What is a Boxcar or Signal Averager?...
The best method to recover a signal from accompanying noise depends
very much on the nature of the signal in question and the required
representation of the result. Lock-in Amplifiers are extremely powerful
signal recovery instruments if the signal is, or can be made to be, an
amplitude modulated AC waveform, where the envelope of the modulation is
the required output. However, where this cannot be done, or where the
rise/fall time of the signal exceeds the available bandwidth, or where the
signal is short lived, other techniques are needed.
A Signal Averager samples the applied signal at a regular sampling rate
and stores the resulting waveform record. It can repeat this process so
that a repetitive input signal can be monitored in exactly the same way on
each new cycle. Each record is added to the sum of the previous records so
that a continuous summation process takes place. Any asynchronous events
(noise) will be reduced in amplitude in relation to the amplitude of the
synchronous events (signal), and hence the summed record represents the
original signal waveform recovered from the noise. In the case of Gaussian
noise, the improvement in signal-to-noise ratio gained from this process
is approximately equal to the square root of the number of summed cycles.
Hence averaging 100 records of an identical event will improve the
signal-to-noise ratio by 10 times.
Two significant types of averager are:
a) The Boxcar Averager
b) The Signal Averager.
The Boxcar Averager uses analog electronics, supported by
digital control, to monitor one discrete point in time on a repetitive
signal. It builds up an average of that point over many cycles before
recording it as a value. It may then move on to a different (later) point
and repeat the process, averaging for the same number of cycles as for the
first point, before recording a second value. In this way it can
"step" across a waveform monitoring it at discrete points to
build up a complete averaged representation of the input signal.
The Signal Averager uses digital techniques to record all of the
waveform on each cycle. This makes it much more time efficient than Boxcar
systems. Nonetheless the time taken to do the summation does limit the
maximum data throughput unless a dedicated hardware averager is included,
as in the Eclipse instruments.
Boxcar systems are particularly well suited to averaging a single point
in time repetitively. As an example, the amplitude of one peak of a
spectrum, derived from a repetitively swept monochromator, could be
averaged easily and recorded as a function of time using a Boxcar system.
This technology can also give good time resolution, with better than 1 ns
being possible. Signal Averagers can provide maximum time resolutions of a
similar level, but are better suited to waveform recovery and to
monitoring short lived phenomena due to their better time efficiency.
|
is
a trademark of AMETEK, Inc.