• State
  • Locked Locked
  • Replies 6 replies
  • Subscribers 17 subscribers
  • Views 937 views
  • Users 0 members are here
Support feedback
Options
Options
Related

This thread has been locked.

If you have a related question, please click the "Ask a related question" button in the top right corner. The newly created question will be automatically linked to this question.

CC430 + small helical antenna (dn038) frequency deviation

Dragos Ionescu
Intellectual 680 points


Hi,

I have followed indications from DN038 to build my own device. 

Here are the important settings exported from SmartRF:

/* RX filter BW = 135.416667 */
/* Device address = 0 */
/* Deviation = 4.760742 */
/* TX power = 0 */
/* Sync word qualifier mode = 30/32 sync word bits detected */
/* Modulated = true */
/* Preamble count = 4 */
/* Manchester enable = false */
/* CRC autoflush = false */
/* Packet length mode = Variable packet length mode. Packet length configured by the first byte after sync word */
/* CRC enable = true */
/* Modulation format = 2-FSK */
/* Address config = No address check */
/* Channel spacing = 124.969482 */
/* Channel number = 3 */
/* Whitening = false */
/* Data format = Normal mode */
/* Base frequency = 869.399994 */
/* Data rate = 9.59587 */
/* Packet length = 255 */
/* Carrier frequency = 869.774902 */
/* PA ramping = false */

And here is the result taken from a spectrum analyzer while the device is kept in continuous TX.

Since I'm not an RF engineer I cannot qualify the result. What I can notice is a small (?) deviation of the resonant frequency by approx (869.786-869.775) 11KHz. Is it OK? Since my RX filter is 135Khz wide, it looks OK for me but I would like to know the opinion from an RF engineer.

If it is not OK, what should I do to correct/improve? I have the recommended placeholder for the ANT1 component on the PCB. I don't have a network analyzer.

 

Thanks

 

  • 0
    TI__Guru**** 317180 points

    You are using a deviation equal to 4.76 kHz. To be able to see if the spectrum is as expected in detail a lower RBW than 2 kHz should be used (300 Hz) and a span equal to ~40 kHz. For your modulation format, datarate and deviation the spectrum should look like something like this:

  • 0
    TI__Mastermind 39860 points

    11 kHz shift in the carrier frequency is ok. Note that the frequency will also change with temperature and aging. This must also be taken into account. With 9.6 kbps data rate and 4.8 kHz deviation you can use a lower than 135 kHz RX filter BW and still allow for crystal inaccuracies. Lower RX filter BW improves sensitivity and close-in selectivity. 9.6 kbps data rate, 4.8 kHz deviation, 2-FSK will give a 99% occupied BW of 21 kHz (RX filter BW thus needs to be wider than 21 kHz to fit the transmitted signal). Assuming +/-20 ppm total crystal error and that in the extreme case the RX and TX units will be in opposite directions, the RX filter BW should be wider than 21 kHz + 4 x 20 ppm*868 MHz = 90 kHz (see DN005, www.ti.com/lit/swra122 for more details). Suggest you change the RX filter BW to 101 kHz. This gives a theoretical improvement in sensitivity of 10*log(135k/101k) = 1.3 dB (or approx 15% longer range).

    As for the output power. The plot does not show if the output power is good or not. Or put in another way - if the antenna is well matched or not. Since you do not have a network analyzer it is hard to optimize the design, but you could try to measure output power at different frequencies, e.g +/-20 MHz and +/-10 MHz from the intended carrier, to check if the antenna resonates at the right frequency

  • 0 in reply to Sverre
    Intellectual 680 points

    Hi Sverre,

    The problem is that I cannot assume 20ppm crystal error because the device should qualify in -40 +85 C temperature range. I'm using Ansen crystals (good quality, same as TI is using on prototyping boards) but they cannot provide 20ppm tolerance at that temperature range.

    So, I assumed 30ppm error, just to stay on the safe side.

    By using the formula from DN005 with 868MHz, 30ppm, 9.6kbps, the resulted RXBW is 123.48 witch has been changed automatically by SmartRF in 135kHz.

    Do you think that I can use another (lower) value?


    Thanks,

    Dragos

  • 0 in reply to Dragos Ionescu
    Intellectual 680 points
    I've just reviewed the emails from Ansen and things are a bit different. They qualify the crystal for 20ppm at -40+85 and 10 ppm at -20+70. So, 20ppm in theory should work. But what about aging? The device should work in theory 10 years. That's why I choosed (based on the expected yearly derating founded in datasheet) 30ppm.

    Thanks
  • 0 in reply to Dragos Ionescu
    TI__Mastermind 39860 points
    Dragos: first of all, had to correct previous post as the RX filter requirement calculation was wrong. Sorry about that.
    20 ppm crystal: RX filter BW > 21 kHz + 4 x 20 ppm x 868 MHz = 90 kHz
    30 ppm crystal: RX filter BW > 21 kHz + 4 x 30 ppm x 868 MHz = 125 kHz

    An alternative to using 135 kHz RX filter BW is to program 81 kHz and enable frequency offset compensation (register FOCCFG). With 80 kHz programmed BW and FOC_LIMIT = +/-BW/4 you get 80 kHz +/-20 kHz (total of 120 kHz).
  • 0 in reply to Sverre
    TI__Mastermind 39860 points
    The aging is given in ppm/year. The frequency error will typically decrease exponentially, so aging is typically only a problem during the first year - unless the crystal is pre-aged to remove most of the aging. A typically spec is ±3-5ppm/year for the first year and half of this the next year and so on. I have also seen some vendors specifying the aging for the first year and the total after 10 year (e.g. the 32 MHz crystal NX2016 from Epson has +/-3ppm/1 year and +/-10 ppm max/10 years). Ask the crystal vendor for details regarding aging.