For the longest time, the narrow-band modulated signals we used for RF transmissions were adequate for our frequency-divided applications like commercial broadcasting and two-way radio communication. Then late in the 80s and continuing in the 90s, the desire for wider bandwidths to deliver more information began to skyrocket driven by the development of vector-modulated communication systems and pulse-compression radars.
These technologies brought about the need to analyze wide bandwidth signals while maintaining sufficient dynamic range. There is a fundamental trade off for the signal analyzer between the sample rate of its digitizer and the number of effective bits captured. (Effective bits are the number of significant bits representing the captured signal above the noise and distortion of the analyzer itself.) At opposite ends of the signal analysis instrument continuum, between bandwidth and dynamic range, sits oscilloscopes with their wide bandwidth capability and spectrum analyzers with their high dynamic range.
Oscilloscopes, in the past a time domain tool used mostly for digital signal analysis, now have sample rates of up to 80 GSample/second making them an excellent platform for RF analysis. Signals up into the Ka band can be characterized without the need for downconversion which can add local oscillator (LO) phase noise, mixer impairments, as well as amplifier gain and phase distortions. Figure 1 shows an oscilloscope used with Vector Signal Analysis (VSA) software to characterize a 10 GHz pulse that is modulated with a linear FM (LFM) chirp 2 GHz wide.
Figure 1 – Evaluation of a wideband LFM chirp using VSA Software running on an oscilloscope
Often when making measurements on radar signals we might want to find and measure a small signal in the presence of a much larger one. Although the modulation of the pulse shown above is well characterized with the oscilloscope and VSA software, a spectrum analyzer would provide better results in this case. Figure 2 shows the display of a spectrum analyzer running the same VSA software we to used with the oscilloscope above. Here we are analyzing the transient properties of an amplifier as it turns on. Note the much greater amplitude range and detail of the measurement. As mentioned above, the analysis bandwidth of spectrum analyzers is limited with the maximum available being about 140 MHz.
Figure 2 - Evaluation of amplifier switch-on using VSA Software running on a spectrum analyzer.
There are some basic things we can do to further reduce the noise floor and improve the dynamic range of the spectrum analyzer further. At microwave frequencies, losses mount quickly with cable length, quality, switching, etc. In some situations, the most effective and least expensive way to reduce the noise level is to use better or shorter cables and move the analyzer closer to the signal to be measured.
Here are some additional actions we can take to reduce the noise floor and improve the dynamic range of our spectrum measurement:
Reduce the analyzer attenuation
Add a preamp (which can be included with the analyzer)
Reduce the resolution bandwidth (RBW) of the analyzer
Add external filtering
No single measurement platform will solve all application challenges. Modern signal analysis platforms, such as oscilloscopes and spectrum analyzers, are continually improving not only in terms of bandwidth and dynamic range specifications but also by employing advanced features to enhance test time, accuracy, and ease-of-use.
About the Author:
John Hansen is currently a senior application engineer for Agilent Technologies’ Electronic Measurements Group. He has more than 20 years of experience in system engineering and new product development within the wireless, microelectronics and defense industries. At Agilent, he has been responsible for the launch of new high frequency microwave signal generator products and is currently involved in market analysis and generation of technical content for the aerospace & defense markets. Prior to joining Agilent, Hansen worked at Hughes Network Systems, where he participated in the development of terrestrial cellular and satellite communication products as an engineering test manager.