Finding frequency from oscilloscope trace? - UK Vintage Radio Repair and Restoration Discussion Forum

Martin G7MRV · 7th Jul 2013, 9:46 pm

A bit embarrasing to have to ask this, but my brains gone to sleep on this one!

Ive just measured what is supposed to be a 10MHz line on my oscilloscope. Now, If I've got it right, 10MHz should be one 'cycle' per division at 0.1uS/div. Now, my scope doesnt do 0.1uS/div, instead I've had to use 0.2uS/div. In which I would expect I think to see 2 cycles per division if the signal was 10MHz, is this correct?

What I'm actually seeing is 3 cycles per division at 0.2uS/div. Now, this is where the old bonce has really let me down! Can someone tell me what that is in MHz please?

Is there a simple formula that will get me from MHz to uS easily?

Wish my maths was better

Martin G7MRV

Station X · 7th Jul 2013, 9:55 pm

Is the timebase control set to the CALibrate position?

jimmc101 · 7th Jul 2013, 10:04 pm

Frequency = Cycles/Seconds (Hz) or Cycles/uSeconds (MHz)

so 3 cycles in 0.2uS = 3/0.2uS = 15MHz

If this is your 10MHz reference, there is no way it can be that far out; as Station X has said check that the timebase control set to the CALibrate position.
If it's not in the CAL position then the timebase could easily be running at 0.3uS/div giving 3cycles/div for a 10MHz input.

Jim

Dickie · 7th Jul 2013, 10:04 pm

Is there a simple formula that will get me from MHz to uS easily? - Yes, one is the reciprocal of the other, eg 1MHz equals 1microsecond.

Martin G7MRV · 7th Jul 2013, 10:23 pm

Just been back out to check the 'scope. The 'SWP VAR' control is turned right around to the 'CAL' position, and clicked into that position (its the only thing I can see with CAL on it and its beside the timebase selector!)

The 0.2uS and 0.5uS positions both have a marking beside them saying 'MAG UNCAL', not sure if this means that the timebase could be inaccurate?

The scope is an old Hitachi V-212 if that helps anyone.

I can feed the frequency counter with it tomorrow for an electronic second opinion!

In most of my work the scope has only ever been an indication, so im afraid im not very up on using them for accurate measurements!

Richard, thanks for the note on reciprocals. I tried a few numbers and think it makes sense to me now

Herald1360 · 7th Jul 2013, 11:05 pm

"MAG UNCAL" sounds like the Y amp is running out of bandwidth for signals that need those sweep speeds. Maybe the timebase is too.

Does your 10MHz input make sense on the slowest speed you can count its cycles on?

Martin G7MRV · 7th Jul 2013, 11:16 pm

I'll try it again in the morning. Its a 20MHz scope so would have thought a 10MHz signal should have been manageable. I think there might be various controls to magnify the trace, whether these have any good effect on the timebase I dont know, time to dig the manual out i think.

Just realised as well that I had the probe in x10 mode, so my voltage reading needs correcting as well!

fetteler · 8th Jul 2013, 1:16 am

Quote:

Originally Posted by Martin G7MRV

A bit embarrassing to have to ask this, but my brains gone to sleep on this one!

Instead I've had to use 0.2uS/div. In which I would expect I think to see 2 cycles per division if the signal was 10MHz, is this correct?

Martin G7MRV

Yes it is

Cheers,
Steve.

majoconz · 8th Jul 2013, 4:03 am

A "10MHz" scope means that the vertical amplitude of the trace at 10MHz will be not less than half (6dB) of the response at some other frequency, maybe 1MHz. It doesn't infer that the trace will be locked and the timebase accurate unfortunately! That will be a pre-production prototype of course, the manufactured version a few years old may be, well, you know.....

Martin G7MRV · 8th Jul 2013, 9:59 am

Thanks for all the hints chaps, I have a much better idea what I'm doing now!

