Comparing Two Frequencies Using Digital and Analog Scopes - Page 1
Forum section: electronics / testgear

author: German_EE
author_rank: Super Contributor
timestamp: January 25, 2020, 08:06:45 pm
content: I suspect that this is a case of 'pilot error' so bear with me.I want to compare two audio frequencies, one at exactly 2 KHz and one somewhere between 1.98 and 2.01 KHz, so I feed them into my analog scope. Locking the sync to the 2 KHz signal on channel 1 I see the signal on channel 2 drift slowly by, so I know that I'm close.Now I switch to my digital scope, a Rigol 1054Z. The same signals are fed in and I see the 2 KHz signal locked on the screen but the second signal is a mess, a screen full of overlapping waveforms that only becomes stable when it is EXACTLY 2 KHz. I never see the slow trace drifting left or right.What am I doing wrong?To save anyone mentioning it, Lissajous figures have the same problem, a slowly rotating image on the analog scope and a mess of lines on the 1054z.

author: KaneTW
author_rank: Frequent Contributor
timestamp: January 25, 2020, 08:15:09 pm
content: I've never used a Rigol so I don't know if there's something specific to it but:Load preset settings so you start from a known baselineCheck sample rateCheck persistenceI've done something similar with a video signal earlier this week and it worked perfectly on my RTB2004, so not sure what's going on there.

author: philipz
author_rank: Contributor
timestamp: January 25, 2020, 10:05:58 pm
content: I hope you keep your analog scope? Few years ago I found the same you did with Rigol 1052. I was shocked because very often need and use this method to calibrate different generators. One-two years later acquire Agilent scope and found that with right scope (no matter digital or analog) method is working.

author: thinkfat
author_rank: Unknown
timestamp: January 25, 2020, 10:21:37 pm
content: I've done the same with the same scope, just with two 10MHz sine waves. It normally works flawlessly.Can you check your acquisition memory settings? Put it on "Auto".

author: Someone
author_rank: Super Contributor
timestamp: January 25, 2020, 11:05:36 pm
content: Quote from: KaneTW on January 25, 2020, 08:15:09 pmGood advice, the effects described by the OP are the same that are caused by aliasing.

author: DaJMasta
author_rank: Super Contributor
timestamp: January 25, 2020, 11:31:12 pm
content: Check sample rate will likely fix it, but adjusting memory depth can help as well as it usually scales with sample rate by default.You are describing signals 30Hz apart, though, so this could be update speed related. The non-triggered signal will be scrolling by quite quickly, especially if you're zoomed in, so it could be that the redraw rate on the screen is low enough with your memory settings that it will look choppy, at best. If you move the signals closer, something like 1.998kHz and 2.001kHz, the beat frequency is much slower so you should be able to see the slower movement even if the screen update is limited.Regular persistence mode, even at a very low setting, may actually help here, as a single update will have more captures worth of data and should show you the smooth speed differential even when they're far apart... but to be sure it's being properly captured, the easy choice would be zooming out a bit and then single shot capturing. That should at least verify that everything's as expected.

author: bob91343
author_rank: Super Contributor
timestamp: January 26, 2020, 01:27:10 am
content: Stick with the analog for this purpose. You can also display a Lissajous pattern to see the comparison.You didn't say why you are doing this. If you are trying to adjust the two to be equal, the Lissajous is probably the best way. If you want to measure the difference, you can count the drift while holding a stopwatch. At 2000 Hz, one complete revolution corresponds to one Hertz, and the time it takes tells you the frequency difference.In other words, if the trace goes around one full cycle in 10 seconds, the frequency error is 1/10 Hz. For greater difference the Lissajous is inconvenient. A counter will solve it easily, of course.

author: SilverSolder
author_rank: Super Contributor
timestamp: January 26, 2020, 08:26:42 pm
content: Just tried this on an older digital scope (Agilent 54622D).It shows the second wave smoothly moving, but only when it is within about 5Hz of the other signal. More than that, and it becomes a "mess".

author: NoisyBoy
author_rank: Frequent Contributor
timestamp: January 27, 2020, 03:56:06 am
content: I know this does not directly address your question. But if you do this on a regular basis, a counter with Ratio A/B function will give you accuracy many magnitude above what a scope can do for you. Besides, the displayed result is much more intuitive than counting waves going across, or spinning on your screen.Or just drive it with a GPSDO, and tune the frequency to whatever you desire.

author: TimFox
author_rank: Super Contributor
timestamp: January 27, 2020, 05:42:43 pm
content: If adjusting for a 1 : 1 ratio, an x-y Lissajous figure on a real analog cro gives instantaneous indication, which greatly speeds the adjustment, compared with a counter.