The PXIe-5840 has two configurable RF paths comprised of an independent vector signal analyzer (RF input) path and vector signal generator (RF output) path. The following high-level block diagram represents the PXIe-5840 RF input path.



The PXIe-5840 RF input path is AC coupled and covers a frequency range of 9 kHz to 6,000 MHz center frequency. For frequencies below 120 MHz (low frequency), RF input signals are directly sampled via analog-to-digital conversion. Frequencies greater than 120 MHz (high frequency) are translated to baseband via a single stage direct conversion (I/Q) downconverter, allowing for wide instantaneous bandwidth.

High Frequency RF Input Path

The high frequency subsystem of the RF input path consists of a programmable and filtered front end that feeds into a direct conversion downconverter. Once at baseband frequencies, the signal is sampled with an analog-to-digital converter (ADC) capable of up to 1,000 MHz of complex equalized bandwidth.

RF attenuation greater than 70 dB can be nominally configured through the combination of switchable attenuation and programmable step attenuators. RF attenuation is automatically chosen in conjunction with amplification stages to provide optimal reference levels over the support range of the device.

A switchable pre-amplifier is also configurable in the front end of the device to further optimize the signal path for noise figure. This pre-amplifier can be configured for bypass, forced inline, or automatically configured based on application needs. The configurable range of reference levels may vary based on which option is selected for the pre-amplifier.

Low Frequency RF Input Path

The low frequency subsystem of the RF input path is used to directly sample RF input signals below 120 MHz. The RF input signal is provided via a frequency selective diplexer to programmable gain components before being filtered and sampled with an analog-to-digital converter.

The programmable gain elements at low frequency consist of a switchable amplifier and a programmable attenuator. When disabled, the switched amplifier is bypassed by a 10 dB attenuator. The programmable attenuator is capable of 31 dB attenuation in 1 dB steps.

Refer to the RF Path Transitions topic for more information on using the low frequency RF input path.

Cal In Path

The RF input subsystem has internal calibration capability in both the high frequency and low frequency paths. Cal In is used in conjunction with Cal Out from the RF Output subsystem to facilitate steps within the following VIs or functions:
  • NI-RFSA Self-Calibrate VI or niRFSA_SelfCalibrate
  • NI-RFSG Self-Calibrate VI or niRFSG_SelfCal
  • NI-RFSA Self-Calibrate Range VI or niRFSA_SelfCalibrateRange
  • NI-RFSG Self-Calibrate Range VI or niRFSG_SelfCalibrateRange

RF Input Signal Path

The RF source signal takes the following path from the PXIe-5840 front panel to the PXI Express controller.

  1. The RF signal is connected to the RF IN front panel connector.
  2. The RF signal is passed through the frequency‑selective path of the front end diplexer to either the high frequency or low frequency path.
  3. The signal is amplified or attenuated based on the reference level to the required mixer level, based on available gain in the selected RF path. The amplification and attenuation maintain optimized dynamic range and avoid signal saturation or ADC overflow.
  4. The RF signal is frequency‑translated based on the signal path used:
    • When in the high frequency path, the attenuated or amplified signal is mixed with a 0° phase and 90° phase LO signal at the tuned center frequency, translating the resulting signal into a pair of I and Q signals 90° out-of-phase from each other. These I and Q signals are then conditioned and sampled by an ADC converter.
    • When in the low frequency path, the attenuated or amplified signal is directly passed to signal conditioning and the ADC without frequency translation.
  5. The translated signal is sampled by the ADC at 1,250 MS/s. The resulting signals are accessible through LabVIEW FPGA I/O nodes for further processing or transfer to the host computer.

RF IN Signal Gain and Attenuation

The RF IN signal path chooses gain and attenuation settings based on the requested reference level.

The gain definition measured in calibration is adjusted and applied according to the following equation:

RF Input Gain (dB) = Complex PowerADC(dB) - Real PowerRF IN Terminal(dB)

The resulting gain scales the acquired data to reference back to the RF IN front panel connector. The reported gain scales both I and Q evenly. If you compute power by reading I/Q data and running a power spectrum or Fast Fourier Transform (FFT), scale the gain by a gain compensation value of 0.5 or reduce the power result by 3 dB. If you use Spectrum Mode, NI-RFSA scales the power for you.