Measurement Computing WavePort rev.3.0 - Manual (Page 32)

4-4 Hardware and Operation Reference 09-29-00 WavePort User’s Manual
Excitation Source
A built-in, programmable excitation source stimulates dynamic systems for transfer function measurements,
and serves as a reference signal for calibration. The excitation source includes a sine/random waveform
generator, a programmable gain amplifier (PGA), a DC-level DAC, and a phase-lock loop. The phase-lock
loop synthesizes the frequency of a fixed-amplitude sine wave and controls 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.
MODULE 1 and MODULE 2 provide constant current to bias ICP® transducers. Two current levels
(2 mA or 4 mA) with voltage compliance of 27 V can be selected via software. The bias current is sourced
through the center conductor of a coaxial lead and returns to the dynamic signal MODULE by way of the
outer conductor. The output impedance is larger than 1 M
Ω and presents virtually no loading effect on the
transducer’s output. For applications that do not require bias, the current source can be removed from the
BNC input by opening a relay contact.
The Dynamic Signal MODULE’s current sources are applied to (or removed from) the input in
channel-groups of two.
For MODULE 1 these are: [CH1-1 / CH1-2], [CH1-3 / CH1-4], [CH1-5 / CH1-6] and [CH1-7 / CH1-8].
For MODULE 2 these are: [CH2-1 / CH2-2], [CH2-3 / CH2-4], [CH2-5 / CH2-6] and [CH2-7 / CH2-8].
High-Pass Filter – for MODULES 1 & 2
Every Dynamic Signal MODULE’s signal input 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) – for MODULES 1 & 2
The High-Pass Filter 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 Dynamic Signal MODULES measure only
bipolar signals up to 5 V peak.
Programmable Low-Pass Filter Phase Equalizer – for MODULES 1 & 2
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 [ and FILTER CLK. IN] for MODULES 1 & 2
Most 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 that
determines the filter’s 3dB cut-off frequency. 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.
FILTER CLK. IN - MODULE 1 and MODULE 2 each have a BNC labeled FILTER CLK. IN (Filter
Clock In). These BNC connectors provide 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 “External Clock” mode, cut-off frequency is set to the input frequency divided by 50.
Simultaneous Sample and Hold – for MODULES 1 & 2
All MODULE 1 and MODULE 2 channels are sampled simultaneously, after which the WAVEBOOK
Module measures each output at 1 µs/channel until all channels are digitized. The time-skew between
sampling on all channels is 150 ns.
When a Dynamic Signal MODULE’s SSH channel is 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.