Subject: Re: Problem with PLL MB15E07SL
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Date: Tue, 26 Nov 2002 16:30:54 GMT
NNTP-Posting-Date: Tue, 26 Nov 2002 11:30:54 EST
Organization: Cox Communications
"Karsten Friedrichs" wrote in message
> I am currently working for my diploma.
> I am using Fujitsus MB15E07SL controlled via a 3Volt Pic.
> I am designing a PLL for 1674 to 1733 MHz. My VCO is a ROS1900V
> My Synthesizer works fine in the range from 1674 to 1689 then freezes and
> continues working when sitching to 1727 MHz. My step is 1MHz.My
> is exacly designed
> as the Fujitsu People described it. I checked many things:
> -DC Voltage to open LOOP. -> VCO ozillates at any desired frequency
> -check DATA Word, LE and Clock for working and not working freuquencys ->
> all OK
> What could it be ?????????
> What could I check ??????
"Freezes" doesn't convey much information. Does the VCO freeze at one
frequency, or does it stop oscillating? Or does it just get very, very cold?
If it stops oscillating with the loop closed, but oscillates fine with the
loop open, one theory is that the VCO input is out of range. That should
only happen if the phase detector saturates one way or the other, and that
should only happen if one of the phase detector inputs disappears. Since
you're feeding 1.7GHz around your board, there could be problems with the
If the frequency is freezing at a fixed frequency, then I'd guess you've got
injection locking. If it is, then the frequency the VCO freezes at will be a
multiple of some other frequency you're using - often the PLL reference
frequency. Wolaver gives a reasonably good description of injection locking.
IIRC, Wolaver's conclusion is that to avoid injection locking, the
feedthrough from the reference input to the VCO input needs to be on the
order of 60dB down. In your case, the feedthrough could be happening in the
Fujitsu chip, but it could also be happening on your board, especially if
you've routed your reference frequency or another clock line anywhere close
to your loop filter / VCO input.
-- Mike --