Oscilloscope 10MHz clock input to sync sample clock with target clocks? - Page 1
Forum section: electronics / projects

author: Smokey
author_rank: Super Contributor
timestamp: January 23, 2015, 06:32:18 am
content: How close to 10MHz does the external clock source need to be? What happens if you enter some appreciably lower or higher frequency?I was reading about some side channel power analysis attack stuff like the ChipWhisperer and they were talking about how you can get better results at lower sample rates by syncing the analog acquisition clock to the target clock. Would that be possible to do, maybe with a lot of clock up/down converting magic, on a bench-top scope using the 10MHz clock input?If the scope has to have something relatively close to 10MHz and the target clock is way off from that, would it be possible (and effective) to take the buffered target clock and use that as the clock source to like a DDS chip to convert it to something around 10MHz and then feed that into the scope? Something like that would probably need a feedback loop to keep the scope clock around 10MHz, but it sounds like it might work in theory at least.Thoughts?

author: jpb
author_rank: Super Contributor
timestamp: January 23, 2015, 04:53:04 pm
content: Do many scopes have 10MHz sync inputs ? I've not come across any - you seem to get stuck with the internal timebase.On other instruments such as counters and AWGs the manuals state how close the external 10MHz reference must be to be used. This depends on the internal reference, the more accurate the internal reference (eg OCXO vs TCXO) the closer the external 10MHz has to be to 10MHz for lock to occur in the PLL.If you try and use a source that is too far away for the PLL to lock I imagine the instrument would just ignore it and use its own reference or it may be upset and refuse to operate at all.

author: Smokey
author_rank: Super Contributor
timestamp: January 23, 2015, 09:08:50 pm
content: Rigol:The Rigol 4000 series is the lowest cost I think that has the Reference Clock input.Agilent/Keysight:3000X don't appear to have it, but3000T series and above does appear to have 10MHz input.Tek:MSO5000, DPO5000 Series have it. Not sure if lower models do.I could have got that wrong since I just sorted by price and then looked at the data sheets instead of the full manuals. If a cheaper scope has the external reference input and doesn't list that as a feature in the data sheet, then that technical writer needs to lose his job

author: ve7xen
author_rank: Super Contributor
timestamp: January 23, 2015, 10:10:21 pm
content: Doesn't Nyquist tell us that the exact sample points don't matter, assuming you're ultimately interested in interpreting the signal as a continuous, time-variant voltage? I guess there may be some value in trying to use the scope as a data acquisition system synced with the events of interest, but can't you use the external trigger to kick this off?It's my understanding that the external timebase input is generally there as a time reference, not for synchronization.Might be possible though, the specsheet will (at least for counters etc.) specify the frequency tolerance for the external reference. It will certainly have to be close to the nominal frequency (multiples might work depending on the PLL).

author: Smokey
author_rank: Super Contributor
timestamp: January 23, 2015, 11:38:42 pm
content: People use the external reference input for at least two reasons.1) To offer a better clock than the internal source for jitter reasons2) To sync up multiple instruments so the data collected from each one is synced and can be interpreted together.Here is a good PDF from Lecroy with a graphic on page 7 showing why you would want the clocks synced between two instruments.http://cdn.teledynelecroy.com/files/manuals/lcxxx-cktrig-om-e.pdfIn order to get more data out than just the frequency of a repetitive signal, you need to sample considerably faster than the base Nyquist (spell check tried to turn this into NyQuil) rate. The type of signals involved in power analysis are short transients. Measuring these values require either lots of over sampling or because they are coincident to a fixed clock, sampling at the clock.What I'm proposing is a little bit of both. Using the highish sample rate of moderately inexpensive modern oscilloscopes in the 1-2Gs/s range and locking the sample clock to the target clock to ensure one of the samples is taken at the optimal time right on a target clock edge. It sounds like the target frequency of DPA signals for software processors is low enough that 1Gs/s is fine, and you probably wouldn't see any difference from an unlocked clock until the target clock approaches the scope sample rate so it's kind of irrelevant, but still an interesting through experiment.Thinking more about this today, I guess my real question is,Is there a fixed phase relationship between the output of a clock converter like a PLL or DDS and the input reference that is driving the PLL or DDS. If there is a fixed phase relationship then you might be able to sync an oscilloscope to a target clock buffered and scaled using the external reference in.

author: ConKbot
author_rank: Super Contributor
timestamp: January 24, 2015, 02:31:48 am
content: I'd think that most devices with a 10 MHz input dont use it directly. They would go into a Phase Locked Loop, to multiply the frequency to whatever frequency the device needs internally, or to just 'buffer' it. A PLL can only 'pull' its VCO so far up and down, so if you 1GS/s scope, there is going to be a 1GHz VCO which is disciplined by the PLL to be locked to 100x the 10 MHz input. Once the VCO runs out of frequency swing, the VCO would unlock, the instrument (should) detect this and revert back to the internal timebase. But yes, when it is locked there is a fixed phase relationship in a PLL. In the 100X example above, assuming no phase delay in any of the dividers or the phase frequency comparitor( a very bad assumpltion, but it will just add a fixed phase offset), every 100th rising edge of the 1GHz signal would line up with the rising edge of the 10 MHz signal.Could you feed it a 9.995 MHz input to slow the sample clock to 999.5 MS/s? Maybe. 999.99 MS/s? probably. 900 MS/s? probably not. That lecroy document specs +-5% for their equipment (guess they dont have a lot of faith on the internal time base)The good news is that the instrument should know if its locked to the external reference and let you know