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Frequency Divider for older Oscilloscopes??
bdunham7:
--- Quote from: tggzzz on July 19, 2021, 08:02:15 am ---"I want to observe a signal that is between 990MHz and 1000MHz. What is the minimum sampling frequency that I need to use?".
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So I might respond by asking you to define the term "observe" and also ask whether "between 990 and 1000MHz" means the entire spectrum of the signal falls between those bounds. Would you respond or send me packing? :) Obviously sampling scopes can display anything that they can get a stable trigger on with an arbitrarily low sample rate if you are patient, but that doesn't mean they are cheap or easy to build.
--- Quote from: Jorge Ginsberg on July 19, 2021, 11:18:06 am ---what kind of waves do you expect to find between 900 Mhz and 1000 Mhz? do you expect to find square waves? do you expect to find saw teeth?
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A lot can happen in a nanosecond these days....
Jorge Ginsberg:
--- Quote --- A lot can happen in a nanosecond these days....
--- End quote ---
Yes... but are you sure that what you are seeing is exactly what is happening?
When you measure signals in the microwave region you can never believe the waveform you are seeing.
Cables... connectors... even reflections of possible waves in the cabinet or shields ... all can alter the signal you are looking at.
When you open up a UHF circuit that is tucked inside a shield and try to observe some waveform, you are altering everything.
believe me... above 500 or 1 Ghz you work with spectrum analyzers, not oscilloscopes.
In the most extreme case, it might be necessary to observe some trigger pulse that lasts several nanoseconds... but there you are not talking about frequency but time. That trigger pulse could be repeated at the rate of one pulse every millisecond. And someone could say "the frequency of the pulse is 1 kHz". Nothing more wrong. NEVER confuse times with frequencies when it comes to laboratory measurements.
tggzzz:
--- Quote from: Jorge Ginsberg on July 19, 2021, 11:18:06 am ---
--- Quote from: tggzzz on July 19, 2021, 08:02:15 am ---Here's a simple question that I have asked interviewees in a job interview...
"I want to observe a signal that is between 990MHz and 1000MHz. What is the minimum sampling frequency that I need to use?".
Those answering 2GS/s faced an uphill struggle.
(And I'm not surprised that the OP hasn't had the courtesy to respond)
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Dear TGGZZZ... what kind of waves do you expect to find between 900 Mhz and 1000 Mhz? do you expect to find square waves? do you expect to find saw teeth?
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It is irrelevant, but you can consider a modulated signal.
--- Quote ---When you work in the high UHF zone and enter the microwave zone, ALL waves are already sine waves.
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Er, that's true in any frequency band.
If you don't like 900/1000MHz, divide it by 1000 or 1000000 and the question is equally valid.
--- Quote ---An oscilloscope is useless at those frequencies.
--- End quote ---
There are 110GHz real time scopes commercially available. The Infiniim XUR series starts at 5GHz and goes to 110GHz albeit with only a 5bin ENOB. I can't afford one.
Now, how do you think Keysight implements it? With a >220GS/s 5 bit ADC? I doubt it (but haven't bothered to research the implementation), and I suspect the reason is a direct consequence of correctly answering my question.
Whether a scope is the right tool is a separate issue; frequently it isn't.
--- Quote ---At very high frequencies, a resistor can behave like a coil and a capacitor can behave like a delay line. Transistors already behave differently. The "h" parameters are no longer used in the design, but the "s" parameters are used.
At very high frequencies all the electronics change.
Intel uses UHF and microwave engineers for the development of its chips. Imagine a microprocessor running at 3.6 Ghz.....
A good technician or engineer in audio or industrial electronics becomes a perfect ignoramus when working above 1 Ghz.
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True but irrelevant to the point.
--- Quote ---It is true that to observe correctly a 100 Mhz square wave you would need a 1000 Mhz oscilloscope, otherwise the square wave would be very distorted in its rising and falling edges; but it is never necessary to "look" at 1000 Mhz waves. In the vast majority of cases it is enough to know if that wave exists and what is his amplitude. For example, people who buy the Tektronix MDO3000 oscilloscope, whose bandwidth is 1Ghz, do not do it to see 1 Ghz waves but to be able to see correctly the 100 Mhz waves, because when you look at a 1 Ghz wave, the only thing you see is a sinewave...
Those of us who work on UHF or microwave radio links only use spectrum analyzers. By looking at the spectrum you know if you have just a sine wave or if you have a more complex wave. In that frequency range it doesn't matter the shape of the waves, the only thing that matters is their harmonic content.
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My question is important for far more than scopes.
tggzzz:
--- Quote from: bdunham7 on July 19, 2021, 01:55:55 pm ---
--- Quote from: tggzzz on July 19, 2021, 08:02:15 am ---"I want to observe a signal that is between 990MHz and 1000MHz. What is the minimum sampling frequency that I need to use?".
--- End quote ---
So I might respond by asking you to define the term "observe" and also ask whether "between 990 and 1000MHz" means the entire spectrum of the signal falls between those bounds. Would you respond or send me packing? :)
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No, I wouldn't send you packing. Nothing special about the term "observe", use "see", "visualise", "measure" etc. Yes, all the energy does lie within those bounds; that's the key point :)
--- Quote ---Obviously sampling scopes can display anything that they can get a stable trigger on with an arbitrarily low sample rate if you are patient, but that doesn't mean they are cheap or easy to build.
--- End quote ---
True if the signal is repetitive, but for a non-repetitive signal you have to capture it all on the fly (i.e. realtime).
tggzzz:
--- Quote from: joeqsmith on July 19, 2021, 02:07:00 pm ---I would ask if the waveform was repetitive.
--- End quote ---
Non-repetitive. You need to capture it alll as it occurs.
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