Figure 1. NI PXIe-5665 Vector Signal Analyzer
1. High-Level Architecture
When building a vector signal analyzer (VSA), a superheterodyne architecture is often the optimal design for image rejection and dynamic range. The NI PXIe-5665 is based on a superheterodyne RF analyzer architecture combined with a software-defined modular implementation. Figure 2 displays the three components of this modular architecture: the NI PXIe-5603/05 downconverters, the NI PXIe-5622 IF digitizer, and the NI PXIe-5653 local oscillator (LO) synthesizer.
Figure 2. The NI PXIe-5665 RF signal analyzer comprises three components.
2. Modular Architecture
The multistage topology of the NI PXIe-5665 provides image rejection of the RF signal with no ambiguity. Image rejection is achieved with a lowpass filter (LPF) that limits the RF signal at the input of the first mixer. Because the frequency of the first LO is greater than the highest possible tuned RF frequency, the LPF also helps reduce the LO reradiation from the RF input. Note that the operating frequency range of the NI PXIe-5665 spans from 20 Hz to 3.6 GHz; the first IF must be above that range. At the individual downconversion stages, each mixer produces an image response when an undesired signal is present at the RF input. Each image response appears at a frequency 2 X IF away from the tuned RF frequency. An NI PXIe-5603/05 downconverter uses filters to block these undesired images, thus providing optimum spurious-free dynamic range (SFDR). In the next few sections, learn about some of the methods used in the individual components of the NI PXIe-5665 that contribute to its high quality and performance.
Enhancements on the NI PXIe-5603/05 Downconverters
Figure 3. NI PXIe-5605 Downconverter
An NI PXIe-5603/05 is a three-stage superheterodyne RF downconverter below 3.6 GHz and is 2 stage from 3.6 GHz to 14 GHz. The module accepts input frequencies from 20 Hz up to 14 GHz and downconverts them to a fixed IF that is digitized on the NI PXIe-5622. The NI PXIe-5653 provides extremely low-phase noise LOs to an NI PXIe-5603/05 for this purpose.
Calibration Techniques and the Use of a Built-In High-Precision Cal Tone
Correct calibration of RF instrumentation and setup is a challenge, especially when you consider environmental factors such as temperature drift, calibration across the entire RF path, and use of external high-precision instrumentation. In a production test environment, regularity of calibration and access to test equipment can be additional concerns. With insightful calibration techniques from NI, you can maintain high-quality calibration even when individual components of the NI PXIe-5665 are removed or replaced. The modular calibration is especially advantageous when multiple NI PXIe-5665 analyzers are phase synchronized for a higher channel system (for example, multiple input, multiple output (MIMO) systems) or when swapping modules. Each component, even in a multichannel architecture, maintains its own calibration constants, which are combined in the NI-RFSA driver at a system level.
Every NI PXIe-5665 RF analyzer is individually calibrated for accurate frequency and amplitude response at the factory. Each is shipped with a calibration certificate that verifies NIST-traceable accuracy levels. Errors are stored as calibration constants on the onboard EEPROM of the individual NI PXIe-5665 components. The NI-RFSA driver uses these calibration constants to calculate and apply corrections to your analysis based on the spectrum of interest.
You can self-calibrate the individual components through NI Measurement & Automation Explorer (MAX) or RFSA soft front panels. During self-calibration, an NI PXIe-5603/05 measures the reference source and compares the resulting measurements to a value stored in the EEPROM. An NI PXIe-5603/05 features a high-precision tone signal to correct for losses on your receiver by comparing the tone value stored on the NI PXIe-5603/05 EEPROM with a recent measured value. As seen in Figure 4, you can programmatically disconnect the RF input and connect the cal tone to the entire signal path instead. You can use the known amplitude and frequency of the cal tone to calibrate the entire instrument.
Furthermore, NI achieves world-class phase and magnitude accuracy by using equalization to accommodate for group delay. Group delay is a phenomenon where frequencies traveling through the nominal frequency of a filter (typically the center) are faster than at the edges of the filter, causing nonlinear phase and amplitude responses. The speed of the various signals traveling through the filter is known as velocity of propagation or velocity factor. Equalization is used to account for the differences in velocity of propagation through the filters on the NI PXIe-5665. The onboard cal tone is AM modulated at 612.5 MHz. A high-precision phase detector is then used to measure the nonlinear velocity of propagation or group delay through the filter and then equalized to minimize the group delay. Figure 4 plots the group delay through a filter in red and the equalized group delay in green.
Figure 4. The equalized group delay is shown in green.
Equalization corrects for magnitude and phase nonlinearity in filters, which leads to drastic improvements in measurements such as error vector magnitude (EVM). Figure 5 shows the effect of equalization on a WCDMA EVM measurement. The difference is clearly visible in the constellation plot.
