Hi,

When I use the EVAL ADF4350 program it almost automatically sets the registers for minimum MOD value in Register 1 and the corresponding Int and Fract values in Register 0. Mathematically I can get the same frequency output by keeping the MOD value constant and varying Fract.

For example, if I want to get 2351.3 MHz, Eval can only set MOD=100 and Fract=13. 13 being an odd number and a prime number.

However, if I want 2351.2, while MOD=100 and Fract=12 would seem logical, EVAL program choses MOD=25 and Fract=3 or it can actually choose MOD=50, Fract=6 also.

Are there phase noise or spurious output related issues associated with these choices. Logically it would seem if MOD=100 gives a particular spur performance, it will be consistent with frequencies no matter what the Fract/MOD.

Thanks,

GRB

Hi,

The reason the software defaults to the lowest common denominator is that the spurs will usually appear at the fractional channel or PFD/MOD offset. With this in mind it stands to reason that the smaller mod values are at a greater offset from the carrier than larger mod values.

The loop filter attenuates fractional spurs and the greater the offset is from the carrier then the more filtering and attenuation of spurs occurs.