Clock issues

[josha]

Hi everyone, hope someone can help

I've had a clock running in a TDA1514a based player for a while and as part of applying a DEM mod I've been poking about with the scope.

The clock provides 11.2896 plus /2 & /4 - I was trying to get /8 from somewhere but that's another story.

The issue here is that while the /2 and /4 are both reasonably square and 5V peak to peak the 11.2896 is a sine wave of 2v peak to peak.

Is that normal?

Also when I touch the 11.2896 output with the probe I get distortion, again - is this normal?

Apologies if these are simple questions, I've googled and searched here.....

Thanks in advance, Josh

[marce]

Scope probes load the clock line and have to be factored in when viewing a signal, they alter the charateristic impedance.

Probing High-Speed Digital Designs
http://www.tek.com/Measurement/App_N...ng/probing.pdf
http://www.isotest.es/web/Soporte/Fo...%C3%B1ales.pdf

[josha]

Thanks Marce, much appreciated. I've had a look over the first two (the third ones blocked, I'll have a look when I get home) and it seems this could certainly explain the distortion as I'm using a fairly basic capacitive probe.

I'm still not sure why I can see the 5v p-p on the /2 /4 but only 2v p-p on the main clock, it's a big difference that surely can't be explained by the probe alone.

I'm not sure it adversly affecting the cdp but it does seem a bit odd.

Cheers, Josh.

[Ken Newton]

In addition to the probe loading effects identified by marce, another issue might be the bandwidth of your o-scope. High frequency clock signals will start to look sinusoidal and show reduced amplitude if your scope doesn't have sufficient bandwidth. This effect would definitely occur in observing an 11.2896MHz clock, should you have a 10MHz or even a 20MHz scope, for example.

The traditional rule of thumb is that your scope needs 10x the bandwidth of the clock signal you are observing to maintain the waveshape and amplitude. None of which means that a lower bandwidth scope isn't still very useful for verifying the existence and frequency of such clock signals, which are often enough to know for troubleshooting purposes.

[josha]

Good point Ken, there's a lot to consider here it seems. I'm using a 20MHz scope, I get a nice clean looking sine wave from the 1x of the clock but at just under half the amplitude of the /2 and /4, could this be a product of the bandwidth limitation?

[Ken Newton]

Originally Posted by josha

Good point Ken, there's a lot to consider here it seems. I'm using a 20MHz scope, I get a nice clean looking sine wave from the 1x of the clock but at just under half the amplitude of the /2 and /4, could this be a product of the bandwidth limitation?

Yes, I would say that it probably is, especially if the 11.2896MHz clock signal divided by four looks a little better than when divided by two. In effect, the insufficient scope bandwidth is low-pass filtering the clock signal, changing the on-screen waveshape to a sinusoid and attenuating the amplitude.

My guess is that there isn't anything wrong with your circuit or equipment. You probably have been observing the effects of a too low scope bandwidth relative to that 11MHz clock. I have a 60MHz Tektronix scope and clearly observe the same sort of effects you describe when I view clock signal frequencies of half the scope's rated bandwidth. It's usually enough for me to know, however, that the clock signal is actually there and of the correct frequency, two facts which you still can well determine even though the waveshape and amplitude may be quite wrong.

[josha]

Well, that's a relief!

I think the fact that clock looks exactly half the amplitude of the /2 and /4 fooled me into thinking I had a problem, add to this the shape and despite having a perfectly happy cdp I though I had a problem!

I really appreciate your help, I find myself learning more, and having more respect for the people involved in this hobby every day.

Now....back to the soldering iron!

clock.JPG

clock-2.JPG

clock-4.JPG

[Ken Newton]

josha, one more thought. It may be that the output amplitude of the 11.2896MHz clock generator is, indeed, lower in amplitude than the /4 and /2 signals out of the clock divider logic. Clock oscillator circuits drive high frequency signals into the capacitive load of whatever they are connected to, which often represent quite a low impedance at the multi-MHz frequencies involved. This can and often does load down the actual output signal level of these clock generators. So, while at least part of the reduced amplitude you observe is undoubtedly due to insufficient o-scope bandwidth, it may not all be due to that. In any event, it sounds to me like all is well inside your player.

Regards,
Ken