A guide to the methodology of
very high speed signal capture and generation.
DataQuest Solutions have built up years of experience related to the requirements
for very high-speed (MHz) signal capture and waveform generation. To capture or generate
signals at these speeds using cards within a computer based system need not be a
complicated process, certainly no more complicated than at slower sampling
speeds, however it does bring with it some extra considerations. What
follows is an explanation of these for someone new to this field or for a
person who wishes for an update as to the associated technology.
When using analogue to digital (A/D) conversion the binary data that represents
the captured signal needs to be transferred, stored and processed,
careful consideration has to be made as to the bandwidth limitations of the computers
interface with the card. This applies equally to where the
programmer needs to do the reverse (i.e. generate a waveform), or indeed when
using a digital I/O card and dealing with logic signals.
Handling data associated with relatively low speed signals of 1MHz is well within the
capabilities of many types of interface, such as USB and Ethernet however
once the waveform speed goes above about 1MHz then internal computer fitment is
the most common interface to handle the greater data bandwidth requirements and
this is the type we will consider here.
DataQuest Solutions tackles the demands of very high-speed data transmission
with the PC by using our range of "Spectrum" instrumentation cards which are
suitable for the PCI, PCI-X and PCI-Express interfaces. Most computer owners are
familiar with PCI , its been around for many years, but the PCI-X version
may be less familiar, but can be readily obtained on many motherboards and
gives over twice the bandwidth for data transmission. Note - the parallel PCI-X
interface should not be confused with PCI-Express (PCIe), it is a completely different
interface and uses a very high speed serial data transmission. Choosing PCI,
PCI-X or PCI-Express is an important consideration and as everything
hinges around the ability of the chosen slot interface to have sufficient bandwidth
this is an important point to investigate. Here are some typical transmission
speeds with our Spectrum M2i range cards:-
PCI: 100 Mbytes/sec, PCI-X: (high speed PCI) 200 Mbytes/sec, PCIe
(PCI-Express): 130 Mbytes/sec.
Note this is not the theoretical maximum, which is higher,
but taking into effect the currently available interface
chips and system overheads. Also in PC operating systems the memory
management is based on memory pages (normally 4 kByte = 4096 Bytes), during
continuous data transfer data is allocated into these pages and different addresses
have to be accessed, this slows down the process of streaming data.
There is a way to improve this by utilising the drivers for our Spectrum
range of high-speed cards, so raising the aforementioned transmission speeds by about 25%.
Our Engineers at DataQuest Solutions will be pleased
to provide more information on this.
The newest interface, PCI-Express, offers the greatest opportunities in terms
of higher speeds in the future, indeed with our Spectrum range of cards PCIe
provides the advantage of allowing multiple cards to be used in the same PC without
having to share available motherboard transfer bus bandwidth. To choose the best slot
type for your application, determine the required bandwidth by taking the number
of samples per second you wish to record and note the number of bits the signal data is digitised into.
This value will depend on the A/D or D/A converter resolution of your chosen card,
this will usually be 8 bit (1 byte) or 16 bit (2 bytes).
(Note that the 12 bit or 14 bit converter also uses 2 bytes as it uses a
16 bit "word" to hold the digitised value). Now convert this into Mega bytes,
with care, as a "Mega" byte is not 1 million bytes, it is in fact 1024*1024=1048576bytes
as there are 1024 bytes in a computer kilo byte!
For example to digitise a single waveform with an 8 bit (1 byte) converter at 100 million
samples per second requires this bandwidth:
100M samples / 1048576 = 95.367M bytes per second.
This is probably OK even for the standard PCI bus, however if the same
calculation is done with a 12 to 16-bit converter the result will be double and using
our example above a PCI-X bus will be certainly required for constant data streaming,
unless you are willing to limit your overall acquisition period and rely more
on the available instrumentation cards memory. Having the ability to use PCI-X interface is
therefore of great benefit. This slot is commonly found on server motherboards,
but these can be in a standard format for fitment into an office type of PC.
When data is transferred to or from a card using one the interfaces just mentioned,
one of the problems is coping with the temporary effects of PC operating system "housekeeping",
where it grabs priority over the card data transfer. To cope with this, memory
installed on the cards is used as a temporary FIFO (First In First Out) buffer,
so that if a brief interruption does occur during data transfer with the PC,
no loss of data will occur. This FIFO buffer holds the extra data until the card
has full access again to the PC bus and memory again.
