Calculating Source and Capture Bandwidth for Digital Pattern Instruments

Overview

This whitepaper discusses how to calculate the bandwidth requirement per Digital Pattern Instrument when sourcing or capturing a waveform. Depending on the configured source or capture waveform the bandwidth requirement will vary. Therefore, it is important to calculate the required bandwidth for each source or capture waveform to make sure it does not exceed the bandwidth limitations of the Digital Pattern Instrument, the PXIe chassis, and the controller. For more information on source and capture waveforms please see the “Source and Capture” section in the Digital Pattern Help.

Calculating Source Bandwidth

The below equation is used to calculate the bandwidth requirements per Digital Pattern Instrument based on the vector rate and the effective transfer size of the source waveform. The calculated bandwidth requirement can then be checked against the Source Waveform Bandwidth Limitation to confirm that the Digital Pattern Instrument can properly generate the configured source waveform at the specified vector rate.

 

Source Waveform Bandwidth Limitation:

The PXIe-6570 can maintain a source bandwidth up to 50 MB/s for continuous sourcing.

 

Calculating Required Source Bandwidth:

Required Source Bandwidth per Digital Pattern Instrument = Vector Rate × Effective Transfer Size

 

Equation Terms:

Vector Rate (MHz)

  • Inverse of the period at which vectors with the "source" opcode execute.

Effective Transfer Size (bytes)

  ≤ NI-Digital 16.0.1 ≥ NI-Digital 17.0
Parallel Waveform # of Sites × # of Source Pins per Site

Site Unique: # of Source Pins per Digital Pattern Instrument

Broadcast: # of Source Pins per Site

Serial Waveform (# of Sites × tobyte(Sample Width))/(Sample Width) (# of Sites × tobyte(Sample Width))/(Sample Width)

# of Sites

  • When specifying “Broadcast” for the Data Mapping setting in the Source Waveform Configuration, use 1 for this term. Otherwise, this term equals the number of sites the source waveform is used for.  

# of Source Pins per Site (bytes)

  • Number of source pins per site rounded up to 1, 2, or 4 bytes. Each pin is considered one bit. For example, 4 source pins per site would be considered 4 bits which rounds up to 1 byte.

# of Source Pins per Digital Pattern Instrument (bytes)

  • Number of source pins per Digital Pattern Instrument rounded up to 1, 2, or 4 bytes. Each pin is considered one bit. For example, 4 source pins per Digital Pattern Instrument would be considered 4 bits which rounds up to 1 byte.

tobyte(Sample Width) (bytes)

  • Sample width specified in the Source Waveform Configuration rounded up to 1, 2, or 4 bytes.

Sample Width

  • Sample width specified in the Source Waveform Configuration.

 

 

Calculating Capture Bandwidth

The below equation is used to calculate the bandwidth requirements per Digital Pattern Instrument based on the vector rate and the effective transfer size of the capture waveform. The calculated bandwidth requirement can then be check against the Capture Waveform Bandwidth Limitations to confirm that the Digital Pattern Instrument can properly acquire the configured capture waveform at the specified vector rate.

 

Capture Waveform Bandwidth Limitations:

The PXIe-6570 has a PCIe Gen2 x4 connection which has a theoretical bandwidth limitation of 2000 MB/s. However, around 1600 MB/s can practically be achieved. Additionally, the bandwidth limitations of the PXIe chassis and controller must be considered. See the specifications for the specific chassis and controller for more information.  

 

Calculating Required Capture Bandwidth:

Required Capture Bandwidth per Digital Pattern Instrument = Vector Rate × Effective Transfer Size

 

Equation Terms:

Vector Rate (MHz)

  • Inverse of the period at which vectors with the "capture" opcode execute.

Effective Transfer Size (bytes)

  ≤ NI-Digital 16.0.1 ≥ NI-Digital 17.0
Parallel Waveform 4 × # of Sites ∑ (# of Capture Pins per Site)
Serial Waveform (4 × # of Sites)/(Sample Width) (∑ (# of Capture Pins per Site))/(Sample Width)

# of Sites

  • Number of sites the capture waveform is used for.

# of Capture Pins per Site (bytes)

  • Number of capture pins per site rounded up to 1, 2, or 4 bytes. Each pin is considered one bit. For example, 4 capture pins per site would be considered 4 bits which rounds up to 1 byte.

Sample Width

  • Sample width specified in the Capture Waveform Configuration.