Re: [Openbts-discuss] 3GPP 05.05 4.6.1 GMSK modulation compliant

[Jongjay L. <jon...@ho...>]

Hello David,

Thank you for your quick response.
I measure directly from TX port from RFX900/USRP1, clock source is Symmetricom X72 which feed to Cyclone III FPGA PLL generating 52Mhz.

What is RangeNetworks 5150 measured phase error ?

Jongjay

  ----- Original Message ----- 
  From: David Burgess 
  To: Jongjay Liou 
  Cc: ope...@li... 
  Sent: Wednesday, April 11, 2012 10:56 AM
  Subject: Re: [Openbts-discuss] 3GPP 05.05 4.6.1 GMSK modulation compliant




  There are a lot of things that can cause the phase error you describe, but all of them have more to do with your clock source and RF transmit path than with OpenBTS itself.  Phase noise in the oscillator itself is one possible problem (this is different from frequency stability). Overdriving the transmitter output can definitely cause this problem.  Power supply noise is a possible cause, too, depending on the design of the radio.  What radio are you using?




  On Apr 10, 2012, at 7:43 PM, Jongjay Liou wrote:


    Hello community,

    Measured with Agilent E4406A, I found openBTS GMSK modulation cannot constantly maintain phase error within 5°, and peak deviation sometime overshoot greater 20°, even when I use Rubidium oscillator as source for 52Mhz clock.

    Can this be intrinsic behavior of SDR or coding in sigProcLib/FPGA? Is there any means to improve GMSK modulation to meet 3GPP 05.05 spec requirement? Commercial BTS phase error is usually within 1°and very stable.

    Thanks!



    Jongjay




  David A. Burgess, CEO
  Range Networks, Inc.
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  cell +1 707 208 2622
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