NI 5670 IQ Rates

In Arb waveform and Script modes, the NI 5670 supports two IQ rates:

• 100 megasamples per second (MS/s)— default. This setting provides the best possible antialiasing performance, up to 20 MHz of real-time bandwidth, and the best time-domain resolution of the signal. This setting is recommended for most applications.
• 50 MS/s — This setting has several specific situations in which it is useful. These situations are discussed in more detail in the following section.

CW generation mode ignores the IQ Rate (S/s) property or the NIRFSG_ATTR_IQ_RATE attribute setting and always uses an IQ rate of 100 MS/s.

When to Use a 50 MS/s Rate

Two main cases exist in which the IQ Rate property or the NIRFSG_ATTR_IQ_RATE attribute setting of 50 MS/s may be useful:

1. Very large waveforms — setting the IQ rate to 50 MS/s halves the memory required to store the same time duration of a given waveform (compared to an IQ rate of 100 MS/s). For example, using an AWG module equipped with the 256 MB memory option, 1.28 s of data sampled at 100 MS/s (equivalent to 2²⁷ samples) can be stored in the AWG module onboard memory. By contrast, approximately 2.56 s of data sampled at 50 MS/s (equivalent to 2²⁸ samples) can be stored in the same amount of memory.
   
   

   Note  Not all available onboard memory can be used for waveform storage. The onboard memory is shared between waveforms and instructions that indicate how the waveform is to be generated.

2. Speed optimization — setting the IQ rate to 50 MS/s can improve speed performance if you need to resample the signal. The resampling process can consume significant CPU time, and larger resampling ratios (final IQ rate / initial IQ rate) require more resampling time. Therefore, resampling to 50 MS/s is less time-consuming than resampling to 100 MS/s.

Implications of Using a 50 MS/s Rate

Setting the IQ Rate property or the NIRFSG_ATTR_IQ_RATE attribute to 50 MS/s has the following implications:

• The NI-RFSG driver must place the IF carrier frequency at 18 MHz ± 1 MHz to avoid aliasing. Forcing the IF carrier frequency to that point implies that the NI-RFSG driver does not have the freedom to pick an IF carrier frequency that could optimize the waveform size if phase continuity is enabled.
• Output signal bandwidth must be < 5 MHz to avoid aliasing.
• Close-in phase noise is higher.