Datasheet

LTC6409
15
6409fa
R
S
50Ω
V
S
R
F
LTC6409
100Ω
100Ω
R
F
C
F
R
I
R
I
R
T
R
L
50Ω
C
BA
6409 F10
1dB
LOSS
IDEAL
4:1
IDEAL
1:4
1dB
LOSS
+
–
C
F
R
T
applicaTions inForMaTion
Figure 8. Definition of IMD3
Figure 9. Graphical Representation of the
Relationship between IMD3 and OIP3
amplifier, implying that each single tone is 1V
P-P
, result-
ing in V
PDIFF
= 0.5V. Using R
L
= 50Ω as the associated
impedance, P
O
is calculated to be close to 4dBm.
As seen in Equation (5), when a higher impedance is used,
the same level of intermodulation distortion performance
results in a lower intercept point. Therefore, it is impor-
tant to consider the impedance seen by the output of the
LTC6409 when working with intercept points.
Comparing linearity specifications between different am-
plifier types becomes easier when a common impedance
level is assumed. For this reason, the intercept points
for LTC6409 are reported normalized to a 50Ω load im-
pedance. This is the reason why OIP3 in the Electrical
Characteristics table is 4dBm more than half the absolute
value of IMD3.
If the top half of the LTC6409 demo board (DC1591A,
shown in Figure 12) is used to measure IMD3 and OIP3,
one should make sure to properly convert the power seen
at the differential output of the amplifier to the power that
appears at the single-ended output of the demo board.
Figure 10 shows an equivalent representation of the top
half of the demo board. This view ignores the DC-blocking
and bypass capacitors, which do not affect the analysis
here. The transmission line transformers (used mainly
for impedance matching) are modeled here as ideal 4:1
impedance transformers together with a –1dB block. This
separates the insertion loss of the transformer from its
ideal behavior. The 100Ω resistors at the LTC6409 output
create a differential 200Ω resistance, which is an imped-
ance match for the reflected R
L
.
As previously mentioned, IMD3 is measured for 2V
P-P
dif-
ferential peak (i.e. 10dBm) at the output of the LTC6409,
corresponding to 1V
P-P
(i.e. 4dBm) at each output alone.
From LTC6409 output (location A in Figure 10) to the input
of the output transformer (location B), there is a voltage
attenuation of 1/2 (or –6dB) formed by the resistive divider
Figure 10. Equivalent Schematic of the Top Half of the LTC6409 Demo Board
P
S
POWER
FREQUENCY
IMD3 = P
S
– P
O
∆f = f2 – f1 = f1 – (2f1 – f2) = (2f2 – f1) – f2
6409 F08
P
S
P
O
P
O
2f1 – f2 2f2 – f1f1 f2
IMD3
1×
IIP3
P
O
OIP3
P
OUT
(dBm)
P
IN
(dBm)
6409 F10
P
S
3×