We need to distinguish between sensitivity, signal handling capability and dynamic range. The corresponding specs for an SA are DANL (Displayed Average Noise Level), 1dB CP (1dB Compression Point - the input level where the gain decreases by 1dB due to signal limiting/clipping) and 1st order dynamic range, which is usually defined as the difference between CP and DANL. But one can easily make mistakes there…
Quite obviously, we can increase the CP as much as we like by adding internal and/or external attenuation. With 20dB attenuation, the CP will increase by 20dB as well. Of course, there is only so much power a certain attenuator can handle and the internal ones will usually only tolerate up to +30dBm at best, but it depends whether these attenuators are mechanically or electronically switched (be careful, without studying the specs, +20dBm would be a reasonably safe value). With the appropriate external attenuators, we can indeed go up as high as +50dBm and even more. But the attenuator will raise the noise floor by the amount of its attenuation.
We also can increase the sensitivity close to the limits of thermal noise by means of a preamplifier, once again either internal or external. With a low noise, high gain external preamp (NF <3dB), we can theoretically make any SA perform a DANL of <-160dBm/Hz. But the preamp will lower the CP by its gain at least – some preamps might even have a lower output CP than the input CP of the SA, thus further reducing the total system CP.
So even though we can have a very low noise floor as well as a very high CP, we cannot have them both at the same time. Consequently the 1st order dynamic range is the difference between CP and DANL within one setup. The whole point of a dynamic range definition is the ability to analyze weak signals in presence of strong ones. For example, if you want to verify the harmonics of your newly designed HF-PA to be <120dBc, you either need an SA that has a dynamic range in excess of 120dB or you have to use some tricks, like using a notch filter to suppress the carrier.
To cut a long story short, it doesn’t mean a thing if someone shows the maximum sensitivity of their gear – simply because it still tells absolutely nothing about the dynamic range.
Here’s an example with the SA44.
With 0.8Hz RBW, preamp off and 10dB attenuation, the noise floor is about -133dBm, which appears at least 6dB higher than the R&S FSEA30 in the similar situation shown by Muxr in Reply #128 (SA44_Noisefloor_REF-80dBm_ATT10dB)
![](https://www.eevblog.com/forum/testgear/which-spectrum-analyzer-would-you-get/?action=dlattach;attach=248353)
Still not too bad, but what is the dynamic range? What is the maximum signal it can handle, i.e. measure with reasonable accuracy? Since the reference level is set to -80dBm, we are limited to some 53dB anyway, since we don’t want to count on measurements outside the display range.
All in all, this measurement is almost meaningless for judging the capabilities of the instrument. In a situation like the one above, we would certainly use the preamplifier and the noise floor would drop by more than 20dB – and that would make perfect sense, since the preamp easily copes with signals in the range of -80dBm and lower, of course.
So we now set the reference level to +10dBm (SA44_Noisefloor_REF+10dBm_ATT10dB)
![](https://www.eevblog.com/forum/testgear/which-spectrum-analyzer-would-you-get/?action=dlattach;attach=248355)
Oops – the noise floor has increased by some 12dB to -121dBm, but now at least we can hope to measure signals up to +10dBm. Let’s try -10dBm first (SA44_-10dBm_REF+10dBm_ATT10dB)
![](https://www.eevblog.com/forum/testgear/which-spectrum-analyzer-would-you-get/?action=dlattach;attach=248357)
This works – I’ve just switched the vertical scale back to 10dB/div, because the noise floor isn’t visible anymore anyway, due to the strong phase noise of the analog synthesizer signal. Anyway, reference level and attenuation remain unchanged and we get an accurate measurement, so this already covers a dynamic range of -10 - -121 = 111dB.
Now let’s try +10dBm (SA44_+10dBm_REF+10dBm_ATT10dB)
![](https://www.eevblog.com/forum/testgear/which-spectrum-analyzer-would-you-get/?action=dlattach;attach=248359)
This still works, although we can see a slight hint on gain compression, as the marker reading is already 0.1dB low. This is in accordance with my previous experiences, that this analyzer has a 1dB compression point (at the mixer input) somewhere at +1.5dBm. With 10dB attenuation, it would be about +11.5dBm of course.
This gives a total first order dynamic range of +11.5 - -121 = 132.5dB.
Not too bad, but still nowhere near the R&S FSEA30.