AD831 Analog Devices, AD831 Datasheet - Page 7

• Manufacturer Part Number: AD831
• Description: High-Performance, Low Distortion 500-MHz Mixer
• Manufacturer: Analog Devices

REV. B
THEORY OF OPERATION
The AD831 consists of a mixer core, a limiting amplifier, a low
noise output amplifier, and a bias circuit (Figure 17).
The mixer’s RF input is converted into differential currents by a
highly linear, Class A voltage-to-current converter, formed by
transistors Q1, Q2 and resistors R1, R2. The resulting currents
drive the differential pairs Q3, Q4 and Q5, Q6. The LO input is
through a high gain, low noise limiting amplifier that converts
the –10 dBm LO input into a square wave. This square wave
drives the differential pairs Q3, Q4 and Q5, Q6 and produces a
high level output at IFP and IFN—consisting of the sum and
difference frequencies of the RF and LO inputs—and a series of
lower level outputs caused by odd harmonics of the LO fre-
quency mixing with the RF input.
An on-chip network supplies the bias current to the RF and LO
inputs when these are ac coupled; this network is disabled when
the AD831 is dc coupled.
OSCILLATOR
LOCAL
INPUT
INPUT
RF
BIAS
LOP
LON
RFN
RFP
VN
VP
11
10
6
7
AMPLIFIER
LIMITING
CURRENT
BIAS
Q2
Q3
Figure 17. Simplified Schematic Diagram
Q4
1k
R4
20
R1
18mA TYP
36mA TYP
Q7
IFP
AP
R3
26
1k
R5
20
R2
Q5
19
20
Q6
3
2
Q1
18mA TYP
IFN
AN
–7–
When the integral output amplifier is used, pins IFN and IFP
are connected directly to pins AFN and AFP; the on-chip load
resistors convert the output current into a voltage that drives the
output amplifier. The ratio of these load resistors to resistors
R1, R2 provides nominal unity gain (0 dB) from RF to IF. The
expression for the gain, in decibels, is
where
is driven fully positive or negative.
5k
5k
1
2
4
2
BIAS
is the amplitude of the fundamental component of a square wave
is the conversion loss
is the small signal dc gain of the AD831 when the LO input
CURRENT
MIRROR
12mA TYP
50
20
VP
G
1
dB
50
20
20 log
BIAS
50
10
27mA TYP
A
4
O
36
1
2
50
2
16
17
18
OUT
VFB
COM
AD831
Equation 1