CASSPER takes a different tack, using the power of DSP. It identifies RF emitters (not just for frequency and power level), in spite of ambient RF noise--and without a pricey anechoic range chamber. It can also let you readily perform A/B comparisons over a range from 200 Hz all the way up to 1 GHz.

Hard-to-identify radiation sources can also be located with pinpoint accuracy. Imagine being able to zero in on a leaky cable, or a harmonic-laden clock line, or a nearby transmitter that's emitting a spurious signal.

Thanks to it being PC-hosted and small in size, a CASSPER system can be sited conveniently at various points in your enterprise or in the field. As such, it seems like a natural for incoming component and subassembly qualification testing. You can run your RF tests on-site if the system you're developing isn't in your lab, or doesn't fit in your lab, or in an anechoic chamber, for that matter.

Your manufacturing technicians can just as readily use it in post-production sample testing. Little special test expertise is required.

The CASSPER box also promises to relieve backlogs from over-scheduled test resources. Moreover, while chamber-bound third party test sessions typically demand as many as three days to come up with results, the little CASSPER can usually do the same tests and come up with meaningful results in half a day or so.

The Virtual Chamber

In the past, one way around the ambient hash and noise problem was to perform your compliance or RFI tests in a remote area, or inside an expensive, and oftentimes large, shielded anechoic chamber or screen room. Now you enter what CIS/ETC calls a Virtual Chamber.

Indeed, the Virtual Chamber is the beauty of the CASSPER system. You can run accurate RF emission tests, even in the presence of annoying ambient signals in the spectrum of interest. Ambient cancellation is the secret.

In fact, you can now make measurements at test sites that have been proven to be overrun with ambient signals. That means in-place testing can even be performed on the factory floor, for example, or in other previously unsuitable locations.

Windows and LabVIEW

The key to making this happen, in part, is CASSPER's use of a point-and-click Microsoft Windows environment running National Instruments' popular LabVIEW software. The CASSPER system uses these PC-hosted applications, along with DSP hardware, in a frequency-synchronized phase-locked multi-port receiver.

The dual-channel receiver's time- and frequency-synchronized channels can simultaneously record signals at multiple locations. The box's high-speed DSP then cancels interfering ambient signals, while accurately estimating EUT signal strengths. Moreover, the system relates radiating signals to their sources. That's pretty neat.

In use, multiple sensors (antennas or perhaps current transformers) are patched to CASSPER's hardware enclosure, with automatic port multiplexing based on the operating frequency of the sensors. Each of the system's two channels (A and B) has up to four front panel 50-ohm input ports. At minimum, you'd connect two sensors, one for each channel. For ambient cancellation, one sensor measures both EUT and ambient radiation, while the other measures ambient radiation.

Prior to actually making any measurements, the system creates files describing the sensors you've chosen. It also stores related data, such as antenna factors, transfer impedance, and insertion loss metrics.

For source localization, one sensor measures the problem signal and the other probes for its origin. The EUT and ambient signals can be CW, or AM or FM, or digitally modulated. Regardless of signal type, CASSPER cancels the ambient signals and recovers the EUT signal. Even multipath distortion won't phase the CASSPER (no pun intended).

In some cases, two signals that have the same frequency aren't necessarily related. They may (or may not) be from the same source. No problem for CASSPER. It can localize RF sources by identifying measurements that are related and from the same source.

If you use CASSPER for ambient cancellation, an antenna is placed at some distance d in front of your EUT. The antenna is then connected to Channel A. Naturally, it receives both EUT and ambient emissions. However, a second antenna is also placed further away--at a distance of approximately ten times d. This becomes your reference antenna; it's connected to Channel B. It records the total ambient signal content. By the way, both antennas are generally pointing in the same direction and have the same polarization.

CASSPER's time- and frequency-synchronized receive channels simultaneously record the radiated field strength at the two locations. The correlated ambient signals from the two received time series are then canceled, creating a virtual third measurement channel. This measurement contains signals unique to Channel A, and therefore represents the EUT.

If you use the system to perform source localization, the EUT antenna used for the ambient cancellation test setup can remain in the same position, connected to Channel A, receiving both EUT and ambient emissions. For board-level testing, the EUT antenna can be replaced with a sensor such as a current clamp, and the reference antenna connected to Channel B can be replaced with an E or H near-field probe.

However, in-place testing of a large EUT may be more practical if two antennas are used instead. Either way, the choice and location of sensors are not as important as they are for ambient cancellation.

CASSPER also provides several nice control functions for its Ambient Cancellation mode. Automatic port switching between multiple sensors, for example, accommodates band breaks. The system also lets you select limit levels; that's useful when making FCC and CISPR measurements where threshold-cancellation is only performed on signals above a specified limit line.

You can also operate the system in peak, quasi-peak, or average detection modes. CASSPER can also run single or continuous-sweep scans, and with selectable bandwidth resolution.

The Usual Suspects

The system can also automatically create what CIS/ETS dubs a Suspect Table. It's a generated list of data points that are equal to, or exceed, a current limit line. The system also offers graph legend and graph markers, and packs a useful data export function that will let you port your gathered data to Word files and Excel spreadsheets.

Because CASSPER's receiver channels are time- and frequency-synchronized, the system simultaneously records the field strength at the two locations. Phase relationships between the two signals can be measured, and a coherence value of 0 to 1 can be assigned.

A coherence value of 1 would indicate that the two signals are related and are from the same source, pinpointing the emissions source. This remains true even if it's one of several emitters radiating at the same frequency and amplitude. This kind of information, and the ability to actually determine the culprit source, can't usually be gathered using conventional instrumentation. It puts the CASSPER in a class by itself.