
if
c
IF
A
if
c
Non Inverted Conversion
Inverted Conversion