Flexible Software

You can operate the USB-5680 as a benchtop instrument or as an

automated measurement device. For benchtop use, the soft front

panel (see Figure 2) helps you configure multiple instrument settings

on the fly. This program runs in Windows Vista, Windows XP, and

Windows 2000 operating systems.

As Figure 2 illustrates, the soft front panel not only returns power

measurements but also helps you configure settings such as autozero,

number of averages, and predetermined power offsets.

In addition, you can fully program the USB-5680 with the NI-568x

driver in languages such as LabVIEW, LabWindows/CVI, C, C++, and

.NET. Using the NI-568x driver, you can fully control the instrument and

integrate it into existing automated test applications. A screenshot of

the API is shown in Figure 3.

Absolute Accuracy

Power meter linearity is one of the most important characteristics of a

power meter because it greatly influences absolute accuracy. In addition,

linearity is dependent on the power level of the signal you are

measuring. As Table 1 illustrates, you can use the power meter in its

most linear region when power levels are less than 18 dBm. Thus, the

sensor is able to provide more accurate measurements at this power

level. Linearity across all power levels is illustrated in Table 1.

Note that sensor linearity is only one of many factors that

contribute to the absolute accuracy of an RF power measurement.

Because impedance mismatch (standing wave ratio, or SWR) and

noise can also add uncertainty, it is important to consider all of these

characteristics when determining overall measurement uncertainty.

National Instruments recommends downloading the Power Uncertainty

absolute measurement accuracy.

With the uncertainty calculator, you can enter signal characteristics

such as power, frequency, and signal type. You also can enter

measurement characteristics such as number of averages and aperture

mode. In addition to providing the absolute accuracy, the uncertainty

calculator highlights individual sources of error. As Figure 5 shows,

you can attribute an absolute accuracy of 4.45 percent to a wide range of

factors, the most significant of which include sensor nonlinearity and

mismatch uncertainty.

Note that the USB-5680 uses an internal auto-zero calibration routine

to reduce errors associated with zero offsets. To do this, an internal

source is used to calibrate the power

meter, and external connectivity

is not required.

Power level <18 dBm

±0.13 dB

Power level

≥18 dBm

±0.18 dB

Table 1. Sensor Linearity According to Power Level

2

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Figure 3. NI-568x Driver API Screenshot

Figure 4. USB-5680 Uncertainty Calculator Screenshot

Figure 5. Expanded Sources of Error from the USB-5680 Uncertainty Calculator

6 GHz True RMS Power Meter with Bus-Powered USB Connectivity

Figure 2. Soft Front Panel Screenshot