RF Vector Signal Analyzers: Say “No” to Poor Performance in Microwave Measurements

RF signal analyzers with frequencies up to 14 GHz offer industry-leading performance, but they are not only in short supply, but also bulky, expensive, and slow. The NI PXIe-5665 high-performance RF not only offers a frequency range from 20 Hz to 14 GHz, but also has the advantages of high performance and speed. The instrument has an average noise floor of -165 dBm/Hz, a phase noise of 129 dBc/Hz at an 800 MHz tone (10 kHz offset), and 0.1 dB absolute amplitude accuracy, making it one of the highest performing analyzers in the industry. The NI PXIe-5665 is ideal for semiconductor and mobile phone test applications, not only for lower frequency modulation measurements, but also for testing up to 14 GHz third and fourth harmonics. In addition, the NI PXIe-5665 can test C, X, and Ku bands, which can be used in radar, satellite, and radio communications.

Figure 1. The NI PXIe-5665 is a modular RF vector signal analyzer that includes a digitizer, downconverter, and synthesizer.

Low-band and high-band architectures

The NI PXIe-5665 is a modular RF vector signal analyzer that includes the NI PXIe-5622 digitizer, the NI PXIe-5605 wideband RF downconverter module, and the NI PXIe-5653 synthesizer. The NI PXIe-5665 can operate in two independent frequency bands that can differ by up to 14 GHz. Frequencies below 3.6 GHz are considered low-band, and frequencies between 3.6 GHz and 14 GHz are considered high-band. The dual-band architecture of the NI PXIe 5665 provides greater image interference and dynamic range within the total frequency range of the instrument.

The low-band features a three-stage superheterodyne downconversion architecture. The NI PXIe-5605 upconverts the RF signal to a higher intermediate frequency (IF) and then downconverts it to a frequency that can be digitized. The three-stage topology provides a clear display of image interference from the RF input signal. In the high-band signal path, the NI PXIe-5605 is a two-stage downconverter. The only attenuation in this band comes from the mechanical step attenuator, which provides 75 dB of attenuation variation in 5 dB steps. An optional preselector (YIG-tuned filter) is used to filter out unwanted images and is placed before the first mixer in the signal chain. In the IF stage, we can adjust the IF gain in 1 dB steps to optimize the power level of the digitizer. The ability to apply larger IF gain to smaller tones is effective for measuring multitone signals, preventing the larger tones from dominating the resolution of the DAC. The last IF stage features a switched filter bank that can further optimize the third-order intercept point (IP3) for higher-level measurements such as adjacent channel power ratio.

Low phase noise for measurement accuracy

The NI PXIe-5665 has an ultra-low phase noise of 129 dBc/Hz at 10 kHz offset with an 800 MHz tone. The NI PXIe-5653 synthesizer has three low phase noise local oscillators (LOs) to achieve ultra-low phase noise. Phase noise is one of the most important specifications for RF signal analyzers. For example, the NI PXIe-5665 can measure an error vector magnitude as low as 0.33% on a 256-QAM modulated signal. With the preamplifier on, the NI PXIe-5665 has an average phase noise of -165 dBm/Hz. The low noise floor allows for the measurement of weak signals that are difficult to measure with other signal analyzers. The combination of high frequency range and low noise floor makes the NI PXIe-5665 an ideal choice for glitch and harmonic testing. 50 MHz real-time bandwidth measurements can be used on the module as a measurement standard, such as the 20 MHz version in 3GPP LTE.

Figure 2. Even in the 12 to 14 GHz range, the NI PXIe-5665 has an average noise floor of -142 dBm/Hz and amplitude accuracy of ±0.25 dB, making it ideal for testing and detecting small, high-frequency signals.

Calibration Tone and Use of YIG-Tuned Filters

The NI PXIe-5605 features an onboard, high-precision calibration tone. The calibration tone accurately corrects for receiver loss by comparing the tone value stored in the device's EEPROM with the most recent measurement. The NI PXIe-5605 also provides a bandpass preselector (YIG-tuned filter) that selectively includes the RF input signal path when downconverting signals greater than 3.6 GHz.

When making general measurements, a preselector can suppress signals that could interfere with or reduce the accuracy of the measurement. The magnetic field generated by the electromagnet controls the center frequency of the preselector. To account for nonlinear and thermal characteristics that may cause the center frequency of the preselector to change, the NI-RFSA driver uses a preselector tuning curve as part of the device's self-calibration process and uses internally generated calibration signals and algorithms. Using the NI PXIe-5665 and preselector in spectrum monitoring and electromagnetic compatibility applications can screen out unwanted images.

Harmonic testing using RF meter mode

Harmonic and spur test measurements are often used to characterize power amplifiers and other radio frequency integrated circuits (RFICs). The NI PXIe-5665 can be used well for these test measurements . Even in the 12 to 14 GHz range, the NI PXIe-5665 has an average noise floor of -142 dBm/Hz and an amplitude accuracy of ±0.25 dB, making the test and measurement results of high-frequency small signals more ideal.

Using RF Table Mode on the NI PXIe-5665 allows you to quickly switch between frequencies, significantly reducing sweep time. RF Table Mode allows you to predefine a list of frequencies or reference levels, among other parameters, and translate them into microcode, eliminating the need for time-intensive software calls from the instrument to the PC and vice versa.

Spectrum Monitoring with NI FlexRIO

The NI PXIe-5665 analyzer connects to the NI FlexRIO module and plugs into the PXI chassis. Data can be written to the field programmable gate array (FPGA) on the NI FlexRIO module in real time on the PXI backplane through peer-to-peer streaming. The onboard FPGA on the NI FlexRIO module can also be programmed using the NI LabVIEW FPGA Module, which has nanosecond decision-making capabilities in data processing. The NI PXIe-5665 has real-time processing capabilities and can be used as a real-time spectrum analyzer. The NI PXIe-5665 streams its acquired data to the NI FlexRIO for real-time processing across the entire bandwidth. In addition, triggers can be sent from the real-time processing-based NI FlexRIO module to other instruments in the PXI chassis. The combination of instruments and NI FlexRIO can build a system to solve highly complex test problems that traditional boxed instruments cannot solve.