RF2516
The schematic of the loop filter is:
RESNTR+
L
RESNTR-
L
LOOP FLT
4 kΩ
The transfer function is:
s ⋅ τ2 + 1
-------------------------------------------
F(s) = R2 ⋅
s ⋅ τ2 ⋅ (s ⋅ τ1 + 1)
where the time constants are defined as:
C1 ⋅ C2
⎛
⎝
⎞
⎠
------------------
τ2 = R2 ⋅ C2 and τ1 = R2 ⋅
C1 + C2
The frequency at which unity gain occurs is given by:
1
------------------
ωLBW
=
τ1 ⋅ τ2
This is also the loop bandwidth.
If the phase margin (PM) and the loop bandwidth (ωLBW) are known, it is possible to calculate the time constants. These
are found using the equations4:
sec(PM) – tan(PM)
1
--------------------------------------------------
-----------------------
τ1 =
and τ2 =
ωL2BW ⋅ τ1
ωLBW
Rev A17 060712
11-9
RFMD [ RF MICRO DEVICES ]
