EEVblog #396 - Bode Plotting on Your Osciloscope

[PA4TIM]

The most important thing you do not have this way is a logarithmic vertical axis.
A filter repons looks very impressive in Volts but mist times yoy want it in dB. A -40 dB response is not extreme but lineair you will ave teouble seeing details over -10 dB

Second problem is insertion loss. You must scale the screen without the filter, not on the peak of the filter. ( or you use the cursors or the DSO and know the amplitude of the generator.

[EEVblog]

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...

If you are talking about the guy himself, sure, have a fuss over it. But when you are referring to the engineering term itself, I think it's perfectly acceptable to say it the way your particular culture has evolved to say it in it's language.
I was taught "bow-ed" as I say it in the video, and for me that's the Australian pronunciation of it. I've been saying it that way for more than 20 years, and it's obviously been around at least a generation before that. And that is also how most of my colleagues over the years have pronounced it too.
I learnt it before the internet and communications revolution, and before you could easily hear countless other people from around the world say it in Youtube videos. So it's only natural that the pronunciation of it will evolve in different directions. That's how language works, it's a living creature.
So I don't think anyone can come along and say that a particular cultures (or even sub groups within cultures) pronunciation is wrong. If that's how it's evolved, that's how it's evolved, it's not wrong, just different. Learn to live with it 
There are many people saying I'm flat out wrong. Bullshit. They are just anal retentive's quoting wiki as if it's the stone tablets, and haven't heard the aussie version before. They need to get out more 

Dave.

[chicthomson]

I remember having similar grief over the pronunciation of Swiss Mathematician Leonhard Euler's surname, and I have a cobber mate who pronounces warez as Wah-ress. Makes it sound like a second rate western shootem up game!

[Fraser]

Isn't human communication about understanding what another person is meaning. I personally never try to correct someones pronunciation of a word...the Americans have many words that they say differently to me but that is the joy of international communication. If I don't understand a pronunciation I just ask for clarification of what was meant.

Now as for soldering being pronounced soddering in the USA..... I have no idea why, but I do know what is meant by it.

Fraser

[SeanB]

I have a tough enough time with how people pronounce my first name, let alone worrying over correct pronunciation.........

[Omicron]

If you are talking about the guy himself, sure, have a fuss over it. But when you are referring to the engineering term itself, I think it's perfectly acceptable to say it the way your particular culture has evolved to say it in it's language.

That can be a slippery slope though. I can perfectly imagine some versions having evolved so far away from the original that it becomes difficult to understand one another. How does a Chinese person pronounce it? Or a Japanese person? I remember a particular occasion where I was on the phone with a Japanese guy to resolve some support issue with a piece of medical equipment. And although he was speaking english and it was actually quite good, it was still difficult to understand him at times because even though the terminology he used was perfectly familiar, his pronunciation of some familiar engineering terms was just "off" enough to throw me off kilter (my native language is Dutch). Love it or hate it but we live in a globalised world these days and clear communication is more important than ever...

[notsob]

AND for those monty pythonites out there

throatwarblermangrove

is pronounced

luxuryyacht

[Rick]

Try visual analyzer. great program

Ok now I see. Thanks.

[JuiceKing]

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?

I tried the experiment again, but this time I added an outboard pulse generator to introduce a delay equal to the time it takes for the tape to move from the record head to the playback head. It turns out that the delay is 27msec for this particular tape deck, and dialing that in gives a nice edge-to-edge Bode plot.

Interestingly, the tape deck (a Nakamichi 680ZX) has rising response with frequency for low (-20dB) signal levels and falling response for high signal (0dB) levels. It's flattest overall at around -10dB.  White noise on the FFT on my Rigol scope shows this same trend. That means that low-level ambient sounds get emphasized even when the overall impression of tonal balance seems natural. Could this be the secret of the legendary "Nakamichi sound"? The deck is supposed to be within +/- 3dB from 20-22,000Hz at -20dB, so I don't think this is what it's supposed to be doing. But despite these problems on the bench, the deck sounds very good in the stereo.


Still not sure how to quantify wow and flutter.

- Ken

[jancumps]

I tried the experiment again, but this time I added an outboard pulse generator to introduce a delay equal to the time it takes for the tape to move from the record head to the playback head. It turns out that the delay is 27msec for this particular tape deck, and dialing that in gives a nice edge-to-edge Bode plot.

Interestingly, the tape deck (a Nakamichi 680ZX) has rising response with frequency for low (-20dB) signal levels and falling response for high signal (0dB) levels. It's flattest overall at around -10dB.  White noise on the FFT on my Rigol scope shows this same trend. That means that low-level ambient sounds get emphasized even when the overall impression of tonal balance seems natural. Could this be the secret of the legendary "Nakamichi sound"? The deck is supposed to be within +/- 3dB from 20-22,000Hz at -20dB, so I don't think this is what it's supposed to be doing. But despite these problems on the bench, the deck sounds very good in the stereo.


Still not sure how to quantify wow and flutter.

- Ken

Ken, can you show the plot?

[JuiceKing]

I tried the experiment again, but this time I added an outboard pulse generator to introduce a delay equal to the time it takes for the tape to move from the record head to the playback head. It turns out that the delay is 27msec for this particular tape deck, and dialing that in gives a nice edge-to-edge Bode plot.

Interestingly, the tape deck (a Nakamichi 680ZX) has rising response with frequency for low (-20dB) signal levels and falling response for high signal (0dB) levels. It's flattest overall at around -10dB.  White noise on the FFT on my Rigol scope shows this same trend. That means that low-level ambient sounds get emphasized even when the overall impression of tonal balance seems natural. Could this be the secret of the legendary "Nakamichi sound"? The deck is supposed to be within +/- 3dB from 20-22,000Hz at -20dB, so I don't think this is what it's supposed to be doing. But despite these problems on the bench, the deck sounds very good in the stereo.


