Help with loop filter for FM transmitter

So right now I am building an FM audio transmitter based on the ADF4360-8. I have the evaluation board for this device and what I am working on right now is changing the loop filter that they included on the board with my own that will work best for audio modulation. What they have included on the board is a third-order, passive, low pass filter. They say the filter was was designed for a bandwidth of 40kHz. So from what I've read, it seems like if I want to have the audio transmitter have good bass-response in the audio, I have to make the loop bandwidth as small as possible (essentially as small as the lowest audio frequency I want to transmit?) So if that's true, the 40kHz that they have isn't going to work. I did some calculations using some software and I chose the option for third-order, passive because I want to replace what is on the board. The problem is that when I set loop bandwidth to 50Hz (which I think is acceptable for my audio). The values of the capacitors go up way too high - unrealistically - they don't make SMD capacitors of that size.

So what am I doing wrong? Do I need to choose a different loop bandwidth (maybe it's OK to choose a higher?) Do I need to use a different topology or an active filter for this purpose instead? I would rather keep the same design because then I can test on this board, but if it is necessary to choose a different, let me know.

Thank you and best regards!

The loop filter is for slowing the response to changes from the charge pump. It is true that if you modulate at very low frequencies, the PLL will 'fight back' to try keeping the frequency constant but that depends on how you apply the modulation.

The device is for creating a programmable fixed frequency, how are you applying the audio modulation to make it FM?

Brian.

I plan on AC coupling into the VTUNE pin of the chip.

This may be adequate if you are operating at a fixed frequency, but not if you want it to be tuneable over the whole commercial FM band with the same modulation sensitivity. You will find that the modulation sensitivity is higher at the bottom end of the band than at the top end. In other words, your received transmission audio will be much louder at the low end and soft (under-modulated) at the highest RF frequency.

You will either have to adjust and compensate the modulation audio level against frequency, or employ a second separate varactor stage modulator.

Not sure if that is the case with this particular device because it does have multiple tuning voltage steps/segments throughout the band and I think it was a design to keep it more linear.

Anyway, can anyone comment on the type of loop filter and bandwidth?
I also had a quick question: The only thing I didn't understand when calculating the loop filter was the phase margin. I left it default at 45deg. What is this and should it be modified for this application?