AD8343 Analog Devices, AD8343 Datasheet - Page 15

• Manufacturer Part Number: AD8343
• Description: DC-to-2.5 GHz High IP3 Active Mixer
• Manufacturer: Analog Devices

This measurement can also be made using the ATN 4000 Series
Multiport Network Analyzer. This instrument, and accompa-
nying software, is capable of directly producing differential
measurements.
At low frequencies and I
ance seen at ports INPP and INPM of the AD8343 is low
(~5 Ω in series with parasitic inductances that total about 3 nH).
Because of this low value of impedance, it may be beneficial to
choose a transformer-type balun that can also perform all or
part of the real value impedance transformation. The turns ratio
of the transformer will remove some of the matching burden
from the differential “L” network and potentially lead to
wider bandwidth.
At frequencies above 1 GHz, the real part of the input imped-
ance rises markedly and it becomes more attractive to use a 1:1
balun and rely on the “L” network for the entire impedance
transformation.
In order to obtain the lowest distortion, the inputs of the AD8343
should be driven through external ballast resistors. At low frequen-
cies (up to perhaps 200 MHz) about 5 Ω per side is appropriate;
above about 400 MHz, 10 Ω per side is better. The specified RF
performance values for the AD8343 apply with these ballast
resistors in use. These resistors improve linearity because their
linear ac voltage drop partially swamps the nonlinear voltage swing
occurring on the emitters.
In cases where the use of a lossy balun is unavoidable, it may be
worthwhile to perform simultaneous matching on both the input
and output sides of the balun. The idea is to independently
characterize the balun as a two-port device and then arrange a
simultaneous conjugate match for it. Unfortunately there seems
to be no good way to determine the benefit this approach may
offer in any particular case; it remains necessary to characterize
the balun and then design and simulate appropriate matching
networks to make an optimal decision. One indication that such
effort may be worthwhile is the discovery that the adjustment of
a post-balun-only matching network for best gain, differs apprecia-
bly from that which produces best return loss at the balun’s input.
A better tactic may be to try a different approach for the balun,
either purchasing a different balun or designing a discrete network.
For more information on performing the input match, see “A
Step-by-Step Approach to Impedance Matching” in the section
covering the AD8343 evaluation board.
Input Biasing Considerations
The mixer core bias current of the AD8343 is adjustable from
less than 5 mA to a safe maximum of 20 mA. It is important to
note that the reliability of the AD8343 will be compromised for
core currents set to higher than 20 mA. The AD8343 is tested
to ensure that a value of 68.1 Ω ± 1% will ensure safe operation.
Higher operating currents will reduce distortion and affect gain,
noise figure, and input impedance (Figures 10 and 11). As the
quiescent current is increased by a factor of N the real part of
the input impedance decreases by N. Assuming that a match is
maintained, the signal current increases by √N, but the signal
Γs
=
(
2
×
S
11
(
−
2
S
−
21 1
S
21 1
)
(
)
(
−
S
−
22
O
S
22
= 16 mA, the differential input imped-
−
S
−
12
S
12
)
+
)
(
+
1
(
−
1
S
−
11
S
11
−
S
−
21 1
S
21 1
)
(
)
(
+
S
+
22 2
S
22
− ×
)
S
12
)
voltage decreases by √N, which exercises a smaller portion of the
nonlinear V–I characteristic of the common base connected
mixer core transistors and results in lower distortion.
At low frequencies where the magnitude of the complex input
impedance is much smaller than the bias resistor values, adequate
biasing can be achieved simply by connecting a resistor from
each input to GND. The input terminals are internally biased at
1.2 V dc (nominal), so each resistor should have a resistance
value calculated as R
be well matched in order to maintain full LO to output isola-
tion; 1% tolerance resistors are recommended.
At higher frequencies where the input impedance of the AD8343
rises, it is beneficial to insert an inductor in series between each
bias resistor and the corresponding input pin in order to mini-
mize signal shunting (Figure 24). Practical considerations will
limit the inductive reactance to a few hundred ohms. The best
overall choice of inductor will be that value which places the
self-resonant frequency at about the upper end of the desired
input frequency range. Note that there is an RF stability con-
cern that argues in favor of erring on the side of too small an
inductor value; reference section on Input and Output Stability
Considerations. The Murata LQW1608A series of inductors
(0603 SMT package) offers values up to 56 nH before the self-
resonant frequency falls below 2.4 GHz.
25
20
15
10
16
20
12
5
0
8
4
0
20
20
TOTAL SUPPLY CURRENT
40
40
TOTAL SUPPLY CURRENT
60
60
BIAS
80
80
= 1.2/I
INPUT IP3
R3 AND R4 –
P1dB
GAIN
100
100
R3/R4 –
INPUT RF = 900MHz
OUTPUT IF = 170MHz
LO LOW SIDE INJECTION
BIAS
INPUT RF = 900MHz
OUTPUT IF = 170MHz
LO LOW SIDE INJECTION
120
120
. The resistor values should
140
140
NOISE FIGURE
160
160
AD8343
180
180
200
200
90
80
70
60
50
40
30
20
10
0
100
90
80
70
60
50
40
30
20
10
0