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Srdjan Milenkovic s.milenkovic at limemicro.comOn 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.