Re: ADF5355 - Good Enough for 24 GHz!

Brian Flynn GM8BJF

Hi Paul,
Regarding the inputs and outputs. You can have a complimenmtary drive for the referemce hence the two SMA sockets . The onboard reference is done that way, . I have used just one to feed a 100 MHz reference to the chip. The data sheet claims there are advatages in PN performance to using a complimentary drive for the reference but I cannot say I noticed any. I used the REF- connector, added a bridge to R8 and disconnected  L1. The fundamental VCO outputs are also complimentary and I  have only ever used one.I mainly used the "OB" output as that is the frequency doubled output.

As the boards arrive you will find the PN performance disapointing.  The main source of the noise is "baseband" noise from the LT voltage regulators. It shows on a spectral plot as large noise shoulders around the carrier. I attach a plot of what mine looked like before adding bypass caps. The noise is within the bandwidth of the PLL and modulates the VCO and so appears on the output. Until you bypass that and get rid of it you cannot see the influence of more subtle noise sources on the PLL In my experience  a "green" Board and a "black" board .both showed similar behaviour.
Hope that helps.

73s, Brian GM8BJF.

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