Author Topic: EEVblog #396 - Bode Plotting on Your Osciloscope  (Read 31601 times)

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Online EEVblog

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EEVblog #396 - Bode Plotting on Your Osciloscope
« on: December 08, 2012, 08:00:33 am »


Dave.
 

Offline Bored@Work

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Re: EEVblog #396 - Bode Plotting on Your Osciloscope
« Reply #1 on: December 08, 2012, 08:37:27 am »
Us old farts used to throw in a germanium diode and a small cap to get the amplitude. And faster sweep so the CRT showed something at all (or longer camera exposure to capture the amplitude on photo).
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Offline KedasProbe

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Re: EEVblog #396 - Bode Plotting on Your Osciloscope
« Reply #2 on: December 08, 2012, 10:35:15 am »
With the formula feature you could probably make the vertical axis a dB scale.
And you could probably use it also to make a curve to add a cursor that shows you the frequency (mainly for log).

Dave, want to try that?
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Re: EEVblog #396 - Bode Plotting on Your Osciloscope
« Reply #3 on: December 08, 2012, 10:49:07 am »
With the formula feature you could probably make the vertical axis a dB scale.
And you could probably use it also to make a curve to add a cursor that shows you the frequency (mainly for log).

Yeah, maybe. Not all scopes have that sort of capability though.
Like Tek scopes have an envelope detect mode that could be useful here.
Might do a follow-up video with various things.

Dave.
 

Offline Pentium100

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Re: EEVblog #396 - Bode Plotting on Your Osciloscope
« Reply #4 on: December 08, 2012, 11:02:42 am »
This is how you are supposed to align the IF transformers of a radio. Connect a sweep oscillator to the input, scope to the output and adjust the cores until the output looks like it is supposed to (bandpass) and is centered on the stated frequency (this mostly applies to FM tuners, as for AM you would only want the peak response, instead of a flat response over 200kHz range).
 

Offline marmad

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Re: EEVblog #396 - Bode Plotting on Your Osciloscope
« Reply #5 on: December 08, 2012, 12:04:10 pm »
Very nice, Dave - a great tip. It seems so obvious once you point it out - like all good ideas  :-+
 

Offline JuiceKing

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Re: EEVblog #396 - Bode Plotting on Your Osciloscope
« Reply #6 on: December 08, 2012, 12:39:22 pm »
I like it!  :D Thanks Dave for this and so many fascinating videos!

As it happens, I was doing just this same experiment last weekend: making a bode plot to measure frequency response of an 30+ year-old Nakamichi cassette recorder. (It was out of spec over the full audio range, but amazingly good up to 10Khz. TDH measured < 1%, too!)

I ran into a problem, though. The deck has 3 heads--one for erase, one for record and one for playback. The tape is pulled across the three in that order, so when you record, the input signal is recorded to freshly erased tape and is immediately available for real-time monitoring. Very neat.

But, there's a small delay between these actions because the tape travels across them at a rate of 1 7/8 inches/sec. This caused me some grief: the playback head was picking up up the sweep, alright, but it was delayed from the signal generator sync by the time it takes for the tape to move from the record head to the playback head.

I could find no way to adjust my analog oscilloscope to compensate for this, so I had bode plots that "wrapped" around the screen and looked pretty lame.

Any ideas of how--with a plain-old function general/sweep and plain-old analog scope--to get the nice end-to-end bode plot in a situation like this, where the filter introduces significant delay between the input and output?

And, while I'm at it, let me ask you if you have any ideas of how to measure wow and flutter with simple instruments like this. There I was totally stumped! (And this is where the old tape deck probably has the most trouble, in fact.) That would be a very interesting topic for another video. (Forgive me if you already did this years ago and I missed it!)

Thanks again for the cool tutorial, Dave!

