Why the shape of spike in spectrum analyzer is different with ideal spike?

Thread Starter

PersianEngineer

Joined Aug 15, 2013
19
Question:
Why the shape of spike in spectrum analyzer is different with ideal spike?
Ideally we have learnt that the spike is a vertical line in frequency domain but in the real world it's not exactly so even if we give it a perfect e.g. sine wave. What is the reason?
 

alfacliff

Joined Dec 13, 2013
2,449
the shape of the "spike " dsplayed is controlled by the bandwidth of the signal. a cw carrier is a smalll spike, an fm broadcast station is a broad spike. and these days a tv station looks like a "haystack" due to all the carriers it contains.
also, the badnwidth setting on the analyzer eill affect it, as well as the sweep rate, and the displayed bandwidth.
 

LvW

Joined Jun 13, 2013
840
I suppose you are referring to an analog SA, correct?
In this case, there is indeed no "spike" - even not for a clean sinusoidal.
Instead , the screen shows the shape of the last IF bandpass filter.
By the way - this is also the reason for an upper search frequency limit because this filter needs a certain time to respond to the search signal.
 

MrChips

Joined Oct 2, 2009
19,159
In theory, a sine wave produces a delta function (a spike) in the frequency domain if the sample record is infinitely long.

In practice, a spectrum analyzer has a time limited record length. The resolution of the frequency spike is inversely proportional to the record duration. Hence you will experience spreading of the frequency spike.
 
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