Re: PLL question

From: John Smith (kd5yikes_at_mindspring.com)
Date: 01/13/05 

Date: Thu, 13 Jan 2005 15:40:34 GMT

"Tp" <eg8980@kolumbus.fi> wrote in message
news:cb4763ac.0501122314.7a67cf6@posting.google.com...
> Hello!
>
> I'm building a frequency multiplier using a PLL 4046. I'm thinking to
> use microcontroller as a dividing element in the feedback loop. I'm
> using a phase comparator II. There is a few us delay in dividing
> process due to microcontroller and I'd like to know does is cause any
> problems.
>
> Tp

I'm doing the same thing. I am using a 1 MHz reference oscillator
(ECS-100AC), an MM74HC4046N, and a couple of counters in a Cypress
microcontroller. I divide the 1 MHz down to 10 kHz. My 4046 VCO is running
about 1.6 MHz and is divided by 160 for the 4046's phase comparator II. I
only care about the frequency of the 1.6 MHz signal, not the phase.

The only problem I've had is phase noise (jitter). I have determined that
the 1 MHz reference is rock steady. The 4046 VCO is very steady, but only if
I disable the 1 MHz oscillator and the micro and drive the VCO frequency
control with a fixed DC. When the loop is closed, the VCO control voltage is
steady, indicating no loop oscillations. I conclude that there is 1 MHz
noise or microcontroller noise getting into the 4046 somewhere (Vcc is
bypassed at each chip).

It appears that I need to get the circuit off the breadboard (the type where
one plugs the component into holes with spring contacts) and put it on a
piece of copper ground plane and bypass the chips with short-leaded caps. I
have been surprised to learn that even a half inch of component leads can
cause problems even at this low frequency.

Anyway, be aware that you can have phase noise problems if you are not
careful with your layout, bypassing, or filtering.

Good luck.

John

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