LMS TxLO noise

Andrey Sviyazov andrey.sviyazov at fairwaves.ru
Fri Jul 20 17:14:00 UTC 2012


Hi Thomas.

I couldn't wait your reply and start to implement 10kHz BW of PLL.
You can find here two pictures with results.
First of all I found that PLL tuning algorithm doesn't work properly at low
PLL BW.
VCOCAP register (r0x19) should contain higher value at least for +3 units
(to make CAP lower), otherwise freq's above 950MHz never locked.
Note, that you should read and change register 0x19 after autotuning.
Second, you can see some noise difference for the 925MHz because of used
DIV=8 of  the VCO, instead of DIV=4 for 942 and 960MHz.
Third, you can see PLL noice dependance with the charge pump current
(r0x16).

On the other two pictures you can find which components have to be changed.
Thomas, please make one more measure of LO noise and jitter at PLL BW=10kHz
by your instrument.
We need to know, is it real to reach modulation accuracy of 1.5 degrees RMS
or impossible, just because of LMS PLL have bigger jitter.

Best regards,
Andrey Sviyazov.



2012/7/19 Andrey Sviyazov <andrey.sviyazov at fairwaves.ru>

> Thomas.
> I've calculate few PIF filters for PLL by the ADIsimPLL and found that RMS
> phase jitter in case of 5kHz and 50kHz only 0.33 and 0.46 degrees.
> It is very small difference, so are you sure that I should implement 10kHz
> bandwidth filter as in USRP?
> Actually it is absolutly not an problem for me to implement it, but I have
> to be sure.
> I've calculated it for 10kHz bandwidth, noise plot will be around
> 75-80dBc/Hz @ 10kHz and 105-110 dBc/Hz @ 100kHz offset.
>
> Best regards,
> Andrey Sviyazov.
>
>
>
> 2012/7/19 Andrey Sviyazov <andrey.sviyazov at fairwaves.ru>
>
>> Alexander.
>> I think that few pulses of LMS hardware reset (LMS_NRST-1 and LMS_NRST-2
>> pins) will be much easily than all registers checking.
>> BTW, are there reset pulses after power up or not?
>>
>> Best regards,
>> Andrey Sviyazov.
>>
>>
>>
>> 2012/7/19 Alexander Chemeris <alexander.chemeris at gmail.com>
>>
>>> Guys, please keep track of all LMS configuration values which are not
>>> set to default during power on and we'll add them to our manual
>>> initialization script.
>>>
>>> On Thu, Jul 19, 2012 at 6:56 PM, Andrey Sviyazov
>>> <andrey.sviyazov at fairwaves.ru> wrote:
>>> > Sergey.
>>> > I'll play around GPS power and RTC supply, thank you for your help.
>>> >
>>> > Thomas.
>>> > About r0x16 please be sure that sometimes it isn't set to default value
>>> > after power up (I saw it few times).
>>> > Also I think that possible those bad "power on reset" gave us unstable
>>> > result of LMS autocalibration.
>>> >
>>> > Best regards,
>>> > Andrey Sviyazov.
>>> >
>>> >
>>> >
>>> > 2012/7/19 sergey kostanbaev <sergey.kostanbaev at gmail.com>
>>> >>
>>> >> Which gives ~1.5khz cut-off, that's strange
>>> >>
>>> >> Also I'd check GPS VDD and AVDD nosies
>>> >>
>>> >>
>>> >> On Thu, Jul 19, 2012 at 6:41 PM, Andrey Sviyazov
>>> >> <andrey.sviyazov at fairwaves.ru> wrote:
>>> >>>
>>> >>> 1kOhm resistor and 0.1uF capacitor already exist between DAC and VC
>>> >>> input.
>>> >>> Of course I'll increase capacitance or resistance to be sure about VC
>>> >>> pin.
>>> >>>
>>> >>> Best regards,
>>> >>> Andrey Sviyazov.
>>> >>>
>>> >>>
>>> >>>
>>> >>> 2012/7/19 sergey kostanbaev <sergey.kostanbaev at gmail.com>
>>> >>>>
>>> >>>> No :) But it may cause.
>>> >>>>
>>> >>>> I'd check all the line from FPGA to DAC to VCTXCO because VC pin is
>>> >>>> really sensible to any noise.
>>> >>>>  - caused by power supply of DAC.
>>> >>>>  - output noise of DAC.
>>> >>>>  - algorithm of changing values
>>> >>>>
>>> >>>> At first I'd inspect VC pin at VCTXCO and try to filter it with
>>> cut-off
>>> >>>> frequency 1-0.1 Hz
>>> >>>>
>>> >>>>
>>> >>>> On Thu, Jul 19, 2012 at 6:29 PM, Andrey Sviyazov
>>> >>>> <andrey.sviyazov at fairwaves.ru> wrote:
>>> >>>>>
>>> >>>>> Sergey.
>>> >>>>> Is it really so frequently (32k/s updates at 1pps reference)?
>>> >>>>>
>>> >>>>> Best regards,
>>> >>>>> Andrey Sviyazov.
>>> >>>>>
>>> >>>>>
>>> >>>>>
>>> >>>>> 2012/7/19 sergey kostanbaev <sergey.kostanbaev at gmail.com>
