Frequency and Microwave
Capture, store and analyse most challenging GHz signals
By Oliver Rovini, technical director at Spectrum Instrumentation S
pectrum Instrumentation has launched a new series of ultrafast digitizers offering an impressive combination of up to 10 GS/s sampling speed, 12-bit vertical
resolution, and 12.8 GB/s data streaming over the PCIe bus. The one and two channel cards also feature up to 3 GHz bandwidth and up to 16 GB (8 GSamples) of on-board memory. The five different variants of this M5i.33xx series are designed for engineers and scientists dealing with today’s most challenging GHz- range electronic signals.
Application 1: Analysing quadrature- modulated communication-signals The M5i.33xx-x16 series digitizers, with over 3 GHz bandwidth and 10 GS/s maximum sampling rate, are well matched to a broad range of RF and high-speed digital applications. Communications measurements are an application area where the M5i digitizers can be employed. Most communications systems use a variety of quadrature modulation schemes to efficiently encode data. Figure 1 illustrates an analysis of an 8PSK modulated 1 GHz carrier. A 20 μs length of the acquired 8PSK signal is shown in the upper left trace in the SBench 6 display. Below that trace is the frequency spectrum of the signal. The spectrum shows a
Ultrafast 10 GS/s sampling, high resolution and market-leading streaming: M5i digitizers are designed for optimal GHz signal acquisition and analysis.
peak at the carrier frequency of 1 GHz with its modulation envelope. The third harmonic of the carrier at 3 GHz is visible and attenuated by about 36 dB from the carrier peak. The bottom centre trace shows an expanded view of the spectrum. Cursors measure the offset of the closest modulation sideband to the carrier frequency. The cursor readout, shown in the info panel to the left, reads the sideband offset
as 160 MHz from the carrier frequency. The modulation envelope for an unfiltered pulsed waveform would have a sin(x)/x shape. The 8SPK signal has been low pass filtered with a raised root cosine filter with a bandwidth of 20 MHz. This is shown in the expanded spectrum view in the bottom right trace. The cursors measure the nominal bandwidth of the filter. The frequencies above the 20 MHz cutoff
Figure 1: The time and spectral analysis of a 1 GHz carrier quadrature modulated by an 8PSK signal. The upper left trace is the acquired 8PSK signal. Traces to the right are horizontal zooms of that trace. The low left trace shows the spectrum of the signal with expanded views of it to the right.
are eliminated from the modulated signal spectrum so the sidebands appear only within 20 MHz of the carrier and sampling nulls. The top centre trace is an expanded view of the acquired time signal. The ripples are due to the data modulation. The spacing between two adjacent narrow peaks provides insight to the data rate of 40 Mbaud. The 160 MHz spacing between the modulation sidebands indicates an additional sampling process at four times the data rate, or 160 MHz. Looking at the highly expanded view of the 8PSK signal in the top right trace you can see the granularity in signal between phase breaks. The cursors are set to measure the time period between phase breaks, and it turns out to be 6.2 ns, or a frequency of 160 MHz. So, the 40 Mbaud modulation is band limited to 20 MHz and sampled again at 160 MHz before being broadcast.
The acquired RF carrier was demodulated externally to SBench 6, using proprietary vector signal analysis software, and then the resultant in-phase and quadrature components are re-imported back into SBench 6 for additional analysis and display. Figure 2 provides an example of the results. The I component is shown in the upper left trace, the Q component is shown below the I component.
The 8PSK signal encodes three bits into every symbol producing eight possible data values per symbol. The I and Q values translate into phase and magnitude information. The phase and amplitude value of each of these states can be shown in a plot of the I signal versus the Q signal, what is known as a constellation diagram. The state transition or trajectory diagram (in the right-hand trace) shows the transition paths between data states. Each trajectory starts and ends at one of the eight data states. The data states occur at eight phases of 0, 45, 90, 135, 180, 225, 270, and 315 degrees. The state transition diagram provides a quick way of evaluating the 8PSK signal generation. Asymmetry and skew of the underlying constellation indicate errors in signal generation.
30 June 2023 Components in Electronics
www.cieonline.co.uk.uk
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