Power Meters Make Connections Via USB

Jan. 23, 2008
For many applications, USB-enabled RF/microwave power meters can provide all the measurement capability and increased flexibility compared to conventional instruments.

For many applications, USB-enabled RF/microwave power meters can provide all the measurement capability and increased flexibility compared to conventional instruments.

Craig Allardice
Systems Consultant, Satori Technology Ltd.
e-mail: [email protected]

Tony Lymer
Principal Engineer
Satori Technology Ltd.,
1 Biggar Business Park, Market Road, Biggar, Scotland, ML12 6FX, United Kingdom;
+44(0) 1899 229 332, e-mail: [email protected], Internet: www.satori-technology.com.

Microwave and RF measurement equipment features simplified remote control by means of universal serial bus (USB) connections with an external personal computer (PC) or network. For testing RF power, this remote measurement capability can service a wide range of applications for users from technicians to engineers.

Since a USB power meter/sensor combination does not require a parent instrument other than a host computer, it can provide substantial weight and space savings over the conventional arrangement of a sensor and separate power meter. Often, a USB power meter can operate from the power supplied by the host computer through the USB cable, resulting in a power savings (Fig. 1).

A USB power meter eliminates the need to cope with interchangeable power sensors and cable lengths, as well as calibration of each power sensor against a precision power reference source. Of course, USB cables are produced in a variety of lengths but since the information flow along a USB cable is digital rather than analog, the problems associated with variable cable insertion loss, noise and radio-frequency interference (RFI) are reduced. In some applications, a USB system can replace automated systems connected by general-purpose interface bus (GPIB) cables. These heavier cables, introduced more than 30 years ago, suffer more severe length restrictions than USB cables. Practical runs between instruments for USB cables are around 5 m (16.5 ft) while for GPIB cables, the practical limit without extender hardware is about 2 m (6 ft.). In fixed test installations, there is a growing tendency to use Ethernet local-area-network (LAN) connectivity, sometimes in the form of LAN Extensions for Instrumentation (LXI) equipment; however, this solution also has limitations due to the inability of LAN cables to carry DC power.

In selecting a USB or other power meter, it should be remembered that there are two basic types of RF power measurement, for average power and for peak power, and not all power meters can measure both types of power. Average power measurements provide a single number averaged over a period of time, defined by the measuring instrument, to describe what is, in its simplest terms, the heating effect of the signal. Peak power or "pulse profiling" is usually a measurement of "power vs. time," not simply a measure of the highest power level reached over that time.

Peak power measurements can also be used to derive average power as well as statistical functions of power, such as a complementary cumulative distribution function (CCDF) of power. This may be a better alternative to average power measurements if the signal is pulse modulated and the duty cycle is low. Under these circumstances, the value of the average power may be too small for accurate measurements, or the measured average power may fluctuate because the time averaging period is not appropriate for the signal. Average power measurements are also unreliable if the signal duty cycle is not constant. This may happen in systems that involve an automatic request for repeat of erroneous data (ARQ) and a finite probability of errors.

While power meters offering peak power measurement are more versatile, those offering only average power generally cost less and, in the case of USB instruments, use less power from the host computer. This lower power consumption results in longer battery life, yet while still providing accurate measurement capability (see table).

Often, there is a need for measurement system distances greater than the 5 m standard limit of USB connections. Nearly all laptop computers, desktop computers, and some personal digital assistants (PDAs) offer USB sockets. USB connectivity can also provide multiple sensor systems with relatively low power consumption. This is important in mobile installations to preserve battery life. Occasionally, there is a need for very long ranges. By combining USB and Internet technology, this too has become possible. Some measurements are even made between countries or continents. While power meters appear in the examples given, other sensors, from keypads to cameras, could replace or augment them.

Using off-the-shelf technology, multiple sensors can be connected to a single computer using a standard USB hub. Typically, as many as eight USB sockets are provided, occupying a single USB port on the host computer. Many USB hubs can also augment the power supplied along USB by the host computer using an external power supply. Cascading multiple hubs permits as many as 127 devices to be connected at any one time. Using as many as five serially connected hubs, the length of cable can also be extended to 30 m (98 ft) although running USB at this length is impractical for all but temporary solutions.

For longer distances, a single USB connection may be increased using a USB extender. Category 5 (LAN) cable with RJ-45 connectors can be used to connect the two adaptors and establish connection distances to around 50 m (164 ft) long. Some extenders have only 20 mA of current consumption. A hub may be used if more ports are needed, or to inject additional DC power, but some units provide four USB ports at the remote end of the connection. Fiber-optic cable is also readily available as a connecting medium, although both ends of the optical cable must be powered separately. The use of fiber-optic cable can be advantageous in areas where extra copper is undesirable, for example, in antenna ranges. The connection distance may be increased to 500 m (1,640 ft) using multimode fiber and 10 km (6.2 miles) using single-mode fiber. Examples of all of these types of USB extender units are made by Icron Technologies Corp. (www.icron.com).

Where access to a LAN is available, a LAN-to-USB bridge or adaptor, with a static Internet Protocol (IP) address, such as AnywhereUSB(TM) can be used (Fig. 2). This gives access to readings across countries without the need for a host computer at the measurement site.

A solution directed primarily at the maintenance and service engineer is to use a PDA with a USB power meter. At the Autotestcon conference and exhibition (Baltimore, MD, September 2007), Satori Technology Ltd. demonstrated a power sensor connected to and powered from an HP iPAQ hx4700 Pocket PC with Microsoft Windows(R) Mobile 5.0 operating system (Fig. 3).

Custom solutions may be necessary when a very high level of reliability is required and this cannot be achieved with consumer electronic equipment. A typical application might be a remote broadcast site. Such sites may comprise a number of main and reserve transmitters (Fig. 4).

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