Signal capture and data transfer route to PC RAM and Hard disk
Note for a waveform generator card transfer will be in the opposite direction
All of our Spectrum M2i cards have at least 256Mbytes of memory as
standard and can be upgraded to 4 Giga bytes, but for most situations the
standard memory used as a FIFO buffer has enough room to allow loss free transfer.
On the PC the best place to store data, at least temporarily, is PC RAM.
Even Giga bytes of PC RAM are of a relatively low cost, so it makes for an economic
as well as a fast and efficient system. This brings us to an important point.
The majority of MS Windows based PC systems are still 32 bit and as such can work
with up to 4 Giga Byte of installed PC RAM, but the operating system will take up at
least 0.5 Giga bytes of this and in reality of the remaining address space only 2
Giga bytes might actually be available for signal data! This can be extended
however with special options at boot up, but don't rely on more than about
3 Giga bytes in total. Fortunately with the advent of 64 bit Linux and
Windows operating systems - for which we can supply drivers, these
restrictions need no longer apply and the programmer can work with as much
memory as his motherboard can hold - well beyond 4Gbytes and increasing all
the time.
Acquired signal data will usually end up being stored hard disk drive (HDD)
and with the Spectrum M2i/M3i card driver it is even possible to do this real
time with the signal capture going on, however note that the HDD is a
bottleneck to really fast data capture. Even the best SATA drive might only
work to 50 to 60 Mbytes/sec and perhaps only half this rate in many cases.
One way around this is with a RAID system using two or more SATA devices
working in parallel, where simultaneous access to multiple disks allows data
to be written to or read from a RAID array faster than would be possible
with a single drive. Having two drives can nearly double the speed and this
set up is called RAID 0. The implications we have discussed to data transmission
similarly apply to many card types and it could also be that a number of cards of
could be synchronised and working together in the same PC, perhaps a combination
of A/D, D/A and digital I/O. All Spectrum cards can be used in this way.
Going to the front end of our instrumentation card, signal capture or
signal generation needs to be controlled and this is normally done through
triggering. This allows a signal to be captured (or generated) only when it
is needed, cutting down the amount of data required and thus data rates.
This can be software controlled, simply by a screen click, but at mega
sample rates triggering is normally controlled by looking at the amplitude
of the actual signal, or a separate external digital pulse, or indeed a
digital pattern (should the application relates to the use of a digital I/O
card). Time stamping of the trigger is often very useful here, particularly if
there are multiple trigger events, so events and data can be correlated later.
Spectrum hardware has this option.
So we know something of very high-speed signal data, its storage and control
and related hardware issues, but it is the software driver that allows the
user to set parameters and have overall control of the system. Programming
of the installed card(s) can be with many types of text code. It all comes
down to the drivers supplied with the instrumentation. The Spectrum
drivers are very versatile allowing text code programming in many types of
code, most commonly Visual Basic, Delphi and C++, the latter most
recommended for best performance and versatility, with many examples
provided to get the programming underway. Other third party packages can be
used too, these being LabVIEW™, LabWindowsCVI™, MATLAB™,
VEE™, DASYLab™ or MS Excel. Every Spectrum card purchased
comes with SBench6™, a menu driven signal capture and analysis package
complete with a scope type display and easy access to board programmable options,
allowing a basic working system to be up and running within a few minutes.
This is a very good way to get familiar with a new system or for more demanding
applications where study of large data sets is required. For more information
about software and programming options for our Spectrum high-speed instrumentation
please click here
For more details and advice on putting together a ultra high speed system,
DataQuest Solutions are happy to discuss your applications. Please also
visit our Web site with its
links to datasheets and operation notes for an extensive range of PCI/PCI-X and
PCI-Express cards, plus the PXI and compact PCI chassis mounted versions, we
provide resolutions 8 to 16 bit, with sampling rates from 1000 to 1 Giga samples/second.
Click here for more information about PCI-X.
Click here for more information about PCI-Express.
© Dataquest Solutions 30.01.08. Updated 16.03.10