LTC5548
14
5548f
For more information www.linear.com/LTC5548
applicaTions inForMaTion
Figure 5. Simplified LO Input Schematic
The RF port is broadband matched to 50Ω from 2GHz to
14GHz with a 0.15pF shunt capacitor (C1) located 1.4mm
away from the RF pin. The RF port is 50Ω matched from
2GHz to 10GHz without C1. An LO between –6dBm and
6dBm is required for good RF impedance matching. The
measured RF input return loss is shown in Figure 4 for
IF frequencies of 240MHz, 1890MHz and 4GHz with low
side LO.
The RF input impedance and input reflection coefficient
versus RF frequency is listed in Table 1. The reference plane
for this data is Pin 5 of the IC, with no external matching,
and the LO is driven at 7.5GHz.
Table 1. RF Input Impedance and S11 (at Pin 5, No External
Matching, LO Input Driven at 7.5GHz)
FREQUENCY
(GHz)
INPUT
IMPEDANCE
S11
MAG ANGLE
2 34.3+j28.9 0.37 99.6
3 49.4+j24.7 0.24 77.4
4 57.2-j3.8 0.08 –25.8
5 37.7+j4.4 0.15 157.4
6 43.4+j2.2 0.07 160.2
7 46.2-j1.9 0.04 –152.3
8 47.8-j1.1 0.02 –155.0
Table 1. RF Input Impedance and S11 (at Pin 5, No External
Matching, LO Input Driven at 7.5GHz)
9 48.8+j0.6 0.01 152.8
10 46.1+j9.1 0.10 107.8
11 35.8+j3.2 0.17 165.2
12 16.3+j4.1 0.51 169.5
13 10.9+j2.3 0.64 174.5
14 12.9-j3.5 0.59 –171.4
LO Input
The mixer’s LO input, shown in Figure 5, consists of a
single-ended to differential conversion, high speed limit
-
ing differential amplifier and an LO frequency doubler.
The LO amplifier is optimized for the 1GHz to 12GHz LO
frequency range. LO frequencies above or below this fre
-
quency range may be used with degraded performance.
The LO frequency doubler is controlled by a digital voltage
input at X2 (Pin 8). When the X2 voltage is higher than
1.2V, the LO frequency doubler is enabled. When X2 is left
open or its voltage is lower than 0.3V, the LO frequency
doubler is disabled.
The DC voltage at the LO input is about 1.6V. A DC block
-
ing capacitor (C4) is required.
LTC5548
LO
X2 V
CC
5548 F05
X2
8 9
11
LO
Z
0
= 50Ω
L = 3.55mm
C4
C3
