A very general troubleshooting question... on PLL/Frequency Synth.

[guitar_199]

I've been studying these circuits lately and have discovered that there is one thing that I do not have a good sense for.

When you take a scope and look at the reference oscillator test point or the VCO test point....what kind of level do you expect to see? Are we talking microvolts....or higher? In other words you are just checking to see if the oscillators are working.....what kind of level signal are we talking (in general of course!)?

Thanks!

Bob

 

[nomadradio]

Umm, "reference oscillator test point"?

Something you would probably find only on a really old PLL CB. Not a standard thing that would be the same on every radio.

"VCO test point" probably refers to the VCO's DC tuning voltage. That would be a DC voltage that changes in step with the selected channel frequency. Typically depends on the supply voltage to the PLL chip. Usually either 8 Volts or 5 Volts DC depending on the type PLL chip. The actual tuning range from minimum to max will vary from one manufacturer's design to the next.

Some older Cybernet-made radios used a sort of reverse-voltage tuning. The cathode of the varactor diode would be connected to the regulated positive-supply voltage. The anode of the varactor would be fed from the tuning voltage. What makes this "reverse" is the the higher the tuning voltage from the PLL chip, the lower the actual voltage difference that's placed across the varactor diode. Makes a higher tuning voltage from the PLL tune a lower frequency. This also causes the binary code feeding the PLL chip to be backwards. The highest binary value feeding into the PLL is the lowest freuqency. The binary codes count down as the channel frequencies go up. Most every CB made since the mid-1980s has the varactor diode's anode grounded. The cathode terminal has the DC tuning voltage fed to it. This makes the frequency go up as the tuning voltage rises. Binary codes also rise as frequency goes up.

Not all radios are created equal. The specific design quirks of a CB radio's PLL section can vary from one radio type to the next.

73

 

[guitar_199]

Well, it is a pretty old Cobra 21 GTL that used to belong to my father in law. :et me upload a couple of shots from the Sams Photofact for it. The upper is the 10.24 reference oscillator (notice TP10) and the lower is the VCO (notice TP3).

I'm simply trying to insure that both oscillators are running but do not know what signal amplitude to expect. Is it on the order of microvolts? millivolts? volts? Just so I can apply the probe and say... "yup that one looks good".

Putting it another way, if you had a dead radio and were probing with a scope to make sure that they were running.... what level p-p would make you happy?

 

[Handy Andy]

How well do you trust the reticular graph scale of your scope?

Per your Sams, we know we already have limits on the possible levels the scopes probe could have.

No more than power supply, and no less than 1 V - per the PLL to Varactor voltage - per your Service Manual channel you test on - be it Channel 1, 40 or the Mid-band 20.

As a good rule of thumb, I set my scale to Volts, multiplier 10 - gives you 10V starting range - which you can back off until you see a waveform you can use for tuning.

On most (to me IMHO) Test points for Signal - there is also the issue of "offset" which if you want AC only - just remember to set Scope accordingly. For if you don't - your signal may drift or skew upwards and you have to recenter the gun (older CRT ray-trace) or make sure the input is AC only after you lost the waveform if you are trying to get PLL to lock in.. Don't always presume the Probe is insulated tip from DC - unless you are using it for RF only (True RF probe) - then the AC value will pass but should keep DC at bay - even then - loading may be an issue so be prepared for it.

Another aspect people don't always recognize is the errors you get in assembly - this isn't from you, not the customer even - it's from the factory.

I'm sure as you look around the board, some TP are the raised ends of resistor legs of the "Stand up" and they are silkscreened.

In rare cases, but it does happen, you can scope the TP (like TP3) and get some offset voltage problems because you set your test probe on the resistor - but the resistor is INSERTED/INSTALLED the wrong way. Check the radios' silkscreen too, for the proper orientation of the Resistor - if you test the wrong side of the lead thinking you are at the right Test Point, you may be there - but the Test point Resistor or cap - you're testing, you inadvertently are probing the wrong leg and now you have different results than what you expected - either by loading effects or by voltage presence affecting the readings.

You bring up a good point and more than once I've been stung by someone or factory - making changes to cover up a fix, or carelessness from a previous repair - and they put a resistor oriented in reverse than what it should be..

 

[guitar_199]

In my case, I am not wanting to qualify the oscillator output as perfect so I am really not even concerned with measuring it....per se.... just want to know if it is alive. The only issue is... I have no idea what level it should be. So I am not looking for a detailed "what level should I measure"..... just an ......"ohhh those things are usually 1 to 2 v p-p" or something like that. Just a sloppy indication of what a working reference osclilator or VCO looks like.....

 

[Handy Andy]

That's what I mean, it's relative.

And if you don't know - play it safe...

No sense in destroying inputs for the sake of seeing a P-2-P value clear enough...

 

[guitar_199]

ok.

 

[kopcicle]

The simplistic view is does it have a DC voltage that changes with vco slug adjustment? Then does it track the channel selector predictably?

Remember I'm a hack

If not then a component by component search is necessary.

 

[Bobcat]

I understand how PLLs work ... Use phase difference to create a control voltage that tunes in the VCO.

I'm just trying to get a handle on ... If I put my scope probe on a buffered test point on the VCO..... Would I see a great big signal? Or would I see a little bitty tiny signal?

That is all I am looking for...

 

[Handy Andy]

Good question to ask - but think of that scenario as - what does the PLL see in return?

Then look for the two signals that will mix, one with be from the PLL the other with be the local oscillator. They get combined and a signal "loops" back to the PLL to "check it".

The PLL is not going to need much signal level to "Sense" and correct any error or drift - it also doesn't want or need all the stuff that those sections generate - it uses a Low pass filter that attenuates the 17MHz and 10MHz stuff - what is left is usually less than 2MHz - enough of a window for the PLL to check for accuracy of the mixing products beat together to form a heterodyne and it only needs the low-frequency end of it to compare.

Unless you're dealing with tubes, you don't need anything more than 10V - with 2 volt scale being typical.