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!