Spectrum analyzers are expensive pieces of equipment that should be handled with care, especially when it involves applying signals to the RF input. Some new users may not know what “care” means and how to go about ensuring the spectrum analyzer they’re using is protected from overloading and damage. The intent of this article is to describe how not to blow up your spectrum analyzer. Those new to the subject of spectrum analyzers and measurement receivers may first want to review references 1 through 3 which cover some of the basics concerning spectrum analyzers and measuring receivers in general.
RF Spectrum Analyzer Partial Front-End
Figure 1 shows the components of a partial “front-end” to a spectrum analyzer.
It is usually the mixer that is most susceptible to damage, followed by some of the other components identified in Figure 1.
The RF input port (usually located on the front of the spectrum analyzer) is where the signal desired to be measured goes. Before applying any signal to this port, it is prudent to first ask ourselves, “What can we do to this input? and “What can we apply here safely?”
These questions and their answers should be revisited each time we set up the spectrum analyzer to take a new measurement, especially if they are radically different measurements from ones we’re normally used to taking. Complacency in not following these rudimentary steps could spell disaster and embarrassment if we’re found guilty of blowing up an expensive spectrum analyzer. Spectrum analyzer users should also consider the potential loss in revenue due to the cost to repair and test/product development time lost if a backup is not available when a spectrum analyzer is damaged.
RF Input Warning Label
The good news is that preventing such a catastrophe, such as an overloaded RF input port, isn’t that difficult. There is usually some descriptive label near the RF input port that indicates what the input capabilities for the RF input are. This label is identified with the familiar exclamation point inside a triangle warning symbol as shown in Figure 2.
It’s VERY important to carefully read and understand what it says on this label before attempting to measure any signal! The warning label will typically tell you the maximum RF signal levels that it can handle and also the maximum DC voltage levels it can handle.
Maximum RF Input Level
Check the warning label to determine what it indicates as its maximum RF input level. A typical value is +30 dBm (1 Watt). Depending on the strength of the signal you want to measure, you may have to install an external attenuator or RF sampler (if measuring signals with high RF power) to ensure the maximum level seen by the spectrum analyzer is always maintained below its specified maximum RF input level of +30 dBm.
Maximum DC Voltage
Be careful because a lot of microwave spectrum analyzers only allow a maximum DC voltage level of zero volts! Some other RF spectrum analyzers only allow a couple of DC volts maximum (there are exceptions, that allow hundreds of DC volts). The maximum DC voltage the spectrum analyzer can handle is also typically printed on the warning label.
In the situation where zero DC volts is the maximum DC input level you can ensure that the signal you want to measure is AC coupled. This is accomplished by inserting a coaxial DC block in front of the RF Input port to ensure no DC voltage can “get through” to damage the mixer and other front-end components.
It is probably also prudent to confirm the maximum DC voltage level and maximum RF input levels in the spectrum analyzers instruction manual before applying any signal. There may be more information found in the instruction manual and if there is a discrepancy between the front label warnings and the instruction manual, a call to the manufacturer is probably in order.
By combining knowledge of the specified input levels to a spectrum analyzer for both the maximum DC voltage level and maximum RF input level, and ensuring that we do not exceed both, we are able to safely operate our spectrum analyzer, accurately measure the signals we want to measure, and have a reliable measurement tool available the next time we need to use it. Trips to the “front office” to explain why we damaged the most expensive piece of test equipment in the facility are also minimized.
References and Further Reading
- “EMI Measurement Receiver Requirements (CISPR 16-1-1),” In Compliance Product Insights, September 2020.
- “Let’s Talk About Real-Time Spectrum Analyzers,” In Compliance Product Insights, September 2019.
- “What Every Electronics Engineer Needs to Know About: Measuring Receivers,” In Compliance Product Insights, September 2018.
- “#51: Basic Spectrum Analyzer Do’s and Dont’s…,” w2aew YouTube Channel, June 19, 2012.