Minutes RF structure meeting 14.11.2008

M. Schuh, M. Vretenar, G. DeMichele, S. Ramberger, R. Wegner, N. Alharbi


PIMS, thermal drifts

  • during operation with SPL duty cycle, PIMS modules will heat up considerably, the frequency will decrease of about 200kHz (worst case)
  • if for any reason, one cell will get a small frequency offset, the power distribution within a PIMS module will change in a direction that will amplify this frequency offset and again the power distribution will change. Is this a drift (convergence) or an instability (divergence)?
  • the strongest effect will occur for a frequency offset in one end cell. So assuming an initial frequency offset of $df_0=+10$kHz in one end cell,
  • the power in this cell will fall down to $p_1=99.2\%$ of the nominal power level (more power will be dissipated in the other cells, the total power level is assumed to be constant all the time)
  • as the power of the end cell is reduced, it cools, shrinks and its frequency increases further, in numbers, to $df_1=+11.57$kHz
  • again the power drops, to $p_2=98.95\% $ and the frequency increases to $df_2=+12.09$kHz
  • after the nth iteration, the frequency offset will be $df_n=df_0\cdot(1+q+q^2+q^3+...+q^n)$ with $q\sim0.20$ in the case of the PIMS hot prototype
  • as q<1, we have convergence and the final frequency offset is $ df_{inf}=df_0/(1-q)\sim df_0\cdot1.244 $
  • curves show the relation between the initial frequency offset and the final frequency offset for each of the 7 PIMS cells. Furthermore, the Tilt sensitivity and max-min/mean voltage errors including the discussed drift effects are given for frequency offsets in each cell. (RolfWegner)

-- MarcelSchuh - 17 Nov 2008

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