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+This directory contains a mixed mode pll, combining
+ngspice and XSPICE circuit blocks.
+The pll consists of the following blocks:
+
+** voltage controlled oscillator:
+vco_sub.cir
+   7 stage ring oscillator with gain cells, CMOS devices
+or
+vco_sub_new.cir
+   vco made from code model d_osc, cntl_array/freq_array data
+   are gained by running test-vco.cir with vco_sub.cir
+
+** digital divider and frequency reference:
+pll-xspice.cir
+
+** phase frequency detector:
+f-p-det-d-sub.cir
+
+** loop filter:
+loop-filter.cir
+   switched current sources as charge pump, 2nd order
+   passive RC filter
+or
+loop-filter-2.cir
+   transistors as switches for charge pump, 2nd or 3rd
+   order passive RC filters
+
+** main simulation control:
+pll-xspice.cir
+
+Two test files are included:
+test-vco.cir simulates vco frequency versus control voltage
+test-f-p-det.cir simulates the phase frequency detector and the loop filter.
+
+The main building blocks are organised as subcircuits.
+
+main simulation control with three reference frequencies:
+pll-xspice-fstep.cir
+   simulates two steps of the reference in one simulation run