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Real-time digitizing oscilloscopes have made a clear step into the RF world when they crossed the double-digit gigahertz bandwidth level last year. Now you can digitize signals with frequencies up to 13 GHz in real time. However, probing methodologies for such high-speed signals are a concern for engineers. Let's preview the types of available probing solutions, the trade offs between them and the practical considerations for ensuring high signal fidelity and accurate measurements.

Agilent introduced a new high-speed active probe architecture with its 7 GHz InfiniiMax series. These probes pioneered a controlled transmission line topology that enabled new levels of performance without sacrificing usability. Later, the company improved the performance to 13 GHz with the introduction of InfiniiMax II series. These probes can also be used on sampling oscilloscopes, spectrum analyzers and network analyzers for true multi-use flexibility. Following are the types of probing solutions this new topology offers:

• Solder-in probe heads offer a semi-permanent connection to the signal under test incorporating solderable nickel wire with either 0.005 inch or 0.007 inch diameter.

  It offers a highly reliable connection for signals that frequently need to be measured. The probe amplifier can be easily disconnected from the probe head that can remain attached to the target device until the signal needs to be measured again. It is a popular probe head due to these attributes. However, it requires easy access to a microscopic soldering station to solder the probe in place the first time. The probe head cannot be moved or removed without access to the solder station.

  Not all probe heads of this type in the industry provide full bandwidth, a flat frequency response, long probe head length and variable angle positioning of the probe head. These characteristics should be considered, as they will dramatically impact measurements and usability.

• Browser probe heads are designed to be positioned by a probe holder and moved from location to location on the target device. It uses more resilient 0.007-inch diameter steel wire. This allows you to quickly inspect signals in various locations. It does not require a solder station for immediate use nor does it require test fixtures to be built into the target system. A user can also inspect signals quickly by hand.

But, using a probe holder, or holding the probe by hand, makes it susceptible to less solid connections to the signal and movement of the target device. Reconnection to a previously probed signal requires the same level of positioning effort each time.

Nevertheless, full bandwidth and flat frequency response remain strong considerations here. Also, the size of the browser probe head, its resultant visibility to the signal under test, the ease or difficulty of adjusting the probe lead span and angles-of-use limitations need to be taken into consideration. I recommend that you try using the various browser solutions before purchasing a system. They are not all created equal.

Differential SMA probe heads can be used on SMA fixtured differential pairs that have been designed in to the target device. They provide easy, reliable and repeatable connection to the differential signal pair and offer the widest, most flexible span between each side of the pair of signals. They require only one signal channel of the test instrument for measuring a differential pair.

Not all target devices can support the space and cost requirements of the SMA connectors. Even if space and budget are available, it is often difficult to determine in advance every signal and every location that will need to be probed.

Likewise, browser probe heads, full bandwidth and flat frequency response continue to be cardinal rules to check for these probe heads as well. Some systems do not offer this type of connection as a probe head, but rather require an entire new probe to support it. This can impact the cost of the probe solution by five to 10 times, so be careful.

Direct probing of signals up to 13 GHz in bandwidth is often a crucial part of the measurement solution for ultrawideband communications, ultrahigh-speed serial buses, aerospace/defense electronics and leading-edge research applications. Research the various probe offerings and don't get caught with the wrong probe solution.

Lon Hintze is the product manager of high-performance oscilloscopes at Agilent Technologies.

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