LMS TxLO noise

sergey kostanbaev sergey.kostanbaev at gmail.com
Thu Jul 19 14:35:15 UTC 2012


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<http://code.google.com/p/umtrx/issues/list?can=1&q=&colspec=ID+Type+Status+Priority+Milestone+Owner+Summary+Modified&cells=tiles>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.
>>>>
>>>>
>>>>
>>>
>>
>
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