Retook the measurements this morning, used the sweep x10 (which I didn't realise did that!) and the 1uS setting and was able to get a nice clean trace, which when allowing for the x10 showed a perfect 10MHz. Also with the scope probe on x1 and not on x10 I could confidently read the level as well (which was 2.2v pk-pk, exactly as it should be). I discovered this probe has no markings on the switch, so that's been rectified as well!!!

mole42uk · 8th Jul 2013, 4:58 pm

I'm glad these things happen to other people as well...

I spend a good fifteen minutes this morning trying to find out why I was reading a 50Hz signal on a 40v DC line.....until I realised that I was using the wrong hook adapter for the scope probe, so the scope probe was't actually connected to anything!

Richard

MotorBikeLes · 8th Jul 2013, 9:43 pm

The "Mag Uncal" seems to mean that on the 0.2 and 0.5 Usecs/div, the magnification can not be treated as accurate. The faster speed ranges on budget scopes tend to be somewhat non-linear, and if you put a fast enough timing signal in, you quickly see the non-linearity especially at the RHS of the trace.
I assume there is a Xs 5 or Xs 10 control on the X shift control. (It may say "Pull for Xs5 for example").
Les.

7th Jul 2013, 9:46 pm	#1
Martin G7MRV Heptode Join Date: May 2008 Location: Selby, North Yorkshire, UK. Posts: 951	Finding frequency from oscilloscope trace? A bit embarrasing to have to ask this, but my brains gone to sleep on this one! Ive just measured what is supposed to be a 10MHz line on my oscilloscope. Now, If I've got it right, 10MHz should be one 'cycle' per division at 0.1uS/div. Now, my scope doesnt do 0.1uS/div, instead I've had to use 0.2uS/div. In which I would expect I think to see 2 cycles per division if the signal was 10MHz, is this correct? What I'm actually seeing is 3 cycles per division at 0.2uS/div. Now, this is where the old bonce has really let me down! Can someone tell me what that is in MHz please? Is there a simple formula that will get me from MHz to uS easily? Wish my maths was better Martin G7MRV __________________ I got food in ma belly and a license for ma telly My Blog - http://g7mrv.blogspot.com

7th Jul 2013, 9:55 pm	#2
Station X Moderator Join Date: Jan 2003 Location: Ipswich, Suffolk, IP4, UK. Posts: 21,288	Re: Finding frequency from oscilloscope trace? Is the timebase control set to the CALibrate position? __________________ Graham. Forum Moderator Reach for your meter before you reach for your soldering iron.

7th Jul 2013, 10:04 pm	#3
jimmc101 Heptode Join Date: Mar 2009 Location: Portsmouth, Hampshire, UK. Posts: 674	Re: Finding frequency from oscilloscope trace? Frequency = Cycles/Seconds (Hz) or Cycles/uSeconds (MHz) so 3 cycles in 0.2uS = 3/0.2uS = 15MHz If this is your 10MHz reference, there is no way it can be that far out; as Station X has said check that the timebase control set to the CALibrate position. If it's not in the CAL position then the timebase could easily be running at 0.3uS/div giving 3cycles/div for a 10MHz input. Jim

7th Jul 2013, 10:04 pm	#4
Dickie Octode Join Date: Sep 2004 Location: St. Albans, Hertfordshire, UK. Posts: 1,478	Re: Finding frequency from oscilloscope trace? Is there a simple formula that will get me from MHz to uS easily? - Yes, one is the reciprocal of the other, eg 1MHz equals 1microsecond. __________________ Regards, Richard, BVWS member

7th Jul 2013, 10:23 pm	#5
Martin G7MRV Heptode Join Date: May 2008 Location: Selby, North Yorkshire, UK. Posts: 951	Re: Finding frequency from oscilloscope trace? Just been back out to check the 'scope. The 'SWP VAR' control is turned right around to the 'CAL' position, and clicked into that position (its the only thing I can see with CAL on it and its beside the timebase selector!) The 0.2uS and 0.5uS positions both have a marking beside them saying 'MAG UNCAL', not sure if this means that the timebase could be inaccurate? The scope is an old Hitachi V-212 if that helps anyone. I can feed the frequency counter with it tomorrow for an electronic second opinion! In most of my work the scope has only ever been an indication, so im afraid im not very up on using them for accurate measurements! Richard, thanks for the note on reciprocals. I tried a few numbers and think it makes sense to me now __________________ I got food in ma belly and a license for ma telly My Blog - http://g7mrv.blogspot.com

