Problem with 10MHz External Reference

We are seeing issues when using a 10MHz external reference for the AD9361, ... when using this reference frequency the BB PLL calibration times-out resulting in a fail.  We have checked the reference clock waveform into the chip and it is a 'Square' wave with a peak amplitude of ~1.1Vpp.


Previously we have used a 40MHz external reference with a similar waveform shape and amplitude and had no issues with calibration or operation.


As further investigation we checked a variety of Reference clock frequencies and found that we could pass the BB PLL calibration only if the reference input frequency was greater than ~13MHz. 


We noticed that there was a post on this forum from April 22, 2015, reporting similar issues, although there was no real resolution to the question posted at that time.  However, that poster made mention that he had better success if the 10MHz waveform was a sine wave, ... We also tried this option and also found that the BB PLL calibration passed if the 10MHz  was a sine wave instead of a square wave. 


Can you please provide some guidance?  Are there any special Clock or PLL settings that are required for a 10MHz reference signal?  Is there some limit to the harmonic content that must be observed for lower reference frequencies?


Thank You.