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ADF4350 Unlocking Problem

Question asked by MattB on Jun 15, 2011
Latest reply on Jul 13, 2011 by MattB

I've recently been using the ADF4350 to tune a narrow band between 144MHz and 152MHz (50KHz spacing, 19.2MHz REF, 10KHz loop filter).  I am seeing this weird phenomenon between when I program this with an FPGA and an external Aardvark SPI controller.  With the Aardvark, the synthesizer seems to always lock fast and hard.  No problems at any frequency.  When I program it using my FPGA, everything locks hard EXCEPT 146MHz and 152MHz (or at least with the 20 or so frequencies I have sampled).  When it doesn't lock, the LD pin seems to go high and low (like a 50% DC square wave with a period of about 30us).  The Vtune, in this case is also ramping up and down between about 0 and 0.3V (well below what it should be...using the aardvark, this voltage is about 1.9V).

 

I would typically start fiddling with the loop filter, but the fact that I can program it well with one device and not the other makes me scratch my head.  For the FPGA, all of the registers except Register 5 are the same for all of the working and non-working frequencies.

 

I'm not sure where to go from here?  Digital line integrity?  Supply line filtering?  Loop filtering?

 

Any insight is highly appreciated!

 

cheers...matt

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