LT5579
16
5579fa
The following examples illustrate the implementation
and performance of the LT5579 in different frequency
configurations. These circuits were evaluated using the
circuit board shown in Figure 12.
1650MHz Application
In this
case, the LT5579 was evaluated while tuned for an
IF of 70MHz and an RF output of 1650MHz. The matching
configuration is shown in Figure 8.
Input capacitors are used only as DC blocks in this ap
-
plication. The
4.7nH inductors and the 120pF capacitor
transform the input impedance of the IC up to approximately
12.5Ω. The relatively low input frequency demanded the
use of 4.7nH chip inductors instead of short transmission
lines.
Closer to the IC input, 47pF capacitors were used instead
of a single differential capacitor (C3 in Figure 1), because it
was found that the addition of common mode capacitance
improved the high side LO performance in this applica
-
tion. The
value of these 47pF capacitors was selected to
resonate with the 100nH inductors at 70MHz. Note that
adding common mode capacitance does not improve
performance with all frequency configurations.
The RF port impedance match was realized with C8 =
1.5pF and L3 = 6.8nH. The optimum impedance match
120pF
1.5pF
1nF
47pF
9.1Ω
100nH
100nH
MABAES0061
4:1
1nF
4.7nH
4.7nH
IF
70MHz
LO
RF
1650MHz
6.8nH
9.1Ω
47pF
5579 F08
Figure 8. IF Input Tuned for 70MHz
RF OUTPUT FREQUENCY (MHz)
1550
GAIN (dB), NF (dB), OIP3 (dBm)
15
20
25
1750
5579 F09
10
5
–5
1600
1650
1700
0
35
OIP3
SSB NF
GAIN
30
LOW SIDE LO
HIGH SIDE LO
T
A
= 25°C
f
IF
= 70MHz
P
IF
= –5dBm/TONE
P
LO
= –1dBm
Figure 9. Gain, Noise Figure and OIP3 vs
RF Frequency with 70MHz IF and 1650MHz RF
Typical applicaTions
was purposefully shifted high in order to achieve better
OIP3 performance at the desired frequency.
Figure 9 shows the measured conversion gain and OIP3
as a function of RF output frequency. As mentioned above,
the output impedance match is shifted towards the high
side of the band, and this is evidenced by the positive slope
of the gain. The single sideband noise figure across the
frequency range is also shown.
Curves for both high side and low side LO cases are
shown. In this particular application, the low side OIP3
outperforms the high side case.
1950MHz Application
In this
example, a high side LO was used to convert the IF
input signal at 240MHz to 1950MHz at the RF output. The
RF port impedance match was realized with C8 = 1pF and
L3 = 4.7nH. As in the 1650MHz case, it was found that
tuning the output match slightly high in frequency gave
better OIP3 results at the desired frequency. The input
match for 240MHz operation is the same as described in
the test circuit of Figure 1.
The measured 1950MHz performance is plotted in Fig
-
ure 10 for both low side and high side LO drive. With this
matching configuration, the low side LO case outperforms
the high side LO. The gain, noise figure (SSB) and OIP3
are plotted as a function of RF output frequency.
