In any multi-channel DAQ or vibration control system, where each channel is capable of measuring unique sensors, it is necessary to quantify how the channels are different from each other. The channels are designed identically, but component tolerances will cause differences between channels.
For example, resistors and capacitors in a circuit will determine the cutoff frequency of a filter. The cutoff frequency of a filter is chosen in the design, but manufacturing of physical components always comes with some tolerance – error that the manufacturer specifies. If a 10 kΩ resistor is chosen in the design and has a tolerance of ±1%, the minimum resistance would be 9,900 Ω and the maximum resistance would be 10,100 Ω. When these resistors are used in different channels, each of them being one of the possible manifestations of the nominal 10 kΩ resistor, the cutoff frequencies of the filters in the channels will not be different. This can be seen clearly in the following two figures.
Figure 1 – Nominal Filter Design
Figure 2 – Real Filter (worst-case)
If all components have a 1% tolerance for a 500 kHz lowpass filter, the cutoff can vary (approximately) from 490 kHz to 510 kHz, assuming the worst-case in mismatch for both components simultaneously.
The cutoff frequency of a first-order filter is where the magnitude has dropped by 3 dB and the phase has shifted by 45 degrees. The cutoff frequency is also where the slope of the phase shift is at a maximum, which means slight changes in frequency near the cutoff result in large changes of phase.
The analog anti-alias filter in our Spider systems has a very high cutoff frequency (relative to the frequency content of the data), which is why our Spider systems have a very good channel-to-channel phase match. With a very high cutoff frequency, the amount of phase shift a signal experiences is minimized. The remaining alias cancellation is handled by the internal decimation filter of the delta-sigma analog to digital converter. For more information on the alias rejection of the decimation filter, see the application note ‘Alias Rejection and Alias-free Bandwidth’.
The next figure shows how phase match can be measured in EDM by using the Frequency Response Function (FRF). The white noise signal was generated using the Spider-80X output channel and the input channels on the same Spider-80X were used to check the phase mismatch. The customized marker in the lower left corner shows the phase shift between each channel with respect to channel 1. For this particular Spider-80X, the maximum phase shift relative to channel 1 is channel 7 – about -0.16 degrees at 20 kHz.
It is possible to link many Spiders together to form a high channel count system, up to 512 channels. The Spider-80Xi is a chassis that contains up to 32 or 64 channels per chassis. All 512 channels are sampled simultaneously and phase match between systems is guaranteed to be less than 1 degree at 20 kHz. This level of precision between modules is achieved with extremely tight tolerances placed on the clocks inside every Spider. With the addition of the Spider-HUB, networking with multiple Spiders is both convenient and precise, providing a modular design as well as fulfilling the IEEE 1588v2 standard – time stamping accuracy is guaranteed to be less than 50 nanoseconds.
