Front-End Choices for RF/Microwave Measurements

Blog Post created by benz on Mar 3, 2017

A few months ago, Keysight’s Brad Frieden and I both wrote about downconversion and sampling, related to wireless and other RF/microwave signals. Brad's article in Microwaves & RF appeared about two weeks before my blog post, though I somehow missed it.

His main focus was on oscilloscopes and improving signal-to-noise ratio (SNR) in measurements of pulsed RF signals. He described the use of digital downconversion, resampling, and filtering to trade excess bandwidth for improved noise floor.

Inside Keysight, debates about scopes versus signal analyzers can become quite animated. One reason: we have slightly different biases to how we look at signals. Engineers in some areas reach first for oscilloscopes, while others have always leaned on spectrum and signal analyzers. It’s more than a general preference for time or frequency domain analysis, but that’s a start.

In test, those distinctions are fading among manufacturers and end users. On the supply side, oscilloscopes are extending frequency coverage into the microwave and millimeter ranges, and signal analyzers are expanding bandwidth to handle wider signals in aerospace/defense and wireless. Happily, both platforms can use the same advanced vector signal analyzer software that provides comprehensive time-, frequency-, and modulation-domain measurements.

On the demand side, frequencies and bandwidths are expanding rapidly in wireless and aerospace/defense applications. That’s why both types of instruments have roles to play.

But if they run the same software and can make many of the same measurements, how do you choose? I’ll give some guidelines here, so that your requirements and priorities guide your choice.

Bandwidth: In the last 15 years, this change has pulled oscilloscopes into RF measurements because they can handle the newest and widest signals. In some cases they’re used in combination with signal analyzers, digitizing the analyzer’s IF output at a bandwidth wider than its own sampler. That’s still the case sometimes, even as analyzer bandwidths have reached 1 GHz, and external sampling can extend the bandwidth 5 GHz! It’s telling that, in his article on oscilloscopes, Brad speaks of 500 MHz as a reduced bandwidth.

Accuracy, noise floor, dynamic range: In a signal analyzer, the downconvert-and-digitize architecture is optimized for signal fidelity, at some cost in digitizing bandwidth. That often makes them the only choice for distortion and spectrum emissions measurements such as harmonics, spurious, intermodulation, and adjacent-channel power. Inside the analyzer, the processing chain is characterized and calibrated to maximize measurement accuracy and frequency stability, especially for power and phase noise measurements.

Sensitivity: With their available internal and external preamps, narrow bandwidths, noise subtraction and powerful averaging, signal analyzers have the edge in finding and measuring tiny signals. Although Brad explained processing gain and some impressive improvements in noise floor for narrowband measurements with oscilloscopes, he also noted that these gains did not enhance distortion or spurious performance.

Multiple channels: Spectrum and signal analyzers have traditionally been single-channel instruments, while oscilloscope architectures often support two to four analog channels. Applications such as phased arrays and MIMO may require multiple coherent channels for some measurements, including digital demodulation. If the performance benefits of signal analyzers are needed, an alternative is a PXI-based modular signal analyzer.

Measurement speed: To perform the downconversion, filtering and resampling needed for RF measurements, oscilloscopes acquire an enormous number of samples and then perform massive amounts of data reduction and processing. This can be an issue when throughput is important.

With expanding frequency ranges and support from sophisticated VSA software, the overlap between analyzers and oscilloscopes is increasing constantly. Given the demands of new and enhanced applications, this choice is good news for RF engineers—frequently letting you stick with whichever operating paradigm you prefer.