Noise affects important performance in spectrum analyzers such as sensitivity, dynamic range, averaged detect, vector-demodulated EVM, signal-to-noise ratio errors ...
In most cases, we take a spectrum analyzer as a receiver. Noise is not directly present in the spectrum analyzer but as a part of the measurement result. It is called the display average noise level (DANL, Display Average Noise Level). Please see the following screenshot from Saluki S3302 series handheld spectrum analyzer, the grey part is the DANL of the current setting.
Figure 1: DANL of S3302 Handheld Spectrum Analyzer
Meaning of DANL
DANL does not represent the actual input signal noise level. Input signal noise goes through the whole instrument and the instrument itself add noise in. With a proper average/VBW/RBW, a noise floor displayed on the screen. In fact, all measurement display of the spectrum analyzer is the result of input signal plus SA internal noise. If the power level of the input signal is not higher than instrument internal noise, then the signal will be overwhelmed in the instrument noise and will not be displayed on the screen. Therefore, the level of DANL reflects the minimum level that the spectrum analyzer can measure and reflects the internal noise level of the spectrum analyzer. DANL determines the sensitivity of a spectrum analyzer.
How to make the DANL lower
Can we eliminate the noise? The answer is NO.
Electronic measurement noise generally comes from two parts: the input signal noise and the measurement system (instrument) noise. Not too much to talk about the input noise. Let have a look at the measurement system noise, it may have caused by the instrument hardware like port, cable, components, and it may also come from software calculation. Beside all this, one of the important noise sources is called "Thermal noise". Thermal noise has nothing to do with the frequency or impedance, it is only related to temperature. Only in the absolute zero 0K, the electronic stop oscillation, then no thermal noise. In normal room temperature (27 degree centigrade), the noise is -174dBm and it can not be eliminated.
To reduce the noise inside the spectrum analyzer, we need to start with two aspects, one is to reduce the overall instrument noise figure, and the other is to reduce the RBW.
From the instrument noise aspect, the spectrum analyzer can add a preamplifier (usually a low noise amplifier), and attenuators (equivalent to amplifying), reducing the noise figure of the instrument. When the pre-amplifier is on and the attenuator is 0, the noise figure of the instrument reaches its minimum value.
From the RBW aspect, please refer to artical Q&A Spectrum Analyzer Question and Attentions ( Part 2 ), Q1. Generally, narrow RBW leads to a lower DANL. ( 1/10 RBW setting leads to 10dBm lower DANL).
Please be kindly noted, VBW has no effect on the average noise level and does not increase the spectrum analyzer's sensitivity.
Figure: Same Input Signal with Different RBW Settings (Screenshot from S3532)
Saluki Spectrum Analyzers
All Saluki spectrum analyzers keep pre-amplifier and antennuator as a standard part to provide the customer a low DANL. Saluki spectrum analyzers all have RBW minimum to 1Hz and the best DANL usually <-160dBm. Now Saluki has the following series of spectrum analyzers