Test & measurement
Star-Hub option is installed by mounting a single piggyback module onto any M5i series cards in the multi-channel system. Using accurately matched and shielded coax cabling, the board distributes the clock to each module and precisely synchronises the trigger event with the system clock.
BURST MODE MEASUREMENTS NMR is a common application where burst mode RF signals are encountered. NMR spectroscopy is an analytical technique used to reveal the chemical composition and molecular structure of a sample. It studies the interaction of RF radiation at selected frequencies with the nuclei of molecules placed in a strong magnetic field. The external magnetic field causes certain nuclei in a molecule to absorb select radio frequencies. The absorbed energy is re-emitted at the substance’s resonant frequency, which reveals the identity and intramolecular relationships within the sample. A conceptual block diagram of an NMR spectrometer is shown in Figure 3. The transmit/receive (T/R) switch controls the function of the RF coil. In the transmit state, a gated RF burst from the power amplifier excites the sample. In the receive state the RF coil picks up the sample’s RF response and feeds it to the receiver. High bandwidth digitizers can be used to study the T/R switch operation, monitoring the RF signal from the power
amplifier, the gate signal, which determines the state of the T/R switch, and the output of the T/R switch. Figure 4 shows a simulated measurement of the T/R switch operation using three M5i.3360- x16 4.7 GHz bandwidth single-channel digitizers, each with a 10 GS/s sampling rate. The units are linked using Star-Hub and Spectrum Instrumentation’s SBench6 acquisition and analysis software is used for control and display. The input to the T/R switch is a 350 MHz continuous sine wave, shown in the upper left grid. The Fast Fourier Transform (FFT) of the input signal, in the lower left grid, shows a single spectral line at 350 MHz. The FFT, filtering, and basic measurements are included in the SBench6 software. The RF excitation frequencies commonly used in NMR range from 10s of MHz to 1 GHz. This is well within the bandwidth of the Spectrum digitizers. Other related applications like radar use higher frequencies. For example, the M5i.33xx-x16 digitizers can be used up to their -3dB bandwidth of 4.7 GHz, which would accept S- band radar signals in the 3 to 4 GHz range. The gate signal, shown in the upper center grid, is a pulse waveform that controls the state of the T/R switch. SBench 6 includes measurement capabilities, shown in the Info panel to the left of the traces, that show the gate pulse train has a frequency of 2.5 kHz (400 μs
period) and a pulse width of 81.9 μs (duty cycle of 20.5 per cent). The digitizer has acquired a 2 ms record length at 10 GS/s. This record includes 20 MS of data for each channel. The digitizer has a standard acquisition memory of 2 GS and an optional memory of 8 GS. The 8 GS memory can record 800 ms at the 10 GS/s sampling rate. Long acquisition records allow multiple cycles to be studied to follow changes in response over time. Even with these long records, there is no loss in time resolution. A horizontally expanded zoom trace compares the details of the gate signal pulse edge with the gated carrier showing the time response of the T/R switch. Additionally, the rise time of the gate signal is measured as 2.5 ns.
The T/R switch output in the upper right grid shows the gated waveform as an RF burst. This pulse modulation changes the RF spectrum as seen in the lower right grid, adding a sin x/x modulation envelope due to the pulse modulation, and decreasing the peak spectral amplitude by about 14 dB, as read by the absolute maximum measurements in the Info panel. In addition to SBench6, the M5i.33xx-x16 series digitizers include a software development kit (SDK) and drivers for Windows and Linux operating systems offering the tools for custom software. The SDK includes detailed documentation and working program examples using the most popular programming languages, such as Visual C++, Delphi, Visual Basic,
VB.NET, C#, Python, Java, Julia, and IVI. Spectrum also supports third-party system software products like LabVIEW and MATLAB.
Figure 2: The Star-Hub option allows up to eight digitizers to be synchronised by sharing a common sampling clock and trigger source.
Instrumentation Monthly March 2024
Continued on page 18... 17
Page 1 |
Page 2 |
Page 3 |
Page 4 |
Page 5 |
Page 6 |
Page 7 |
Page 8 |
Page 9 |
Page 10 |
Page 11 |
Page 12 |
Page 13 |
Page 14 |
Page 15 |
Page 16 |
Page 17 |
Page 18 |
Page 19 |
Page 20 |
Page 21 |
Page 22 |
Page 23 |
Page 24 |
Page 25 |
Page 26 |
Page 27 |
Page 28 |
Page 29 |
Page 30 |
Page 31 |
Page 32 |
Page 33 |
Page 34 |
Page 35 |
Page 36 |
Page 37 |
Page 38 |
Page 39 |
Page 40 |
Page 41 |
Page 42 |
Page 43 |
Page 44 |
Page 45 |
Page 46 |
Page 47 |
Page 48 |
Page 49 |
Page 50 |
Page 51 |
Page 52 |
Page 53 |
Page 54 |
Page 55 |
Page 56 |
Page 57 |
Page 58 |
Page 59 |
Page 60 |
Page 61 |
Page 62 |
Page 63 |
Page 64 |
Page 65 |
Page 66 |
Page 67 |
Page 68 |
Page 69 |
Page 70 |
Page 71 |
Page 72 |
Page 73 |
Page 74 |
Page 75 |
Page 76 |
Page 77 |
Page 78 |
Page 79 |
Page 80
