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PLL demodulator - RF Cafe Forums
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lots of great posts in the archive that ware worth looking at.
Below are the old forum threads, including responses to the original posts.
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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
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
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,
postPosted: Thu Jan 22, 2004 3:52 pm
Joined: Wed Aug 20, 2003 11:47 am
Santa Barbara, CA
Your description of a PLL demod is accurate enough
>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
>because it neutralize the phase
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
PLL demod works quite well. Don't know why folks are afraid to use it.
Thu May 06, 2004 2:49 pm
Joined: Wed Aug 20, 2003 11:47 am
>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
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
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.
Unread postPosted: Wed Sep
15, 2004 4:03 am
where can i find a basic block diagram of a