QThe output frequency is 88MHz-108MHz, channel spacing is 100KHz, reference
frequency is 10MHz. For the loop filter, I would like to use the second passive
configuration (3rd order) in the software ADISimPLL, loop bandwidth is 20Hz to
30Hz, the phase margin can be varied. I don't care about the lock time (I can
accept 200mS lock time), but the FM SNR should be more than 60dB.
From my simulation in the ADISimPLL, I can only get 58dB FM SNR, which is 2dB
less than their requirement. Do you have any solutions to this problem?
The reason for such a narrow loop bandwidth is because using it in the FM
broadcast, and the audio frequency is 20Hz to 20KHz. And I like the high
integration and the low cost of this part.
AThat will happen. The open loop VCO noise at this point according to simulation
is - 60 dBc / Hz. The loop filter filters out any VCO noise inside this loop
bandwidth. But if it is narrow, the VCO noise at 20 Hz (-60 dBc / Hz) will
appear in the final system noise profile. Is it not possible to use a wider
bandwidth? The synthesizer noise is very low inside the loop bandwidth, < -100
dBc. 25 kHz would be a very good choice of bandwidth.