Subtle issue (problem?) with channel_model2

The channel model2 has the potential to lock up the flowgraph if the delay and frequency are provided from the same (two-output) sync_block.  The lockup happens after millions of samples for reasonable values of offset rate.  The frequency input is consumed by the 'VCO' block at a the output​​ rate of the resampler.  However, the timing input is consumed at the input​​ rate of the resampler.    Given a sufficiently long simulation the frequency buffer can become full while the resampler buffer is empty (or vv).  This will take ~ 16k / rate_offset.  I would suggest changing the order of the VCO and resampler.  

Note that there is a slight difference in the output that would have to be accounted for when computing the frequency (negligible for 99.99% of applications).

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Eugene Grayver, Ph.D.
Aerospace Corp., Principal Engineer
Tel: 310.336.1274
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