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Capacitor Choice on Reference Design for cc115l

Fahd Albinali
Intellectual 490 points
Other Parts Discussed in Thread: MSP430F5342, CC115L, CC110L

Hi,

I am using cc115l along with msp430f5342. The radio is configured to operate at 315MHz. Looking at the reference design here http://www.ti.com/lit/zip/swrr081. I see 0.1uF decoupling capacitors across the digital and analog supply (DVDD, AVDD). Why were these values picked and wouldn't it make more sense to go with lower capacitance on the analog side (e.g. 550 pf) in order to have a higher resonance frequency? What would you recommend for capacitor values for the 315Mhz application? Thanks.

Fahd

  • 0
    TI__Mastermind 39860 points

    The decoupling capacitor value is not critical and we decided to have the same value on all supply pins to make it easier for customers. 100 nF is ok to use also at 315 MHz operation.

  • 0 in reply to Sverre
    Intellectual 490 points

    My initial choice was using a few different capacitor values including 0.1uF, 220pF, 2.2pF but that resulted in a really problematic situation where I am seeing a single transmission as determined by the interrupt on GD0 but on the receiver getting the packet at multiple frequencies spaced by 275KHz from 315, so I would actually get the data simultaneously at the following frequencies:

    315.825MHz
    315.55Mhz
    315.275MHz
    315Mhz
    314.725 MHz
    314.45 MHz
    314.175MHz
    313.9MHz
    So, definitely the choice of capacitors values made a difference. Any thoughts why that might have happened?
    Fahd
  • 0 in reply to Fahd Albinali
    TI__Mastermind 39860 points
    In the past we did use different values for the different supply pins, but when we changed them all to 100 nF we saw do difference in performance. Need some more information to comment on your measurements. What is the receiver? Your initial post was related to CC115L, which is a transmitter only, and there is no mention of the receiver. What do you mean by " I am seeing a single transmission as determined by the interrupt on GD0"? Is it the CRC /packet sent signal you have routed to GDO0?
  • 0 in reply to Sverre
    Intellectual 490 points

    Hi,

    I configured multiple radios including 2 cc110L and 1 cc430f54317 to different frequencies, I determined the frequencies looking at a sweep of my spectrum analyzer between 314 and 316Mhz... see attached image for the odd behavior. I configured IOCFG0 with 06 to assert an interrupt when the SYNC word is transmitted and deassert when a packet transmission is completed... The duration of the interrupt  on GDO on the scope is around 3msec which makes sense because I was sending a single BYTE ... no crc at 2.4kbps which should take around 8/2400 = 3.3msec. Please let me know your thoughts. Thanks.

    Fahd

  • 0 in reply to Fahd Albinali
    TI__Guru**** 317180 points
    - Why do you use a external reference on your spectrum?
    - Is this plot showing the CC115L transmitting on what should have been one frequency?
    - Do you send a CW? It could look more like preamble.
    - Could you share your schematic?
  • 0 in reply to TER
    Intellectual 490 points

    - The setup allows for higher accuracy measurement hence the external reference.

    - Yes, this is CC115L transmitting on what should have been 1 frequency and I register values work fine using smartrf and the interrupt is indicating a single packet

    - No, I am sending packets with 16 bit preamble, 2 sync bytes, fixed length of 1 byte of data and no crc

    -  Attached schematic:

    I am considering putting 0.1uF on all digital supply pins and DCOUPL and 620pf on analog, it seems it makes sense to have a higher resonance frequency for the analog pins, what are your thoughts about this approach?

    Thanks.

    Fahd

  • 0 in reply to Fahd Albinali
    TI__Guru**** 317180 points
    Do not change the cap value on the DCOUPL pin. This pin is connected to an internal LDO and you can risk instability if changed.

    Before sending one short packet, send a CW and see if this looks correct. Sending only one packet will give a sinc function in the frequency domain, it's not a good way to evaluate Tx performance.