DEMO CIRCUIT 1464
DEMO CIRCUIT DC1464 QUICK START
QUICK
FOR LTC6412
GUIDE
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LTC6412
800MHz, 31dB Range
Analog-Controlled VGA
DESCRIPTION
Demonstration circuit 1464 features the LTC6412 ana-
log-controlled VGA. The demo board incorporates a va-
riety of passive components to allow for direct connec-
tion to a 2-port network analyzer or other single-ended
50Ω test system for simplified evaluation.
The 1464 demo board factory default configuration is set
for –V
G
control mode in the power-on and amplifier-
enabled state (SHDN=1,
EN=0).
The demo board is easily configured to access the +V
G
control mode and
EN/SHDN
features. Other simple PCB
modifications can accommodate differential input/output
signals and shunt tuning elements for bread-boarding
into larger systems.
Design files for this circuit board are available. Call
the LTC factory.
L
, LTC, LTM, and LT are trademarks of Linear Technology Corporation. Other product
names may be trademarks of the companies that manufacture the products.
QUICK START PROCEDURE
Demonstration circuit 1464 is easy to set up to evaluate
the performance of the LTC6412. Refer to Figure 1 for
proper measurement equipment setup and follow the
procedure below:
1.
Connect SMA cables to the +IN and +OUT ports.
Applied signal should be less than +10dBm peak RF
power to avoid damage in the power-off state.
2.
Apply +3.3V between +V
CC
and GND turrets. Limit
supply current to approximately 150mA.
3.
Apply the desired control signal to either the –V
G
turret or the –V
G
SMA to access the –V
G
pin of the
LTC6412. For this –V
G
control mode, an
increase
in
control signal voltage produces a
decrease
in ampli-
fier gain.
This procedure contains only one critical sequence order.
The user must apply supply voltage before applying the
control signal voltage and remove or turn-down the con-
trol signal voltage before turning down the supply volt-
age. This proper sequence will prevent excessive current
through the ESD diodes from the ±V
G
pins to the positive
supply, V
CC
.
Table 1 shows the function of each SMA connector on
the board. Only J1 and J2 are used in the default evalua-
tion configuration. Other connectors provide flexibility
for fast control signals, differential input/output signals,
and balun loss calibration.
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DEMO CIRCUIT DC1464 QUICK START GUIDE FOR LTC6412
Table 1. DC1464 SMA Connector Descriptions
CONNECTOR
J1 (+IN)
FUNCTION
Differential input connected to input balun for single-ended operation.
Drive from a 50 Ohm signal source. No external termination needed.
Differential output connected to output balun for single-ended operation.
Impedance matched to 50 Ohms for direct connection to a 50 Ohm network/spectrum analyzer input.
Differential input.
Not connected by default.
Resistor R9 can be removed to drive the input balun differentially.
Differential output.
Not connected by default.
Capacitor at C21 can be moved to C9 to drive the differential outputs.
SMA terminal.
Default connected in parallel with –VG turret to the –VG pin of LTC6412.
Used to apply fast control
signals. Move jumper R15 to location R14 and jumper R19 to location R18 to configure for +V
G
control.
Cascaded 1:1 balun thru connection. Used to estimate the loss of the input balun at the frequency of interest.
J2 (+OUT)
J3 (-IN)
J4 (-OUT)
J5 (VGAIN)
J6 (TEST IN)
J7 (TEST OUT)
Figure 1. Picture of DC1464 Top Silkscreen with recommended
default measurement setup connections shown in maroon.
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DEMO CIRCUIT DC1464 QUICK START GUIDE FOR LTC6412
ADDITIONAL INFORMATION
SHUTDOWN AND ENABLE
The factory default configuration is set for the power-on
and amplifier-enabled state (SHDN=1,
EN=0).
The
SHDN
pin is pulled high with a 1kΩ resistor to V
CC
, and
the
EN
pin is pulled low with a 1kΩ resistor to ground.
One can override these board-level defaults by applying
voltages directly to the
SHDN
and
EN
turrets as needed.
–V
G
AND +V
G
CONTROL
The factory default configuration is set for –V
G
control
with the V
GAIN
SMA connector hooked in parallel with the
–V
G
turret. These control inputs can be reconfigured for
+V
G
operation by moving jumper R15 to location R14
and moving jumper R19 to location R18. Clip leads to
the ±V
G
turrets are susceptible to noise pickup and
should be lowpass filtered to avoid AM up-conversion
artifacts. While using the ±V
G
turrets, a 4.7µF capacitor
from the V
GAIN
SMA terminal to ground provides an ef-
fective lowpass filter with a cutoff frequency of 340 Hz.
DIFFERENTIAL INPUTS
The inputs connectors can be configured for differential
drive. Remove the jumper in location R9 to enable dif-
ferential input drive (50 Ohm).
Alternatively, the input balun T1 can be removed com-
pletely and the ±IN signals routed directly to C5 and C8
using 0603 0Ω jumpers across the input balun pads.
TURRET OPTIONS
Turrets to V
CM
, DECL_1, and DECL_2 are usually not
stuffed as they represent internal voltages generated by
LTC6412. Do not connect to these nodes beyond the
recommended bypass capacitors to ground.
DIFFERENTIAL OUTPUTS
The output connectors can be configured for differential
drive. Move capacitor C21 to location C9 to enable dif-
ferential output drive (50 Ohm).
Alternatively, the output balun can be removed com-
pletely with the ±OUT signals routed to LTC6412 output
pins using 0Ω 0603 jumpers across the output balun
pads as shown below. This transformerless configura-
tion requires two additional inductors from the balun
center tap pad to conduct supply current to the LTC6412
output pins. The inductors should choke the RF signal
sufficiently at the frequency of interest while passing DC
current with minimal supply drop, <100mV is recom-
mended. A surface mount 0603 inductor is a good size
choice to bridge the PCB pad gap. Keep the DC blocking
capacitors at C6 and C9 to avoid a DC path to the output
SMA connectors.
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DEMO CIRCUIT DC1464 QUICK START GUIDE FOR LTC6412
Figure 2. DC1464A Schematic
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DEMO CIRCUIT DC1464 QUICK START GUIDE FOR LTC6412
Table 2. DC1464A Parts List
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