Building a Custom Test Multiplexer, with No Engineering Required
Fig: 1: Finished Test Mux
Pulse Instruments is a sister company to PRL. PI makes the world's finest automated test stations for Infrared Focal Plane Arrays, CCDs, and CMOS Imagers.
High performance test systems also require high performance testing themselves, so we needed to build programmable test mux that was performant and flexible enough for us to conduct all the tests we need to run, plus easy-to-use and affordable enough to supply to our international service reps and to our customers. It also had to be rugged enough to travel as checked luggage during system installation and on-site acceptance testing.
Finally, we wanted to build this test mux using all off-the-shelf products from PRL, so that we would not have to do any engineering.
Test up to 16 channels each of high-speed clock driver signals and low-noise DC bias signals. Tests include:
- Clock Driver signals
- Clock high level and low level
- Rise and fall time, fmax
- Voltage sense and current sense
- Low-noise DC Bias signals
- Programmed voltage levels
- Voltage sense and current sense
- Spectral noise density at 10 KHz, 1 KHz, 100 Hz, 10 Hz, and 1 Hz.
Up to 16 signals needed to be routed, programmatically, to one of 4 paths:
- To external measurement equipment, such as a Tektronix oscilloscope, a Keithley DMM, or an HP spectrum analyzer.
- To one of 3 resistive loads.
The two types of signals required very different types of switches:
- The clock driver signals require high-bandwidth, controlled impedance paths to preserve rise time.
- The low-noise DC bias signals required low-loss, isolated paths to preserve DC accuracy and prevent introduction of ground-coupled noise sources.
All testing required:
- A high degree of automation, to ensure consistency in the testing process.
- Flexibility, to allow for testing of many different product models and system configurations.
- Robustness, as the test muxes needed to survive international travel as checked luggage.
Two banks of PRL-854 series multiplexers were integrated into a PRL-MRK-3-1 rack-mount kit.
For the clock driver signals, we selected the PRL-854-RM-SMA, 4:1 RF Multiplexer. We cascaded a set of 5 units to switch any of 16 inputs down to one selected channel, and we used a 6th unit to switch the selected channel either into the measurement path or into one of 3 resistive loads for current measurement on the UUT's ISENSE line. The resistive loads, built from the PRL-PINET Signal Conditioning Kits, are attached to the outside of the PRL chassis, so they can be swapped out as required by the different models of clock driver card:
Fig: 2: Block Diagram (High Speed Side)
For the low-noise DC bias signals, we selected the PRL-854DC, 4:1 Signal/Ground Multiplexer. The same basic configuration was used to switch any of 16 bias signals into a measurement path or into a resistive load.
The PRL rack-mount kit has sufficient module sites to permit integration of both muxes into a single transportable unit. The isolated signal/ground paths of the bias mux prevents noise from the high-speed side from contaminating the sensitive DC measurement side.
Fig: 3: DC Bias Noise Measurement (corrected for 1000x preamplifier)
This solution was drawn on a lunch napkin in 5 minutes, and then implemented on two separate Velcro plates in less than an hour:
Fig: 4: Test Mux, Conceptual Sketch
We subsequently used the PRL-MRK-3-1 kit and the optional mid-panel to integrate both muxes into one transportable unit for repeatability and robustness:
Fig: 5: Test Mux, Internal Assembly
The resulting product, the PRL-4524, is now our standard test mux, used for production test and site acceptance. This model also is available for purchase, and is used by customers for periodic system verification at our installations around the world.
Due to the inherent modularity of the solution, the test muxes can be easily modified or expanded to handle an arbitrary number of input channels by adding additional cascade stages.