DC Offset vs temperature

Srdjan Milenkovic s.milenkovic at limemicro.com
Sat Jul 28 10:33:28 UTC 2012

Hi Alexander,

I understand. It is the same with WCDMA and other FDD modulation 
standards, BTS can not stop transmitting/receiving. However, we can not 
expect temperature to change from -40 to +85 deg C without doing 
anything about it.

LMS is quite robust regarding temperature change. For example, PLL will 
stay locked within +/-20 deg C from the tuning point.

The graph below shows DC/LO leakage calibration stability vs 
temperature. Applications based on OFDM (LTE, WiMAX for example) do not 
need to wary about DC/LO leakage recalibration as DC OFDM carrier is not 
used. Other modulation schemes will have to take care of this. If we 
define the spec of -60dBc LO leakage, the graph below shows that LMS IC 
is OK within +/-10 deg C offset from the calibration point.

BTW, you do not need UmTRXv3 at the moment. You can build a small board 
with temperature sensor alone and interface it back to BB/FPGA.

Best regards, Srdjan

Dr Srdjan Milenkovic

On 28/07/2012 03:15, Alexander Chemeris wrote:
> Hi Srdjan,
> Thank you for the comment.
> Unfortunately, we can't perform regular re-calibration, because a GSM
> base station can't stop transmitting. This means we need a full blow
> temperature sensor and this is a bad news, as we have to create
> UmTRXv3 to add it. This is something you should definitely consider
> for your next-gen chip - an embedded temperature sensor.
> Do you know how sensitive those parameters are to a temperature? I.e.
> should we calibrate adjust them every centigrade or every five
> centigrade?
> On Fri, Jul 27, 2012 at 6:28 PM, Srdjan Milenkovic
> <s.milenkovic at limemicro.com> wrote:
>> Hi Alexander,
>> As I mentioned before, IQ phase error is quite stable over temperature in
>> 900/1800MHz region so let us put it aside. This can be tuned and fixed in
>> production.
>> On the other hand, DC offset and LO leakage are temperature dependent as one
>> would expect. However, they can be calibrated in the product as well not
>> just in the lab. You need just to trigger built in DC cal blocks and
>> optionally use RF loop back to further improve LO leakage. If you want to
>> avoid temperature sensor you have an option to do these calibrations
>> regularly, every 30min-1h for example. This should track the temperature
>> change without having the sensor on board.
>> Our other customers are using both approaches, with and without temperature
>> sensor, depending on application and spec. If they have on board temperature
>> sensor they are using it for multiple purpose such as monitoring BB, PA,
>> etc.
>> Best regards, Srdjan
>> On 26/07/2012 16:48, Alexander Chemeris wrote:
>> On Thu, Jul 26, 2012 at 4:37 PM, Alexander Chemeris
>> <alexander.chemeris at gmail.com> wrote:
>> On Thu, Jul 26, 2012 at 3:31 PM, Srdjan Milenkovic
>> <s.milenkovic at limemicro.com> wrote:
>> Combining Thomas's idea of DC recalibration and, if necessary, making
>> look
>> up table for IQ phase error correction vs temperature should put us in
>> good
>> position to meet the specs.
>> Well, AFAIK, LMS6002D doesn't have a temperature sensor built in and
>> this means we have to install an external one, which again increase
>> BoM cost. Is that's how this issue is solved by your other customers?
>> I wonder is there a way to solve this without introducing a
>> temperature compensation loop.
>> Let me clarify - DC offset re-calibration is (more or less) fine in
>> lab setup, but it's expensive to manufacture if we have to add
>> temperature sensors and put every unit into a temperature camera for
>> calibration.
>> --
>> Regards,
>> Alexander Chemeris.
>> CEO, Fairwaves LLC / ООО УмРадио
>> http://fairwaves.ru

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