RSSI variation in CC2520

Hi,

receiver performance varies with the temperature, but that is reflected in the level of sensitivity of the receiver, not in the RSSI which is a measure of the signal strength.

Relation between sensitivity and temperature can be found in table 5.9 of http://www.ti.com/lit/gpn/cc2520

Thanks,

TheDarkSide

yes, so if sensitivity decreases by say 1.6dB for 40^o increase in temperature then will it be correct to say that the received power will also decrease by same magnitude for same rise in temperature

No that is not the way I would interpret it.

Sensitivity defines the minimum level at which the receiver performs, guaranteeing certain quality conditions typically measured as BER or PER.

As mentioned, receiver performance(measured in terms of BER/PER) change depending on the temperature. This means that the minimum input level at which BER/PER performance are guaranteed at a specific threshold (1%) changes accordingly.

Thanks,

TheDarkSide

Original question: "How does RSSI varies with temperature in CC2520?" 

RSSI plot in CC2520 data sheet is for operation at 25C. For the same input power level the RSSI reading will be higher at cold temperatures and lower at warm temperatures (relative to 25C)

Assume at a specific input power level the RSSI is X dBm @25C
At +125C the RSSI will be typically X -  4 dB for the same input power level
At -40C the RSSI will be typically X + 3 dB for the same input power level

Note: corrected temperature to +125C

Hi Sverre,

Please can you elaborate on how you came up with these numbers regarding RSSI and if they are given somewhere can you give me the reference it will really be very helpful.

Thanks,

Yuvraj

I have access to the CC2530 characterization data and the numbers provided are taken from this. Note: the variation with temperature indicated in previous post are not guaranteed values and should be used as an indication of expected variation only. 

This much variation in receiver power can hamper the functioning of CC2520 so is there a way to avoid this variation?

The RSSI readout is converted to dBm by subtracting an RSSI offset value. At 25C this offset is 76 decimal. For less RSSI variation across temperature you can use different RSSI offset vs temperature.

So does the offset value also changes with temperature and it compensates the variation of RSSI with temperature?Please elaborate this part.

Most customer will use only one RSSI offset and accept a variation in RSSI.

Example 1: Input power of -83 dBm and same RSSI offset (76) value used for all temperatures

Temperature    RSSI readout   RSSI in dBm
+25C              -7                    -7 - 76 = -83 dBm
-40C               -4                    -4 - 76 = -80 dBm
+125C            -11                  -11 - 76 = -87 dBm

If you need to measure the power at the antenna more accurately across temperature you need to find RSSI offset values at other temperatures than 25C also.

Example 2: Input power of -83 dBm and different RSSI offset depending on temperature

Temperature    RSSI readout   RSSI in dBm
+25C              -7                    -7 - 76 = -83 dBm
-40C               -4                    -4 - 79 = -83 dBm
+125C            -11                  -11 - 72 = -83 dBm

Hi Sverre,

Thanks for the explanation. Can I say that the received power at antenna for CC2520 does not changes with change in temperature as both RSSI and offset value vary which cancel out the variation  (according to example 2)?

I saw a paper where they gave variation of RSS for CC2420 that RSS decreases by 3dB for a 40^o rise in temperature from 25C to 65C (Section 2.2)

http://e2e.ti.com/cfs-file.ashx/__key/communityserver-discussions-components-files/158/0714.hotemnets08.pdf

Please can you shed some light on this. Although I am working on CC2520 but I thought that if there is a variation in CC2420 received power then there must be variation for CC2520( not exactly same variation) but the above explanation gives some different answer.

Waiting for your reply.

Thanks,

Yuvraj

In my posts I have assume the same power level at the antenna regardless of temperature. The CC2520 calculates / estimates the power level at the antenna and this calculation / estimation (i.e. RSSI reading) gives different results for the same input power level  for different temperatures (the RSSI is derived from the internal CC2520 gain, which changes with temperature). Example from previous post (-83 dBm at the antenna):

Temperature    RSSI readout 

+25C              -7               
-40C               -4                
+125C            -11                

The RSSI reading is converted to dBm using an RSSI offset value (typically 76) and most customers use this across the entire temperature range. Thus, for the same power level at the antenna regardless of temperature (-83 dBm in this case) the RSSI will report the power level as -80 dB

Hi Sverre,

Thanks for the reply.

I have few more questions

1. For same output power transmitted by transmitting antenna will the received power at receiving antenna(not RSSI) will vary with temperature?

2. Should one really ( not concerned with what most customers do) consider the variation of RSSI offset with temperature?

Waiting for your reply.

Thanks,

Yuvraj

1) Assuming the transmitted output power is constant across temperature the power received at the antenna will also be constant. The RSSI, however, will change with temperature for the same input power because the gain internal to CC2520 changes with temperature.

2) I do not see why you need a very accurate RSSI across temperature. This is, however, application dependent I guess...