From: Chuck Simmons
Organization: You jest.
X-Mailer: Mozilla 4.61 [en] (X11; U; Linux 2.0.33 i586)
Subject: Re: How to perform clock recovery from 1152 kB/s as easy as possible ?
Date: Sat, 07 Sep 2002 14:52:43 GMT
NNTP-Posting-Date: Sat, 07 Sep 2002 07:52:43 PDT
"Thomas S." wrote:
> I have a digital transmission with 1,152 Mbit/s coming out of a
> DECT RF Receiver IC.
> At the moment I simply feed the BB output into a Comparator that is
> adjusted around the
> Rx Data Outputs DC Offset (some 1.2 V). Gives more or less exactly back
> what I have fed into the Transmitter
> but with still alot jitter , as you can imagine.
> My Burst is mostly empty, filled with F0F0.. bytes to achieve DC
> Balancing in TX PLL modulator. So, I should have a rather
> strong Component at 1152/8 = 144 kHz in my Baseband Signal. Can i try to
> filter this out and use it in
> a PLL (x8) to get CLK back ? are there simple ICs available for this ? I
> only found optical SONET CDR ICs yet.
> in my Transmitter i can feed in only "1" and "0" binary , so no AMI
> Codes are possible. I want to avoid Manchester too.
> Maybe someone here made a rather simple Implementation of that .. in DSP
> or uC ?
You will have to use a run length limited code (RLL) or a code that
resynchs at each symbol boundary (teletype codes really). Basically, a
PLL is about all you need for clock recovery if you use an RLL code. The
PLL must be capable of harmonic lock but this is easy. Basically, you
will have to decide on the maximum run length (maximum number of raw bit
clocks without a data transition). Manchester code is very inefficient.
There are much better codes. Encoding 2 user bits into 3 channel bits
was very common in disk drives for years. CD and DVD read channel chips
can do what you want but may be overkill because of other functions.
Also, these chips may be a bit clumsy to program since they expect
certain patterns in the channel (14t in DVD and 11t-11t in CD). Still,
the PLL idea will work.
... The times have been,
That, when the brains were out,
the man would die. ... Macbeth
Chuck Simmons firstname.lastname@example.org