Acquisition:Spectrum:FFT Width Property
- Updated2024-05-08
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Acquisition:Spectrum:FFT Width Property
Short Name: FFT Width
Property of niRFSA
Specifies the FFT width of the device. The FFT width is the effective bandwidth of the signal path during each signal acquisition.
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Note The maximum FFT width when using the PXIe-5622 is constrained to 50 MHz or 25 MHz, depending on the digitizer option you purchased. The maximum FFT width when using the PXIe-5624 is constrained to 400 MHz or 765 MHz, depending on the digitizer configuration. |
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Note You can use the FFT Width property with in-band retuning. For more information about in-band retuning, refer to the Downconverter Center Frequency property. |
NI-RFSA treats the device instantaneous bandwidth as the effective real-time bandwidth of the signal path. The span specifies the frequency range of the computed spectrum. A vector signal analyzer can acquire a bandwidth only within the device instantaneous bandwidth frequency. If the span you choose is greater than the device instantaneous bandwidth, NI-RFSA obtains multiple acquisitions and combines them into a single spectrum. By specifying the FFT width, you can control the specific bandwidth obtained in each signal acquisition. If you read the FFT Width property without setting it, NI-RFSA returns the value of the Device Instantaneous Bandwidth property.
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Note When the Smooth Spectrum Enabled property is set to enabled, this property operates as a maximum FFT width instead of a strict maximum and minimum value. |
Valid Values:
The lower limit for all FFT width supported devices using the PXIe-5622 digitizer is 7.325 kHz. The lower limit for all FFT width supported devices using the PXIe-5624 digitizer is 400 MHz or 800 MHz, depending on the FPGA image that is downloaded upon opening the session to the PXIe-5624 digitizer.
PXIe-5663/5663E—The FFT width upper limit for the PXIe-5663/5663E depends on the RF frequency and on the module revision of the PXIe-5601 as illustrated in the following table. Refer to the Identifying Module Revision topic for more information about determining which revision of the PXIe-5601 RF downconverter you have installed.
| RF Frequency | PXIe-5601 Instantaneous Bandwidth | FFT Width Upper Limit |
|---|---|---|
| 10 MHz to <120 MHz | 10 MHz | 10 MHz (Revision E), 20 MHz* (Revision G or later) |
| 120 MHz to <330 MHz | 20 MHz | 20 MHz (Revision E), 30 MHz* (Revision G or later) |
| 330 MHz to <6.6 GHz | 50 MHz | 50 MHz |
| *National Instruments does not guarantee device specifications if you set the FFT Width property greater than the warranted instantaneous bandwidth specification. | ||
PXIe-5665/5667/5668—The upper limit of the FFT width is the maximum device instantaneous bandwidth.
If you do not set the Device Instantaneous Bandwidth property for the PXIe-5665/5667/5668, NI-RFSA determines the appropriate IF filter to use based on the value that you set for the FFT width and your specific device. Refer to the following tables to determine which IF filter corresponds to each FFT width frequency range for your device.
If you set the Device Instantaneous Bandwidth property, NI-RFSA selects the appropriate IF filter based on the Device Instantaneous Bandwidth property and the FFT Width property determines how much filter bandwidth is used during acquisition. Refer to the Device Instantaneous Bandwidth property to determine which IF filter corresponds to the Device Instantaneous Bandwidth property setting.
| Device | FFT Width Property Value Frequency Range | IF Filter |
|---|---|---|
| PXIe-5603/5665 (3.6 GHz) | ≤300 kHz | 300 kHz IF filter |
| >300 kHz and ≤5 MHz | Through IF filter | |
| >5 MHz | Through IF filter | |
| PXIe-5605/5665 (14 GHz) | ≤300 kHz | 300 kHz IF filter |
| >300 kHz and ≤5 MHz | 5 MHz IF filter | |
| >5 MHz | Through IF filter | |
| PXIe-5667 | ≤30 kHz (IF Conditioning Downconversion Enabled property set to Enabled) | 30 kHz |
| >30 kHz and ≤400 kHz | 400 kHz | |
| ≤400 kHz (IF Conditioning Downconversion Enabled property set to Disabled) | 400 kHz | |
| >400 kHz and ≤1.4 MHz | 1.4 MHz | |
| >1.4 MHz and ≤5 MHz | 5 MHz | |
| >5 MHz and ≤20 MHz | 20 MHz | |
| >20 MHz and 50 MHz (Signal Conditioning Enabled property set to Bypassed) | Bypass | |
| PXIe-5668 | ≤300 KHz | 300 KHz |
| >300 KHz and ≤5 MHz | 5 MHz | |
| >5 MHz and ≤100 MHz | 100 MHz | |
| >100 MHz and ≤320 MHz | 320 MHz | |
| >320 MHz and ≤400 MHz | (high band) 765 MHz | |
| (low band) 320 MHz |
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Note At frequencies greater than 3.6 GHz, the PXIe-5605 provides a typical bandwidth of 47 MHz at –6 dB with the downconverter preselector enabled. The FFT Width property can override the typical bandwidth of the PXIe-5605 up to 57 MHz using an external digitizer and up to 50 MHz or 25 MHz depending on the PXIe-5622 digitizer option you purchased. The increase in bandwidth results in faster signal acquisitions, but amplitude accuracy is decreased for spectrum acquisitions and magnitude and phase accuracy is decreased for I/Q acquisitions. National Instruments does not guarantee device specifications if you set the FFT Width property greater than the warranted instantaneous bandwidth specification. |
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Note When using the PXIe-5606, the 765 MHz IF filter is only available at center frequencies of 3.6 GHz and above. |
Default Value: N/A
Supported Devices: PXIe-5663/5663E/5665/5667/5668
Remarks
The following table lists the characteristics of this property.
| Datatype |  |
| Permissions | Read/Write |
| High-level VIs | N/A |
| Channel-based | No |
| Resettable | Yes |