Analog Devices AD8342 - Manual (Page 15)

AD8342
Rev. 0 | Page 15 of 20
AC INTERFACES
The AD8342 is designed to downconvert radio frequencies (RF)
to lower intermediate frequencies (IF) using a high or low-side
local oscillator (LO). The LO is injected into the mixer core at a
frequency higher or lower than the desired input RF. The
difference between the LO and the RF , f
LO
− f
RF,
(high side) or
f
RF
− f
LO
(low side) is the intermediate frequency, f
IF
. In addition
to the desired RF signal, an RF image is downconverted to the
desired IF frequency. The image frequency is at f
LO
+ f
IF
when
driven with a high side LO . When using a broadband load, the
conversion gain of the AD8342 is nearly constant over the
specified RF input band (see Figure 3).
The AD8342 is designed to operate over a broad frequency
range. It is essential to ac-couple RF and LO ports to prevent dc
offsets from skewing the mixer core in an asymmetrical man-
ner, potentially degrading noise figure and linearity.
The RF input of the AD8342 is high impedance, 1 kΩ across the
frequency range shown in Figure 41. The input capacitance
decreases with frequency due to package parasitics.
2.00 1.00
0 0
0 1G
05352-042 FREQUENCY (Hz)
RE
S
IS
T
ANCE
(k
Ω
)
CAP
ACITANCE
(pF)
1.75
1.50 0.75
1.25
1.00 0.50
0.75
0.50 0.25
0.25
100M 200M 300M 400M 500M 600M 700M 800M 900M
Figure 41. RF Input Impedance
The matching or termination used at the RF input of the
AD8342 has a direct effect on its dynamic range. The charac-
terization circuit, as well as the evaluation board, uses a 100 Ω
resistor to terminate the RF port. This termination resistor in
shunt with the input stage results in a return loss of better than
−10 dBm (relative to 50 Ω). Table 4 shows gain, IP3, P1dB, and
noise figure for four different input networks. This data was
measured at an RF frequency of 250 MHz and at an LO
frequency of 300 MHz.
Table 4. Dynamic Performance for Various Input Networks
Input
Network 50 Ω
Shunt 100 Ω
Shunt 500 Ω
Shunt Matched
(Fig. 40)
Gain (dB) 0.66 3.5 5.3 9.3
IIP3 (dBm) 25.4 22.9 20. 6 18.5
P1dB (dBm) 10.8 8.4 6.3 2.3
NF (dB) 14 12.5 10.2 10.5
The RF port can also be matched using an LC circuit, as shown
in Figure 42.
05352-043
Z L 1k Ω
Z
O
= 50 Ω
f MAIN = 250MHz
50 Ω
3.6pF
100nH
(1000 + j0) Ω
Figure 42. Matching Circuit
Impedance transformations of greater than 10:1 result in a
higher Q circuit and thus a narrow RF input bandwidth. A 1 kΩ
resistor is placed across the RF input of the device in parallel
with the device internal input impedance, creating a 500 Ω load.
This impedance is matched to as close as possible to 50 Ω for
the source, with standard components using a shunt C, series L
matching circuit (see Figure 43).
05352-044
25.0
10.0
10.0
25.0
50.0
100.0
200.0
500.0
500.0
200.0
100.0
50.0
Q = 3.0
1 2
3
4
Point 1(1000.0 + j0.0)Ω Q = 0.0 at 250.000 MHz
Point 2(500.0 + j0.0)Ω Q = 0.0 at 250.000 MHz
Point 3(55.6 − j157.2)Ω Q = 2.8 at 250.000 MHz
Point 4(55.6 − j0.1)Ω Q = 0.0 at 250.000 MHz
Figure 43. LC Matching Example