8
LT5525
5525f
APPLICATIO S I FOR ATIO
WUUU
As shown in Figure 3, the RF input return loss with no
external matching is greater than 12dB from 1.3GHz to
2.3GHz. The RF input match can be shifted down to
800MHz by adding a series 3.9pF capacitor at the RF input.
A series 1.2nH inductor can be added to shift the match up
to 2.5GHz. Measured return losses with these external
components are also shown in Figure 3.
2
OPTIONAL SERIES
REACTANCE FOR
LOW BAND OR
HIGH BAND
MATCHING
RF
+
RF
IN
LT5525
5525 F02
3
RF
–
Figure 2. RF Input Schematic
RF FREQUENCY (MHz)
500
–30
RETURN LOSS (dB)
–25
–20
–15
–10
–5
0
1000 1500 2000 2500
5525 F03
3000
NO RF
MATCHING
SERIES 1.2nH
SERIES 3.9pF
Figure 3. RF Input Return Loss Without and
with External Matching Components
Figure 4 illustrates the typical conversion gain, IIP3 and NF
performance of the LT5525 when the RF input match is
shifted lower in frequency using an external series 3.9pF
capacitor on the RF input.
RF input impedance and reflection coefficient (S11) ver-
sus frequency are shown in Table 1. The listed data is
referenced to the RF
–
pin with the RF
+
pin grounded (no
external matching). This information can be used to simu-
late board-level interfacing to an input filter, or to design
a broadband input matching network.
Table 1. RF Port Input Impedance vs Frequency
FREQUENCY INPUT REFLECTION COEFFICIENT
(MHz) IMPEDANCE MAG ANGLE
50 10.4 + j2.63 0.675 174
500 18.1 + j23.7 0.551 124
700 25.8 + j30.7 0.478 106
900 36.5 + j34.5 0.398 90
1100 48.4 + j33.3 0.321 74
1300 59.5 + j25.7 0.244 57
1500 65.9 + j13.1 0.177 33
1700 65.0 – j1.0 0.131 –3
1900 59.0 – j12.2 0.138 –47
2100 50.2 – j19.0 0.187 –79
2300 41.8 – j22.1 0.250 –97
2500 34.9 – j22.7 0.311 –109
2700 29.1 – j21.9 0.369 –118
3000 23.2 – j19.1 0.435 –130
RF FREQUENCY (MHz)
800
–5
GAIN AND NF (dB), IIP3 (dBm)
0
5
10
15
900 1000
1100
1200
5525 F04
IIP3
25
20
850 950
1050
1150
LOW SIDE LO
HIGH SIDE LO
SSB NF
GAIN
T
A
= 25°C
f
IF
= 140MHz
Figure 4. Typical Gain, IIP3 and NF with
Series 3.9pF Matching Capacitor
A broadband RF input match can be easily realized by
using both the series capacitor and series inductor as
shown in Figure 5. This network provides good return loss
at both lower and higher frequencies simultaneously,
while maintaining good mid-band return loss. The broad-
band return loss is plotted in Figure 6. The return loss is
better than 12dB from 700MHz to 2.6GHz using the
element values of Figure 5.
LO Input Port
The LO buffer amplifier consists of high speed limiting
differential amplifiers designed to drive the mixer core for
high linearity. The LO
+
and LO
–
pins are designed for
