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Frequency Spectrum of a Real Sine Wave 
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#1
Feb1310, 07:40 PM

P: 393

Hi. I'm doing a lab where we hooked up an RF signal generator at 1.25MHz/10dBm to a spectrum analyzer using a 50 ohm wire.
Can anyone explain to me or link me to a place where I can read about why a real sine wave's frequency spectrum is not a pulse? Also, why would with a sweep from 1 Mhz to 4 Mhz the SA show three sine waves separated by multiples of the frequency of the intended sine wave(~10dBw @ 1.25, ~66 dBw @ 2.5, ~87 dBw @ 3.75)? Would there also be lowered amplitude frequencies to the left of the intended frequency? I'm pretty confused, because I haven't taken the signals and systems class that stresses frequency response, FFT, and so on. Also, I have no idea how this hardware works. I just want an informed lab report! 


#2
Feb1310, 09:56 PM

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P: 5,196

I'm not sure why you're seeing those higher frequency harmonics. I wonder if it's some sampling or aliasing issue. 


#3
Feb1810, 03:22 PM

P: 276

Even an expensive sig gen isn't spectrally pure  there are always some sidebands or harmonics which is what you are seeing. If you dig up the sig gen's data sheet, you'll likely see specs defining how big these sidebands can be and likely (if calibrated and working properly) the values you are seeing are below those spec's. If the spectrum analyzer can't measure down to those specs you might not see that however. Assuming otherwise above. There are also artifacts that can be caused by the way swepttuned spectrum analyzers or FFTbased analyzers work that spread the otherwise impulselike frequency spurs to look more rounded. This is usually caused by sweeping too fast relative to the resolution bandwidth  generally using the automatic modes avoid this. Don't go to manual settings on the spectrum analyzer without knowing how. Agilent has a Fundamentals of Spectrum Analysis application note that goes into every you need to know about this. Highly recommended. 


#4
Feb1810, 03:35 PM

P: 393

Frequency Spectrum of a Real Sine Wave
Thanks, guys. I think I turned in a decent report. I talked mainly about the SG's inability to produce perfect sine waves(evidenced by the harmonics spotted) and about how the filter resolution also probably widened the pulse.



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