Still not sure how to quantify wow and flutter.

- Ken

Ken, can you show the plot?

Sure. It's hard to get a good plot off the analog oscilloscope so I set it up again with the DSO. The speed and display resolution of the analog scope make for a much nicer plot, but you can see the same thing with the DSO and it's much easier to get a snapshot image to share. Here the sweep is linear, so the midpoint on the screen is about 10khz. In each I adjusted the vertical gain so that the maximum response is just below the top of the screen.

[jancumps]

I was first thinking that this was the dolby NR kicking in. But if you measure on the output, the dolby impact should be zeroed out.

[JuiceKing]

I was first thinking that this was the dolby NR kicking in. But if you measure on the output, the dolby impact should be zeroed out.

Yes, that's true. I tried with with Dolby in and out and it didn't make much difference.

Turning the tape type or bias the the wrong settings made matters worse, so this is about the best it can do with a cleaning, de-magnitizing and adjustment of user controls. I have the service manual and it's very detailed. But to make adjustments inside you need unobtainium Nakamichi reference tapes, so I'm not mucking with it.

[Slothie]

My 1976 Tex 7806 scope (that I love) and bought for £30 (thats $48 US, $45 AU for all you furriners) has a X voltage output on the back, as I imagine many others do. A few years back I used it with the VCO from my synth to plot the response of some filters I was playing with. Of course, being a computer nerd rather than an EE I didn't know it was a bode plot but at the time I didn't care!!  The output had a highish voltage range so I used an attenuator & opamp to ajust the offset, but youd have to check out your own CRO manual to adjust to suit your VCO.

If your CR 'scope has that output its not hard to knock up a cheap VCO, and even some cheap signal generators have a CV input, so you might not actually need to spend a lot of cash provided youre not too fussed about calibration & such.

[jancumps]

I was first thinking that this was the dolby NR kicking in. But if you measure on the output, the dolby impact should be zeroed out.

Yes, that's true. I tried with with Dolby in and out and it didn't make much difference.

Turning the tape type or bias the the wrong settings made matters worse, so this is about the best it can do with a cleaning, de-magnitizing and adjustment of user controls. I have the service manual and it's very detailed. But to make adjustments inside you need unobtainium Nakamichi reference tapes, so I'm not mucking with it.

This is my take at a Luxman K-112, using a type1 cassette.
Frequency from 28Hz to somewhere close to 20KHz.
Since my generator doesn't have sync out (yet - hacking planned), I used external triggering 50Hz, and adjusted the sweep frequency to somewhat lock in with that trigger. The image slowly scrolled over the x-axis.
Took the picture when the sweep start was at the left.
Yellow marker added to represent how the graph looks in reality.

[jancumps]

A bit unrelated,but still fun:
Transistor curve plotting on te scope.

[nadona]

IanB:
"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  " + 

[vk6zgo]

My 1976 Tex 7806 scope (that I love) and bought for £30 (thats $48 US, $45 AU for all you furriners) has a X voltage output on the back, as I imagine many others do. A few years back I used it with the VCO from my synth to plot the response of some filters I was playing with. Of course, being a computer nerd rather than an EE I didn't know it was a bode plot but at the time I didn't care!!

Back in the day,we called it a "frequency response sweep"
The term "Bode Plot" was reserved for the idealised version with sharp cornered "break points"which was derived mathematically! *

The output had a highish voltage range so I used an attenuator & opamp to ajust the offset, but youd have to check out your own CRO manual to adjust to suit your VCO.

If your CR 'scope has that output its not hard to knock up a cheap VCO, and even some cheap signal generators have a CV input, so you might not actually need to spend a lot of cash provided youre not too fussed about calibration & such.

*It seems that with the modern ability to generate more realistic plots,the difference has blurred to non-existence!

[vvanders]

It's a shame the Agilent DSOX series doesn't include a sweep with their wavegen then you could get the whole thing in a single instrument. I wonder if there's enough ARB points to generate a decent sweep manually.

[Hypernova]

It's a shame the Agilent DSOX series doesn't include a sweep with their wavegen then you could get the whole thing in a single instrument. I wonder if there's enough ARB points to generate a decent sweep manually.

You are behind the curve buddy, The new FW released last month has added the sweep function, AM/FM/FSK all the ones you need. No need for a trigger cable too since sweep from modulation is part of the trigger options.

[free_electron]

Agilent: lightyears ahead of the curve...

[vvanders]

That's odd, don't see the trigger on my DSOX-3014A w/ 2.20 firmware unless you're talking about using the rise/fall trigger. Even then the modulation only goes to a limit of 20kHz.

[edit]
Nevermind, found it on the source setting of the edge trigger, sweet

[EEVblog]

You are behind the curve buddy, The new FW released last month has added the sweep function, AM/FM/FSK all the ones you need. No need for a trigger cable too since sweep from modulation is part of the trigger options.

Someone was thinking!

Dave.

[baljemmett]

Even then the modulation only goes to a limit of 20kHz.

Nope, the modulation goes to a limit of your selected wavegen output frequency.  At least, it did last night when I installed the latest firmware on my 2000-series to have a play...

Edit: whoops, sorry, you're right -- I was thinking of the FM deviation -- the frequency of the modulating waveform does indeed top out at 20kHz.

[geekysuavo]

OK, this is a little late on the draw, but here goes...

With just the DSO2XWAVEGEN you can make Bode plots using a Linux system via USB TMC control. Granted, the dynamic range is limited, but it's better than hand-dialing every frequency and typing the results into a spreadsheet!

Video here:
Code here: http://geekysuavo.github.io/usbtmc-bode

~ Brad.