A Flexible Phase-Locked Loop Synthesizer for Radio-Science and Communication Payload Applications
Simone, L.; Nanni, V.; Barletta, F.; Santoni, A.
Thales Alenia Space Italy

Next generation of Radio-Science equipment's will require flexible Local Oscillator to be used as actuator inside the carrier recovery loop mechanization. Likewise, Frequency Converters requirement for Communication Payload application are becoming more demanding in terms of phase noise, to support high order modulation scheme, and flexibility, in order make possible channel re-location during the satellite operative life.

The paper will address a new Phase Locked Loop (PLL), so-called Flexible Phase Locked Loop (F-PLL) well suited for Radio-Science and Frequency Converter equipment's.

The F-PLL combines a Phase/Frequency Detector (PFD) and a Sampling Phase Detector (SPD) that is used as sample down-converter inside the loop. Moreover, the F-PLL reference is synthesized by a Direct Digital Synthesizer (DDS) that provides the required flexibility in terms of tracking and channelization capabilities.

Main features of the F-PLL are:
1) Optimum phase noise (conversion-based loop, PFD noise floor impact is minimized)
2) Safe and robust acquisition due to the use of a PFD and relevant lock status
3) Frequency flexibility due to the use of DDS approach

In order to optimize the spectral purity, a special digital signal processing techniques based on noise shaping is adopted DDS in order to sensitively improve the relevant spurious free dynamic range around the synthesized frequency. Truncation along the processing path produces spurious at the DDS output, however the DDS used in the F-PLL implements a noise shaping approach based on Error Feedback (EF) algorithm allowing to strongly improve the spurious free dynamic range in the band of interest.

The idea of the EF is to save the errors created in the quantization operation, feeding them back through a separate filter, in order to correct the result at following sampling instants.

A complete breadboard of the F-PLL has been designed, developed and fully tested at Thales Alenia Space - Italy.

In particular, the phase noise performance are quite impressive, being the X-Band integrated figure in the range 10 Hz - 1 MHz nearly equal to 0.8 deg-rms, i.e. a factor 4 of improvement has been achieved with respect to state-of-art performance.