AD8343 Analog Devices, AD8343 Datasheet - Page 12

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

AD8343
CIRCUIT DESCRIPTION
The AD8343 is a mixer intended for high-intercept applications.
The signal paths are entirely differential and dc-coupled to permit
high-performance operation over a broad range of frequencies;
the block diagram (Figure 1) shows the basic functional blocks.
The bias cell provides a PTAT (proportional to absolute tem-
perature) bias to the LO Driver and Core. The LO Driver
consists of a three-stage limiting differential amplifier that pro-
vides a very fast (almost square-wave) drive to the bases of the
core transistors.
The AD8343 core utilizes a standard architecture in which the
signal inputs are directly applied to the emitters of the transistors in
the cell (Figure 7). The bases are driven by the hard-limited LO
signal that directs the transistors to steer the input currents into
periodically alternating pairs of output terminals, thus providing
the periodic polarity reversal that effectively multiplies the signal
by a square wave of the LO frequency.
To illustrate this functionality, when LOIP is positive, Q1 and
Q4 are turned ON, and Q2 and Q3 are turned OFF. In this
condition Q1 connects I
to OUTP. When LOIP is negative the roles of the transistors
reverse, steering I
and gain are possible because at any instant the signal passes
through a common-base transistor amplifier pair.
Multiplication is the essence of frequency mixing; an ideal multi-
plier would make an excellent mixer. The theory is expressed in
the following trigonometric identity:
This states that the product of two sine-wave signals of different
frequencies is a pair of sine waves at frequencies equal to the
sum and difference of the two frequencies being multiplied.
Unfortunately, practical implementations of analog multipliers
generally make poor mixers because of imperfect linearity and
because of the added noise that invariably accompanies attempts
to improve linearity. The best mixers to date have proven to be
those that use the LO signal to periodically reverse the polarity
of the input signal.
sin(ω
PWDN
DCPL
LOIM
LOIP
sig
t)sin(ω
VPOS
LO
INPP
BIAS
DRIVER
t) = 1/2 [cos(ω
LO
to OUTP and I
INPP
COMM
MIXER
CORE
to OUTM and Q4 connects I
Q1 Q2
INPP
sig
t – ω
INPM
LO
Q3 Q4
AD8343
INPM
t) – cos(ω
to OUTM. Isolation
sig
t + ω
OUTP
OUTM
LO
t)]
INPM
In this class of mixers, frequency conversion occurs as a result
of multiplication of the signal by a square wave at the LO
frequency. Because a square wave contains odd harmonics in
addition to the fundamental, the signal is effectively multiplied
by each frequency component of the LO. The output of the
mixer will therefore contain signals at F
5 × F
nents arising from signal multiplication by LO harmonics falls
off with increasing harmonic order because the amplitude of a
square wave’s harmonics falls off.
An example of this process is illustrated in Figure 2. The first
pane of this figure shows an 800 MHz sinusoid intended to
represent an input signal. The second pane contains a square
wave representing an LO signal at 600 MHz which has been
hard-limited by the internal LO driver. The third pane shows
the time domain representation of the output waveform and the
fourth pane shows the frequency domain representation. The
two strongest lines in the spectrum are the sum and difference
frequencies arising from multiplication of the signal by the LO’s
fundamental frequency. The weaker spectral lines are the result
of the multiplication of the signal by various harmonics of the
LO square wave.
FREQUENCY
LO
DOMAIN
DOMAIN
± F
TIME
sig
, 7 × F
LO
± F
sig
FREQUENCY
, etc. The amplitude of the compo-
LO
± F
sig
, 3 × F
SIGNAL
LOCAL
OSCILLATOR
SIG
SIG
LO
LO
LO
± F
sig
,