I was working for an avionics maintenance facility where a variety of military avionics systems were tested, repaired, and overhauled. I was part of the team that provided engineering support for the repair floor. My job was to troubleshoot test software and hardware, provide tooling, and sometimes solve problems with incurable chemical concoctions. I had to solve the problems that no one else could (or wanted to).
On one assignment, I had to handle an unusual problem with a temperature-controlled oscillator that was being used as a radar receiver. The automated test program that was testing the receiver reported sideband amplitudes well above the maximum test limit. The technologists had no idea why.
They removed the faulty oscillator and replaced it with another, but the problem persisted. They removed the oscillator from the receiver and performed a bench test using a power supply and a spectrum analyzer. The bench test produced perfect results on both test oscillators, with all sideband values within limits. The oscillator was then reinstalled into the receiver, and the problem returned.
They experimented on the bench with varying the load and heat cycling the oscillator, to no avail. When the problem came to me, I performed their testing methodology, and it appeared to all of us that the only thing left to examine was the radar receiver power supply. I asked the technologists if they had checked the quality of the power supply. Obviously annoyed that I assumed they hadn't checked the power supply, they assured me that it was nice and flat, like all DC power supplies should be. They went further to say that the ripple value was in tolerance, as well. I felt a bit apprehensive about posing further power-supply inquires.
We then explored the possibility that there might be a grounding issue, since the frequency of the oscillator was quite high. We moved the power supply from the receiver and tried a variety of grounding techniques, with no success. After some time working this job, you get an intuitive sense of where to look.
Sometimes knowing too much and exploring too many options masks the simplicity of the real problem. By asking the basic questions and doing the due diligence in terms of the general state and maintenance history of the oscillator, you would have been on the trail of the problem much earlier. But you're absolutely right that it's always easier to come to this conclusion in hindsight. How come we never learn??!!!
Looks like the solution came from not taking anything for granted. Over and over with these Sherlock Ohms problems and solutions, the answer comes when the most simple (and thus invisible) assumptions are challenged.
One of the first things that jumped out at me was the comment about being "in tolerance." Sometimes I find people that are looking to make a change to this or that for whatever their personal reason/goal is and all they care about is if the parts are "in tolerance". What happened to wanting to make sure the end product was of good quality and made as cost effective as possible?
A classical example of how misleading digital scopes can be.
An analog scope wouldn't have hidden the problem--you'd have had a broad stripe across the display at low sweep speeds. When using digital scopes, I leave them in "envelope" mode most of the time, for just that reason.
As a Calibration Tech for AMD in the beginning of my career, I had seen these problems easily with my analog scopes.
When I worked at Cray, I saw and dealt with SAMPLE and HOLD issues at the GHz level. Issues with the plug-ins for the 7104 to work with greater than 1 GHZ levels were always a problem. Intermittent and harmonic static had to be captured and NO trigger was fast enough!
Today, I ONLY keep analog scopes. The noise may show up as a blur, but that is still better than the scope " proving " that no noise exists...
That " Sample and Hold " issue is the ugly secret that today's scope makers ( including Tektronix ) won't talk about.
The real irony: It cost Cray ~$60k for the 7104 & the plug-ins. That was in 1980s $.
It costs around $1k for the same stuff today. Just spend another $1k to get it recalibrated. New is not always better. Old warhorses still kick @$$...
I've already collected my stuff. Have at it on eBay....
Good point about analog scopes. I cherish my older Tektronix 465B, which I bought on Ebay years ago and had calibrated and a few components replaced. Love that scope. Recently I had to help troubleshoot a signal source. At first it looked OK, but when I stepped through the time/division-switch settings it was obvious the oscillator had noise on the signal. Turned out that the power-supply filter was bad and let 120-Hz noise modulate the oscillator output. I find if I don't look at signals on all of the frequency settings I can miss problems that don't appear when I "think" the problem exists elsewhere.
How true! Turns out the main reason why I didn't look closer the first time was because I was working with techies who already assured me that the DC was fine. I had originally suspected the power supply but assumed that they had checked it ok. Never assume anything.
The more posts I read I am reminded of an old supervisor of mine that whenever he see any data first asks what am I looking at, and how are you measuring it.
