fsk demodulation

Can someone explain or point me to the right link on how a PLL is used to demodulate a FM carrier? Every book I read tells me it can be done but not specific enough on how it is done. I have read Horowitz & Hill: The Art of Electronics 2nd edition and RF Engineering for Wireless Networks by Daniel M. Dobkins.

Hi,

Just browse google, you will find a lot of ressources....

I suppose, it′s not necessary to search special chapters in PLL books about FM demodulation. Because it′s relatively simple.
Remember, that the PLL is able resp. should be able to follow small frequency deviations at the reference input (within the PLL lock range). That means, there is a control signal at the VCO input to tune the VCO correspondingly. This signal, therefore, is proportional to the frequency deviations and, thus, is the demodulated LF signal.

exactly , the use of the PLL it to control the VCO so it track the phase an frequency of the reference signal.
in the demodulation ur reference signal will be the modulated FM signal u want to demodulate, and hence the PLL is locked , ur output is taken from the VCO control voltage.

khouly

PLL FM demodulator

Thanks for the help!

Shouldn't there be a Reference Oscillator as input to the phase comparator in the PLL_FM_demodulation.gif file?

no there is not refernce for the PLL in the FM demodulation

Khouly

Pardon my ignorance but......

How come there isn't a need for PLL reference in FM demodulation?

Is there a need for PLL reference in FSK demodulation?

So basically if the input IF stage of receiver and the VCO has the same phase the output of the phase comparator will be low and if they are out of phase tho output of the phase comparator will be high?

The above discussion is simply assuming a PLL, that's fast enough to follow the momentary input signal. That's a meaningful assumption generally, but other modes of operation are possible. With the fast PLL design, the demodulator output information is represented by the VCO frequency setpoint, as shown in the schematic by vfone.

This also works with FSK, which isn't anything but FM with a squarewave modulation signal. Due to limited transmission channel and PLL bandwith, the demodulated signal isn't exactly a square wave, of course.

A different situation is with phase switching PSK, as e. g. present in GPS signals. It needs a different PLL design (a costas loop) for demodulation.

The modulated input frequency is your reference.

The PLL is constantly tracking the reference input trying to achieve lock. The error voltage of the comparitor reproduces the modulation.

Rod

Pardon my ignorance but......

How come there isn't a need for PLL reference in FM demodulation?

Is there a need for PLL reference in FSK demodulation?

So basically if the input IF stage of receiver and the VCO has the same phase the output of the phase comparator will be low and if they are out of phase tho output of the phase comparator will be high?

hi,

can any1 tell me howz the different look like of the FM modulation and after the PLL, the FM demodulation output is look like how?

Im very blur on this...

Thank You.

The basic theory is pretty easy to understand. The VCO has a tuning voltage that is proportional to its output frequency, so you just look at the tuning voltage when the pll is locked and determine the input frequency.

That is where the simplicity ends, however. In the real world, there are things like noise and jamming signals. What stops the PLL from locking onto the wrong signal? Your IF Filter? Then it had better be a brickwall filter, because most IF filters I have seen can let in a little out-of-band signal, especially if you consider LO drifts, etc.

Also, what happens when there is no signal present? The VCO typically tunes to one end or the other of the power supply rail. Then lets say a weak received signal shows up. What exactly makes the VCO suddenly tune the the middle of its band and lock up? That weak signal burried in noise? You might have to add a sawtooth sweeping circuit on the VCO tune line just to assure re-acquisition.

So unless you have a very strong signal, very good signal to noise ratio, etc, a PLL FM demodulator can be a pain in the butt.

If in this case, do any1 know how to convert this FM demodulation to MathCad?

Thankz

Oh my dear ***, thank you SO much for this answer! I don't know why I couldn't see how that worked. lol I was sitting here pulling out what little hair I have left trying to explain how a PLL would demodulate better than just some random LPF connected with a wire. You've saved my arse on this report I'm writing.