One of the most demanding applications for an Arbitrary Waveform Generator (AWG) system is the requirement to generate a high-speed series of waveforms, such as RADAR signals. In most cases, the Pulse Repetition Frequency (PRF) of the outgoing waveforms needs to be so fast that it is not possible to load the next waveform data from the host PC and re-arm the system trigger with a software command before the next waveform has to be generated.
AWG manufacturers solve this problem using a technique called Multiple Replay (also called Memory Segmentation). In this case, the onboard AWG memory is configured with as many segments as waveforms to be output and a special hardware re-arming feature is added to the AWG card. This allows the board to automatically output on trigger, point to the next waveform and re-arm the trigger totally under firmware control. As a consequence very fast PRF's can be generated.
There are a number of factors to consider when choosing a suitable AWG:
1) The Multiple Replay 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 AWG 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 10.416 MHz when sampling at 125 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 AWG card is also able to stream the waveform data from the host PC. Transferring data to the AWG while the generator is running can reduce the amount of onboard memory required and therefore save you from buying additional AWG 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 Required 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 DAC resolution
= 8.192 MBytes/second for 12, 14 or 16-bit DAC resolution
As both RDR 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 waveform data to the AWG while it is running (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 output the incoming data fast enough (note 2). It is also possible to stream the data from hard disk. The maximum read rate of a configuration using a RAID controller and four S-ATA disks has been measured at 90 to 100 MBytes/s.
Note that the RDR is independent of the AWG's sampling rate. To illustrate this, consider that the RDR of a 14-bit resolution card operating with one active channel at 100 MSamples/s requires 200 MBytes of data from the host PC 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 AWG cards could be used.
A firmware option called Multiple Recording is available for the UltraFast Digitizer 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 all of the UF2 AWG cards.
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 from 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.