PLL Demodulator - RF Cafe Forums
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?
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,
Unread postPosted: Thu Jan 22, 2004 3:52 pm
Joined: Wed Aug 20,
2003 11:47 am
Location: Santa Barbara, CA
description of a PLL demod is accurate enough however,
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.
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
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 11:47 am
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 am
that's right. if there is weak signal it
might not lock. And the analog still spits out some dirty bits.
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
Hope this help.
Unread postPosted: Wed Sep 15, 2004 4:03 am
where can i find a basic block diagram of a pll circuit?