Re: [Discuss-gnuradio] DBSRX coherent problem (Phase Noise)(partial solution! Charge Pump)

[Jason Abele]

On Sat, Nov 13, 2010 at 10:41 AM, tom_wgs <address@hidden> wrote:
>
>
> tom_wgs wrote:
>>
>>
>>
>> tom_wgs wrote:
>>>
>>> Hi all
>>> ...
>>> ..
>>> Thank you for your time and any help would be very much appreciated.
>>> Tom
>>>
>>
>> I have attached a couple additional plots showing the results of taking
>> 128 complex points at 500 kHz sampling rate from channel 2, and dividing
>> it (complex) by the same number of complex points from channel 1
>> (element-by-element division, or complex mixing).
>>
>> ....
>> ...
>> .
>>
>> http://old.nabble.com/file/p30202507/Time%2BDomain%2BFreq%2B%2526%2Bother%2Berrors.jpg
>>
>>  http://old.nabble.com/file/p30202507/RXA_norm_RXB_Time_Domain.jpg
>>
>>
>
> I found a partial solution but am not sure why and if my problem is
> completely fixed.  I found that the Charge pump setting (_set_cp(int) was
> being set to 2 different values at 1880mHz for the 2 receivers.  After
> experimenting the best results come when forcing the values both to 1 for
> all tested frequencies.
>
> The results can be seen below.  Compare this to the results previous
> provided above.   Can anyone provide some information on the charge pump and
> how the settings effect the receiver.
>

Interesting results, there is an application note (max2116 AN02-R0)
that can be requested from Maxim regarding how to select VCOs when
tuning the max2118.

It indicates that the Vtune voltage for the selected VCO should be
read (_max2118_read_regs.adc) and the value used to select the best
charge pump current.  That is what was implemented at:

http://ettus-apps.sourcerepo.com/redmine/ettus/projects/uhd/repository/revisions/master/entry/host/lib/usrp/dboard/db_dbsrx.cpp#L383

Could you set dbsrx_debug to true on line 46 of db_dbsrx.cpp and
rebuild and install UHD.  Then look for output lines like:

DBSRX: final vco %d, vtune adc %d

and see what VCO and Vtune voltage is achieved on each DBSRX.

It is possible that your frequency of choice falls on the edge of a
VCO band for the DBSRX, and thus you get different settings.  But if
the DBSRX is locked, I would not think the difference in charge pump
current would cause varying phase offsets.

Jason