Figure 5. Effect of Equalization on WCDMA EVM
3. Dynamic Range and Noise Floor Improvements on an NI PXIe-5603/05
In the following sections, learn about techniques that you can use on an NI PXIe-5603/05 to drastically improve dynamic range for measuring small adjacent channels effectively. Also discover how to lower the noise floor to measure low-level signals that would otherwise be masked by the noise of the analyzer.
Use of IF Filters
Traditional RF analyzers have incorporated IF filters to implement resolution bandwidth. However, most modern analyzers use a combination of IF and digital filters to provide optimum dynamic range without compromising on speed. An NI PXIe-5603/05 supports multiple IF paths: a narrow bandwidth path of 300 kHz, a 5 MHz path (only on the NI PXIe-5605), and a wider path with a nominal bandwidth of 50 MHz. The narrow path is especially useful for the rejection of distortion harmonics, and the higher bandwidth path is optimized for faster frequency sweeps. Figures 6 and 7 show the effect of the IF filter. A two-tone TOI measurement is made using the NI PXIe-5665. The two tones are separated by 2 MHz. Figure 6 shows an analog IF bandwidth of 50 MHz. The frequency spacing is less than the IF bandwidth, so distortion tones are noticed at ±3 MHz, resulting in an SFDR of -82 dBc.
Figure 6. A Two-Tone Signal Acquired on the NI PXIe-5665 Using the Through Path
Figure 7 shows the same tones but using an IF bandwidth of 300 kHz. Only one tone passes through the IF path at a time, eliminating the intermodulation products and resulting in an SFDR of -90 dBc.
Figure 7. A Two-Tone Signal Acquired on the NI PXIe-5665 Using the 300 kHz Path
You can use the instantaneous bandwidth (IBW) property in the NI-RFSA driver to select an IF path. If the IBW is set to less than 300 kHz, the narrow path is chosen and anything greater than 300 kHz reverts to the 50 MHz path.
Figure 8. Effect of IF Filters on a Multitone Signal
Note that the time taken to acquire the spectrum in Figure 8 is more than the time taken by the acquisition on the left. This is caused by the narrow bandwidth filter at 300 kHz. A narrower bandwidth over the same span implies larger numbers of bins that the span needs to be broken into. This also means that the center frequency has to be retuned every 300 kHz chunks. The primary cause for the difference in the time sweep over the same path is the requirement to retune a larger number of times when using a narrow IF path.
Use of Programmable Gains and Attenuators
To optimize for adjacent channel power, you can choose to apply additional gain for a multitone signal. This property is valid for the 300 kHz IF path and can be used with the “Min Adjacent Channel Power” property node. With this property, you can estimate adjacent channel power and apply that amount of appropriate gain on the adjacent channel. Figures 9 and 10 show the difference between using programmable IF gain and not using it. You can use the IF gain property node to achieve the 2 dB to 3 dB of reduction in the noise floor (shown in Figure 10) to read small signals next to larger ones.
Figure 9. Two-Tone Signal With No IF Gain Applied
Figure 10. Two-Tone Signal With IF Gain Applied Only on All Adjacent Tones
Optional Preamplifier (Below 3.6 GHz)
As mentioned earlier, measuring small signals below the noise floor has been a challenge for engineers. The traditional option was to purchase a bulky, expensive boxed instrument to make the measurement or use an external preamplifier to improve the noise figure of the instrument. An NI PXIe-5603/05 features an onboard optional switchable preamplifier that you can use to improve the noise figure of the input signal. This preamplifier is available only for frequencies less than 3.6 GHz on the NI PXIe-5605 downconverter.
Figure 11. Onboard Optional Switchable Preamplifier on an NI PXIe-5603/05
Figure 12 shows the differences between having the preamplifier turned off and turned on. At 1 GHz, the average noise floor is about -156 dBm with the preamplifier off and -165 dBm with the preamplifier on. You can use the preamplifier to measure small spurs and signals effectively.
Figure 12. Effect of Preamplifier on Noise Floor
4. Enhancements to the NI PXIe-5653
The NI PXIe-5653 LO source is a continuous-wave (CW) generator featuring three low noise sources:
- LO1 generates a signal at a frequency that can vary from 3.2 GHz to 8.3 GHz
- LO2 generates a signal at a frequency of 4 GHz
- LO3 generates a signal at a frequency of 800 MHz
Using the NI PXIe-5653 with an NI PXIe-5603/05 downconverter and the NI PXIe-5622 digitizer generates world-class phase noise. Figure 13 shows the phase noise of the NI PXIe-5665 compared to the Agilent PXA. While the Agilent PXA has slightly better phase noise from 100 to 10 kHz, the NI PXIe-5665 is better close in. Both instruments are similar when it comes to far-out phase noise.
