Measurement Computing WaveBook rev.3.0 - Manual (Page 43)

WaveBook User’s Manual WBK Expansion Options, WBK14 3-11
High-Pass Filter (HPF)
Each WBK14 channel has two independent High-Pass Filters (HPFs) with a 3-dB cut-off frequency (Fc) at
0.1 Hz and 10 Hz. The 0.1-Hz HPF is a single-pole RC filter, and is primarily used to couple vibration
signals. The 10-Hz HPF is a 2-pole Butterworth type and can be used to couple acoustic signals or attenuate
setup-induced low-frequency signals that can reduce the dynamic range of the measurement (for example
when using tape recorders as signal sources).
Programmable Gain Amplifier (PGA)
The HPF removes the DC voltage from the input signal. A PGA amplifies the AC voltage with flat response
up to 500 kHz. Each channel has a PGA with 8 programmable gains (1, 2, 5, 10, 20, 50, 100, and 200) and
a software-controlled DAC for offset nulling. The WBK14 measures only bipolar signals up to 5 V peak.
Programmable Low-Pass Filter Phase Equalizer
The first filter stage is a programmable 2-pole continuous-time low-pass filter. The phase equalizer provides
more than 65 dB alias protection to the next filter stage. In addition, it fine-tunes the phase shift of the
channel to optimize the phase-matching between channels. At calibration, the phase shift of each channel is
measured and stored in an EEPROM that is read at configuration.
Programmable Low-Pass Anti-Aliasing Filter
Most of the signal alias rejection is performed by an 8-pole Butterworth filter. This filter is implemented
with a switch-capacitor network driven by a programmable clock. Each channel has an independent clock
whose frequency determines the 3-dB cut-off frequency of the filter. The switch-capacitor filter provides no
attenuation at the clock frequency—hence, the need for the continuous-time low-pass filter.
Note: The Low-Pass Anti-Aliasing Filter can be bypassed to process signals with a bandwidth higher
than 100 kHz.
The EXT.CLK input provides a path to externally control the cut-off frequency of the Low-Pass
Anti-Aliasing Filter. The input waveform can be TTL or sinusoidal, with an amplitude peak of at least
500 mV. In this mode, the cut-off frequency is set to the input frequency divided by 50.
Simultaneous Sample and Hold
All WBK14 channels are sampled simultaneously, after which the WaveBook measures each output at
1 µs/channel until all channels are digitized. The time-skew between sampling on all channels (up to 72)
is 150 ns, regardless of the number of WBK14s attached to the WaveBook.
When using WaveBook with an SSH channel enabled, the per-channel sample rates are
reduced. The rate reduction is the same as that which would occur if another channel
were added. The per-channel rate (with SSH enabled) is:
1 MHz / (n+1), where n is the number of active channels.
Excitation Source
The excitation source includes a sine/random waveform generator, a programmable gain amplifier (PGA), a
DC-level DAC, and a phase-lock loop (PLL). The PLL is used to synthesize the frequency of a fixed-
amplitude sine wave and control the bandwidth of the random signals. The PGA conditions the signal
amplitude to a value between 0 V to 5 V peak. The DC level of the signal is varied independently of signal
amplitude by a software-controlled DAC from -5 V to +5 V. The DC level of the excitation signal can be
used to balance static loads, while the AC signal provides the dynamic excitation.
Calibration
WBK14 is calibrated digitally, eliminating the need for all potentiometers and manual adjustments.
WaveCal, a provided Windows-based program, simplifies the calibration process.