Why does FM create side bands? - Page 1
Forum section: electronics / beginners

author: ice595
author_rank: Regular Contributor
timestamp: October 15, 2014, 12:31:36 am
content: From what I've learned, a FM signal can be generated by a VCO.Lets say for example, if the VCO is outputting 100MHz when 0v is applied at the input, then the carrier frequency is 100MHzany change in the input voltage to the VCO will cause it to deviate from the resting frequency which is 100MHz.so if a triangle wave is applied to the VCO, it should just sweep from low frequency to high frequency linearly right?and if the output of the VCO is connected to a spectrum analyzer we should see the carrier frequency move back and forth in the frequency scale right?but instead, we get side bands. where do they come from?

author: Skimask
author_rank: Super Contributor
timestamp: October 15, 2014, 12:56:53 am
content: http://en.m.wikipedia.org/wiki/Triangle_wave

author: ice595
author_rank: Regular Contributor
timestamp: October 15, 2014, 01:05:12 am
content: but if the triangle wave is very low in frequency, lets say 1Hz, will we see the 100MHz carrier sweep back and forth once per second on the spectrum analyzer?

author: LightYear
author_rank: Newbie
timestamp: October 15, 2014, 01:52:59 am
content: Quote from: ice595 on October 15, 2014, 01:05:12 amYes, provided the sampling period of the analyser was sufficiently small, the linear variation in frequency introduced by the triangular wave will be negligible. Each successive sample will simply contain a high frequency oscillation at a certain frequency, which varies over time.The sidebands come about when the modulation frequency is much higher - in that case, over any appreciable sample period the signal contains multiple frequencies. It turns out that a sinusoid with a sinusoidally varying frequency can be decomposed into a sum of frequencies that are positive and negative integer multiples of the modulation frequency away from the carrier frequency. These appear as sidebands on the spectral plot.

author: coppice
author_rank: Super Contributor
timestamp: October 15, 2014, 02:16:10 am
content: Are you expecting a single sweeping frequency to come out of the VCO? How could that happen? You are endlessly changing the frequency of the VCO, so you are never generating an exact part of any particular sine wave. It shouldn't be surprising that this means you never get an exact sine wave when you look at the spectrum of the signal. The faster you sweep the generated frequency the less any tiny segment of the generated waveform looks like a sine wave, and the more the spectrum spreads. A detailed analysis of exactly how it spreads requires some maths, but I don't think any maths is needed to grasp the basics of what is going on.

author: Teledog
author_rank: Regular Contributor
timestamp: October 15, 2014, 02:44:13 am
content: VCOs are also infamous for phase noise (depending on the quality of the VCO), hence the oddball sidebands

author: T3sl4co1l
author_rank: Super Contributor
timestamp: October 15, 2014, 03:09:35 am
content: More precisely, the sidebands are always there, you just need a long enough acquisition period and fine enough frequency resolution (which are the same thing, given the capability) to resolve them. A 1Hz sweep has sidebands around 1Hz apart, so on a typical 3kHz BW instrument... how would you know?Tim

author: IconicPCB
author_rank: Super Contributor
timestamp: October 15, 2014, 08:06:24 am
content: And then try to wrap Your brain around the Bessel null