loop filter calculator

Does anyone use this loop filter caculator:
http://my.athenet.net/~multiplx/cgi-...ilter.main.cgi
I wonder whats the meaning of the last parameter :" Desired attenuation from the low pass filter"
This is a 3rd loop filter,should it be 60dB? or the default value 20 dB?

many thanks!

I'm guessing that this is the "additional" attenuation added by the 3rd pole (R1, C3 if I'm reading their schematic correctly) for reference spurs and such.

This is a bit of a dangerous parameter as you can request arbitrarily large attenuation that is not plausible (due to the permissible rolloff rate of a single RC section). Also you want to make sure the rest of the loop response is stable after adjusting this value. This all depends on the ratio of the chosen

I didn't run the calculator, I just loaded to look at it -- this is more generic advice.

Lance

Thank you for reply
The additional 3rd RC pole should mean 20 dB attenuation. I've input some value to caculate, but it seems the time constant results are not reasonable...

Anyway , Lance ,Thanks again !