- Ken
« Last Edit: December 08, 2012, 12:49:16 pm by JuiceKing »
 

Offline SeanB

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Re: EEVblog #396 - Bode Plotting on Your Osciloscope
« Reply #7 on: December 08, 2012, 02:34:13 pm »
Voltage offset in the x lead, a single AA cell and a 1k pot to give a variable floating supply. Adjust to give the right offset to compensate for the delay. Centre frequency then adjust the pot to make it hit the centre line. As to the wow and flutter do the same and increase the X gain a lot and look at the sidebands generated ( intensity gives a rough binning.
 

Offline nixxon

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Re: EEVblog #396 - Bode Plotting on Your Osciloscope
« Reply #8 on: December 08, 2012, 03:28:38 pm »
Nice. With a CRO you would have to increase the horizontal sweep speed to get an image of the whole frequency span simultaneously. Reminds me of w2aew's video from May 19: http://youtu.be/-A_DxsxPdeI

 :-+
« Last Edit: December 08, 2012, 04:11:48 pm by nixxon »
 

Offline krivx

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Re: EEVblog #396 - Bode Plotting on Your Osciloscope
« Reply #9 on: December 08, 2012, 05:46:17 pm »
This seems like it's only have of a Bode plot - you have no information on phase shift. Could you hook up a phase detector between your sweep and filter output and make a phase vs frequency plot?
 

Offline jahonen

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Re: EEVblog #396 - Bode Plotting on Your Osciloscope
« Reply #10 on: December 08, 2012, 06:01:52 pm »
I must comment on using a soundcard. For Windows there is a software called ARTA, which makes nice bode-plots, if one can accept the frequency range limitation. It can also use a two channel input mode where it compensates the imperfections of the driving amplifier, so that input signal is also measured and taken as an reference, so imperfections resulting from that are essentially calibrated out.

But if a 192 kHz soundcard is used, then one can measure up to 80 kHz or so with quite nice dynamic range. Some analog loopback distortion results using a MOTU ultralite mk3: THD IMD.

I used it to plot a loop gain of a power supply feedback loop with a small level matching and input protection circuitry. If one uses high quality sound card, the result can be quite acceptable. The dynamic range was hugely better than what I got using an oscilloscope.

Here are two measurements from a power supply (data plotted with Maple) using that soundcard method (using my work PC integrated soundcard, using 48 kHz sampling):





Regards,
Janne
 

Offline DarkPrince

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Re: EEVblog #396 - Bode Plotting on Your Osciloscope
« Reply #11 on: December 08, 2012, 07:59:42 pm »
I stated it in the YouTube comments but eh. When I got my waveform generator (SDG1020) one of the things I tested right away was a make-shift system like this. The sync-out of my function generator is different than Daves though, unfortunately. It generates a tiny pulse (like 1V for 1nS) right at the start of the sweep and that is it. Hard to trigger on it with a Rigol scope sometimes (better leads could help). Anyway this video gave  me a couple other ideas plus I didn't try switching the function generator to logrithmic so I need to test that. Would have been perfect in our class (signal analysis using op-amps and various filter configurations).

Just thinking out loud, but for someone's comment about detecting phase angle. Couldn't you just take two channels, one before and one after the filter, and subtract them. Granted you need to normalize both inputs so their amplitude is equal... an op-amp on one of the inputs or something. Granted the noise would probably make it unusable. Just thinking out loud.
 

Offline IanB

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Re: EEVblog #396 - Bode Plotting on Your Osciloscope
« Reply #12 on: December 08, 2012, 08:25:07 pm »
I like how it's described as a "poor man's Bode plot".

With a $1300 sig gen and a $12,000 scope you certainly will be poor after getting so equipped  ;D
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Offline free_electron

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Re: EEVblog #396 - Bode Plotting on Your Osciloscope
« Reply #13 on: December 08, 2012, 10:01:21 pm »
That’s why some advanced signal generators actually have marker outputs. My Agilent 3325 can sweep from 0hz to 22MHz and generate up tot 40vpp it has several outputs you connect to a scope.

There is a sweep start output that you connect to the trigger input of the scope. this makes sure the scope runs in sync with the sweep. there is a marker output that you connect to channel two of the scope. this is a 5 volts pp signal and you put the scope at 0.5 volts/div vertical and set the trace in such a way you only see the vertical portion of that pulse.

