PLL demodulator - RF Cafe Forums
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Post subject: PLL demodulator
Unread postPosted: Wed Jan 21, 2004 5:58 am
tell me about how the pll demodulator works? what are the limitatations of the same compare with the conventional
In a narrow band system, what is the significance of the VCO or VCXO in PLL demodulator?
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.
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,
Unread postPosted: Thu Jan 22, 2004 3:52 pm
Wed Aug 20, 2003 11:47 am
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
>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.
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.
Unread postPosted: Thu May 06, 2004 2:49 pm
Joined: Wed Aug 20, 2003
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.
Post subject: dual or multiple
Unread postPosted: Fri May 07, 2004 3:12
that's right. if there is weak signal it might not lock. And the analog still spits out some dirty
Post subject: PLL Demodulator
Unread postPosted: Fri Jun 18, 2004
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.
Unread postPosted: Wed Sep 15, 2004 4:03 am
where can i
find a basic block diagram of a pll circuit?