RF Output Subsystem
- Updated2024-09-19
- 4 minute(s) read
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 output path.
The PXIe-5840 RF output 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 output signals are directly generated via digital-to-analog (DAC) conversion. Frequencies greater than 120 MHz (high frequency) are translated from baseband I and Q signals to the specified RF frequency via a single stage direct conversion (I/Q) upconverter, allowing for a wide instantaneous bandwidth.
High Frequency RF Output Path
The high frequency subsystem of the RF output path consists of a wideband direct conversion upconverter and a front end signal path with programmable RF filtering and gain stages. I and Q baseband signals generated by the DAC are mixed and combined to create an RF signal with up to 1,000 MHz of complex equalized bandwidth centered at the specified RF center frequency. This signal can then be conditioned for both bandwidth and level.
Greater than 80 dB of RF attenuation can be configured nominally through the combination of switchable fixed attenuation and programmable step attenuators. RF attenuation is automatically chosen in conjunction with amplification to provide optimal reference levels over the support range of the device. There are two switchable amplifiers and one switchable high power amplifier.
The switchable high power amplifier is used for generating signals significantly larger than 0 dBm. This amplification stage is automatically selected based on output power level setting and extends the available peak power range to +20 dBm, calibrated. Additional power may be achieved above +20 dBm, however it is not guaranteed to meet absolute or relative accuracy specifications.
Low Frequency RF Output Path
The low frequency subsystem of the RF output path is used to directly generate the requested RF output signals below 120 MHz from the DAC. The RF signal is generated from the DAC through programmable gain stages and transmitted from the RF OUT front panel connector through a frequency selective diplexer. This allows for a seamless transition from low frequency to high frequency paths. Refer to the RF Path Transitions topic for more information about using the low frequency path.
The low frequency path programmable gain stages consist of a 31 dB step attenuator with 1 dB resolution and a switchable amplification stage. The off state of the switchable amplifier has additional fixed attenuation integrated into it. This provides a nominal gain range of +5 dB to -40 dB. Additional attenuation is possible with the DAC full scale configuration settings as well as digital waveform scaling.
RF Output Signal Path
The RF output signal takes the following path from the PXIe-5840 FPGA to the RF OUT front panel connector.
- The I and Q digital signals are sent to the baseband digital-to-analog (DAC) converter through a LabVIEW FPGA I/O node after being written from the host.
- The DAC converts the digital I and Q waveforms into analog I and Q signals at 1,250 MS/s. Internal to the DAC, the output is two times interpolated to 2,500 MS/s.
- Based on the specified center frequency, the analog I and Q signals are conditioned and passed either to the direct conversion modulator high frequency path, or sent directly to the low frequency path for additional level control.
- Depending on the selection of high frequency path or low frequency path, the converted RF frequency is further attenuated or amplified based on the specified output power settings.
- The conditioned RF output signal is transmitted from the PXIe-5840RF OUT front panel connector through a frequency selective diplexer stage.
Using the PXIe-5840 in CW Mode
You can use the PXIe-5840 as a continuous waveform (CW) generator when programming the PXIe-5840 using NI-RFSG. To generate a CW signal, use NI-RFSGConfigure Generation Mode.
Average Power and Crest Factor Considerations
Crest factor is the difference between peak signal power and average power.
The crest factor for a sinusoidal signal, as is used in CW mode, is 3 dB. In other words, the average RMS power of the sinusoid is 3 dB less than its peak instantaneous power. For modulated signals, specifically orthogonal frequency-division multiplexing (OFDM), the crest factor can be much larger, in the order of 10 dB to 12 dB.
Consider both the average power and the crest factor of a signal when you configure the device for generation. The PXIe-5840 supports a maximum specified peak output power. Generating signals beyond specification are not guaranteed to be calibrated or linear. If the peak power is set higher than specified as a result of signal crest factor added with requested average power, severe saturation might occur or the reverse power protection circuitry of the PXIe-5840 may be enabled. Refer to the PXIe-5840 Specifications for more information about output power range specifications.
Related Information
- Quadrature Mixing (Direct Conversion)
Quadrature mixing (direct conversion) allows for the complex modulation or demodulation of signals as represented by two orthogonal signals, I and Q. Quadrature mixing has the added benefit of an increased complex bandwidth that allows for bandwidths up to the sample rate of the DAC.
- RF Path Transitions
The PXIe-5840 supports acquisition and generation of RF signals from to . For both the RF Input and RF Output channels, the transition from low frequency to high frequency is implemented with a passive diplexer scheme. This passes RF energy below 120 MHz to the low frequency path and RF energy above 120 MHz to the high frequency path. Because the diplexer has a frequency response of its own, it is possible to extend RF frequency coverage above or below 120 MHz in both paths.