Spectrum analyzer dynamic range - Page 1
Forum section: electronics / beginners

author: salamander
author_rank: Contributor
timestamp: March 08, 2017, 07:52:28 pm
content: Hi guys. I have a quiz question I can't find an answer to. I believe it's really simple, I'm just unable to find the right solution.The questionQuoteThanks for any help. (I don't expect a direct solution, but at least point me right direction)BTW: I'm not an electronics engineerFSL3 datasheet:https://www.testworld.com/wp-content/uploads/specifications-rohde-and-schwarz-fsl3-fsl6-fsl18-spectrum-analyzers.pdf

author: technogeeky
author_rank: Frequent Contributor
timestamp: March 09, 2017, 05:31:57 am
content: WARNING:I started this off as a general answer to your question because I didn't know the answer. So this is not for you, but for me, future me, and future people to form a better formatted answer to the general question: what is the equation for spectrum analyzer dynamic range? I got bored and stopped before I finished.So a ideal spectrum analyzer has a both a maximum power level and an absolute-minimum power level (or noise floor, or displayed average noise level (DANL)). An ideal spectrum analyzer would have an ideal display, an ideal mixer, and presumably ideal preamplifier, attenuators, and amplifiers.It looks like the equation the overall dynamic range of a spectrum analyzer is:QuoteYou've heard of some of these before:rangedisplayis borne from the fact that the any display device has a maximum and minimum intensity.rangemeasurementis the difference between the largest signal from the most attenuated input setting to the smallest signal from the least attenuated setting.noisespuriousis the amplitude of the spurious tones created by the instrument's amplifiers and mixers.noiseDANLis the Displayed Average Noise Level, ornoise floororsensitivity.*noisephaseis the instability caused by mixing the signal with the inherently unstable local oscillator (LO)HINT: Ignoringrangedisplay, notice the impact from each component of the dynamic range decreases from left to right:rangemeasurement(-151dBm and up from Figure 1)noiseDANL(-135 dBm and up)noisespurious(97 and 90 dB of range above the -135dBm floor)noisephase(the 80 dB of range left over when you set your reference to minimize phase noise)HINT:noiseDANLis the "key"property when measuring two signals that are far apart in frequency (in contrast to phase noise)HINT:noisephasebehaves likethird order inter-modulation(TOI) distortion: it is most significant when the input signal has two tones that are nearby to each other (less than 1 MHz) -- at least for purposes of dynamic rangeEven if we don't fully understand all of these, we use some relationships, hints, and assumptions to simplify our question:First, we work under the assumption that a wide signal is similar enough to two thin signals spaced apart by the width. This is a simplification used in tons of testing procedures, and it's the analysis ofinter-modulation distortion. The two unhighlithed tones f1and f2are the inputs. Ithink, however, that in your specific case we don't need to consider this both because it doesn't mention two tonesandbecause the RBW is only 300 Hz, which is much smaller than 1 MHz.According tothis paper(from which this response is almost entirely plagiarized),afterwe set our reference (mixer level) just right (right where our second-order and third-order distortion curves meet), then the dynamic range is equal to theequalityof these two equations:range3rd= (2/3) * ( DANLactual- TOI )range2nd= (1/2) * ( DANLactual- SHI )TOIa= mixer level - (1/2) (distortion products in dBc)SHIb= mixer level - (distortion products in dBc)DANLactual= DANLat 1 Hz- 10 log ( DANLat your RBW/ DANLat 1 Hz)So, having typed all of this out, I think you have enough to know where to look in themanual you linkedto find the DANL at 1 Hz, then find the right mixer level at 1.2 GHz; converting your answer to the 300 Hz RBW (which happens to be the minimum RBW for that instrument).aThird Order Intercept (point). Also called IP3.bSecond Harmonic Intercept (point). Also called SOI or IP2.*DANL actually has aphysicalabsoluteminimum of -174 dBmderived from its defining equation:DANLmin= kTB.k is Boltzmann's Constant (which has dimensions of  / , I'm sure Idoneed to remind you. I needed to remind me.).T is temperature which has dimensions of ... , and units of .B is bandwidth which has units of , we're going to use .Apparently, if you are bored enough to multiply these together, (or even to just do the dimensional analysis, honestly) you will find that these are in units of power and calculated at room temperature and 1 Hz (a standard for some reason), youreallydoget -174 dBm.