7th Jul 2013, 11:05 pm	#6
Herald1360 Dekatron Join Date: Feb 2007 Location: Leominster, Herefordshire, UK. Posts: 16,535	Re: Finding frequency from oscilloscope trace? "MAG UNCAL" sounds like the Y amp is running out of bandwidth for signals that need those sweep speeds. Maybe the timebase is too. Does your 10MHz input make sense on the slowest speed you can count its cycles on? __________________ ....__________ ....\|____\|\|__\|__\_____ .=.\| _---\__\|__\|_---_\|. .........O..Chris....O

7th Jul 2013, 11:16 pm	#7
Martin G7MRV Heptode Join Date: May 2008 Location: Selby, North Yorkshire, UK. Posts: 951	Re: Finding frequency from oscilloscope trace? I'll try it again in the morning. Its a 20MHz scope so would have thought a 10MHz signal should have been manageable. I think there might be various controls to magnify the trace, whether these have any good effect on the timebase I dont know, time to dig the manual out i think. Just realised as well that I had the probe in x10 mode, so my voltage reading needs correcting as well! __________________ I got food in ma belly and a license for ma telly My Blog - http://g7mrv.blogspot.com

8th Jul 2013, 4:03 am	#9
majoconz Heptode Join Date: Jan 2008 Location: Ashhurst, Manawatu, New Zealand Posts: 571	Re: Finding frequency from oscilloscope trace? A "10MHz" scope means that the vertical amplitude of the trace at 10MHz will be not less than half (6dB) of the response at some other frequency, maybe 1MHz. It doesn't infer that the trace will be locked and the timebase accurate unfortunately! That will be a pre-production prototype of course, the manufactured version a few years old may be, well, you know..... __________________ Cheers - Martin ZL2MC

8th Jul 2013, 9:59 am	#10
Martin G7MRV Heptode Join Date: May 2008 Location: Selby, North Yorkshire, UK. Posts: 951	Re: Finding frequency from oscilloscope trace? Thanks for all the hints chaps, I have a much better idea what I'm doing now! Retook the measurements this morning, used the sweep x10 (which I didn't realise did that!) and the 1uS setting and was able to get a nice clean trace, which when allowing for the x10 showed a perfect 10MHz. Also with the scope probe on x1 and not on x10 I could confidently read the level as well (which was 2.2v pk-pk, exactly as it should be). I discovered this probe has no markings on the switch, so that's been rectified as well!!! __________________ I got food in ma belly and a license for ma telly My Blog - http://g7mrv.blogspot.com

8th Jul 2013, 4:58 pm	#11
mole42uk Nonode Join Date: Aug 2010 Location: Resolven, Wales; and Bristol, England Posts: 2,608	Re: Finding frequency from oscilloscope trace? I'm glad these things happen to other people as well... I spend a good fifteen minutes this morning trying to find out why I was reading a 50Hz signal on a 40v DC line.....until I realised that I was using the wrong hook adapter for the scope probe, so the scope probe was't actually connected to anything! Richard

8th Jul 2013, 9:43 pm	#12
MotorBikeLes Nonode Join Date: Dec 2012 Location: Kirk Michael, Isle of Man Posts: 2,350	Re: Finding frequency from oscilloscope trace? The "Mag Uncal" seems to mean that on the 0.2 and 0.5 Usecs/div, the magnification can not be treated as accurate. The faster speed ranges on budget scopes tend to be somewhat non-linear, and if you put a fast enough timing signal in, you quickly see the non-linearity especially at the RHS of the trace. I assume there is a Xs 5 or Xs 10 control on the X shift control. (It may say "Pull for Xs5 for example"). Les.