Because of the high maintenance needed to monitor and filter spammers from the RF Cafe Forums, I decided that it would
be best to just archive the pages to make all the good information posted in the past available for review. It is unfortunate
that the scumbags of the world ruin an otherwise useful venue for people wanting to exchanged useful ideas and views.
It seems that the more formal social media like Facebook pretty much dominate this kind of venue anymore anyway, so if
you would like to post something on RF Cafe's
page, please do.
Below are all of the forum threads, including all
the responses to the original posts.
Post subject: pll - active loop filter Posted: Wed Sep 28, 2005 7:30 am
Do any one
have a design or example circuit of active loop filter ?
and what are the advantages of the active over the
passive filter and how does it effect the vtune ?
Thank you ....
subject: active filterPosted: Wed Sep 28, 2005 12:51 pm
Active filter is useful in a case when
the tunning voltage from
the IC PLL\Phase detector is not fit to the tunning voltage of the VCO.
case you must use Active filter to amplified the voltage.
but, it will increase your phase noise (~6dB) in the
I dont have an exemple but you may find one at the demo`s from the
RF software like EagleWare
, AWR etc.
Post subject: Posted: Wed Sep 28, 2005 2:32 pm
check out National Semiconductors for starters, in case you already haven't.
I'm sure you can find good
app-notes on this website.
Post subject: active loop filtersPosted: Wed Sep 28,
2005 3:03 pm
1. Active PLL filters look very much like classical op-amp integrators, except there are one or
more added resistors to set the frequency response.
2. One advantage is that you can get higher-order
loops. This gives you the possibility of tracking a changing frequency better. This may also give you more
reduction of the spurious frequencies that appear at a delta of the reference frequency above and below the
oscillator's output frequency. These are sidebands, and are undesired garbage that you try to minimize.
Another advantage that you can (but don't necessarily) get is the ability to drive the phase error to zero.
4. The effect on noise depends on the noise level of the amplifier used in the active filter. There are some
amazingly quiet opamps available these days!
Posted: Mon Oct 10, 2005 12:32 pm
ADISimPLL from Analog Devices is a program that will help you simulate the
effects of using an active loop filter.
The Analog parts are generally better than the national parts I