AD8344
Rev. 0 | Page 15 of 20
The high input impedance of the AD8351 allows for a shunt
differential termination to provide the desired 200 Ω load to the
AD8344 IF output port.
It is necessary to bias the open collector outputs using one of
the schemes presented in Figure 39 and Figure 40. Figure 39
illustrates the application of a center-tapped impedance trans-
former. The turns ratio of the transformer should be selected to
provide the desired impedance transformation. In the case of a
50 Ω load impedance, a 4-to-1 impedance ratio transformer
should be used to transform the 50 Ω load into a 200 Ω
differential load at the IF output pins. Figure 40 illustrates a
differential IF interface where pull-up choke inductors are used
to bias the open-collector outputs. The shunting impedance of
the choke inductors used to couple dc current into the mixer
core should be large enough at the IF frequency of operation as
to not load down the output current before reaching the
intended load. Additionally, the dc current handling capability
of the selected choke inductors needs to be at least 45 mA. The
self resonant frequency of the selected choke should be higher
than the intended IF frequency. A variety of suitable choke
inductors are commercially available from manufacturers such
as Murata and Coilcraft. An impedance transforming network
may be required to transform the final load impedance to 200 Ω
at the IF outputs. There are several good reference books that
explain general impedance matching procedures, including:
• Chris Bowick, RF Circuit Design, Newnes, Reprint Edition,
1997.
• David M. Pozar, Microwave Engineering, Wiley Text Books,
Second Edition, 1997.
• Guillermo Gonzalez, Microwave Transistor Amplifiers: Analy-
sis and Design, Prentice Hall, Second Edition, 1996.
04826-0-042
COMM
8
IFOP
7
IFOM
6
COMM
5
AD8344
Z
L
=
2
0
0
Ω
I
F
O
U
T
Z
O
=
5
0
Ω
+
V
S
4:1
Figure 39. Biasing the IF Port Open Collector Outputs
Using a Center-Tapped Impedance Transformer
04826-0-043
COMM
8
IFOP
7
IFOM
6
COMM
5
AD8344
R
F
C
+
V
S
R
F
C
Z
L
=
2
0
0
Ω
I
F
O
U
T
+
I
F
O
U
T
–
+
V
S
Z
L
IMPEDANCE
TRANSFORMING
NETWORK
Figure 40. Biasing the IF Port Open Collector Outputs
Using Pull-Up Choke Inductors
04826-0-044
0180
30
330
50MHz
50MHz
500MHz
500MHz
60
90
270
300
120
240
150
210
REAL
CHOKES
IDEAL
CHOKES
Figure 41. IF Port Loading Effects due to Finite-Q Pull-Up Inductors
(Murata BLM18HD601SN1D Chokes)
LO CONSIDERATIONS
The LO signal needs to have adequate phase noise characteris-
tics and reasonable low second harmonic content to prevent
degradation of the noise figure performance of the AD8344. A
LO plagued with poor phase noise can result in reciprocal
mixing, a mechanism that causes spectral spreading of the
downconverted signal, limiting the sensitivity of the mixer at
frequencies close-in to any large input signals. The internal LO
buffer provides enough gain to hard limit the input LO and
provide fast switching of the mixer core. Odd harmonic content
present on the LO drive signal should not impact mixer
performance; however, even-order harmonics cause the mixer
core to commutate in an unbalanced manner, potentially
degrading noise performance. Simple lumped element low-pass
filtering can be applied to help reject the harmonic content of a
given local oscillator, as illustrated in Figure 42. The filter
depicted is a common 3-pole Chebyshev, designed to maintain a
1-to-1 source-to-load impedance ratio with no more than
0.5 dB of ripple in the pass band. Other filter structures can be
effective as long as the second harmonic of the LO is filtered to
negligible levels, e.g., ~30 dB below the fundamental. The meas-
ured frequency response of the Chebyshev filter for a 1200 MHz
−3 dB cutoff frequency is presented in Figure 43.
04826-0-045
AD8344
LOIN
3
COMM
4
LOCM
2
R
L
FOR R
S
= R
L
f
C
- FILTER CUTOFF FREQUENCY
R
S
C1 C3
LO
SOURCE
L2
C1 =
1.864
2
πf
c
R
L
C3 =
1.834
2
πf
c
R
L
L2 =
1.28R
L
2
πf
c
Figure 42. Using a Low-Pass Filter to Reduce LO Second Harmonic
