January 6, 2011

Analyze signals for frequency and time

The RSA5000 series lets you see frequency-domain signals over a period of time because it can trigger on characteristics such as frequency rise and fall times.

Martin Rowe, Senior Technical Editor -- Test & Measurement World, 1/5/2011 10:08:23 AM

Spectrum analyzers let engineers view signals in the frequency domain, but today's wireless signals have frequency characteristics that change with time. Some signals switch off to conserve power, others may change carrier frequencies. Thus, a signal isn't continuously visible. With the RSA5000 signal analyzer, you can use tools long used on oscilloscopes to let you see how a signal's frequency spectrum or phase changes with time.


The RSA5000 signal analyzers let you see frequency versys time.

Figure 1. The RSA5000 signal analyzers let you see frequency versys time.
The RSA5000 series consists of two models whose difference is frequency range—the RSA5103A (1 Hz-3 GHz) and the RSA5106A (1 Hz to 6.2 GHz). The instruments have acquisition bandwidths of 25 MHz, with 40 MHz and 85 MHz available as options. The instruments use a downconverter to reduce the carrier frequency of a signal so it's low enough to digitize and convert to the frequency domain through an FFT. The instrument digitizes signals multiple times at its acquisition bandwidth to cover a frequency span of interest. Within those acquisition bandwidths, the signal analyzers RBW (resolution bandwidth) defines how much detail you can see. The smaller the RBW, the more detail is visible.

With the 85-MHz acquisition bandwidth option, the instruments let you see the entire frequency range of wireless signals such as Bluetooth, ZigBee, RFID, and Wireless LAN. You can also use the instruments to view radar signals and for EMI/EMC signal analysis.

Tek RSA5000 showing frequency-hopping signal as a function of time

Figure 2. You can trigger on frequencies and see this frequency-hopping signal as a function of time.
Figure 1 shows a plot of frequency versus time. This screen shows the instrument in DPX mode, which puts 50,000 waveforms/s on the screen. The image shows a slice of frequency within a 10-kHz range centered 16 kHz above a 2.44530 GHz carrier for 10 s. The RBW is 20 kHz. The color indicates a statistical density of frequencies. Red across the center of the screen indicates the frequencies that occur most often. This plot shows frequency anomalies occurring between approximately 6 s and 8 s.

The RSA5000 series lets you see frequency-domain signals over a period of time because it can trigger on characteristics such as frequency rise and fall times. That's much the way oscilloscopes capture rising and falling edges in the time domain. The instruments let you trigger an acquisition on frequency, amplitude, phase, or modulation as a function of time. Modulation types include amplitude, frequency, phase, and RF I/Q. Figure 2 shows an example of a frequency-hopping signal. Here, the signal changes among three frequencies over time.

Prices start at $34,900. Textronix, www.tektronix.com/rsa5000.