Functional testing often requires you to simulate analog and digital signals that a UUT (unit under test) encounters in actual operation. You also may need to emulate control signals and commands. I recently developed a system that tests four digital control board subassemblies used in RF amplifiers. Twelve USB data-acquisition modules built into a test fixture provide the necessary stimulus signals and measurements.

The subassemblies the system tests are two digital automatic loop-control boards that control the power output of the RF amplifier, one power amplifier logic board used for controlling and interfacing between the internal components and subassemblies of the RF amplifier and the internal motherboard, and one motherboard that is the primary interface board for communicating with our customer's interfaces that use Ethernet/LAN, RS-485, and RS-232 protocols.

The I/O signal types consist of TTL signals used as single-ended bits as well as 8-bit bytes that simulate discrete control and communication between subassemblies. The test system needs single-ended and differential-input analog channels to measure distributed power-supply voltages and RF detector outputs.

The tester also must create single-ended and differential analog voltages to dynamically simulate RF detector signals for the controller boards. Because the RF amplifier operates under software control, the tester has Ethernet, RS-485, and RS-232 ports for communicating with its motherboard.

The test system uses three different types of USB data-acquisition modules from Measurement Computing (MCC). The modules mount inside the test fixture, which provides portability among the test stations. That lets us change PCs without having to remove and reinstall PC plug-in cards.

Three MCC USB-DIO96H modules provide the TTL I/O signals. Although you can assign each 8-bit port to be an input or an output port, we needed two of the 96-channel digital I/O modules configured as outputs and a third configured as an input device. To test the TTL I/O signals of the digital-control boards, one DIO module stimulates the UUT's TTL inputs and the other two monitor the UUT's outputs.

Three MCC USB-3112 DACs satisfy the requirement for differential analog signals. To stimulate the differential analog inputs of the UUT, we drive the DACs in pairs. The test system also uses an RS-485 half-duplex interface to query the UUT for the magnitude of the differential analog input signal. This process repeats over a 0-V to 5-V dynamic range in 1-V increments. These signals simulate the analog voltage from RF detectors that logarithmically represent the output power level of an RF amplifier. Software compares the magnitude of the UUT's incoming analog signal against pass/fail criteria and then stores the results in Excel.

Six MCC USB MiniLab multifunction modules provide 30 single-ended analog inputs, one differential analog input, eight single-ended analog outputs, and digital I/O. The single-ended analog outputs simulate current or voltage metering from an external power supply. Differential analog signals control proprietary RF subassemblies over a 0-V to 1-V dynamic range. Differential signals maximize noise rejection.

The NI RS-485 half-duplex interface sends commands to the UUT, telling it to drive its analog outputs across various dynamic ranges. At the same time, the single-ended and differential ADC inputs measure the UUT's analog outputs. The results of a comparison to pass/fail criteria are stored in Excel.

A USB hub in the test fixture connects 12 instruments to a board under test.

All of the USB data-acquisition modules mount inside a 5¼-in. rack-mountable chassis. A self-powered USB hub provides a single-point USB connection to a PC. The UUTs mount on standoffs atop the cover of the chassis. All signals from the USB modules connect to the UUT through standard mating connectors in a wire harness (Figure 1). This figure shows a schematic of the test system.

The test software, written in Agilent Vee 7.5, lets the system test from one to four subassemblies. The operator can choose between testing each subassembly in its entirety or simply testing all signals relative to a specific connector or connectors. The test software automatically programs the UUT's programmable logic devices—FPGAs, microcontrollers, and DSP controllers.

Once the testing begins, the operator can see the test results and pass/fail status for each signal, connector, and subassembly. The overall test time of each subassembly is approximately 5 min. For troubleshooting purposes, the operator can use a manual panel to stimulate and observe the response of a particular signal or pin. Operators can see test data on the PC monitor in real time while the system writes test data to Excel. We store the test data on the local PC's hard drive. When testing is complete, the system copies the data to a remote server for redundant and permanent data storage.

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