Reference Clock Control; Harmonics And Range Of Operation; Channel Standby And Rf Enable - SIGNALCORE SC5506A Operating & Programming Manual

Reference Clock Control

As mentioned above, the primary clock reference for the SC5506A is an onboard 10 MHz TCXO. Should
the user require better frequency stability and/or accuracy, this TCXO can be programmed to phase-lock
to an external source such as an OCXO or rubidium clock. The device can also be programmed to export
its internal 10 MHz clock. To adjust the accuracy of the TCXO as needed (e.g., to correct for long-term
accuracy drift), the user may vary the reference DAC voltage by writing the REFERENCE_DAC_SETTING
register.

Harmonics and Range of Operation

The SC5506A's guaranteed operating frequency range is 100 MHz to 6.0 GHz. However, the device is
capable of tuning as low as 25 MHz and as high as 6.15 GHz. Within these out-of-range regions, the
amplitude may not be within specification but the frequency accuracy will not be affected. At low
frequencies of operation (200 MHz and lower), the harmonics of the signal can potentially be observed as
high as -10 dBc at 0 dBm output. This is due to the limited space available for additional filtering in these
ranges. At lower frequencies, the large physical size of appropriate filters makes it impossible to
accommodate them within the compact form of this device. Furthermore, as the low frequencies are
synthesized through frequency dividers, their output waveforms become more "square" than sinusoidal,
giving rise to higher odd-order harmonics.
The device is specified to a maximum calibrated level of +10 dBm, although the maximum calibrated
output is greater than that in most regions of the spectrum. The accuracy degrades as the amplitude
approaches the compression point due to the linear approximation in the correction algorithm. As a
general rule however, the lower the tuned frequency the higher the achievable output power.

Channel Standby and RF Enable

The SC5506A has independent standby and output enable features for both channels. The user may wish
to place one or both of the channels into standby mode to reduce power consumption and thus lower the
operating temperature of the device under the same environmental conditions. Putting one channel into
standby also eliminates its signal generation, and as a result the remaining operational channel does not
receive any cross-signal contamination, especially when generating low level signals. Although the device
has the option to independently disable the RF output of one channel or the other, this feature does not
shut down the internal synthesizer, so there still exists the possibility of contaminating signals. They are
usually very low but might not be tolerated in some applications. Taking either channel out of standby
requires the device to wait for the power rails to settle and all internal components to be reprogrammed,
usually on the order of one second.
Disabling the RF output moves the frequency to some very low value so that the step attenuators and the
voltage controlled attenuator have the most effective attenuation. Combined, this will push the signal
level below -100 dBm. Enabling the RF output is nearly instantaneous as all components remain active
even when the RF output is disabled.
SC5506A Operating & Programming Manual Rev 2.1.1 10