4 files changed, 16 insertions, 6 deletions
diff --git a/gr-digital/lib/clock_tracking_loop.cc b/gr-digital/lib/clock_tracking_loop.cc
index 0eaa8f29cf..ed8b3efe05 100644
--- a/gr-digital/lib/clock_tracking_loop.cc
+++ b/gr-digital/lib/clock_tracking_loop.cc
@@ -126,8 +126,20 @@ namespace gr {
 
         // Integral arm of PI filter
         d_avg_period = d_avg_period + d_beta * error;
+        // Limit the integral arm output here, as a large negative
+        // error input can lead to a negative d_avg_period, which
+        // will cause an infitine loop in the phase wrap method.
+        period_limit();
+
         // Proportional arm of PI filter and final sum of PI filter arms
         d_inst_period = d_avg_period + d_alpha * error;
+        // Limit the filter output here, for the errant case of a large
+        // negative error input, that can lead to a negative d_inst_period,
+        // which results in an incorrect phase increment, as it is assumed
+        // to be moving forward to the next symbol.
+        if (d_inst_period <= 0.f)
+            d_inst_period = d_avg_period;
+
         // Compute the new, unwrapped clock phase
         d_phase = d_phase + d_inst_period;
     }
diff --git a/gr-digital/lib/clock_tracking_loop.h b/gr-digital/lib/clock_tracking_loop.h
index bc4aee1e6a..e8f0fe7989 100644
--- a/gr-digital/lib/clock_tracking_loop.h
+++ b/gr-digital/lib/clock_tracking_loop.h
@@ -415,12 +415,12 @@ namespace gr {
        * integrator portion of an IIR filter, so T_avg could potentially
        * wander very far during periods of noise/nonsense input.
        *
-       * This function should be called after advance_loop to keep the
+       * This function is called in advance_loop to keep the
        * estimated average clock period, T_avg, in the specified range.
-       * It is set as a separate method in case another way is desired as
-       * this is fairly heavy-handed.
+       * It is set as a separate virtual method in case another way is desired
+       * as this is fairly heavy-handed.
        */
-      void period_limit();
+      virtual void period_limit();
 
       /*******************************************************************
        * SET FUNCTIONS
diff --git a/gr-digital/lib/symbol_sync_cc_impl.cc b/gr-digital/lib/symbol_sync_cc_impl.cc
index e00edeeeb8..c9e44c708b 100644
--- a/gr-digital/lib/symbol_sync_cc_impl.cc
+++ b/gr-digital/lib/symbol_sync_cc_impl.cc
@@ -538,7 +538,6 @@ namespace gr {
             d_inst_clock_period = d_clock->get_inst_period();
             d_avg_clock_period = d_clock->get_avg_period();
             d_clock->phase_wrap();
-            d_clock->period_limit();
 
             // Symbol Clock and Interpolator Positioning & Alignment
             d_inst_interp_period = d_inst_clock_period / d_interps_per_symbol;
diff --git a/gr-digital/lib/symbol_sync_ff_impl.cc b/gr-digital/lib/symbol_sync_ff_impl.cc
index 180204f77e..da14f05282 100644
--- a/gr-digital/lib/symbol_sync_ff_impl.cc
+++ b/gr-digital/lib/symbol_sync_ff_impl.cc
@@ -538,7 +538,6 @@ namespace gr {
             d_inst_clock_period = d_clock->get_inst_period();
             d_avg_clock_period = d_clock->get_avg_period();
             d_clock->phase_wrap();
-            d_clock->period_limit();
 
             // Symbol Clock and Interpolator Positioning & Alignment
             d_inst_interp_period = d_inst_clock_period / d_interps_per_symbol;