lt5546 Linear Technology Corporation, lt5546 Datasheet - Page 9

• Manufacturer Part Number: lt5546
• Description: 40mhz To 500mhz Vga And I/q Demodulator With 17mhz Baseband Bandwidth
• Manufacturer: Linear Technology Corporation

APPLICATIO S I FOR ATIO
extended gain control range of –23dB to 57dB. The V
pin is a very sensitive input because of its high input
impedance and therefore should be well shielded. Signal
pickup on the V
noise floor in the I/Q baseband outputs. It can degrade the
linearity performance and it can cause asymmetry in the
two-tone test. If control speed is not important, 1µF
bypass capacitors are recommended between V
ground.
A fast responding peak detector is connected to the VGA
input, sensitive to signal levels above the signal levels
where the VGA is operating in the linear range. It is active
from –22dBm up to 5dBm IF input signal levels. The DC
output voltage of this detector (IF DET) can be used by the
baseband controller to quickly determine the presence of
a strong input level at the desired channel, and adjust gain
accordingly. Figure 3a shows the simplified circuit sche-
matic of the IF DET output.
I/Q Demodulators
The quadrature demodulators are double balanced mix-
ers, down-converting the amplified IF signal from the VGA
into I/Q baseband signals. The quadrature LO signals are
generated internally from a double frequency external CW
signal. The nominal output voltage of the differential I/Q
baseband signals should be set to 0.8V
depending on the linearity requirements. The magnitudes
of I and Q are well matched and their phases are 90° apart.
Quadrature LO Generator
The quadrature LO generator consists of a divide-by-two
circuit and LO buffers. An input signal (2xLO) with twice
the desired IF signal frequency is used as the clock for the
divide-by-two circuit, producing the quadrature LO signals
for the demodulators. The outputs are buffered and then
drive the down-converting mixers. With a fully differential
approach, the quadrature LO signals are well matched.
Second harmonic content (or higher order even harmon-
ics) in the external 2xLO signal can degrade the 90° phase
shift between I and Q. Therefore, such content should be
minimized. In disable or standby mode, the divide-by-two
stage is powered down. After enabling the circuit, the phase
relation between the IF signal and the baseband (I or Q)
signals can be either 0° or 180°, since the circuit cannot
distinguish between the two subsequent identical sinusoi-
CTRL
U
pin can lead to spurs and increased
U
W
P-P
U
or lower,
CTRL
CTRL
and
INPUT
dal waveforms of the 2xLO input signal. The phase rela-
tion between I and Q is always 90°, i.e. I always leads Q by
90° for f
schematic of the 2xLO inputs. Depending on the applica-
tion, different 2xLO input matching networks can be cho-
sen. In Figure 4, three examples are given. The first net-
work provides the best 2xLO input sensitivity because it
can boost the 2xLO differential input signal using a nar-
row-band resonant approach. The second network gives
a wide-band match, but the 2xLO input sensitivity is about
2dB lower. The third network gives a simple and less
expensive wide-band match, but 2xLO input sensitivity
drops by about 9dB. The IF input sensitivity doesn’t change
significantly using any of the three 2xLO matching
networks.
Baseband Circuit
The baseband circuit consists of I/Q low-pass filters, I/Q
hard limiters (clippers) and I/Q output buffers. The hard
limiters operate as linear amplifiers normally. However, if
a high level input temporarily overloads a linear amplifier,
Figure 4. 2xLO Input Matching Networks for 4a) Narrow Band
Tuned to 570MHz, 4b) Wide Band, 4c) Single-Ended Wide Band
2xLO
V
3.3pF
(4a)
CC
3.8k
3.3pF
39nH
IF
Figure 3a. Simplified Circuit Schematic of the
IF DET Output and Figure 3b. The 2xLO Inputs
> 1/2 • f
(3a)
TO 2xLO
TO 2xLO
2xLO
IF DET
–
+
INPUT
2xLO
. Figure 3b shows the simplified circuit
2xLO
2xLO
V
CC
1:4
(4b)
+
–
240Ω
TO 2xLO
TO 2xLO
1k
INPUT
8k
2xLO
–
+
(3b)
LT5546
56Ω
100pF
(4c)
+
–
400mV
5546 F03
100pF
TO 2xLO
TO 2xLO
8k
5546 F04
5546fa
9
+
–