[DSP-10] PLL Problems

[Gérard Parat / F6FGZ]

Tan Teik Chuan a écrit :

> I performed the PLL diagnostics and got the following readings:
>
> U104-3: 5V
> U104-4: 5V
> U7-16: 4.7V
> Across R44: 0.12V
> U105-1 In: 0.9V
> U106-1 In: 1.3V
> U8-1 In: 1.3V
> U9-1 In: 2.2V
> U104-7: 2.2V (means it is not locked?)

Yes, it is unlocked. If you can have a look with an oscilloscope, you will see
some kind of square waveform.

> U7-11: 4.7V (means it is locked?)

Might be or very close. Again, an oscilloscope trace should show a DC voltage.

> U107 and U108 were programmed correctly.

Good news, we may assume U104 and U7 as well.

> For 124-128Mhz PLL, U104-5 gave voltage about 4.7V.

You are above upper limit of lock range since U104 is unlocked as above
noticed.

> I've tried to
> temporarily connect a ground to the junction of R105/C113

So your command voltage is fixed and you can't use loop and therefore you stay
unlocked.

> , remove the core
> from L102, and even change the number of turns to 15 turns. But, it is still
> unlocked. I haven't try to change the C117 to 6.8pF because I do not have
> one. I measured frequency using a scope at "126Mhz LO". It showed about
> 127Mhz.

Yes, it is what I have done and it falls in the lock range with the L102 coil
screwed to the bottom.
To see the VCO frequency range, you can open the loop as explain before in the
archives or another mean is to vary the division ratio into U104 using UHFA
interface by changing the frequency well under the 144 MHz (there is no
software limit). Just measure the voltage at R105/R104 junction and notice when
it goes from something close to 5V to something close to 0V.
In my case, it showed with C117 at 10 pF, a locking range between 112.7 MHz on
PC screen (lower voltage = 0.31V before seeing a square waveform on U104-7) and
115.4 MHz (upper voltage = 4.76V before seeing a square waveform on U104-7).
You can compute your LO frequency by substracting 19.665 MHz to the frequency
displayed on PC screen. It is assumed that the VCO is not failed of course,
which is your case. With 6.8 pF you might be around 140 MHz or maybe above 150
MHz if you lowered L102. Anyway, step by 1 MHz and you'll see voltage at
R105/R104 varying between 4.7V and 0.3V. Notice the frequency when about 3V at
R104/R105, then compute LO frequency. Some tuning on C117 may do the job to
center the lock range.

> Since U7-11 gave 4.7V and the frequency at "19.68Mhz LO" was about 19.7Mhz.
> Can I assume 19.68Mhz PLL is locked?

If voltage at R54/U6-6 junction is something between 0V and 10V, say 4V, yes it
is.

Hope it helps.

--
73 Gérard F6FGZ