The generator is programmed to swepp and by pressing the ‘Marker’ button on the generator , whenever the internal sweep counter crosses the marker value the immediate fruquency is latched in the generator.

in essense where that vertical line of channel 2 is , is whatever frequency is dispayed on the function generator. the generator changes its display to show you what frequency corresponds to the marker. as the sweep is a pure digital affair the frequency indication is absolute.

this not only allows yo to bode pots but it also allows you to have a marke ron the screen an know the exact frequency the generator is at. so you could find the -3db point ( point where only 70.7% of the amplitude is left ) by hitting the marker left/right buttons an reading the amplitude on the scope. once found you look at the generator and you know the frequency.

these older synthesizers were built especially to do this kind of stuff as spectrum analysers invariably do not have the dynamic range required and DSA’s are bloody expensive. I have another synthesized source that is very ‘pure’ as in no harmonics and has differential output capability including floating above ground.

The 3325 does is not an ARB but a real sine oscillator steered by a DDS circuit driving a pll. So it has excellent linearity at its output.

We frequently use it to find the resonance point of the voice coil for example. This is a tuned LC network but as the resonance point changes depending on the position of the arm you need to measure it at multiple positions to model it.

To do the active filters in the positional servo control loop of the harddisk we have a few of those Stanford SRS756 DSA machines.
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Offline marmad

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Re: EEVblog #396 - Bode Plotting on Your Osciloscope
« Reply #14 on: December 08, 2012, 10:06:35 pm »
I like how it's described as a "poor man's Bode plot".

With a $1300 sig gen and a $12,000 scope you certainly will be poor after getting so equipped  ;D

Well, if it makes you feel any better, here it is almost replicated with a $100 signal generator and an $840 scope :)  My plot isn't adjusted correctly for the screen size in this shot (and I was out of 2.2nF caps so I used 2x 1nF for the low-pass linear test) but you get the idea.
 

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Re: EEVblog #396 - Bode Plotting on Your Osciloscope
« Reply #15 on: December 08, 2012, 10:17:16 pm »
i have to try it... Unfortunately i don't have much time right now....
 

Offline jancumps

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Re: EEVblog #396 - Bode Plotting on Your Osciloscope
« Reply #16 on: December 08, 2012, 11:47:50 pm »
50€ generator and 60€ scope:
(and free camera)

 

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Re: EEVblog #396 - Bode Plotting on Your Osciloscope
« Reply #17 on: December 08, 2012, 11:49:52 pm »
Velleman Electronics introduced me to this set-up rather than my more usual use of a Wobbulator (sweep gen + voltage detector) or mega expensive Spectrum Analyser + Tracking Gen.

Wobbulators have been around for donkies years. they work by using a sweep generator to produce the required frequency sweep at constant amplitude and also output a voltage ramp to drive an internal or external oscilloscope X axis. The output of the DUT is fed to a quality zero bias diode detector or even just a simple Germanium diode detector. the output of the detector drives the Y axis of the oscilloscope. These days you can knock up a modular VCO and ramp generator very cheaply and create a basic RF Wobbulator for a few dollars. Audio wobbulators can also be made but a PC sound card can do it so simply that there isn't a great deal of point.

I bought one of Velleman's PC driven oscilloscopes that used the parallel port for the control input. In the manual it stated that if you bought the Velleman PC controlled function generator, they could share the parallel port control and also perform bode plots. I already had the function generator and sure enough, the PC driven oscilloscope and function generator worked very well for frequency response plotting within the capabilities of the equipment. I recently bought Vellemans combined oscilloscope and function generator that runs off a USB port and that can do Bode plots as well. It cost me only GBP30 in new condition because the seller couldn't get it to work under Win 7 ! Velleman kit can be cheap on the used market, especially the kit that uses parallel port control..... set it up with a cheaply available old laptop like my Dell Inspiron PII-333MHz 3200 and you are in business  :) Don't even think about using USB to Parallel port leads on one of these.... it definitely will not work. Most PCMCIA (PC Card) Parallel ports do work though.

