JAJSS39A November 2023 – March 2024 DAC39RF10EF , DAC39RFS10EF
PRODUCTION DATA
An external VCO can significantly improve in-band and wideband noise, with best phase noise typically being achieved with narrow band VCOs such as Voltage Controlled Crystal Oscillators (VCXO), Voltage Controlled Surface Acoustic Wave Oscillators (VCSO) and Dielectric Resonance Oscillators (DRO). For this design, a Synergy Microwave 8 GHz DRO (SDRO800-8), is used for the wideband noise (-170 dBc/Hz) and low flicker noise corner (-160 dBc/Hz at 1 MHz). DROs are typically available from about 5 to over 25 GHz.
Figure 8-8 shows a block diagram of the synthesizer. An external active loop filter is used to improve noise performance and extend the tuning voltage range required for the DRO (0 to 10 V). The DROs output is split, with one output feeding back into the LMX2820 and the other being sent to the DAC clock distribution network.
In this implementation, the loop bandwidth was set to around 24 kHz, where the flicker noise of the PLL intersects the open loop noise of the DRO. This offers the best overall integrated phase noise. The noise floor is improved by about 10 dB from the fully integrated example since the output buffers were not used and the DRO has exceptional broadband noise of around -170 dBc/Hz.
As with the integrated VCO, the loop filter components can be designed using PLLatinumSim software.