Signal coupling and signal integrity in multi-strip Resistive Plate Chambers used for timing applications

Abstract

We have systematically studied the transmission of electrical signals along several 2-strip Resistive Plate Chambers (RPCs) in the frequency range $f=0.1-3.5$GHz. Such a range was chosen to fully cover the bandwidth associated to the very short rise-times of signals originated in RPCs used for sub-100ps timing applications. This work conveys experimental evidence of the dominant role of modal dispersion in counters built at the 1 meter scale, a fact that results in large cross-talk levels and strong signal shaping. It is shown that modal dispersion appears in RPCs due to the intrinsic unbalance between the capacitive and the inductive coupling $C_m/C_o \neq L_m/L_o$. A practical way to restore this symmetry has been introduced (hereafter `electrostatic compensation'), allowing for a cross-talk suppression factor of around $\times 12$ and a rise-time reduction by 200ps. Under conditions of compensation the signal transmission is only limited by dielectric losses, yielding a length-dependent cutoff frequency of around 1GHz per 2 meter for typical float glass -based RPCs ($\tan \delta|_{glass} = 0.025\pm0.005$). It is further shown that `electrostatic compensation' can be achieved for an arbitrary number of strips as long as the nature of the coupling is `short-range', that is an almost exact assumption for typical strip-line RPCs. Evidence for deviations from the dominant TEM propagation mode has been observed, although they seem to have negligible influence in practical signal observables. This work extends the bandwidth of previous studies by a factor of almost $\times 20$.