Hi I tried the pad on the 53230A. By the time I got to 6 db, the one sigma started to come up. At 4 db … not so bad.
Bob > On Feb 10, 2021, at 6:20 PM, Magnus Danielson <[email protected]> wrote: > > Hi, > > So, a very simple additional test one can do is to insert a 6 dB damping > pad. That effectively reduces the amplitude and also slashed the > slew-rate in half. You can do that for both squarewave and sine with the > same result. Using that, one can get some idea of the noise on the input > as you get two different slew-rates it converts to noise, and then see > how much "inherent" noise the counter has. Test with yet another > amplitude/slew-rate and you can refine that estimate and validate it. > > If you want to have fun, try different trigger directions etc. to see > how well calibrated the inputs are. > > Cheers, > Magnus > > On 2021-02-10 18:18, Bob kb8tq wrote: >> Hi >> >> Back a few years ago, Keysight did one of their PR videos: >> >> https://www.youtube.com/watch?v=4DfLkxVe7Lk >> <https://www.youtube.com/watch?v=4DfLkxVe7Lk> >> >> It shows a fairly simple setup to see what your two channel >> counter is doing. A lot of counters have a built in standard >> deviation function so it applies to more than just the counter >> he’s showing. People have been doing this sort of test for many >> decades. >> >> The one thing he’s not to clear on is what sort of signal levels >> are being used. “Square wave” is about as specific as he gets. >> >> I decided to try the same test with a 100 MHz sine wave at >> 2.5 dbm. Sine wave is nice since it has a well defined slope >> at the zero crossing ( which does matter in this test). It also >> happened to be an available output on the FS-740. >> >> If you repeat the YouTube test with this signal, you get the same >> 13 ps one sigma and 9 ps resolution on a 53230A. That suggests >> to me that this is a “fast enough” signal to be useful. Same test >> gave the same results on a couple of 53230’s. >> >> So … what happens if we try this on a few other counters? >> >> A 53132A comes in right around 140 ps one sigma to give it >> a 100 ps resolution. That’s nice compared to a 150 ps spec. >> >> The “surprise” candidate is the 53220A. One would *think* that >> with a 100 ps resolution spec, it would be 5X worse than the >> 53230A. Not so much. It comes in at 14 ps one sigma and 10 ps >> resolution. ( Usual disclaimers apply …. used counter of >> unknown (China / eBay) origin …. one sample tested…. YMMV ….). >> >> Does this test tell you *everything* about your counter? No >> of course it doesn’t. There are a lot of bits and pieces in there, >> this is only looking at some of them. Does it tell you what you >> will see on a 10 MHz sine wave at 10 dbm? Nope, that’s a >> slower edge. The one sigma is roughly 24 ps on a 53230A. >> >> What it *does* provide is a quick check of how your counter >> is doing. Most of what gets killed in a counter is in the input >> channels (as noted in a previous post). This gives you a fast >> check on how they are doing. It also checks a bit more than >> that …. >> >> To be really useful, you need to run the test with *your* source >> on *your* counter(s). (It does not have to be 100 MHz and 2.5 >> dbm) Note the number(s) you get and file them away. Come back >> a year or three later scratching your head about some readings >> ….. you have something to check against. Checking against spec >> … not so much …. >> >> The key thing in this test as shown in the video *is* the fast edge. >> For a lot of years many of us have simply been using a 10 MHz >> test signal. There’s nothing wrong with doing that. One might >> *guess* that’s what the designers of the 53230 did. Turns out >> that a faster edge is a better edge, even at RF frequencies. >> Who knew …. ( other than anybody who watches YouTube …. :) ) >> >> Could you extend this test by playing with various levels to come >> up with a good idea of what to expect at this or that level >> and this or that frequency? Sure you could …. >> >> Might a faster signal produce better results? I have a nasty >> suspicion that the video was done with a signal that was >> fast enough to show the best numbers possible on the ‘230. >> That’s just a guess ….. >> >> Does the 53220 “surprise” also carry over to the 53210? >> There’s no way to know. This test simply does not work >> on a single channel counter. The 53180 is in the same >> “twilight zone”. >> >> How does a recently aligned and fully functional 5370 >> compare on a “fast edge” test. We have lots of data with >> 10 MHz…. Same question on the SR-620 and CNT-90. >> >> Grubby details on the setup: >> >> Both inputs AC coupled / 5V range >> Auto Level turned off on both channels >> Channel A set to 1 M ohm >> Channel B set to 50 ohms >> Both trigger levels set to zero volts >> Measure TI from A to B >> My coax was about 2.5 ns long. >> External reference applied to all counters >> External ref not coherent with the test signal. >> >> It took you longer to read all that nonsense than it will to >> run the test :) >> >> Fun !!! >> >> Bob >> _______________________________________________ >> time-nuts mailing list -- [email protected] >> To unsubscribe, go to >> http://lists.febo.com/mailman/listinfo/time-nuts_lists.febo.com >> and follow the instructions there. > > _______________________________________________ > time-nuts mailing list -- [email protected] > To unsubscribe, go to > http://lists.febo.com/mailman/listinfo/time-nuts_lists.febo.com > and follow the instructions there. _______________________________________________ time-nuts mailing list -- [email protected] To unsubscribe, go to http://lists.febo.com/mailman/listinfo/time-nuts_lists.febo.com and follow the instructions there.
