WiSense Radio Configuration

The CC1120  radios on  the  WSN1120L  and  WSN1120CL  are configured to operate in the  865-867 MHz license free band in India. The modulation used is GFSK (Gaussian FSK).  The  data rate is configurable (example – 1.2 kbps, 10 kbps, 20 kbps and so on). Higher data rate translates to higher channel bandwidth (receiver filter bandwidth) which translates to higher channel noise (lower SNR) seen in the receiver resulting in reduced range.

Carson’s rule states that  nearly all (~98 percent) of the power of a frequency-modulated signal lies within a bandwidth  “BWsig” where

BWsig = dr + 2*fdev

Where  “dr” is  the data  rate and “fdev” is the frequency deviation around the carrier frequency.

Taking into consideration crystal inaccuracy, the minimum channel bandwidth “BWchann” required comes  to –

BWchann >  BWsig +  4*XTALppm*fRF

-Or –

BWchann >  dr + 2*fdev + 4*XTALppm*fRF

where “XTALppm”  is the total accuracy of the crystal including initial tolerance, temperature drift, loading, and ageing.

and

“fRF” is the carrier frequency which will be 866 MHz (at the center of the 865-867 MHz license free India band) in our application.

Modulation index for  FSK modulation (h) is given by –

h = 2*fdev / dr

for example, if deviation is +/- 4 kHz and the data rate  is 1.2 kbps, then the modulation index is (2*4000) / 1200 = 6.666.

The chosen frequency deviation “fdev” will impact sensitivity and occupied bandwidth in opposing directions. For a fixed RX filter BW and data rate, higher deviation will give better sensitivity;

Theoretically, there is an optimum separation/data-rate setting if you simultaneously minimize the receiver filter bandwidth. Every halving of receiver filter bandwidth increases sensitivity with 3 dB whereas sensitivity vs separation/data-rate decreases with about 1.5-2.5 dB per halving down to a certain limit where the loss increases very fast.

The CC1120 operates well at modulations indexes of 0.5 and above.

Let us look at some settings for the CC1120 Radio in a WiSense LPWMN  node.

Crystal specs

• Crystal frequency: 32 MHz
• Crystal tolerance:  +/- 10 ppm @ 25 deg C
• Crystal operating range: -40 deg C to +85 deg C
• Crystal frequency vs temperature variation: +/- 20 ppm
• Crystal frequency aging: +/- 1 ppm per year (max)

 

Setting #1:

Radio configuration

• Modulation: 2-GFSK
• Bits  per symbol:  1
• Data Rate: 1.2 kbps
• Frequency  Deviation: +/- 3.997803 kHz
• Carrier frequency: 865-867 MHz
• Channel  BW (RX filter BW): 25 kHz
• Feedback to PLL feature: Enabled

BWsig = 1200 + 2*3998 = 9196

Taking crystal inaccuracy (+/- 10 ppm) in consideration,

BWchann = 9196+ 4*10*(10^-6)*866*(10^6) 

BWchann = 43836 Hz  or 43.84 kHz

Modulation index = (2*3998) / 1200 = 6.66 (Minimum for CC1120 is 0.5, higher the better)

TI recommended setting for this configuration is 25 kHz which is  less than 43.84 kHz. The  CC1120  has a feature called feedback to PLL (F2BPLL) which increases the effective RX filter bandwidth without degrading the noise bandwidth and hence  the sensitivity. Feedback to the PLL “increases the RX filter” and thus allow for a less accurate crystal to be used. The noise bandwidth, and hence sensitivity, will not increase when enabling this feature. Enabling feedback to PLL increases BW from “programmed RX filter BW” to “programmed RX filter BW +/- RX filter BW/4”. For example, when RX filter BW is configured as 25 kHz, the effective RX filter BW increases  to  25 +  (25/2) = 37.25 kHz. The only downside of using  the F2BPLL is the need for a longer preamble length (of 4 bytes) instead of say just 2 bytes.

For best sensitivity we need to use the lowest RX filter BW possible. So an effective RX  filter BW of 37.25 kHz is a good compromise for this application.

In addition, each WiSense node’s carrier frequency frequency  error (due to the crystal’s initial crystal tolerance  of +/- 10 ppm) is removed by

• Configuring  the radio to output an un-modulated continuous  wave (CW) signal and
• Using a spectrum analyzer to accurately measure the frequency of this CW signal and thereby the offset from expected value and
• Configuring the static frequency offset register on the radio with the offset measured above
• This offset frequency value (in  HZ) is stored on each node in an EEPROM

Now any variation from desired frequency will only be because of temperature changes and aging.

Setting #2:

Radio configuration

• Modulation: 2-GFSK
• Bits  per symbol:  1
• Data Rate: 10 kbps
• Frequency  Deviation: +/- 5.004883  kHz
• Feedback to PLL feature: Enabled
• Channel  BW (RX filter BW): 25 kHz
• Carrier frequency: 865-867 MHz

BWsig = 10000 + 2*5005 = 20010 HZ

Taking crystal inaccuracy (+/- 10 ppm) in consideration,

BWchann = 20020 +  4*10*(10^-6)*866*(10^6)  

BWchann =  54660 =  54.6 kHz 

Modulation index = (2*5010) / 10000 = 1.002 (Minimum for CC1120 is 0.5)

Since feedback to PLL feature is enabled, effective rx  filter bw is  25000 * 1 .5 = 37.5 kHz.

In addition, the carrier frequency frequency  error (due to the crystal’s initial crystal tolerance  of +/- 10 ppm) has been removed as explained  above. We tested this configuration and it is  working well.

Setting #3:

Radio configuration

• Modulation: 2-GFSK
• Bits  per symbol:  1
• Data Rate: 20 kbps
• Frequency  Deviation: +/- 10.009766 kHz
• Channel  BW (RX filter BW): 50 kHz
• Feedback to PLL feature: Enabled
• Carrier frequency: 865-867 MHz

BWsig = 20000 + 2*10009 = 40020 HZ

Taking crystal inaccuracy (+/- 10 ppm) in consideration,

BWchann = 40020 +  4*10*(10^-6)*866*(10^6)  

BWchann =  74660 =  74.6 kHz

Modulation index = (2*10009) / 20000 = 1.0009 (Minimum for CC1120 is 0.5)

Since feedback to PLL feature is enabled, effective bandwidth is  40020 * 1 .5 = 60.03 kHz.

In addition, the carrier frequency frequency  error (due to the crystal’s initial crystal tolerance  of +/- 10 ppm) has been removed as explained  above. We tested this configuration and it is working well.

References:

• https://e2e.ti.com/support/wireless_connectivity/proprietary_sub_1_ghz_simpliciti/f/156/p/470774/1691274
• https://e2e.ti.com/support/wireless_connectivity/low_power_rf_tools/f/155/t/180039
• https://en.wikipedia.org/wiki/Frequency_modulation#Modulation_index
• CC1120  data sheet  (www.ti.com)
• https://www.eea.epson.com/wp-content/uploads/2016/09/September-2016-PCS-MASTER-BOOK.pdf
• http://www.mouser.com/ds/2/137/A-128_en-931416.pdf
• http://www.ti.com/lit/an/swra122c/swra122c.pdf