# AD9956 PLL active loop filter

Question asked by fiftyfive on Feb 2, 2011
Latest reply on Feb 4, 2011 by KennyG

I've inherited an AD9956 frequency up/down ramping design that works successfully using 3V commercial VCOs with 3V max tuning voltage; the loop filter is a simple series R-C in shunt with the charge pump output. At the outset, let me mention that I'm new to the 9956 and have some, not a lot, of PLL design experience.

We're migrating to 5V VCOs for better availability, higher output power, and wider tuning range. Simplistically then, a Gain~2 op amp circuit needs to be added to the loop filter to bump tuning voltage, with corresponding changes in filter R-C. In practice of course there's more to it than that, more sophisticated things can be done with an active filter, and op amps bring their own baggage like higher frequency poles.

My basic question is what if any PLL design tools does ADI have for this class of parts? Didn't really see anything on the site and PLLSim doesn't list the 9956. I'm just getting the PLL synthesis module activated in our copy of Genesys and can build the PLL out of datasheet and VCO parameters if that's a good way to go.

The engineer from whom I inherited the first design did a great job with his Mathcad equations, parameters and calculations; he designed the PLL from the ground up, admirably. Here are his assumptions:

DDS Ramp

Flow-Fhigh = 2110-2170MHz

Up-ramp time = Down-ramp time = 7usec (more on that later)

PLL

Kvco = 49MHz/V

I charge pump = 4mA => Phase det gain = 0.637mA/rad

Phase det comparison freq = 25MHz => N = 2140MHz/25MHz = 85.6 => 85 truncated

Natural freq = 515kHz (based on transient response presumably to handle ramp time (?))

Damping factor = 0.7 (ditto)

Design Results

The loop filter values that came out of his passive filter calculations are C=220pf R=2kohm (rounded to std values).

3dB Loop BW~1MHz.  Phase margin ~ 50deg.

I'm guessing an active filter in a 1MHz loop bandwidth could be challenging due to the op amp's poles. Suggestions and guidelines about op amps and filters are welcome, including how feasible this is. While the documents show 7usec up/down, I don't know if the system actually runs that fast - or needs to. On my end I'll check this.