The demands of recording a fast series of waveformsOne of the most demanding applications for a measurement system is the requirement to record a high-speed series of waveforms, such as RADAR return signals. In most cases, the Pulse Repetition Frequency (PRF) of the incoming waveforms are so fast that it is not possible to store the digitized waveform and re-arm the system trigger with a software command before the next waveform appears.
Digitizer manufacturers solve this problem using a technique called Multiple Recording (also called Memory Segmentation). In this case, the onboard Digitizer memory is configured with as many segments as waveforms to be recorded and a special hardware re-arming feature is added to the Digitizer card. This allows the board to automatically record, store and re-arm the trigger totally under firmware control and as a consequence very fast PRF’s can be recorded without data loss.
There are a number of factors to consider when choosing a suitable Digitizer board:
1) The Multiple Recording trigger re-arming time will dictate the maximum signal PRF that can be recorded. In the case of the UltraFast UF2 series of PCI bus Digitizer cards this specification is less than 4 clock cycles (note 1). This combined with a minimum segment size of 8 samples (note 1) leads to a maximum PRF of 16.6 MHz when sampling at 200 MS/s. Larger segments and/or slower sample rates clearly reduce the PRF, as does the addition of a pre-trigger.
2) Older Memory Segmentation and Multiple Recording designs could not be used in conjunction with a pre-trigger, so that the first few samples of each waveform were lost. The UF2 cards allow a pre-trigger of between 4 samples and 16kSamples to be selected.
3) Calculate the required onboard memory = number of waveforms * samples/waveform * number of active channels. Remember that 8-bit A/D’s only occupy 1 Byte/sample, while 12/14/16-bit A/D’s will occupy 2 Bytes/sample.
4) Check if the Digitizer card is also able to stream the recorded data to the host PC. Off-loading the data while the measurement is running can reduce the amount of onboard memory required and therefore save you from buying additional Digitizer memory. If the card supports streaming, you will need to calculate the Digitized Data Rate (DDR) of the incoming signals. This figure has to be lower that the PCI bus continuous transfer rate, otherwise data loss will occur.
To calculate the Digitized Data Rate = number of channels * samples/waveform * PRF (trigger frequency)
i.e. = 1 channel * 2048 samples * 20 kHz = 4.096 MSamples/sec
This corresponds to:
= 4.096 MBytes/second for 8-bit A/D resolution
= 8.192 MBytes/second for 12, 14 or 16-bit A/D resolution
As both DDR values are lower than the maximum PCI bus continuous transfer rates (approx. 100 MBytes/s for 33MHz/32-bit PCI and 225 MBytes/s for 66 MHz/32-bit PCI), then it is possible to continuously transfer the digitized data to the host PC for storage (note 3).
The maximum PCI rates do depend on a number of factors: PCI bus design, no other activity on the bus, use of c/c++ or Delphi program and storage to PC RAM. Using programs such as MATLAB, VEE, DASYLab, etc that are not optimized for data transfer will significantly reduce the host-PC’s ability to receive the incoming data fast enough (note 2). It is also possible to stream the data to hard disk. The maximum write rate of a configuration using a RAID controller and four S-ATA disks has been measured at 105 MBytes/s.
Note that the DDR is independent of the Digitizers sampling rate. To illustrate this, consider that the DDR of a 12-bit resolution card operating with one active channel at 100 MSamples/s generates 200 MBytes of data per second. In this example, the 33 MHz/32-bit PCI bus would be too slow for streaming, while the newer 66 MHz/32-bit PCI bus used on the UF2 series of Digitizer cards could be used.
A firmware option called Multiple Replay is available for the UltraFast Arbitrary Waveform Generator cards. Furthermore, the UltraFast Digitizer and AWG cards can be synchronized by a common sampling clock to create a complete test system.
1. These figures are valid for most of the UF2 digitizer cards. However, some cards use two ADC’s on a single channel in a ping-pong manner in order to achieve the fastest sample rates, interleaving the samples of each ADC. The re-arm time when the cards are used in the interleave mode is then 8 samples and the minimum segment size is 16 samples. The cards that use interleaved ADC’s at the fastest sample rates include:
| UF2-2030, UF2-2031 : | at rates above 100 MS/s to 200 MS/s |
| UF2-3015, UF2-3016 : | at rates above 80 MS/s to 160 MS/s |
| UF2-3022 : | at rates above 50 MS/S to 100 MS/s |
| UF2-3025, UF2-3026 : | at rates above 100 MS/s to 200 MS/s |
The re-arm time and minimum segment size is also different for the UX series of 3U PXI/CompactPCI cards and UC series of 6U CompactPCI cards. Please refer to the respective datasheets on this website or contact us for more information.
2. The latest version of the LabVIEW driver that we offer as a cost-option is now able to transfer data at up to 200 MBytes/s. No sorting, calculations or display are possible to achieve this speed.
3. The UF2 series cards use hardware controlled Scatter-Gather mode: data is transferred to the PC RAM with no need for software intervention. As long as there is room in the programmed buffer, data is transferred even if the PC is busy with other tasks.
