USB Devices Simplify RF/Microwave Testing

A growing number of RF test functions, including signal generation and control, power measurements, and signal switching, are now available by plugging a USB device into a personal computer.

Computer-controlled test equipment once evoked images of racks of instruments connected to a "technical" computer via the general-purpose interface bus (GPIB). While GPIB-controlled gear is still a staple of many automatic-test-equipment (ATE) applications, newer test interfaces, such as the Universal Serial Bus (USB), are quickly gaining ground for their ease of use and flexibility. In fact, the availability of a growing number of measurement functions under USB computer control is making the true desktop AE system under laptop control a reality. Telemakus is one of several companies now offering high-frequency measurement functions through 6 GHz that take advantage of a laptop or personal computer (PC) for its processing and display capabilities.

The company is typical of the relatively small firms involved in USB test equipment development. It currently offers several flavors of microwave power meters with sensors, synthesized signal generators, digital attenuators, and two RF/microwave singlepole, double-throw (SPDT) switches. In the simplest configuration, several signal sources can be controlled by a computer running a simple graphical user interface (GUI) program to perform multitone testing. In more elaborate setups, digital attenuators can be combined with frequency synthesizers, a switch, and a power meter to provide high-speed pulsed testing with precise automatic-level-control (ALC) capability.

All of the firm's instruments fit the familiar form of a USB "dongle," and all are compatible with the USB 2.0 standard (Fig. 1). USB was originally introduced in 1996 as version 1.0, with a data transfer rate of 12 MB/s, as a single solution to the multitude of connectors then in use at the back of personal computers. The standard was developed by leading computing companies, including Compaq, Digital Equipment Corp., IBM, Intel, and Microsoft. The USB 2.0 standard was reached toward the end of 2001 with a data rate of 480 MB/s, with additional companies, such as Hewlett-Packard and NEC, lending support to the new standard. The USB 3.0 standard, which is being implemented in some early products with estimated transfer rates of about 1000 MB/s, will be backward compatible with USB 2.0. In a USB system, multiple devices are connected through hubs, with one hub serving as the main or root hub. As many as 127 USB devices, including hub devices, can be connected to a single controller.

Telemakus offers several building blocks for a rudimentary USB ATE system. For example, the company's model TEA4000-7 digital attenuator provides a total attenuation range of 0 to 31.75 dB, controllable with 7-b, 0.25-dB resolution over a frequency range of 50 MHz to 4 GHz. In addition to the selected attenuation, the device does suffer some signal attenuation, with maximum rated insertion loss of 2.4 dB. The digital step attenuator can handle average input power levels to +23 dBm and peak input power levels to +30 dBm, and delivers a third-order intercept point of +59 dBm. It draws 150 mA at +5 VDC (from a USB port) and is equipped with SMA male and female connectors. The USB 2.0 interface is by means of a Mini A USB connector. When combined with one of the company's power meter/sensor units, it can form a simple USB-controlled ALC or automatic-gain-control (AGC) loop. The attenuator is supplied with simple-to-use GUI software as well as software drivers for integration into ATE programs, such as LabView. The USB attenuator contains 0.5 GB Flash memory for documentation or test data.

As for power measurement equipment compatible with the frequency range of the digital attenuator, the company's model TED6000-50 power meter/sensor is a true root-mean-square (RMS) power meter in the size of a USB memory device (Fig. 2). It covers a frequency range of 50 MHz to 6 GHz with a 50-dB dynamic power measurement range, from -40 to +10 dBm. It promises 0.5 dB power measurement accuracy and, like the digital attenuator, draws 150 mA at +5 VDC from a USB port. For power measurements at higher frequencies, the firm also supplies the model TED10200-45 with frequency range of 9.3 to 10.2 GHz and the model TED14500-45, which operates from 14.0 to 14.5 GHz. Both of these higher-frequency power meter/ sensor devices are rated for input power levels to +15 dBm with 0.5 dB power measurement accuracy.

All three of the USB power meters are calibrated versus frequency and power at the factory, with calibration factors loaded into each device's 0.5-GB of Flash memory; no zeroing or further calibration is required. When a higher level of accuracy (than standard) is required, the devices can be loaded at the factory with custom calibration tables. Temperature correction is achieved by using an internal temperature sensor. As with the digital attenuator, the power meter/ sensor units can be controlled by means of built-in drivers or with a free utility software program running on any laptop or PC with Windows XP or Windows Vista operating system (Fig. 3).

For generating test signals, the firm currently offers two options: models TEG1000-10 (700 to 1000 MHz) and TEG2700-4 (1800 to 2700 MHz). Each is stabilized by its own internal 10-MHz crystal reference oscillator and can tune with 1-kHz resolution, although the tuning step size can be programmed with standard steps of 1 MHz, 100 kHz, 10 kHz, and 1 kHz, or custom steps from 1 kHz to full range. The GUI (Fig. 4) supports the control of multiple signal generators for multitone testing.

The lower-frequency synthesizer delivers +10-dBm minimum output power while the higher-frequency source generates at least +6 dBm output power and typically +10 dBm output power. The spurious levels for both sources are typically -80 dBc, with second harmonics of typically -20 dBc, and third harmonics of typically -15 dBc. The single-sideband (SSB) phase noise is typically -105 dBc/ Hz offset 100 kHz from the carrier.

The company offers two USB-controlled SPDT switches, model TES6000- 30 and TES3000-60, for applications from 100 MHz to 6 GHz. The reflective TES6000-30 switch features 30 dB isolation at 4 GHz and less than 2 dB insertion loss. The return loss is 20 dB at 4 GHz. It hits 1-dB compression with +33 dBm input power (it is rated for maximum input power of +29 dBm when switching), and has a third-order intercept point of +49 dBm at 6 GHz. It achieves a 10-to-90-percent RF switching time of 33 ns and draws 150 mA at +5 VDC. Fully terminated model TES3000-60 provides 60-dB isolation at 1 GHz with less than 1 dB loss.

The TES6000-30 is supplied with an SMA male RF common connector and SMA female connectors on the RF1 and RF 2 ports. It has a menu-selectable pulse function that switches it at a constant pulse width of 10 microseconds and repetition rate of 1 kHz (Fig. 5). Together, the USB instruments form a starting point for a complete USB measurement setup. Telemakus LLC, 13405 Folsom Blvd., Suite 502, Folsom, CA 95630; (916) 458-6346, FAX: (916) 983-8713, e-mail: [email protected], Internet: For sales inquiries, contact the distributor RFMW Ltd., at (877) 367-7369, Internet:

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