[time-nuts] PLL suggestions

[Richard (Rick) Karlquist]

You need to determine your requirements for phase
noise.  You also need to coordinate this PLL with
the phase noise of the 3.125 GHz PLL, which will
need its own VCO.

The best VCXO's I have found are from Abracon,
COTS in distribution (DigiKey, etc) for only
about $25. Lucky for you that 125 MHz is a
standard frequency for them. At 3.125 GHz there are several
vendors such as Z-comm to browse.  You need
to find one with 3.125 GHz output frequency and then
it needs to be low phase noise.  May not be
available.  It's just the luck of the draw
as to what they choose to make.

Any PLL chip with a divider and on board phase
detector will give only mediocre performance
(which may be good enough for you).  And it
will require a microcontroller to load the
gazillion registers onboard (no strapping).  For
high performance, you need a conventional
multiplier from 10 MHz to, say, 120 MHz and
then mix down to 5 MHz and use a mixer as
a phase detector.  You can divide the 10 MHz
by 2 using a 74AC74.

You decide: performance vs cost/simplicity.

Rick N6RK


On 12/31/2019 8:01 AM, Dan Kemppainen wrote:
> Hi All,
> 
> We've got a project going on where we need a 125MHz clock that should be 
> locked to 10MHz. I'm considering an on board 10MHz oscillator and 
> external 10MHz input to utilize an external standard to lock the 125MHz 
> which would PLL a 125MHz source.
> 
> We've identified a few VCXO's that seem to be low noise and are small 
> enough. The 10Mhz is easy enough. However my stumbling block is a simple 
> PLL.
> 
> What I'm looking for is a simple, small PLL chip. Something with 
> external analog loop filter components to allow easy tweaking would be 
> preferred.
>