Oscillator 67

Besides a wide FTR, inductive tuning can also pr ovide the beneﬁt of isolated

DC noise from the tuning element.

The loads on transformer for inductive tuning can be categorized into

three types: resistor [131], capacitor [147], and inductor [145]. Wide FTR is

then achieved by controlling the value of the load. However, traditional loaded

transformer topologies suﬀer from various limitations. For example, resistor-

loaded transformer has a nonlinear tuning-curve with larg e eﬀective K

V CO

,

which can make PLL diﬃcult to lock [13 1]. Capacitor-loaded tr ansformer s uf-

fers from a narrow FTR due to the limited tuning range and poor quality factor

of the varactor at high frequency region [147]. Inductor-loaded transformer re-

quires the use of multiple number o f transformers, which constrain the eﬀective

number of sub-bands due to layout size and design complexity [145].

4.2 Frequency Tuning by Loaded Transformer

4.2.1 Inductive Tuning Analysis

The mechanism of lo aded transformers applied for inductive tuning can be

explained by Figure 4.1. The loaded transformer is utilized to tune the eﬀective

inductance (c

eff

) in a LC-tank, while C

t

consists of the total capacitance in

the LC-tank. Note the 3 types of loaded transformer s can all be approximately

equalized to a RC tank and analyzed with the same equivalent circuit as shown

in Figure 4 .1.

The transformer is as sumed to be ideal with coupling factor k, and with

L

1

and L

2

as the primary inductance and secondary inductance, re spectively.

The equivalent circuit w ith l

eq

and R

eq

can then be calculated as

L

eq

= L

1

×

R

2

[1 − ω

2

CL

2

(1 − k

2

)]

2

+ ω

2

L

2

2

(1 − k

2

)

2

R

2

(1 − ω

2

CL

2

) [1 − ω

2

CL

2

(1 − k

2

)] + ω

2

L

2

2

(1 − k

2

)

R

eq

=

R

2

L

1

[1 − ω

2

CL

2

(1 − k

2

)]

2

+ ω

2

L

1

L

2

2

(1 − k

2

)

2

Rk

2

L

2

.

(4.1)

Thus the oscillation frequency becomes

Figure 4.1: Equivalent circuit model f or inductive tuning of loade d

transformer.

68 Design of CMOS Millimeter-Wave and Terahertz Integrated Circuits

ω =

1

p

L

eq

C

t

. (4.2)

For a resistor or inductor-loaded transformer, the FTR of the equivalent

circuit can be estimated by considering the two extreme conditions of R in

Figure 4.1:

(

L

eq max

= L

eq

(R → ∞) = L

1

×

1−ω

2

CL

2

(1−k

2

)

1−ω

2

CL

2

L

eq min

= L

eq

(R → 0) = L

1

1 − k

2

.

(4.3)

By substituting (4.3) into (4.2), the FTR for LC-tank oscillation frequency

can be obtained:

ω

min

= ω (R → ∞) =

r

ω

2

1

+ω

2

2

−

√

(ω

2

1

+ω

2

2

)

2

−4ω

2

1

ω

2

2

(1−k

2

)

2(1−k

2

)

ω

max

= ω (R → 0) =

ω

1

√

1−k

2

, (4.4)

where ω

1

=

1

√

L

1

C

t

and ω

2

=

1

√

L

2

C

. As s hown in Figure 4.1, ω

1

and ω

2

represent the resonant frequencies at the primary side and the secondary side

of the transformer, respectively.

Note that ω

1

is pre-determined by pa rameters o f the transformer and the

LC-tank, while ω

2

would be aﬀected by the load. By deﬁning ω

2

= αω

1

, whe re

α>0 is the ratio between two re sonant frequencie s, we can further analyze the

value based on diﬀerent α va lue s. Since

∂ω(R→∞)

∂α

stays positive for all α values,

by taking the extreme conditions for α, the range for can be estimated as

ω (R → ∞) ≈

ω

2

, 0 < α ≪ 1

ω

1

, α ≫ 1.

(4.5)

According to (4.5), when ω

2

is much higher than ω

1

or equals ω

1

, indicat-

ing negligible dep endence between value of ω (R → ∞) and the load. However,

as ω

2

drops below ω

1

, ω (R → ∞) is dec reased, approaching the value of ω

2

in-

stead. This is actually tie mechanism for frequency-tuning of capacitor-loaded

transformer.

The eﬀect of ω

2

value on the quality factor for the eﬀective LC-tank must

be considered, which can be easily derived from (4.1) as

Q

eq

=

R

eq

ωL

eq

=

R

ωL

2

×

1 −

ω

2

ω

2

2

[1 −

1 − k

2

ω

2

ω

2

2

]

k

2

+

ωL

2

R

×

1 − k

2

k

2

.

(4.6)

Note that here the loss fro m the transformer and the LC-tank is not in-

cluded in the calculation and Q

eq

quantiﬁes the additional loss coupled from

transformer load into the LC-tank. As (4.6) shows, this coupled loss is con-

tributed by two portions. When R >> ωL

2

, the ﬁrst item on the right-side

Start Free Trial

No credit card required