Diagnosing PLL that won't lock

crutschow

Joined Mar 14, 2008
25,123
Sorry I don't have the patience to read and digest that long thread. ;)

However, one reason for a a PLL not locking is that the VCO frequency is not within the range of the input frequency (or multiple thereof if there's a frequency divider in the loop).
Has anyone checked the VCO frequency to see if it seems correct?
 

MisterBill2

Joined Jan 23, 2018
6,330
It seems that this is an existing design that used to work, but now it no longer functions correctly. A sawtooth voltage on the oscillator control line indicates that either it is in a search mode, or that the oscillator is not working right, or that the phase detector or the filter have changed.
Since it apparently was a working commercial product, my first guess is that the oscillator circuit has changed, although it may also be that the reference oscillator circuit has a problem. The last PLL circuit that I had to repair had an intermittent tuning diode in the oscillator circuit. The voltage controlled oscillator portion does seem to be where many of the problems are found.
 

danadak

Joined Mar 10, 2018
4,057
Could be loop filter component has lifted ground connection so it
is no longer properly filtering output of phase detector. Or a element
of the phase detector has changed its value dramatically. Like a leaky
cap, carbon R that has shifted, absorbed moisture.


Regards, Dana.
 

MisterBill2

Joined Jan 23, 2018
6,330
Could be loop filter component has lifted ground connection so it
is no longer properly filtering output of phase detector. Or a element
of the phase detector has changed its value dramatically. Like a leaky
cap, carbon R that has shifted, absorbed moisture.


Regards, Dana.
Since the output of the loop filter is claimed to be a sawtooth waveform that tends to indicate that the filter portion is functional. But since we have no more information about the rest of the system it is challenging to do much more than guess.
 

danadak

Joined Mar 10, 2018
4,057
Since the output of the loop filter is claimed to be a sawtooth waveform that tends to indicate that the filter portion is functional. But since we have no more information about the rest of the system it is challenging to do much more than guess.
Google PLL output waveforms, you will see many variants in waveshape.
Depended on filter, first order, second order, if it is integrator dominant then yes,
triangle wave dominant feature. but if lead/lag whole other matter. And it
matters what phase detector implementation is.

Regards, Dana
 

Thread Starter

Ylli

Joined Nov 13, 2015
965
Thank you all for the answers. I realize it is a long thread over on AK. Let me summarize:

1. After some troubleshooting, it was found that the voltage on the tuning line had a sawtooth of approximately 5 V p-p at 20 Hz on it. The measured DC however, did match the 9 - 20 volts from the low end to the high end of the band as in the receiver test spec.

2. I had the poster open the loop by removing a resistor between the loop integrator and the tuning line. That did leave some bulk cap and a trap on the tuning line. Checked for leakage on the tuning line, found none. External power supply on tuning line allowed receiver to be tune over it's normal range.

3. Poster has good scope. PLL chip has the divided outputs available as test point. Found the divided reference frequency at output at 1 KHz. Divided output of LO signal (after tweaking the externally applied tuning voltage) could also be adjusted to 1 KHz. Phase detector output is questionable, but that has always been difficult with the loop open.

4. Only a couple posts beyond this. https://audiokarma.org/forums/index.php?threads/looking-for-assistance-on-pioneer-sx-d7000-tuner-issue-long.883443/page-3 . PLL pin 11 is the divided LO output, pin 16 is the phase detector output , and pin 19 is the divided reference frequency output.
 
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Thread Starter

Ylli

Joined Nov 13, 2015
965
Capture2.JPG Capture1.JPG Capture.JPG
R1 dot on ground should not be there. Grounding base of Q19 made no difference.

Edit - I see I cut off a resistor in the 3rd pix. Q42 has a 6.8K resistor from collector to +32 volt supply.
 
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MisterBill2

Joined Jan 23, 2018
6,330
Perhaps the input signal to one of the divider strings is not adequate amplitude. A divider not working quite right will certainly cause problems. And looking at small parts of a system makes following it a challenge.
 

Thread Starter

Ylli

Joined Nov 13, 2015
965
Yup, got the SM.

Perhaps the input signal to one of the divider strings is not adequate amplitude.
The output from the reference divider looks quite clean. Got the impression from the poster that the divided LO output was clean too, but I don't think he posted a picture. Will double check that issue.
 

bertus

Joined Apr 5, 2008
20,568
Hello,

Is the PLL producing the sawtooth in FM and AM mode?
There are buffer fets at the inputs of the prescaler.
There is also a buffer fet between the prescaler and synthesyser:

pioneer_sx_d7000_PLL.png
This is the principle schematic.

Bertus
 

Thread Starter

Ylli

Joined Nov 13, 2015
965
Yes, it behaves the same in both AM and FM modes. Here is what the poster is seeing on the tuning line. Pin 61 is a test point on the tuning line. First is with the scope AC coupled, second is with the scope DC coupled.
Capture.JPG Capture1.JPG
 

bertus

Joined Apr 5, 2008
20,568
Hello,

Are the signals on the pins 12 and 20 of the synthesizer clean?
Is the signal on pin 16 of the synthesizer also showing a sawtooth?

Bertus
 

MisterBill2

Joined Jan 23, 2018
6,330
Just because the output looks OK does not mean that it is counting correctly. If the input is not right there can be several different problems and still have a nice waveform at the output.
 

bertus

Joined Apr 5, 2008
20,568
Hello,

When the signal is shifting due to the changing tuning voltage, I would expect the signal on pin 12 to jitter.
The signal at pin 20 should be stable, as that is the reference signal.

Bertus
 

Thread Starter

Ylli

Joined Nov 13, 2015
965
Are the signals on the pins 12 and 20 of the synthesizer clean?
We haven't look at those. I assumed that since the digital outputs are clean, the analog inputs were clean and sufficient.

Is the signal on pin 16 of the synthesizer also showing a sawtooth?
If I understand the posters responses correctly... With the tuning voltage set so that the LO is running below the frequency set on the receivers digital display, the output at pin 16 is low with no output pulses. As the tuning voltage is increased to get the LO running above the digitally set frequency, positive going pulses are observed, with the pulse width varying from narrow to wide (or vice versa). Single sweep scope picture attached. The waveform seems to repeat about every 8 ms - but I would imagine this changes depending on the difference between the reference and LO frequencies. When the loop is closed, the sawtooth on the tuning line has a period of around 50 ms (20 Hz).

Capture.JPG

In my experience, the behavior I am seeing is indicative of leakage on the tuning line. We did check that with an ohmmeter, but we did not test it for current draw with a higher tuning voltage applied - may be a diode breaking down. May have the poster try that.
 

MisterBill2

Joined Jan 23, 2018
6,330
It almost looks like some power supply ripple on the tuning voltage line. Have the DC supply voltage lines been checked with a scope to verify that they are free of ripple and noise? That should have been done quite early in the diagnostic sequence.
 
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