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 Post subject: Distorted square wave
PostPosted: Jan Sat 13, 2018 4:31 pm 
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I have a Lucent KS-24019 L108 rubidium oscillator (Reference Frequency and Timing Generator) which has a 10 MHz test point and a 10 MHz reference out. The 10 MHz reference out signal goes into a 10 MHz reference in port on the crystal oscillator portion of the redundant system. A few days ago, the output of the 10 MHz reference out became non-functional (the 10 MHz test point still puts out a nice square wave). Looking at the signal on the oscilloscope, I get the output shown in the photo. The upward spikes correspond to the rising edge of the square wave and the downward spike to the falling edge. Unfortunately, I do not have the block diagram or schematic for the instrument. What would cause the square wave to be distorted like this? Any ideas on how to approach this problem?


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 Post subject: Re: Distorted square wave
PostPosted: Jan Sat 13, 2018 7:25 pm 
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When you lose the horizontal (steady-state) portion of a square wave...

It's usually because a coupling cap opened up.
There are other possible causes, but caps are always the (my) first suspect.

Your scope photo shows a nice clean positive spike at the leading edge of the square wave, and a nice clean negative spike at the falling edge. Amplitudes are quite uniform.

That's exactly what one expects when a square wave is passed through a very small capacitance.

- Leigh

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 Post subject: Re: Distorted square wave
PostPosted: Jan Sat 13, 2018 8:11 pm 
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That certainly makes sense.. but without a schematic or at least a block diagram, I'm afraid I'm up a creek without a paddle :cry:


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 Post subject: Re: Distorted square wave
PostPosted: Jan Sat 13, 2018 8:51 pm 
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Don_S wrote:
That certainly makes sense.. but without a schematic or at least a block diagram, I'm afraid I'm up a creek without a paddle :cry:
Hi Don,

The creek is short, and not wide.

You need look only at the circuitry between the BNC with good output and the one with bad output.
There are not going to be many parts between them, only a few of which are capacitors.

Also, you could have a loose or broken connection that has the same effect as a bad cap.
I would start at the bad BNC and work backwards.
I assume you have a 'scope. That should find the problem.

- Leigh

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 Post subject: Re: Distorted square wave
PostPosted: Jan Sat 13, 2018 10:40 pm 
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Turned out to be an easy fix..

Checked all capacitors visually (under an inspection microscope) for signs of damage - found none. Checked the BNC connector - the solder joint of the center pin to the PCB looked suspect. De-soldered and then re-soldered the connection and voila! back in business again. Thanks, Leigh!


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 Post subject: Re: Distorted square wave
PostPosted: Jan Sat 13, 2018 11:36 pm 
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Wonderful, Don. Congrats.

- Leigh

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 Post subject: Re: Distorted square wave
PostPosted: Jan Sat 13, 2018 11:56 pm 
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That waveform is what I've seen when a square wave is coupled through an undersized cap, you see it charge and discharge on each reversal. You had enough stray capacitance across the broken solder joint.

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 Post subject: Re: Distorted square wave
PostPosted: Jan Sun 14, 2018 4:52 am 
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It was pure luck that it was the BNC connector and not one of the SMD capacitors. I don't know if I would have been able to find the bad capacitor!


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 Post subject: Re: Distorted square wave
PostPosted: Jan Sun 14, 2018 5:00 am 
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Don_S wrote:
I don't know if I would have been able to find the bad capacitor!

Touch an AC-coupled 10X scope probe to each side of the capacitor.

Should be the same waveform both sides (may be a bit different, but not much).

- Leigh

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