PXIe-5668 Theory of Operation

NI RF Vector Signal Analyzers (NI-RFSA 18.1) Help

Edition Date: June 2018

Part Number: 372058U-01

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The PXIe-5668 vector signal analyzer comprises the PXIe-5624 digitizer, PXIe-5606 RF signal downconverter, and the PXIe-5653 synthesizer/LO source. The PXIe-5668 operates in the following distinct frequency bands up to 26.5 GHz:

  • Low Band—RF Frequencies ≤ 3.6 GHz
  • High Band—RF Frequencies > 3.6 GHz and ≤ 26.5 GHz

The following block diagram represents the PXIe-5668.


Low Band Operation (Center Frequency < 3.6 GHz)

When the PXIe-5668 center frequency is less than 3.6 GHz, the PXIe-5606 upconverts the RF signal to a higher intermediate frequency. If the IF filter bandwidth is 100 MHz or less, then the signal passes through two more downconversion stages before being digitized. For IF filter bandwidths greater than 100 MHz, the signal passes through only one more downconversion stage.

The PXIe-5606 RF front end includes a mechanical step attenuator, which is variable up to 75 dB in 5 dB steps. When the PXIe-5606 is operating in the low band signal path, the solid-state attenuator provides an additional 31 dB of attenuation, variable in 1 dB steps. A preamplifier (if desired) can be enabled to reduce the system noise floor. The RF input is AC-coupled by default, but you can also configure the input to be DC-coupled using NI-RFSA.

Notice Notice  The PXIe-5606 has no internal DC block, which allows the PXIe-5606 to make measurements at frequencies as low as 20 Hz. High-frequency components in the PXIe-5606 can be damaged when DC signals are applied directly to the RF IN connector of the PXIe-5606. NI is not responsible for damage resulting from improper signal connections.

The PXIe-5606 ships with a 2.92 mm DC block attached to the RF IN connector to prevent damage to the device when a DC input signal is present. You must remove the DC block to make measurements using frequencies less than 16 kHz because the series capacitive reactance of the device increases as the RF input frequency drops from 16 kHz to 20 Hz. NI recommends that you keep the DC block attached to the RF IN connector for all measurements at frequencies greater than or equal to 16 kHz to maximize the accuracy of the device.
Notice Notice   To reinstall the DC block on the PXIe-5606 RF IN connector, use a torque wrench set to a maximum torque of 1.65 N · m (15 lb · in.). Using more than the recommended amount of torque may damage the RF IN connector. NI is not responsible for damage resulting from improper signal connections.
Notice Notice  If the RF attenuation is set to 0 dB with the DC block attached, an input DC voltage greater than ±2 V may damage the internal components of the downconverter. To prevent damage to the downconverter, ensure that the DC voltage at the RF IN connector is between –2 VDC and +2 VDC. With the DC block removed, the maximum allowed voltage is 0 VDC.

High Band Operation (Center Frequency > 3.6 GHz)

In the high band signal path, the PXIe-5606 acts as a two-stage downconverter for IF filter bandwidths less than or equal to 100 MHz. For larger IF filter bandwidths, only a single stage downconversion is required. The only available attenuation in this frequency band is the mechanical step attenuator, which provides 75 dB of attenuation, variable in 5 dB steps. An optional preselector assists in filtering unwanted frequency components from the RF input before the signal reaches the first mixer.

Note Note  For frequencies greater than 7.55 GHz, the PXIe-5668 doubles the LO1 signal to facilitate the high-frequency signal downconversion using the available LO1 output signal of the PXIe-5653. For frequencies greater than 15.85 GHz, the PXIe-5668 quadruples the LO1 signal to facilitate the high frequency downconversion using the available LO1 output signal of the PXIe-5653.

IF Operation

You can adjust the IF gain of the system so that the IF signal appears at or near the full-scale power of the digitizer by adjusting the reference level. The final IF stage has a switched filter bank that allows you to further optimize the system third-order intercept point (IP3) to make advanced measurements, such as adjacent channel power ratio (ACPR). The switched filter bank provides a 300 kHz filter, a 5 MHz filter, a 100 MHz filter, and a 320 MHz filter. For frequencies above 3.6 GHz, the PXIe-5668 provides a 765 MHz wide filter. For more information about PXIe-5668 IF filter paths, refer to PXIe-5606 Downconverter IF Filter.

LO Operation

The PXIe-5653 supplies a low phase noise LO, which is shown at the bottom of each of the PXIe-5668 block diagrams. You can use the PXIe-5606 LO outputs to daisy chain multiple downconverters with a single LO source. Using the same LO source is useful for phase-coherent signal acquisition applications, such as multiple input, multiple output (MIMO) systems. When using this configuration, every PXIe-5606 is tuned to the same RF frequency.

The IF signal leaves the RF downconverter through the IF OUT connector and enters the IF IN connector on the PXIe-5624 IF digitizer, which is shown at the right of each PXIe-5668 block diagram. The PXIe-5624 digitizes the IF signal and digitally downconverts it to baseband, resulting in complex I/Q data stored in the digitizer onboard memory. The I/Q data is then transferred to the host PC for processing.


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