What are your needs tomorrow?
Trade-offs affect the future as well. Test engineers cannot only consider today’s needs, because test devices usually aren’t reconfigurable. If you buy a unit that works for a current project, next year may bring another design that has more expansive requirements and will need a separate test system. And yet, that second unit likely could have covered the current test cases, so overly narrow economics can also be self-defeating.
In some cases, the additional expense of greater coverage might be negligible. For example, if you want to test transceivers in the frequency range of 900 MHz to 2 GHz, a customized test system might cost virtually the same as one that would cover 698 MHz to 3 GHz because the latter could use more standard parts, gaining off-the-shelf cost efficiencies.
Consider the amount of attenuation you will need on connections. There are typical ranges, such as from 0 dB to 95 dB in 1 dB steps up to 6,000 MHz, or 0 dB to 127 dB in 1 dB steps up to 3,000 MHz. The more you can contain your testing attenuation needs into typical ranges, the more likely the test system will use less expensive standard components.
Each port in transceiver test equipment will represent one RF signal for one of the radios being tested. Each antenna, with radios often in a shielded enclosure to control the testing environment, is connected to the port through a cable.
There are three types of configurations you may find in transceiver test equipment:
- Full fan-out;
- Limited fan-out;
- Hub fan-out.
Full fan-out is the most flexible because it offers a fully meshed matrix. It is also the most expensive because it requires the most RF components. In a full fan-out configuration, there is an attenuator for each possible path between radio pairs. If you have 12 ports, there are (12 x 11)/2, or 66, possible two-way paths, each requiring a programmable attenuator. With 6 ports, there are (6 x 5)/2, or 15, possible paths, and, so, 15 programmable attenuators.
In a limited fan-out, each port connects to a specific subset of other ports to either side. If you take a 12 port box and have an 8 limited fan-out design, each of the 12 ports will connect to the four immediately above it and the four immediately below. That would reduce the number of paths needing attenuators to 48. The more ports, the more economically attractive a limited fan-out design can be. A 36-port full fan-out box would need 630 programmable attenuators. Switch to a 36- port 12 limited fan design and the number of programmable attenuators you need is now only 216, a savings of about two-thirds. A limited fan-out can work, if in real-world use, the radios would be spread out geographically far enough so that not all would directly communicate.