Improving LMS6002 phase noise

Srdjan Milenkovic s.milenkovic at limemicro.com
Sun Jul 22 11:11:55 UTC 2012


On 21/07/2012 16:28, Alexander Chemeris wrote:
> Hi Srdjan,
>
> On Sat, Jul 21, 2012 at 4:33 PM, Srdjan Milenkovic
> <s.milenkovic at limemicro.com> wrote:
>> Hi all,
>>
>> As far as I am aware, you are currently discussing how to improve LMS6002
>> PLL phase noise. Below are some inputs from my side which may help.
> Yes, here are some pictures of phase noise we have at UmTRX right now:
> http://lists.osmocom.org/pipermail/umtrx/2012-July/000030.html
>
> It looks like they're 5-12dB higher then data I saw in your
> temperature measurement report. First thought is that this could be
> due to a clock source. Did you use your EVB board for those
> measurements?
Yes, we used Lime EVB, 30.72MHz TCXCO. However, using 26MHz instead of 
30.72MHz TCXCO should not affect phase noise so much (5-12dB). Do you 
have an alternative 26MHz TCXCO with better PN? As you quite rightly 
mentioned, you are probably limited by TCXCO PN at the moment.

>
>> Apart from playing with PLL registers we have two additional options.
>>
>> 1. Use clean TCXCO which provides 4 times higher reference followed by
>> divide by 4 to generate PLL reference clock. Recently, we experimented with
>> 30.72*4 MHz TCXCO. Dividing its output by 4 before going into LMS chip we
>> have got improvement of 6dB in phase noise plateau region.
>>
>> 2. Current PLL loop filter has been designed to cover the whole LMS
>> frequency range hence using kind of mid value for Kvco. We can customize the
>> loop filter for a particular band.
> Well, we're working with GSM and it has only four main bands:
> GSM-850:  824.2–849.2 UL / 869.2–894.2 DL
> E-GSM 900:  880.0–915.0 UL / 925.0–960.0 DL
> DCS-1800:  1,710.2–1,784.8 UL / 1,805.2–1,879.8 DL
> PCS-1900:  1,850.2–1,909.8 UL / 1,930.2–1,989.8 DL
>
> It would be great if we could optimize for the use in these bands
Let us first try to recover 5-12dB PN as mentioned above. Redesigning 
loop filter will not help at all if we are limited by TCXCO.





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