dim:faq:baseband_be
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+ | ====== Baseband back end and bunch-to-bunch coupling ====== | ||
+ | **...Being able to knock out a single bunch is very important for us. I have never really understood why it is OK to use a 250(nbsp)MHz amplifier for knocking out a 500(nbsp)MHz bunch.** | ||
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+ | Let's think about the process of single-bunch kick generation in time domain. We start with a 2 ns pulse with fairly fast rise and fall time (∼400(nbsp)ps). Rise time of a baseband device with first-order RC response is | ||
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+ | < | ||
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+ | Amplifier with the 3(nbsp)dB bandwidth of 250(nbsp)MHz should have rise time of around 1.4(nbsp)ns. That's a total of 1.46(nbsp)ns, | ||
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+ | Next we have the striplines. Response of 1(nbsp)ns long stripline has rise and fall times of 2(nbsp)ns - here is an illustration for 2(nbsp)ns rectangular drive pulse: | ||
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+ | {{ : | ||
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+ | Total rise and fall times are < | ||
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+ | In practice, typical bunch-to-bunch isolation in the back-end when all is said and done is 12-20(nbsp)dB. With 12(nbsp)dB bunch cleaning gets pretty difficult. iGp12 has a few tricks to improve the performance even in these fairly tough situations. First of all, for cleaning out a bunch in the middle of the train, we turn off feedback for that | ||
+ | bunch and turn on the excitation. The feedback is still on for the neighbors. As a result, excitation coupled in the back end is rejected by the feedback, consistently with the loop gain. Secondly, there is a three tap shaping filter in the iGp12 output. This filter allows one to pre-distort the amplifier drive so that bunch-to-bunch coupling is | ||
+ | minimized. In recent commissioning at [[machines: | ||
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+ | To summarize, you don't need 500(nbsp)MHz of bandwidth. In fact, since rise times add quadratically, | ||
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+ | --- // |