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1
Description
2
Initial Configuration
2.1
Power Supply
3
Signals
3.1
Input Signals
3.2
Output Signals
3.3
TGC Outputs
Description
The Texas Instruments VCA8500BOARD evaluation module makes it easy to evaluate the performance of
the VCA8500 variable gain amplifier. The evaluation module operates in three modes: Default Power Up,
PC Control, and Direct Control. Default-Power-Up mode does not require any external clocking or data
input to enable the operation of the VCA8500. The functionality in Default-Power-Up mode is limited to
factory-programmed modes. PC-Control mode uses the included software to operate all of the functions of
the VCA8500. The software was written on Windows™ XP and its operation has been verified with
Windows™ XP. Direct-Control mode uses external control and data signals to control the functions of the
VCA8500.
The VCA8500BOARD has the following features:
•
Easy testing of the VCA8500
•
Single-ended input
•
Three modes of operation:
–
Default Power Up
–
PC Control
–
Direct Control
The VCA8500BOARD has provisions for six separate power supplies:
•
+5 V: Positive VCA Supply
–
This positive 5 V supplies the CW and VCA circuits. (P2)
•
+3.3 V: Positive VCA Supply
–
This positive 3.3 V supplies the LNA, Digital circuit and VCA Outputs . Ensure that all three pads of
W1 solder jumper are shorted. (P2)
•
+5 V/–5 V: Supplies for all other circuitry
–
These supplies all other +5 V/-5 V requirements onboard. (P4)
•
+15 V/–15 V: Supplies for CW Amplifiers
–
These supplies are optional for the CW amplifiers only. To drive these amplifiers with +15 V/–15 V,
W2 and W3 solder jumpers should be open. (P3)
The input signals are applied to SMA connectors J1 to J8. The input impedance of the VCA is 8 k
Ω
. The
inputs are ac-coupled into the LNA of the VCA8500 through 1-
µ
F capacitors and do not attenuate the
input signal.
The outputs of the evaluation module are located at SMA connectors J21 through J28 for the TGC
outputs, and J10 to J19 for the CW outputs. The outputs from the VCA8500 are buffered with operational
amplifiers.
The differential TGC outputs are buffered by operational amplifiers (OPA842) in a differential configuration
with a differential gain of 1. When testing the VCA8500BOARD, outputs J21 through J28 (TGC outputs)
must be terminated into 50-
Ω
loads such as those of a spectrum analyzer. This termination results in the
proper matching for the output amplifiers, and no loss has to be taken into account.
2
VCA8500BOARD
SLOU216 – January 2008
