Improving LMS6002 phase noise

Srdjan Milenkovic s.milenkovic at limemicro.com
Mon Jul 23 09:30:32 UTC 2012


Hi Andrey,

Below is the loop filter for 100kHz loop bandwidth, 26MHz reference 
clock, 400uA charge pump current. It has been tested and should give you 
around -90dBc/Hz plateau.



C1 	R2 	C2 	R3 	C3

9.01E-11 	2.46E+03 	1.42E-09 	3280.93 	2.56E-11



Loop filter components for 10kHz bandwidth, 26MHz reference clock, 400uA 
charge pump current are below:

C1 	R2 	C2 	R3 	C3

9.01E-09 	2.46E+02 	1.42E-07 	328.09 	2.56E-09


There should not be any issue with loop stability even with 10kHz 
filter. However, PLL settling time is increased so you have to slow down 
VCOCAP auto-tune.

I would recommend to use 100kHz filter.

Regards, Srdjan

On 22/07/2012 20:53, Andrey Sviyazov wrote:
> Hi Srdjan.
>
> First of all thank you for support.
>
>     We do not have register dump. However, Lime GUI project file used
>     in this experiment is attached. You can use GUI File->Open Project
>     option to import it. I see Ichp and Ichp offset currents are
>     different from defaults but these still do not justify 5-12dB
>     worse PN in your reports. You can give it a try though before
>     changing TCXCO.
>
>
> We will try to adjust Ichp and offset first off all. Thank you for hint.
>
>     *Test Description:*
>
>       * DC MAX applied through analogue inputs, DACs off
>
> It is quite important detail too.
>
>       * TXVGA1 and TXVGA2 at max gain
>
> Similar.
>
>       * Loop filter redesigned for 100kHz loop bandwidth and 40MHz
>         reference
>
>       * Icp and Icp offset optimized at 25 deg. Same set up used at
>         all other temperatures
>
> Please inform us values of components for 100kHz BW filter.
> But we are forced to use one clock 26MHz for all because of target price.
>
>       * Cap code and VCO picked up by PLL tune routine
>
> I implemented 10kHz BW filter for PLL and found that frequency locking 
> become unstable because of calculated VCOCAP value a bit lower then 
> required (too high capacity).
> If VCOCAP incremented manualy (after auto-tuned) then no any problems.
> What do you think about it.
> And could you please inform me values for 10 kHz BW filter, just to 
> compare with my calculations.
>
> Best regards,
> Andrey Sviyazov.
>
>

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