Frank,
No you don't have to worry about the phase oft he signal going into the 151
because the phase detector is edge triggered and the delay is constant. Worse
case the frequency may be off by one count of the divide by n but it is getting
late so that may not be true either. If it is a problem then add another stage
of 04 in series.
I would think the loop under damped if it is oscillating wildly. (About the
frequency of the reference into the phase detector) This measn each sample of
the phase detector results in over compensation or driving the VCO too far. The
next sample results in over compensation it the reverse direction of the
control voltage. I would try increasing the R or C. This will reduce the change
in frequency per sample so the phase detector can eventually lock.
Say the control voltage is swinging slowly at less than phase detector
reference frequency. This could mean the phase detector can never drive the vco
far enough per sample to lock. This would mean it is over damped and the phase
detector can't catch a lock because there is a big lag. This case requires
reducing the R and C value. You could put a pot in place of the R and tune it
across the range while observing the VCO control voltage stability.
Power supply ripple on the VCO can drive a loop crazy.
Designing a good synthesizer is the hardest part of a home brew RX project.
Good luck. Surf around the web I have run into design software that will help
you understand what is going on. Rohde has some good books and then there is
Floyd Garner's Bible which I'm sure is out of print.
The 152 has a lock output pin and maybe your chip has one. This is another
thing you can watch to learn what is going on.
If the divide by N or divide by R are wrong you will never lock. Be sure you
have the right data accounting for the external divider. Also make sure the
divider is working properly. This is an easy test. Just disconnect the vco
control voltage ans first set it to a low voltage then the high rail. Make sure
the divider works at both ends of the range.
Say you are not sure of the divider. Drive the VCO control voltage with a
variable supply and watch the phase detector. Yuo will see it go almost stable
at one voltage as you go past it.
When things are right you will have a lock and the two outputs of the phase
detector will each take turns making small pulses to hold the vco on frequency.
Then you can fine tune the filter.
hope this helps Good Luck. Frank WA1GFZ
Frank <[EMAIL PROTECTED]> wrote:
--- In [email protected], FRANCIS CARCIA <[EMAIL PROTECTED]>
wrote:
>
> Look at the vco control voltage with a scope. Say it is slammed against a
> rail high or
ground. This means the reference divider or the divide by N are not set up
properly.
> If it is oscillating then the filter is wrong. surf old motorola for special
> functions for
Yes, the VCO voltage is jumping up and down on the scope. I hooked up a bicolor
led
on the double ended PLL error signal pins (phi_R phi V).
>applications information. The applications will tell you how to determine the
> constants you need. Usually you set the filter corner frequency at 1/10 the
reference into the phase detector for starters. I did a 145152 in my homebrew
rx. fc
Ok, I'll test out the BW=1/10 ref frequenency (i.e. crystal freq divided by R).
Another question, I made my own prescaler in the sense I took the VCO buffered
by a
NPN follower, followed by a 74AC04 NOT gate with a feedback resistor, another
NOT
gate and finally a 74AC163 to make my prescaler to finally go in to the
MC145151,
which internally has its own NOT gate with feedback resistor. Should I be
worried
about the phase of the divided down VCO output coming into the MC145151?
Frank
>
> Frank <[EMAIL PROTECTED]> wrote:
> Hello,
> I would like some generic rule of thumb rules for a MC141515 single modulus
> PLL
> using a simple 1st order RC filter. The datasheet gives the bandwidth of the
> loop as
a
> function of the other parameters, but I don't know what it should be. How
> much
> bandwidth is enough, all I know is that my circuit does not lock? In reality,
> for an RC
> filter I need to know these values as a function of all the rest (Kphase,
> Kvco, N).
> Any advice, to teach me to fish please?
> Frank
>
>
>
>
>
>
> [Non-text portions of this message have been removed]
>
[Non-text portions of this message have been removed]
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