[time-nuts] More Noise Floor Nonsense

Wed Feb 10 23:20:24 UTC 2021

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
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