Effect of temperature on WiSense nodes

All WiSense sensor nodes use low cost RF crystals for the radio (CC1120 or CC1101). The crystal oscillator generates the reference frequency for the radio’s frequency synthesizer, as well as clocks for the ADC and the digital part. For example, the WSN1120L/WSN1120CL nodes use a 32 MHz cystal (FA-128-32MHz from Epson).

If the crystal frequency is incorrect, the transmitter carrier frequency and the receiver LO frequency will also be incorrect. The crystal frequency error is due to  initial tolerance, capacitive loading errors, ageing, and temperature drift.

Example: If the crystal frequency has an error of +/- X ppm (parts per million) the RF frequency  also has an error of +/- X ppm. As an example, if the crystal error is say +10 ppm and the CC1120 is programmed for a carrier frequency of 866 MHz, there will be an error in the carrier frequency of 866 *(10^6)* 10 * (10^-6) = 8.66 kHz.

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.

If both the transmitter and receiver crystal accuracy is ±10 ppm and the CC11xx is
programmed for a carrier frequency of 866 MHz ,

BWchann > BWsig + 4 * XTALppm * fRF = BWsignal + 4*10*(10^-6)*866*10^6 Hz.

Let us take a narrow band application with raw data rate of 1.2 kbps using 2-GFSK modulation and  frequency deviation of 4 kHz.

BWsig = 1200 +  2*4000 = 9.2  kHz

BWchann > 9.2 kHz +  4*8.66 kHz

BWchann > 34.64 kHz

You can easily see  that  the channel bandwidth requirement is  dominated by the +/- 10 ppm crystal tolerance figure !.  Note that as channel bandwidth (programmed as  the  RX  filter bandwidth parameter in radios  such as  CC1101  and  CC1120) increases, radio range decreases. So it is important to keep the channel bandwidth to the minimum required to carry the modulated RF signal.

 

Temperature has a significant effect on crystal frequency. In our  tests using WSN1120L nodes, we saw large carrier frequency drifts when subjected to different temperatures.

The snapshot below shows the carrier frequency (865.999687 MHz) at 21 deg C (room temperature).

rssi_866m_21degc

 

The snapshot below shows the new carrier frequency (866.5 MHz) at 6 deg C (node inside a refrigerator)

rssi_866m_6degc

Frequency drifts because of environmental conditions (such as temperature) are big problem for narrow band channels (channel bandwidth around 25 kHz or less).  If channel spacing is low, RF signal on adjacent channels can overlap. Frequency drift between two nodes (on the same channel) can cause the nodes to loose communication if the transmissions fall outside the programmed RX filter bandwidth (on each node).

In narrow band radio devices (such as WiSense nodes) are installed outdoors, they should be inside weather proof enclosures to minimize frequency drift due to environmental conditions.

One approach is to use a temperature sensor mounted near the RF crystal and tune the radio carrier frequency based on the temperature.

Another  (more complicated) approach is to use a tighter tolerance crystal on the coordinator node’s radio and let the other nodes  in the network track the coordinator’s frequency using the CC11XX automatic frequency compensation feature.

Why not use crystals with tighter tolerance across a wide temperature range? The simple reason is cost.

The CC1120 radio supports  low cost crystals (32 MHz) as  well as  temperature compensated crystal oscillators (32 MHz TCXO). On the CC1120, either a crystal can be connected to XOSC_Q1 and XOSC_Q2, or a TCXO can be connected to the EXT_XOSC input. The problem is that a TCXO  can cost more than the total  cost of a WSN1120L/WSN1120CL  node !!.

 

References:

 

Posted on October 30, 2016, in Uncategorized. Bookmark the permalink. Leave a comment.

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