Products > Test Equipment
Pocket-Sized 6 GHz 1 TS/s ET Scope
SJL-Instruments:
--- Quote from: joeqsmith on February 07, 2024, 01:52:43 pm ---Shown with channel1 boundary set to +/-50mV, 7.5ns start of collection, 1ns sweep range, 1ps resolution. Test signal at 7GHz. THD is around -22dB, down in the muck.
I am curious on the cause of the DC shift? The test signal is CW, so I expect something in your front end is doing some rectification that is somehow tied to the trigger. Granted, I know I am pushing this thing to look at VERY small signals at much higher frequencies than you spec it to and pushing that start of collection way early.
To be clear, by no means am I suggesting this is a problem. I just want to understand what is going on internally.
--- End quote ---
Yep, this is why we spec the start time at 11 ns. ;D
The DC shift is not due to rectification and should be independent of the signal. We're actually reasonably sure it can be compensated for. The only reason we haven't is that we're not 100% sure it won't change with temperature, aging, etc.
Internally, the threshold voltage for the Channel 1 comparator must be rapidly switched from the trigger voltage to the CDF query voltage. The compensation network for this isn't perfect, and you're seeing the residual transient. You could avoid this problem this with two comparators and a resistive splitter, but that draws more power, increases the noise floor, and opens up matching issues.
This is also why decreasing the CDF search range improves the trigger sensitivity.
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Perhaps a good middle ground for the software is to show data down to 8 ns by default, for all channels except CH1.
joeqsmith:
Thanks for the added information on the DC.
I wouldn't limit it in software, or if you do, set it to your absolute minimum (76486). I would rather have it be documented that 11 is the recommended start of acquisition and that it may be possible to use lower values with reduced performance. Let the user decide if it is useful to them or not but don't lock them out.
For a default power up, it doesn't matter to me as I will save the setup for my personal use. It would be nice if it supported an initialization file where, if present, the software would automatically load these settings without the need to do it manually. It would be no different than your other setup files, except for a unique name that your software looks for.
Shown with 7GHz 70mVp-p signal applied to Ch1. Other channels have ground caps inserted. Manual doesn't mention isolation. Is this coupling from Ch1 to Ch2 normal?
SJL-Instruments:
--- Quote from: joeqsmith on February 08, 2024, 01:38:23 am ---I wouldn't limit it in software, or if you do, set it to your absolute minimum (76486). I would rather have it be documented that 11 is the recommended start of acquisition and that it may be possible to use lower values with reduced performance. Let the user decide if it is useful to them or not but don't lock them out.
--- End quote ---
We are not planning to set any limits if extended timebase settings and/or power-user mode are enabled. We trust the user to understand the limitations and consult the manual.
But in the default mode, we will still keep a limit at 11 ns (at least on CH1), to prevent confusion if CH1 appears distorted.
The exact minimum delay is unit-dependent and subject to process variation. 8 ns is at the 2 sigma level (~95% of units), and 8.5 ns is ~3 sigma.
--- Quote from: joeqsmith on February 08, 2024, 01:38:23 am ---For a default power up, it doesn't matter to me as I will save the setup for my personal use. It would be nice if it supported an initialization file where, if present, the software would automatically load these settings without the need to do it manually. It would be no different than your other setup files, except for a unique name that your software looks for.
--- End quote ---
Easy enough to implement (except on macOS, for reasons...). We'll add this to the Windows + Linux versions.
--- Quote from: joeqsmith on February 08, 2024, 01:38:23 am ---Shown with 7GHz 70mVp-p signal applied to Ch1. Other channels have ground caps inserted. Manual doesn't mention isolation. Is this coupling from Ch1 to Ch2 normal?
--- End quote ---
This coupling is normal and increases at higher frequencies. Later units also have a revised top case which improves the crosstalk.
Do note that the CH2 waveform you're seeing is below the "nominal" RMS noise floor. :)
joeqsmith:
When looking at the start of collection, what determines the minimum value before it reports an underflow? The manual states:
--- Quote ---Error Response: WARNING: Delay underflow -inf This error occurs when the requested value is outside the range of the delay generator and cannot be achieved. If you are certain the requested delay falls within acceptable bounds, contact us for
a diagnosis.
--- End quote ---
Above it states:
--- Quote ---Undefined behavior may occur if the requested delay is lower than 11 ns (D110000) or higher than the holdoff minus 25 ns. It is the user’s responsibility to stay within these bounds.
--- End quote ---
The term bounds must be referring to different boundary conditions as it does not appear to be constant for a fixed holdoff time. Can you provide the equations? 7.7ns seems to work in all of my test cases so far.
The document does mention the delay generator in 2.3 but doesn't mention how it ties into the underflow warning.
--- Quote ---Later units also have a revised top case which improves the crosstalk. Do note that the CH2 waveform you're seeing is below the "nominal" RMS noise floor. :)
--- End quote ---
I dare say pretty much everything I have looked at over the last two days was outside of nominal. :-DD
Showing 8.5GHz signal applied with varying amplitudes. Start of collection was set to 7.7ns and a 1ps resolution. Indeed channel 2 doesn't exhibit the same tailing as channel 1. Based on this, I plan to remove 3ns worth of coax.
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I suggest as you make changes to the hardware, you consider sending a set of replacement parts for this unit anyone working on a review so they remain current. For a while, I was thinking to have you ship a production unit that you bless as a gold standard to have as a reference. I no longer feel that will be useful. After all of my testing and our conversation, I'm now fairly confident that the electronics on this particular unit represent production.
joeqsmith:
Let it run overnight and saw no problems using the 7.7ns start of collection. Showing both the time domain and distortion.
Drift is dominated by open loop YIG, not the scope.
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