PLL question

[Tp]

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

 

[Andrew Holme]

Tp wrote:

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.

The delay will cause phase shift. It's not a problem if the delay is
constant: it just means the VCO edges won't coincide with reference
edges - they'll be slightly offset.

A variable delay (e.g. due to interrupts, jitter on the CPU clock or
concurrent processing) would cause phase modulation (and therefore
frequency modulation) on the VCO.

If your microcontroller has a dedicated counter, which is clocked by
the VCO, and which toggles its output without software intervention,
then you should be fine.

 

[Joerg]

Hi John,

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.

That's the ticket. I have never seen any RF stuff work well on these
plug-in boards. Therefore, we do not have any of these boards in the
lab. What I found to work really well are little islands of parts on
copper clad. Wainwright strips are used a lot but cut up pieces of
copper clad and some glue would also do. It doesn't look pretty and the
more you touch the copper the uglier it becomes. But those kinds of
prototypes can operate nicely into the GHz range.

Regards, Joerg

http://www.analogconsultants.com

 

[John Smith]

"Joerg" <notthisjoergsch@removethispacbell.net> wrote in message
news:ClAFd.10132$wZ2.5558@newssvr13.news.prodigy.com...

Hi John,

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.


That's the ticket. I have never seen any RF stuff work well on these
plug-in boards. Therefore, we do not have any of these boards in the lab.
What I found to work really well are little islands of parts on copper
clad. Wainwright strips are used a lot but cut up pieces of copper clad
and some glue would also do. It doesn't look pretty and the more you touch
the copper the uglier it becomes. But those kinds of prototypes can
operate nicely into the GHz range.

Regards, Joerg

http://www.analogconsultants.com

Hi, Joerg -

Yes, that's the way I normally do it when higher frequencies are involved.
This time I was led astray by the 1.6 MHz maximum frequency. Stupid me. I
wasn't thinking about the nanosecond rise and fall times of all three
devices.

John