Information Technology Reference
In-Depth Information
C
5
2
C
W
L
W
L
2
where, C
ox
is the gate oxide capacitance per unit area, (W
p
, W
n
) are the widths and
(L
p
, L
n
) are the lengths of the M
p
and M
n
respectively. The VCO's oscillation
frequency is inversely proportional to sum of the charging and discharging time (t
dis
)
and is given by [13]
Nt
t
I
1
f
NC
V
DD
3
Fig. 1.
Conventional Current-Starved Ring Voltage Controlled Oscillator
In eq. (3), the number of stages N, V
DD
are initially specified for the ring VCO. C
tot
and I
bias
can be varied to obtain the desired frequency tuning. In the present work, I
bias
in the replica bias stage is varied to tune the frequency of oscillations. By varying I
bias
,
the time t
ch
(=t
dis
) in each stage of the oscillator varies and therefore, frequency tuning
is obtained. I
bias
is a function of control voltage (V
control
). Thus, the frequency range
and linearity of the VCO depends upon the variation of I
bias
with V
control
. Also, the
loop gain K
VCO
of the VCO depends on the slope of I
bias
-V
control
characteristic.
Therefore, a constant K
VCO
across the tuning range of the VCO requires a constant
I
bias
-V
control
slope. The variation of I
bias
with V
control
can be obtained using large signal
analysis of the bias stage (shown in fig. 1). Assuming current sources M
pb
, M
ps
and
current sinks M
nb
, M
ns
are matched respectively, for V
control
>
V
tn
the current I
bias
varies linearly with V
control
till control voltage reaches V
bias
+V
tn
as M
nb
and M
pb
operate in the saturation region. For V
control
>
V
bias
+V
tn
, M
nb
enters linear region and
M
pb
still remains in saturation region, under this condition I
bias
in the bias stage is
governed by
ʲ
V
V
V
V
2
V
DD
V
V
4
2
Since M
pb
remains in the saturation region for the entire range of V
control
, the non-
linearity in I
bias
variation is attributed to M
nb
operating in the linear region. V
tn
and V
tp
are the threshold voltages of NMOS and PMOS transistors respectively.