Figure 13. Phase Noise Comparison Between the NI PXIe-5665 and the Agilent PXA
YIG Speed Modes
Based on your test requirements, you can programmatically select the behavior of the local oscillator YIG main coil drive. This property adjusts the dynamics of the current driving the YIG main coil. The two options are the following:
- Normal: Adjusts the YIG main coil for an under-damped response
- Fast: Adjusts the YIG main coil for an over-damped response
Normal mode provides better frequency accuracy at the expense of tuning time. The phase noise (on the NI PXIe-5653) is also better in normal mode than in fast mode. Table 1 shows the advantages and disadvantages of using each mode.
(1 GHz Step)
|Frequency Accuracy||Phase Noise (10 KHz)|
|Normal||9.3 ms||0.1 ppm||-138 dBc/Hz|
|Fast||2.3 ms||1 ppm||-130 dBc/Hz|
Table 1. Advantages and Disadvantages of Using Normal or Fast Mode
RF List Mode
With RF list mode, you can create a list of RF configurations before testing begins. This way, the instrument can progress rapidly through the list without the overhead associated with updating the RF configuration. Figure 14 shows a visual representation of this.
Figure 14. Performing Deterministic RF Configuration Changes With RF List Mode
To learn more about list mode, read the white paper titled Reducing Test Time With RF List Mode.
5. Production and Testing
Each module on the NI PXIe-5665 is sealed and shielded to prevent outside noise from affecting the accuracy of the instrument. Every module also undergoes rigorous emissions testing to ensure that internal components do not affect adjacent modules. You can fill up an entire 18-slot chassis with NI RF instruments and be guaranteed that the specifications will hold true. The NI PXIe-5665 is also tested for a temperature range of 0 °C to 55 °C. NI recommends that you perform a self-calibration if the temperature in the test environment changes by more than 5 °C.
6. Phase-Coherent Channels
With the unique modular architecture of the NI PXIe-5665, you can share a single LO with multiple downconverters and digitizers for a tightly synchronized phase-coherent system. Phase coherency guarantees that two devices are locked to the same frequency with a constant phase offset between them. If you are working with standards that need multiple antennas, such as 802.11n and LTE, you can use multiple NI PXIe-5665 analyzers locked to a single LO for multichannel acquisition systems. Figure 15 shows a two-channel phase-coherent system where one LO is shared with two NI PXIe-5603/05 downconverters and two NI PXIe-5622 digitizers. The 100 MHz OCXO from the NI PXIe-5653 is exported to the CLK IN of the first NI PXIe-5603/05. The first NI PXIe-5622 digitizer then exports this clock to the CLK IN of the second NI PXIe-5622 digitizer. All three LOs from the NI PXIe-5653 are also shared across both NI PXIe-5603/05 downconverters. You can use this configuration for multiple phase-coherent channels even across multiple chassis.
Figure 15. Multiple Phase-Coherent Signal Analyzers Locked to a Single LO
7. Flexibility With Open Field-Programmable Gate Array (FPGA) Programming
You can use the NI PXIe-5665 analyzer with NI FlexRIO modules that plug into a PXI chassis. With peer-to-peer streaming, you can stream data in real time over the PXI backplane to the programmable FPGA on the NI FlexRIO modules. Figure 16 shows the implementation of a real-time spectrum analyzer using the NI PXIe-5665 and NI FlexRIO. All of the data acquired by the NI PXIe-5665 is transferred to the FPGA on the NI FlexRIO module using peer-to-peer streaming. You can also use the NI LabVIEW FPGA Module to program the onboard FPGA on the NI FlexRIO modules and process data with nanoseconds of decision-making capabilities. With this real-time processing capability, you can use the NI PXIe-5665 as a real-time spectrum analyzer by streaming all of the data acquired by the NI PXIe-5665 over the entire bandwidth to the NI FlexRIO module for real-time processing. Furthermore, you can send triggers from the NI FlexRIO module based on the real-time processing to other instruments in the PXI chassis. Use this combination of instruments with NI FlexRIO to create a system that can solve highly complex test problems that are impossible for traditional boxed instruments.
Figure 16. An Implementation of a Real-Time Spectrum Analyzer Using the NI PXIe-5665 With NI FlexRIO Using Peer-to-Peer Streaming
The NI PXIe-5665 is the best-in-class RF signal analyzer on the market. This world-class instrument is the culmination of technologies and inventions in all stages of development, starting from the use of a modular superheterodyne architecture in the design stage to the use of novel packaging techniques in the assembly and production stage. Not only is the NI PXIe-5665 the highest accuracy signal analyzer with world-class performance (phase noise of -129 dBc/Hz and noise floor of -165 dBm), but it is also about 20 times faster than rack-and-stack instruments for automated test. Furthermore, you can use this instrument with the programmable onboard FPGA on NI FlexRIO modules for applications that cannot be solved by traditional rack-and-stack instruments.