Been there, done that. I use the LFW (Last Fiddled With, or at least thats the "nice" version of it) method in evaluating problems. How many times have we all heard "oh yes, I did so-and-so, but that had nothing to do with this problem"?
And not just o-scopes, but in my old age I take no readout as the word of God. A very recent example is one of the staff engineers told me an eight watt air to ground radio was outputting 1 watt and had a 1/4watt reflection using a Bird watt meter. It came back from the repair facility no trouble found. I measured it myself with an attenuator and spectrum analyzer and found full power. He used the wrong slug in the meter!
Also I get a kick out of the guys here who use 1 hertz resolution on a 500MHz signal, then have the nerve to tell me "the oscillator is off frequency by 1234Hz." I pull out the spectrum analyzer spec on freq drift/accuracy. I get the "Gee, I never thougt about that." answer. Same with the LSD on digital meters. Read the accuracy specs!
Had a non-tech guy yesterday come in an he reached out to touch a circuit card I just fixed. I told him "Don't touch that or you'll get hurt." He asked "Will it electrocute me?" I replied, "No, its low voltage, but if you break it I WILL HURT YOU!"!
I love my digital instruments, but have to learn their limitations. Great story, one that I have lived countless times over the past (almost) half century.
Ha...! Yeah...I know what you mean. I had one where a techie was testing an inductor using the ringing test on a Sencore RCL meter and found that all the devices were failing. Turns out that the manual stated that the ringing test was not appropriate to test inductors of that value.
Another one was where someone had spec'd an intrinsically safe milliohm meter to be used in fuel tanks. Turns out that the accuracy wasn't good enough to make the application measurement. He commented on the fact that he hadn't considered the accuracy specs prior to recommending the meter. Wha?? It all worked out in the end. The nota bona here is that no one is perfect, including me. This is why all aircraft maintenance requires that there be an independent inspection post repair.
Reminds me of another coworker who was trying to get an inverter chip to work and became really annoyed that he couldn't get any output. He fought with it most of that day only to realize that he had grounded the chip...boy did he feel foolish...to this day, he and I still use "did you ground the chip?" question to poke fun when either of us does something stupid or misses the obvious. It happens.
Now, where's my pencil...?? Over my ear...duh!! :-)
The first thing that came to mind is, if there was a lower frequency waveform on the power supply line, why couldn't they see it even at a short time base? One end or the other of the wave would have wandered from baseline. Is it because they were using a digital scope? One of the persistent problems I see is that digital scopes do not represent noise correctly. You either don't see it or it looks horrendous but isn't really there. They need a good combo scope, oh, wait, you mean almost nobody makes them anymore???
Then that doesn't say much for that very expensive digital scope. It was definitely mis-leading them. For the life of me, I can't figure out how you can have 2V pp of noise and not see it. How can you have that amount of noise and see, on your very expensive digital scope, a dead flat line??? Case in point, we had one customer complaining about a 25 hz oscillation coming out of our system. It's an eddy-current displacement system and the only frequency normally on this system is the 500khz to the sensor. As it turned out, their (also very expensive) digital scope was aliasing the residual 500khz carrier. As they changed the time base, the frequency of the 'noise' changed until they got to 500khz.
I too have been burned by digital scopes and aliused signals. A new analog scope is hard to come by but I got a Tektronix 2466 used. It usually spots the real problems when the digital scope doesn't.
As long as you really understand how to use them. Knowing their weaknesses, an analog 'scope will always have a place on my bench.
Any instrument can "lie" to you if you're not careful about how you set it up for the measurement. It also pays to ensure the instrument is appropriate for the measurement you are trying to make. This point was made earlier on but bears repeating.
IT is easy to get results that lead to incorrect conclusions when measuring with a scope. The challenge with digital scopes is that they can be misused in many more ways. It is not a fault of the scope, except for the ones that don't allow an adequate amount of bandwidth reduction.Sometimes it is very handy to nt have the 20MHZ bandwidth displaying all of the ambient noise, when what I want to see is the ten KHZ noise from a system. It is really hard to read the value of a signal that has an eighth-inch thick fuzzy trace of 17MHz noise from the ambient, when I know that the noise I want to read accurately is in the hundresds of Hz.
But with all of the features, it can be challenging to even understand how to use some digital scopes. That certainly does lead to problems and incorrect readings.
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