This application note details the design of a complete 12GHz, ultra-low phase noise fractional-N phase locked loop (PLL) with external VCO. It consists of a high performance fractional-N PLL (MAX2880), op-amp based active loop filter (MAX9632), and 12GHz VCO (SYNERGY DXO11751220-5).
Figure 1. 12GHz ultra-low phase noise fractional-N PLL based on MAX2880.
This high performance PLL can be used to generate a mixer local oscillator (LO) frequency or an ADC/DAC clock for applications in microwave point-to-point systems, test and measurement equipment, or automotive radars.
Phase noise performance data and detailed setup guide are included in this app note.
PLL overall phase noise is usually dominated by the PLL's inband phase-noise floor and VCO phase noise. To build an ultra-low phase-noise PLL, the designer needs to select a low-phase noise VCO and PLL. The MAX2880 is capable of achieving an inband phase noise floor of -229dBc/Hz for integer mode and -225dBc/Hz for fractional low-noise mode.
Table 1 and Figure 2 show the phase noise performance of the circuit at 12GHz and different modes of the MAX2880.Table 1. 12GHZ Phase Noise Summary
|RMS Jitter (Integer Mode )||89||femtosecond|
|RMS Jitter (Fractional Low-Noise Mode )||92||femtosecond|
|RMS Jitter (Fractional Low-Spur Mode )||140||femtosecond|
|SSB Integrated Phase Noise (Integer Mode )||-46.7||dBc|
|SSB Integrated Phase Noise (Fractional Low Noise Mode )||-46.2||dBc|
|SSB Integrated Phase Noise (Fractional Low Spur Mode )||-42.7||dBc|
|VCO Tuning Range||0.5-15||V|
Figure 2. 12GHz phase noise plot.
A MAX2880 evaluation kit (EV kit) and a 12GHz external VCO (Synergy DXO11751220-5) are required for this design.
Note: the Synergy VCO is not included in the MAX2880 EV kit, and some modifications of the EV kit are required.
Table 2. MAX2880 EV Kit Modifications
|C1 (nF)||Open||10||Remove passive filter|
|R2A (ohm)||Open||100||Remove passive filter|
|C2 (nF)||Open||100||Remove passive filter|
|R3 (ohm)||Open||47.5||Remove passive filter|
|C3 (nF)||Open||1||Remove passive filter|
|R4 (ohm)||Open||0||Remove passive filter|
|R27 (ohm)||0||Open||Connect active filter|
|R26 (ohm)||4300||Open||Active filter|
|C4 (pF)||15||Open||Active filter|
|C101 (pF)||150||Open||Active filter|
|R102 (ohm)||560||Open||Active filter|
|C102 (pF)||8200||Open||Active filter|
|R9 (ohm)||Open||16.9||disconnect onboard VCO|
|R9 to NC1||16.9||Open||connect external VCO|
|JP4 2 pin jumper||Open||Connect||power down onboard VCO|
|JP5 3 pin jumper||connect center pin to 5P0_VCC||Open||pamp input+ bias|
Integer Low-Noise mode: 003C0000,06000001,6F00CE22,00002503,00000004,00000005
Fractional Low-Noise mode: 003C0000,06000001,0F00FFFA,00000303,00000004,00000005
Fractional Low-Spur mode: 003C00C8,06000001,6F00CE22,00000B03,00000004,00000005
Figure 3. Jumper position.
Figure 4. Detail setup diagram.