• State Resolved
  • Locked Locked
  • Replies 3 replies
  • Subscribers 51 subscribers
  • Views 430 views
  • Users 0 members are here
Support feedback
Options
Options
Similar topics

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.

CC2541 parasitic capacitance

Peter Borenstein
Mastermind 8695 points
Other Parts Discussed in Thread: CC2541

I would like to observe the trace capacitance for my clock crystals. I want to make sure the crystals have the correct load capacitance. The traces give different readings on a fully populated board (excluding clocks) vs a raw PCB.

Should I be measuring parasitic capacitance on a fully populated board? Does a CC2541 with no power give meaningless results when measuring?

  • 0
    TI__Guru 56590 points

    Hello Peter, 

    I would say you should measure on an un-populated PCB. But how do you measure this? There is a very practical approach also which is to initially assume around 1-2 pF on your traces (unless they are very different from the ref. design). After you populated you load capacitors, then enable CW on your board and look at the frequency on a spectrum analyzer. If the offset is to big try to increase or decrease the size of the load capacitors until you get the lowest offset possible.

  • 0 in reply to Eirik V
    Mastermind 8695 points

    Hi Eirik,

    How does the 32MHz impact the 2.4 GHz carrier frequency? If I use HCI commands to output CW on channel 0 and I see 2403MHz instead of 2402, what needs to change? Does the 32MHz control the CW? 

    I measured the capacitance using an L/C meter from Almost All Digital Electronics. I measured between the crystal contacts and ground. My top layer is mostly ground poor so its easy to tape the leads in a fixed configuration (tape) and find a ground contact. 

    The BOM of the Keyfob and Sensortag and the designs appear to assume 8pF. The Sensortag uses 12pF load caps for a 10pF 32MHz crystal (assume 8pF trace). The Keyfob uses 15pF caps for a 12.5pF 32.7KHz crystal (assume 10pF trace). This seems too high and the CC2541 datasheet doesn't mention pin capacitance. 

    I observed trace capacitances of 3-4pF on a raw PCB, and 8-10pF with all components populated.

  • 0 in reply to Peter Borenstein
    TI__Guru 56590 points

    Hello Peter,

    The 32 MHz oscillator is multiplied up to become the actual source of the RF carrier at 2.4 GHz. This means that any PPM offset on the 32 MHz oscillator will result in the same PPM offset on the RF carrier.

    3-4 pF seem very reasonable.

    The two load capacitors form a series connection through ground and hence the total load capacitance from the capacitors only is half the value of a single one. I have given a detailed explanation for this in another thread as well:

    http://e2e.ti.com/support/wireless_connectivity/f/155/p/332131/1158815.aspx#1158815