VCA8500BOARD

BOARD DEV FOR VCA8500

1 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

2 Initial Configuration

2.1 Power Supply

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 supply 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. The user does not need them connected unless using CW Mode. To drive these amplifiers with +15 V/–15 V, W2 and W3 solder jumpers should be open. (P3)

3 Signals

3.1 Input Signals

The input signals are applied to SMA connectors J1 to J8. The input impedance of the VCA is 8 kΩ. The inputs are ac-coupled into the LNA of the VCA8500 through 1-μF capacitors and do not attenuate the input signal.

3.2 Output Signals

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.

3.3 TGC Outputs

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.

3.4 CW Outputs

Figure 1 shows the CW output circuit. This circuit is duplicated for each CW output

The CW outputs are current outputs and require a current-to-voltage conversion in order to view the outputs with an oscilloscope or spectrum analyzer. The THS4021 operational amplifier is used as the current-to-voltage converter.

When using the VCA8500, the CW outputs need a compliance voltage of +2.5 V. The circuit in Figure 1 shows how the compliance voltage is applied to the VCA8500 device through a 220-μH inductor. The CW output circuit allows the noise of the CW path to be measured. This configuration is the reason for the large feedback resistor in the output amplifier circuit. Before summing more than two CW channels, ensure that the output amplifier does not exceed its maximum output capability. To increase the output capability of the THS4021 ensure that W2 and W3 are open and supply ±15 V to P3.

4 Microcontroller

The VCA8500BOARD contains a microcontroller to access internal functions. The microcontroller provides the evaluation module the ability to be controlled by software running on a PC. The microcontroller is run from its own internal oscillator set to 12 MHz. The only external clock signals are those used to communicate to the VCA8500 via the serial data clock line, and the embedded clock signal in the RS-232 communications circuit. The RS-232 clock operates only when data is being sent from the personal computer (PC) to the microcontroller. The serial communication from the PC to the microcontroller is set to 19200 kbps (baud rate). This baud rate is set internally on the microcontroller and is configured automatically in the PC software.

5 Modes of Operation

The three modes of operation for the VCA8500BOARD are Default Power Up, PC Control, and DirectControl.

5.1 Default-Power-Up Mode

If no external source or PC control is available, the VCA8500 powers up in a default mode, which sets the VCA8500 to TGC mode with these settings, PGA Gain 20 dB, Clamp Level Disabled, and Filter Bandwidth 15 MHz.

5.2 PC Control Mode

This mode requires P10 to be shorted using surface-mount jumpers. This allows the VCA to interface with the microcontroller. Switch S1 is not installed, and the settings are hard-wired to PC-Control mode. The VCA8500 is controlled by the software running on a PC. At this point, the serial cable included with the VCA8500BOARD must be connected to the PC serial port. The program called VCA8500EVM then sets the proper baud rate for communication with the VCA8500BOARD. By using a PC to control the VCA8500, the entire device functionality is available for testing. The software provides the ability to select any of the clamp ranges, filter bandwidth, PGA gains settings, direct an input to any CW output, or power down the chip completely.

5.3 Direct Control Mode

Remove solder jumpers from P10, and connect the external source to DIN, CLK, DOUT, and CS. These are SMA connectors J30, J31, J32, and J33, respectively. This allows for direct control of the VCA8500 with an external control source such as a digital word generator.

6 VCA8500BOARD Full-Access Software

The TI website includes full-access installation software, data sheet, and EVM schematic

6.1 Loading the Software

Download the zip file from the TI website at http://www.ti.com/product/vca8500. . Select the folder called Installation Software, and double-click on Setup.exe. The installation wizard guides you through the rest of the installation process.