SIGNALCORE SC5309A, Hardware Manual, Page: 20 / 49

©2018 Rev 2.0
19 Functional Description
The LO Module
The local oscillator (LO) module contains three local oscillators, the reference clocks, and the
calibration EEPROM as shown in
Figure 5 .
Figure 5. Block diagram of the local oscillator
The Reference Clocks
The base clock of the downconverter is a 10 MHz Voltage Controlled Temperature Controlled
Crystal Oscillator (VCTCXO) with an initial accuracy better than 500 ppb once the device has
reached a stable temperature. Its initial accuracy is set at the factory via an on-board 14-bit voltage
reference DAC. This DAC is accessible for dynamic accuracy calibration. The other reference is a
100 MHz Voltage Controlled Crystal Oscillator (VCXO) which is phase locked to the base reference
whenever an external reference source is not used.
When an external reference is selected as the base clock by enabling the device to phase lock to it,
the effect only occurs when the presence of this reference is detected. In other words, although
the device is programmed to lock externally it will not attempt to do so until the reference signal
is detected at the input reference port. Notice, both the reference clocks will attempt to lock to
the external source. Having the VCXO lock directly to the external source has the advantage of
utilizing the close-in phase noise of the source; it is best to assume that the external source is
superior to the internal base. Although the internal VCTCXO is not used when an external reference
100M PLL
REF DETECT
10
To Signalchain Module
ENABLE
LO3 LO2
LO1
TCXO PLL
Programmable
component
REF
IN
REF
OUT
Fixed
component
LO Section
Reference
Section
CAL EEPROM