[time-nuts] More Noise Floor Nonsense

[Bob kb8tq]

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 <magnus at rubidium.se> 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
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