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CC1101 TX

jacky kong1
Prodigy 30 points
Other Parts Discussed in Thread: CC1101

Hi,

I am using the IC CC1101 applicatication circuit shown in its specification to make a 868MHz Transiver unit. Please see the photo above: the yellow line is from my module at TX mode and the green is from another vendor's module (CC1101 also) at TX mode.

My questions are as below:

1)Why do the two modules have RF power at the marks of 2 and 3 at TX mode? Is it normal?

2)How to decrease them?

3)When the two modules are changed to RX mode, the vendor's module has a higher sensitivity than mine at the same test conditions.  Please be noted from the photo above that the vendor's module has lower RF power at the marks of 2 and 3 than mine. Does the receiving sensitivity at RX mode has the direct relation with the RF power at the marks of 2 and 3  at TX mode in the same module with CC1101?

Please kindly help.

Many thanks

Jacky

  • 0
    TI__Mastermind 39860 points

    There will be spurs at N x reference frequency (= N x crystal frequency/2), which are also folded around the carrier. Using a 26 MHz crystal there will be a spur at 871 MHz and, when operating at 869.1 MHz also a spur at 868.2 MHz. The closer the  N x reference frequency spur is to the carrier, the higher it will be in amplitude. For operation at 869.1 MHz you should use a 27 MHz crystal frequency as this will move the  N x reference frequency spur further away from the carrier (this is also mentioned in the data sheet). 

    The spurs will not degrade sensitivity. 

  • 0 in reply to Sverre
    Prodigy 30 points

    Many thanks for your quick replies, but I still have some questions as below.

    1) How to reduce the RF power amplitude of the spurs using 26MHz crystal? I tried to change the values of the inductors in the balun filter, the RF power amplitude of fundamental 869.1MHz was down 3dB, but that of the spurs still remained.

    2) I adjusted the values of inductors in the balun filter and the LC filter in my CC1101 TRX module to get the maximum fundamental RF power output at TX mode when the ANT terminal was connected with spectrum analysizer during design. I assume its RX sensitivity should be the best. Is it right? How to improve the RX sensitivity of the module PCBA during design in the case that other test conditions and software setting parameters aren't changed and only the LC values in the balun filter and LC filter may be changed?

    Looking forword to getting your answers for them urgently.

     

     

  • 0 in reply to jacky kong1
    TI__Mastermind 18780 points

    Changing the filter and balun will not affect the spur level as they are too close to the carrier. As Sverre said, to reduce them, when operating at 869 MHz, you must change your xtal frequency.

    When optimizing your output power and sensitivity, you must also look at the harmonics and current consumption. Our ref design is designed for optimal overall performance and to get the best possible performance, I would highly recommend to copy our ref design. Wire wound inductors also gives the best performance for the frequency band you are working with. The CC1101 868 MHz is robust, changing the balun I will not believe gives you much improvement. The point between the balun and filter is 50 ohm so if you need more filtering, I would try to tune the filter components. See more about the filter in DN017 http://www.ti.com/lit/an/swra168a/swra168a.pdf. Remember to add spaceholders for tuning components for your antenna in your design. Changing your sw settings will not require HW tuning.

  • 0 in reply to CHS
    Prodigy 30 points

    Hi CHS,

    Many thanks for your quick reply. It is very helpful for me.

    BR,

    Jacky