The WaveExpert NRO 9000 and SDA 100G are the first instruments to combine the high bandwidth and accuracy of a sampling oscilloscope with the speed and flexibility of a real-time instrument. These are the first products in the new instrument class called Near Real Time Oscilloscopes (NRO), which eliminate most of the constraints of traditional sampling oscilloscopes.

The principle feature that differentiates these oscilloscopes from conventional sequential sampling oscilloscopes is the coherent interleaved sampling (CIS) acquisition mode. This acquisition mode phase locks the scope’s sample clock to the declared bit rate of the input signal. The digitizers, which are sampled at near 10 MS/s in CIS mode, can be 14-bit highresolution devices. The CIS timebase permits the capture and display of very long serial data waveforms without the need for an external pattern trigger. Since the digitizers are clocked at up to 10 MS/s the update rate for the scope is very high compared to conventional sequential mode sampling scopes, which update at a 0.1 MS/s rate.

One of the benefits of LeCroy’s use of the CIS mode is the ability to analyze waveforms with all the powerful mathematical tools usually only available in LeCroy’s realtime oscilloscopes. This is a unique feature in sampling scopes.

One of the most powerful analysis tools available in LeCroy oscilloscope’s is the fast Fourier transform or FFT. Figure 1 shows the spectrum of the PPG-E135 PRBS data generator. This WaveExpert module generates PRBS data patterns at baud rates of 2.5, 5, and 10 Gb/s. In this figure we see the spectrum of a PRBS7 data pattern running at 10 Gb/s. The spectrum consists of a large number of spectral lines spaced at the pattern rate of 78.8 MHz (10 Gb/s/127). These have a sin(x)/x envelope and the first null is the 10 GHz clock component. Note that the dynamic range of this spectral measurement is about 104 dB.

Figure 1:

The spectrum of a PRBS 7 data pattern exhibiting an 84+ dB dynamic range.

Figure 2 shows an expanded view of the averaged FFT of a filtered PRBS 7 data pattern clocked at 10 Gb/s. The portion of the spectrum shown exhibits a near-linear rolloff in amplitude due to a 150 MHz lowpass filter. The filter response provides a variety of spectral components with varying amplitudes. Amplitude cursors measure the amplitude dynamic range of 94 dB. This dynamic range exceeds all real-time scopes by more than 30 dB due to the use of a 14-bit digitizer in the LeCroy WaveExpert series scopes. A 14-bit converter offers a dynamic range of 84 dB on a single-shot acquisition. Keep in mind that to take advantage of wide dynamic range, the waveform must be repetitive with known clock rate and pattern duration, a requirement of any sampling scope.

Figure 2:

The averaged spectrum of a PRBS 7 data pattern running at 10 GB/s after filtering through a 150 MHz low-pass filter showing a dynamic range in excess of 94 dB

The long memory available in CIS acquisition has an added advantage in the frequency domain where it allows for better frequency resolution in the FFT. The resolution bandwidth of the spectrum in Figure 2 is 500 kHz with a spectrum span of 640 GHz for an FFT transform length of 1.28 MS. Common sequential sampling scopes do not offer anywhere near the 512 MS of acquisition memory available in the WaveExpert line.

Another advantage of the coherent acquisition is that signal components that are not synchronous with the input clock are attenuated. The screen image in Figure 3 shows a comparison of the PRBS 7 signal by itself and with a 500 MHz interfering signal summed into the PRBS 7 waveform. The upper trace (M3) is the time domain trace of the PRBS signal. Immediately below that is the spectrum of the PRBS signal (Trace F1). Trace M1 is the time domain display of the PRBS7 waveform with an additive 500 MHz sine.

Figure 3:

A comparison showing that the effects of a 500 MHz interfering signal are minimized in the CIS acquisition mode

The spectrum of the summed waveform (trace M2) shows minimal effects from the 500 MHz components. Almost all the effect is seen as a raising of the spectrum noise floor. This occurs because the 500 MHz signal is not synchronous with the PRBS pattern and clock rate. The result is that the 500 MHz signal appears as noise, is spectrally distributed, and does not appear as a line structure in the FFT. The 500 MHz sine, if it were synchronous with the 10 Gb clock rate, would appear as a spectral line at the center of the FFT display.

In summary, CIS mode provides faster acquisitions, longer acquisition memory, and the ability to analyze acquired waveforms using the finest analysis tools available in the oscilloscope world.