October 13, 2015
Spectrum monitoring is often used as the first part of an interference hunting process. The current move towards small cells, as well as the interference and cell edge issues inherent in LTE, only increases the need for a clean spectrum and greater use of advanced spectrum monitoring tools.
There are many spectrum monitoring tools available today, which makes deciding on the proper solution a challenge. In this post, we offer a few guidelines for selecting a spectrum monitoring instrument. It is important to pay attention to specifications and features. Missing or sparse specifications are a warning of a low-quality instrument.
Third Order Intercept (TOI) - Instruments with poor TOI will be more susceptible to generating spurious responses (spurs) internally that may be confused with real, external signals. A good TOI figure is in the 15 to 20 dBm range, with two –20 dBm tones and no attenuation. Be wary of instruments that specify TOI with >0 dB attenuation, because this makes the TOI number artificially high.
Phase Noise - Excessive phase noise appears as an elevated noise floor near any strong signal, and may mask a weak signal at a frequency near a strong signal. A good phase noise specification, such as –100 dBc/Hz at 1 GHz, means that this effect will be minimized.
DANL - Instruments with a poor Displayed Average Noise Level (DANL) may not see a low-level signal due to a high noise floor. A good DANL is quite helpful when setting up a spectrum monitoring station or network, ensuring that you can effectively monitor signals over a wider area. An example of a good DANL specification is less than –160 dBm in a 1 Hz RBW. Remember that when setting up a monitoring network, having a noise floor or DANL that is 3 dB better may mean that you need only have half as many monitoring stations.
Dynamic Range - A large dynamic range allows you to see a small signal when a large signal is also present. This is essential if the signal you are interested in is small and close in frequency to a large signal. Unfortunately, dynamic range for a spectrum analyzer can be specified in many different ways with wildly different numbers. One example of a very useful dynamic range specification is “greater than 106 dB in a 1 Hz RBW,” using the method “2/3(TOI-DANL).”
Overload Indicator - It is important that your instrument indicates when its front-end is experiencing overload conditions. Overloads may generate spurious signals that appear to be legitimate, and these signals are made worse by poor TOI. If you are alerted when an overload is occurring, you can add attenuation, external band-pass filters, or even move to a different location. Otherwise, you may waste a lot of time investigating signals that are generated inside the instrument itself!
Multi-step Attenuator - A wide range attenuator with many different possible attenuation values is a fundamental tool when monitoring spectra. Some instruments have few or no attenuation settings, making it difficult to see low-level signals after enabling the attenuator. Coarse attenuation selections also make it more difficult to deal with input overloads.
Preamplifier - Weak signals may need preamplification to become visible. The difference can be dramatic, lowering the apparent noise floor by about 10 dB. It is important that the preamplifier be wide range, preferably equal to that of the instrument. Certain handheld spectrum analyzers (figure 1) limit the maximum attenuation when using the preamp to help you avoid setups that degrade the dynamic range of the instrument.
Burst Detect - Burst Detect is a feature found on Anritsu handheld spectrum analyzers that makes spotting or even locating intermittent signals much easier and faster. It is quite helpful to have a way to see bursty signals quickly because many of the current generation of digital signals are bursty in one form or another.
Video Bandwidth Control - A Video Bandwidth (VBW) filter is a fast way to average the displayed data and lower the noise floor. A user-adjustable VBW filter is an important feature when selecting a tool to monitor in the current RF environment.
Field Strength Measurements - If coverage or exposure limits are important, you will want to measure in field strength units. This capability normally includes an automatically calculated correction for the antenna gain factors, as well as automatically converting measurements into field strength units such as Watts per Meter Squared. This is a real time saver if accurate, repeatable field strength measurements are required.
Spectrum Recording – Not all spectrum measurement devices allow traces, spectrograms, and events to be saved. If you are in a position where it is necessary to compare or report on measurements, the ability to store traces, either normally, through save-on-event, or within a spectrogram, will be important.
A free spectrum monitoring application note is available for download. It contains more information on features to look for in a monitoring tool, as well as techniques you can employ to locate interference.