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PLL demodulator - RF Cafe Forums

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Below are all of the forum threads, including all the responses to the original posts.


Siva
Post subject: PLL demodulator
Unread postPosted: Wed Jan 21, 2004 5:58 am

Can anybody tell me about how the pll demodulator works? what are the limitatations of the same compare with the conventional one?
In a narrow band system, what is the significance of the VCO or VCXO in PLL demodulator?


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Guest
Post subject:
Unread postPosted: Wed Jan 21, 2004 4:56 pm

The PLL demodulator works in a similar way to a regular PLL loop. The only difference is that the VCO, which is in the in the feedback loop is set to the carrier frequency.

The input to the demodulator is a frequency modulated (FM) IF signal, which is fed to a phase detector together with a VCO frequency that is locked to the carrier frequncy.

The purpose of this demodulator is to demodulate FM, hence the frequency deviation (from the carrier) - which is the modulated information forms a correction voltage, and the loop follows this deviation which is actually the information being demodulated.

The VCO has to be very accurate in terms of temperature and stability.

The advantage of PLL demodulator is that it can improve the sensitivity of a Receiver by up to 3dB, because it neutralize the phase interference.

Hope this helps,
Itay Reiss


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kpainter
Post subject:
Unread postPosted: Thu Jan 22, 2004 3:52 pm
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Joined: Wed Aug 20, 2003 11:47 am
Posts: 47
Location: Santa Barbara, CA
Your description of a PLL demod is accurate enough however,

>The advantage of PLL demodulator is that it can improve the sensitivity >of a Receiver by up to 3dB
I have never heard this and in fact, my experience is that PLL demods are much worse than discriminators.


>because it neutralize the phase interference.
This doesn't make sense. If the job of the PLL is to track the incoming frequency deviation (noise is also deviation, is it not) how can a PLL descriminate the signal from the noise. The PLL cancels the noise of the VCO only. In fact, a large phase change (i.e. noise) on a weak signal can throw the loop out of lock and it is no longer a demodulator.


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guest
Post subject:
Unread postPosted: Wed May 05, 2004 4:27 pm

the PLL demod works quite well. Don't know why folks are afraid to use it.


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kpainter
Post subject:
Unread postPosted: Thu May 06, 2004 2:49 pm
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Joined: Wed Aug 20, 2003 11:47 am
Posts: 47
Location: Santa Barbara, CA
>the PLL demod works quite well. Don't know why folks are afraid to use it.

I didn't say they didn't work. Just not as well as a discriminator as they can come unlocked in low SNR situations - a discriminator doesn't. At issue here was the assertion a PLL cancels noise of the of the incoming signal. If you believe that, I suggest you look into how a dual point modulator works.


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guest
Post subject: dual or multiple
Unread postPosted: Fri May 07, 2004 3:12 am

that's right. if there is weak signal it might not lock. And the analog still spits out some dirty bits.


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Guest
Post subject: PLL Demodulator
Unread postPosted: Fri Jun 18, 2004 1:03 pm

The loop filter of a PLL used as a demodulator must be designed properly to work in a low Carrier-to-Noise situation. However, when designed properly, PLL's allow the demodulator to function at lower CNR's than discriminators, due to an effect called "threshold extension". That allows the output SNR curve to continue down at only a dB/dB as CNR gets small, until the CNR gets close to the theoretical minimum and then the output SNR "falls off the cliff". If the modulating signal is band-pass (no DC component), then the VCO long-term stability isn't so important. The phase noise (short-term stability) is critical, as it shows up as an error voltage, just as modulation does. VCXO's usually have a maximum frequency at which their frequency can be changed - this has to be taken into account if a wide-band signal is being demodulated.

Hope this help.


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whatanohm
Post subject:
Unread postPosted: Wed Sep 15, 2004 4:03 am

where can i find a basic block diagram of a pll circuit?



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