AD831
The mixer has two open-collector outputs (differential cur-
rents) at pins IFN and IFP. These currents may be used to pro-
vide nominal unity RF-to-IF gain by connecting a center-tapped
transformer (1:1 turns ratio) to pins IFN and IFP as shown in
Figure 18.
IF OUTPUT
Low-Pass Filtering
MCLT4-1H
VPOS
IFP
20
18mA TYP
2
VP
1
IFN
18mA TYP
A simple low-pass filter may be added between the mixer and
the output amplifier by shunting the internal resistive loads (an
equivalent resistance of about 14
Ω
with a tolerance of 20%)
with external capacitors; these attenuate the sum component in
a down-conversion application (Figure 20). The corner fre-
quency of this one-pole low-pass filter (f = (2
π
RC
F
)
–1
) should
be placed about an octave above the difference frequency IF.
Thus, for a 70 MHz IF, a –3 dB frequency of 140 MHz might
be chosen, using C
F
= (2
× π ×
14
Ω ×
140 MHz)
–1
≈
82 pF, the
nearest standard value.
C
F
=
1
1
=
89.7 f
2
π
fR
C
F
1
20
19
LOP
11
LOCAL
OSCILLATOR
LON
10
INPUT
Q3
Q4
Q5
Q6
5kΩ
2
C
F
3
RFP
RF
INPUT RFN
6
7
LIMITING
AMPLIFIER
Q2
R4
1kΩ
R5
1kΩ
Q1
4
AN
50Ω
GND
5kΩ
5
IFN
VP
IFP
50Ω
AP
COM
18
R1
20Ω
R2
20Ω
VN
RFP
VFB
17
VP
BIAS
VN
BIAS
CURRENT
Q7
R3
26Ω
36mA TYP
6
OUT
16
7
RFN
VN
VP
9
AD831
Top View
VN
15
8
BIAS
14
LON
10
Figure 18. Connections for Transformer Coupling to the IF
Output
Programming the Bias Current
LOP
11
VP
12
GND
13
Because the AD831’s RF port is a Class-A circuit, the maxi-
mum RF input is proportional to the bias current. This bias cur-
rent may be reduced by connecting a resistor from the BIAS pin
to the positive supply (Figure 19). For normal operation, the
BIAS pin is left unconnected. For lowest power consumption,
the BIAS pin is connected directly to the positive supply. The
range of adjustment is 100 mA for normal operation to
45 mA total current at minimum power consumption.
3
2
1
20
19
Figure 20. Low-Pass Filtering Using External Capacitors
Using the Output Amplifier
The AD831’s output amplifier converts the mixer core’s dif-
ferential current output into a single-ended voltage and provides
an output as high as
±
1 V peak into a 50
Ω
load (+10 dBm).
For unity gain operation (Figure 21), the inputs AN and AP
connect to the open-collector outputs of the mixer’s core and
OUT connects to VFB.
3
2
1
20
19
AN
50Ω
4
IFN
VP
IFP
50Ω
AP
COM
18
AN
50Ω
4
IFN
VP
IFP
50Ω
AP
COM
18
GND
GND
5
VN
RFP
VFB
17
5
VN
RFP
VFB
17
6
OUT
16
6
OUT
16
IF
OUTPUT
7
RFN
VN
VP
9
AD831
Top View
VN
15
VPOS
7
1.33kΩ
BIAS
14
0.1µF
8
RFN
VN
VP
9
AD831
Top View
VN
15
8
BIAS
14
LON
10
LON
10
LOP
11
VP
12
GND
13
NOTE ADDED
RESISTOR
LOP
11
VP
12
GND
13
Figure 19. Programming the Quiescent Current
Figure 21. Output Amplifier Connected for Unity Gain
Operation
–8–
REV. B
AD [ ANALOG DEVICES ]