I have been using a Wavetek 1002 wobbulator for years when setting up IF filters and amplifier responses. Spectrum Analysers with a TG option are my preffered tool these days though.

Fraser
« Last Edit: December 09, 2012, 10:34:16 am by Aurora »
 

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Re: EEVblog #396 - Bode Plotting on Your Osciloscope
« Reply #18 on: December 09, 2012, 01:08:50 am »
This is how you are supposed to align the IF transformers of a radio. Connect a sweep oscillator to the input, scope to the output and adjust the cores until the output looks like it is supposed to (bandpass) and is centered on the stated frequency (this mostly applies to FM tuners, as for AM you would only want the peak response, instead of a flat response over 200kHz range).

I did a video on this a few months ago...
https://www.youtube.com/#/watch?v=-A_DxsxPdeI
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Offline Rick

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Re: EEVblog #396 - Bode Plotting on Your Osciloscope
« Reply #19 on: December 09, 2012, 02:41:43 am »
I must comment on using a soundcard. For Windows there is a software called ARTA, which makes nice bode-plots, if one can accept the frequency range limitation. It can also use a two channel input mode where it compensates the imperfections of the driving amplifier, so that input signal is also measured and taken as an reference, so imperfections resulting from that are essentially calibrated out.

But if a 192 kHz soundcard is used, then one can measure up to 80 kHz or so with quite nice dynamic range. Some analog loopback distortion results using a MOTU ultralite mk3: THD IMD.

I used it to plot a loop gain of a power supply feedback loop with a small level matching and input protection circuitry. If one uses high quality sound card, the result can be quite acceptable. The dynamic range was hugely better than what I got using an oscilloscope.

Here are two measurements from a power supply (data plotted with Maple) using that soundcard method (using my work PC integrated soundcard, using 48 kHz sampling):



Regards,
Janne

Dave did not elaborate on the sound card method. Dave's method seems straightforward after explanation (I may have even seen it in my past life). Could you explain it a bit how you do it with a sound card? There is something I miss there.
 

Offline T4P

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Re: EEVblog #396 - Bode Plotting on Your Osciloscope
« Reply #20 on: December 09, 2012, 04:17:43 am »
Try visual analyzer. great program
 

Offline free_electron

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Re: EEVblog #396 - Bode Plotting on Your Osciloscope
« Reply #21 on: December 09, 2012, 06:05:48 am »
Make a real detector circuit using some opamps. That gets rid of the diode drop. Bab pease had a nice active rectifier using opamps that didn t use an external diode. He used two mosfets and some comparators to make ideal diodes. Forgot the appnote. ...
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Online EEVblog

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Re: EEVblog #396 - Bode Plotting on Your Osciloscope
« Reply #22 on: December 09, 2012, 06:25:49 am »
50€ generator and 60€ scope:

There is s prize for whoever gets it working with string and two tin cans!  ;D

Dave.
 

Offline Wytnucls

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Re: EEVblog #396 - Bode Plotting on Your Osciloscope
« Reply #23 on: December 09, 2012, 08:32:10 am »
Not quite strings and cans, but I'm working on it!
Rigol DS1052E and DG1022

« Last Edit: October 05, 2015, 06:38:39 pm by Wytnucls »
 

Offline whatchitfoool

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Re: EEVblog #396 - Bode Plotting on Your Osciloscope
« Reply #24 on: December 09, 2012, 09:26:16 am »
For those that care, it isn't pronounced "bow-ed".
As a concrete reference, The wiki article of the man has the name as "boh-de"  or "boh-dah"

This caused a heated debate a while back in one of my university classes.  We ended up getting an email response from one of his children saying neither of those pronunciations are quite right. Hendrik had some french heritage as well as dutch, but the name never took on any of it on in america; it was trimmed and mangled over the years.  "bow-day"  is probably the correct way to say it, but "boh-dah" followed by "boh-de" is close enough for the family.
The e is supposed to have the è  (Bodè) going on. It is almost never done, but i have seen it like that in a book or 2.

The class wasted a ridiculous amount of time on this...
 


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