AD8344
Rev. 0 | Page 14 of 20
AC INTERFACES
The AD8344 is a high-side downconverter. It is designed to
downconvert radio frequencies (RF) to lower intermediate
frequencies (IF) using a high-side local oscillator (LO). The LO
is injected into the mixer core at a frequency greater than the
desired input RF frequency. The difference between the LO and
RF frequencies, f
LO
− f
RF,
is the IF frequency, f
IF
. In addition to
the desired RF signal, an RF image will be downconverted to the
same IF frequency. The image frequency is at f
LO
+ f
IF
. The con-
version gain of the AD8344 decreases with increasing input
frequency. By choosing to use a high-side LO the image fre-
quency at f
LO
+ f
IF
is translated with less conversion gain than
the desired RF signal at f
LO
− f
IF
. Additionally, any wideband
noise present at the image frequency will be downconverted
with less conversion gain than would be the case if a low-side
LO was applied. In general, a high-side LO should be used with
the AD8344 to ensure optimal noise performance and image
rejection.
The AD8344 is designed to operate using RF frequencies in the
400 MHz to 1200 MHz frequency range, with high-side LO
injection within the 470 MHz to 1600 MHz range. It is essential
to ac-couple RF and LO ports to prevent dc offsets from skew-
ing the mixer core in an asymmetrical manner, potentially
degrading linear input swing and impacting distortion and
input compression characteristics.
The AD8344 RFIN port presents a 50 Ω impedance relative to
RFCM. In order to ensure a good impedance match, the RFIN
ac-coupling capacitor should be large enough in value so that
the presented reactance is negligible at the intended RF fre-
quency. Additionally, the RFCM bypassing capacitor should be
sufficiently large to provide a low impedance return path to
board ground. Low inductance ceramic grade capacitors of no
more than 330 pF are sufficient for most applications.
Similarly the LOIN port provides a 50 Ω load impedance with
common-mode decoupling on LOCM. Again, common grade
ceramic capacitors will provide sufficient signal coupling and
bypassing of the LO interface.
04826-0-040
0180
30
330
10MHz
500MHz
60
90
270
300
120
240
150
210
Figure 37. IF Port Reflection Coefficient from 10 MHz to 500 MHz
IF PORT
The IF port uses an open collector differential output interface.
The NPN open collectors can be modeled as high impedance
current sources. The stray capacitance associated with the IC
package presents a slightly capacitive source impedance as in
Figure 37. In general, the IFOP and IFOM output ports can be
modeled as current sources with an impedance of ~10 kΩ in
parallel with ~1 pF of shunt capacitance. Circuit board traces
connecting the IF outputs to the load should be narrow and
short to prevent excessive capacitive loading. In order to main-
tain the specified conversion gain of the mixer, the IF output
ports should be loaded into 200 Ω. It is not necessary to attempt
to provide a conjugate match to the IF port output source
impedance. If the IF signal needs to be delivered to a remote
load, more than a few centimeters away, it may be necessary to
use an appropriate buffer amplifier to present a real 200 Ω load-
ing impedance at the IF output interface. The buffer amplifier
should have the appropriate source impedance to match the
characteristic impedance of the selected transmission line. An
example is provided in Figure 38, where the AD8351 differential
amplifier is used to drive a pair of 75 Ω transmission lines. The
gain of the buffer can be independently set by choosing an
appropriate gain resistor, R
G
.
04826-0-041
COMM
8
IFOP
7
IFOM
6
COMM
5
AD8344
AD8351
+
–
R
F
C
+
V
S
R
F
C
Z
L
=
2
0
0
Ω
+
V
S
+
V
S
200Ω R
G
Z
L
Tx LINE Z
O
= 75Ω
Tx LINE Z
O
= 75Ω
Figure 38. AD8351 Used as Transmission Line Driver and Impedance Buffer