>>> >>>>>>
>>> >>>>>>
>>> >>>>>>
>>> >>>>>> On Thu, Jul 19, 2012 at 6:10 PM, Andrey Sviyazov
>>> >>>>>> <andrey.sviyazov at fairwaves.ru> wrote:
>>> >>>>>>>
>>> >>>>>>> Hi all.
>>> >>>>>>>
>>> >>>>>>> I've found spurs on the LO noise plot at  ~33kHz and ~66kHz
>>> offset
>>> >>>>>>> when GPS antenna used and position locked.
>>> >>>>>>> This spurs begin grow up when GPS just near to lock position and
>>> >>>>>>> after locking spurs stopped to grow as you can see at picture.
>>> >>>>>>> If thereafter GPS disconnected then noise coming back to normal
>>> plot.
>>> >>>>>>> I think that it is result of 32768Hz clock in the GPS module
>>> EB-230,
>>> >>>>>>> but can't understant how it can impact to 26MHz clock or VCO.
>>> >>>>>>
>>> >>>>>>
>>> >>>>>> As another idea it can be caused by continues GPS correction lead
>>> to
>>> >>>>>> DAC changes to the VCTXCO.
>>> >>>>>>
>>> >>>>>>>
>>> >>>>>>> Please tell me something who know.
>>> >>>>>>>
>>> >>>>>>> Thomas.
>>> >>>>>>> Was GPS antenna connected when noise measured like on the picture
>>> >>>>>>> which you sent us (also attached here)?
>>> >>>>>>> I would to know because of anomal peak near to 30kHz offset too.
>>> >>>>>>>
>>> >>>>>>> Best regards,
>>> >>>>>>> Andrey Sviyazov.
>>> >>>>>>>
>>> >>>>>>>
>>> >>>>>>>
>>> >>>>>>> 2012/7/19 Andrey Sviyazov <andrey.sviyazov at fairwaves.ru>
>>> >>>>>>>>
>>> >>>>>>>> Sorry, that was meant to be sent to the mailing list :)
>>> >>>>>>>>
>>> >>>>>>>> Best regards,
>>> >>>>>>>> Andrey Sviyazov.
>>> >>>>>>>>
>>> >>>>>>>> ---------- Forwarded message ----------
>>> >>>>>>>> From: Andrey Sviyazov <andreysviyaz at gmail.com>
>>> >>>>>>>> Date: 2012/7/18
>>> >>>>>>>> Subject: LMS TxLO noise
>>> >>>>>>>>
>>> >>>>>>>> Hi Thomas.
>>> >>>>>>>>
>>> >>>>>>>> Here forwarded my last e-mail with noise plots when I stopped
>>> work
>>> >>>>>>>> around it at first time, please see below.
>>> >>>>>>>>
>>> >>>>>>>> Please try to play around Tx PLL charge pump current (register
>>> 0x16)
>>> >>>>>>>> for better RMS phase stability.
>>> >>>>>>>> I think we should reach 1 degree or below.
>>> >>>>>>>>
>>> >>>>>>>> Alexander gave me the second UmTRX board and after checking and
>>> >>>>>>>> fixing all known hardware issues I've got roughly the same LO
>>> noise plot.
>>> >>>>>>>> Possible Robin had no time to fixing all of our issues, so check
>>> >>>>>>>> them all please.
>>> >>>>>>>> And also check please what type of TCXO installed on your board.
>>> >>>>>>>>
>>> >>>>>>>> Best regards,
>>> >>>>>>>> Andrey Sviyazov.
>>> >>>>>>>>
>>> >>>>>>>> ---------- Forwarded message ----------
>>> >>>>>>>> From: Andrey Sviyazov <andreysviyaz at gmail.com>
>>> >>>>>>>> Date: 2012/4/13
>>> >>>>>>>> Subject: Re: LMS TxLO noise
>>> >>>>>>>>
>>> >>>>>>>> Hi all.
>>> >>>>>>>>
>>> >>>>>>>> There is progress with LMS PLL :)
>>> >>>>>>>> Pictures are attached here.
>>> >>>>>>>> t was discovered that 80 kHz spurs come from Ethernet, or rather
>>> >>>>>>>> from the ET1011.
>>> >>>>>>>> I unknowingly put the choke between transistor of 1V regulator
>>> and
>>> >>>>>>>> analog power 1V.
>>> >>>>>>>> As a result, the regulator has become unstable and oscillated
>>> 80 kHz
>>> >>>>>>>> with amplitude of 200 mV, which climbed into the LMS PLL.
>>> >>>>>>>> To correct this problem L46 should be replaced by jumper on all
>>> alfa
>>> >>>>>>>> version PCB's.
>>> >>>>>>>>
>>> >>>>>>>> Also I just played with current in the PLL loop, shown on the
>>> >>>>>>>> picture for clarity.
>>> >>>>>>>> Proved to be the optimal current 1,9 mA (you should write 0x93
>>> in
>>> >>>>>>>> the register of 0x16).
>>> >>>>>>>> But, I think, for the RxPLL will be better use of the current
>>> 2.4
>>> >>>>>>>> mA, because the nearest noises more important for Rx (you
>>> should write 0x98
>>> >>>>>>>> in the register 0x26).
>>> >>>>>>>>
>>> >>>>>>>> Best regards,
>>> >>>>>>>> Andrey Sviyazov.
>>> >>>>>>>>
>>> >>>>>>>>
>>> >>>>>>>
>>> >>>>>>
>>> >>>>>
>>> >>>>
>>> >>>
>>> >>
>>> >
>>>
>>>
>>>
>>> --
>>> Regards,
>>> Alexander Chemeris.
>>> CEO, Fairwaves LLC / ООО УмРадио
>>> http://fairwaves.ru
>>>
>>
>>
>
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