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Reference design doesn't work as expected

Alexey Petrov
Prodigy 250 points

Hello community!

I developed a 868 MHz RF device based on reference design "868 MHz Monopole PCB Antenna" (Design Note DN024). I copied the design as close to the original as possible and expected it to be compatible with SmartRFCC1110-868 module from "CC1110DK-MINI-868" development kit.

Two my modules work together normally, but when I change one of them to SmartRFCC they don't work - sent packets are not received. I connect both modules (my and SmartRFCC) to SmartRFStudio and using Continuous TX on one side and Continuous RX on another I can see that max RSSI level  presents on the receiving end when a carrier frequency on the transmitting end is ~100kHz higher (another RF parameters are identical on both ends). So it may be supposed that the reason is in different resonances of module's antennas. But a bandwidth of antennas is much wider than 100kHz, so different resonance frequencies can't be the reason. Am I right? Probably there is another reason. Could anybody help me in this situation. 

Thanks in advance.

Alexey

  • 0
    TI__Mastermind 18780 points

    Hi,

    Just so I am sure I have understood this correctly:

    1) You have made your own design based on our reference design. Two modules of this design communicate well, and the range is as expected. By the way, have you done any conducted measurements on this design? Output power, current and sensitivity is as expected? Have you checked that your carrier frequency is at the expected frequency or what is the frequency offset? If the frequency offset is large compared to what you expect, you should investigate to make sure that you have the correct crystal load.

    2) You also have the CC1110DK MINI kit. This kit works well. However, you are not able to get modules from this kit to communicate with modules with your referenence design. Are you running the link SW example that follows the MINI kit or have you made your own SW? This SW uses a low datarate and thus also a low RX filter bandwidth. What happens if you change the settings and increase the RX filter bandwidth? What is the distance between the modules? If they are too close, you will risc saturating the receiver.

    3) You say that when checking your module and the kit module, the RSSI is higher 100 kHz away expected, is this test done with the kit module as the receiver? To check the resonanse of the antenna, a good way is to do a frequency sweep with the transmitter and keeping the output power the same. Looking at this with a spectrum analyzer, the output power at the resonance frequency will be at the highest output power. This however assumes that your transitter is sending at the expected frequency. Note that the antenna performance is very dependent of the size of the ground plane and also if your modules are encapsulated, handheld etc. If this is changes on your design, the antenna migth need tuning, see more in AN058 http://focus.ti.com/lit/an/swra161b/swra161b.pdf 

     

    Charlotte

  • 0
    Expert 1670 points

    My first guess would be that your crystal oscillator is at a slightly different frequency than the TI module. Obviously measuring the RF output frequency with a spectrum analyzer would be best to verify you're transmitting at the proper frequency; but in lieu of that, perhaps just measuring the frequency of your crystal vs. the TI crystal with an oscilloscope and make sure they're exactly the same. I've accidentally shifted frequencies like you're describing by using improper loading caps on the crystal (or poorly routed traces). Make sure you have the proper type of crystal and proper loading.

    You're right to assume the antenna probably isn't the issue. It's very unlikely the resonance of the antenna would give a large RSSI difference over only 100khz. 

    Have you tried shifting your transmit frequency by the 100Khz and see if it's able to communicate with the TI module? You could try setting a very wide bandwidth and set FOCCFG FOC_LIMIT to 11b and see if they communicate then; but that may not prove much...

    -Chad C.