Tektronix 465 restoration

[Herald1360]

Originally Posted by Skywave

'Bandwidth' - in the sense of electronic equipment that uses analogue amplifiers and attenuators - is essential an analogue metric. Therefore, an analogue measuring technique is the only reliable method of accurately determining that bandwidth.

Not sure I'd agree with that statement as an absolute.

One can say that analogue measurements may be approximately correct whereas digital ones may be precisely wrong.

[Skywave]

Originally Posted by Skywave

'Bandwidth' - in the sense of electronic equipment that uses analogue amplifiers and attenuators - is essential an analogue metric. Therefore, an analogue measuring technique is the only reliable method of accurately determining that bandwidth.

Originally Posted by Herald1360

Not sure I'd agree with that statement as an absolute.

Analogue measurements: I repeat from that post of mine:

Originally Posted by Skywave

That statement is subject to the accuracy and resolution of the test equipment used plus how the various extra items of equipment (if necessary) are chosen, configured and used to make that assessment.

That clearly qualifies as a restriction, qualifying the range and scope of that statement. Therefore, that statement is not 'absolute'.

Al.

[mhennessy]

By "analogue", are you differentiating between sine wave and pulse measurements? I ask, as pulses were mentioned earlier. Of course, a pulse is analogue

Using pulse testing to determine rise time - hence bandwidth (or more strictly, the high frequency -3dB point, as not all amplifiers work down to DC) - is perfectly valid, but there is an important assumption made about the 0.35 constant - that the roll-off response is single-pole. That's normally a reasonable assumption for an analogue 'scope, say, but not so for a digital 'scope where there is often steeper filtering ahead of the ADC to minimise aliasing. Tek recommend using 0.45 on a modern DSO.

Of course, sine wave measurements could be done first to establish if a DUT ('scope or otherwise) does have a single-pole response, and then the pulse test (using 0.35) could then be done if desired (although by this stage you'd already know the -3dB point, so it's arguably a bit redundant). Or you could do both sets of tests to determine what the constant should be for the particular DUT.

Moving laterally only slightly*, impulse testing is an important field of measurement today, thanks to the enormous processing power we all have sat on our desks. As a perfect impulse contains energy at all frequencies, an inverse-FFT on an impulse can derive a frequency response plot in much less time than it would take you to do a sine wave sweep (or spot frequencies). That's how the majority of all loudspeaker testing software works, because you can do various time-domain tricks to eliminate the effects of reflections in the room (few can afford to test loudspeakers in anechoic chambers these days), and unlike electronic devices, you can't test loudspeakers or microphones with fixed-frequencies sine waves - even in anechoic chambers.

* I realise that loudspeaker testing might seem to be quite a way from amplifiers, etc, but it's a useful real-world example to validate the theory.

[Argus25]

The way to accurately check the scope's bandwidth is with a leveled sine wave generator (Tek plug ins are good).

I use the Tek SG503 and 504 for my 2465b's which goes to 1GHz , it has a leveling head. The SG503 though is good for the 465 series scopes. I have a 464 and two 466's. It still requires that a 50 Ohm precision cable is used along with a good 50R terminator/attenuator to reduce any spurious responses. Then it is easy to check them for uniform response across their full bandwidth and find the -3dB point.

I have have been surprised and disappointed at the way a number of 464 and 465 scopes have been set up by calibration houses, clearly the people adjusting them had little knowledge of the principles of vertical amplifiers and frequency response adjustments as they apply to oscilloscopes.

Because of this I wrote an article about vertical amplifiers in the 464 and how to adjust them, which applies to many scopes, and how the various compensation networks should be set, it might be of interest to those new to the calibration of scope vertical amplifiers:

http://worldphaco.com/uploads/TEKTRO...ILLOSCOPES.pdf

For example it is possible to use a differential probe to help in the calibration of the vertical output board's settings, versus the presets on the main signal board and inadvertently having those presets compensate for frequency response errors on the output board downstream, which results in a less uniform overall response. But there was nothing on this in Tek's manual.(see remarks under "standardizing the drive to the vertical output amplifier")

[Skywave]

Originally Posted by mhennessy

By "analogue", are you differentiating between sine wave and pulse measurements? I ask, as pulses were mentioned earlier. Of course, a pulse is analogue

My answer to your first Q. is "Yes".

"A pulse is analogue": in the strict interpretation of that, you are correct, of course, since the world in which we live is indeed 'analogue'! However, you and I well know that in the world of electronics, it is almost universally accepted that the word 'analogue' is restricted to refer to continuous sine waves, even if they contain harmonics; 'pulse' waveforms are similarly usually limited to refer to waveforms that are distinctly non-sinusoidal and usually have a distinct 'pulse' nature: rising edge; flat top; falling edge. Of course, a 'pulse' type waveform can have rising and falling edges that are so slow and a decidedly non-flat top that the waveform is 'halfway' between 'pulse' and 'analogue', (e.g. line flyback pulse in a T.V.), but that point is not relevant to this thread.

Originally Posted by Argus25

The way to accurately check the scope's bandwidth is with a levelled sine wave generator.

Thank you! My point entirely!

Al.

[Herald1360]

That's one horse suitably flogged to death

[turretslug]

By 'scope jockeys?

[dave cox]

Just doing 'levelled sine wave' testing will miss any nastiness in the phase response. In such a case knowing the -3db point would not be very useful as any non-sine wave you look at could have been horribly distorted even though the scope appears to have enough bandwidth.

dc

[mhennessy]

Originally Posted by Skywave

My answer to your first Q. is "Yes".

"A pulse is analogue": in the strict interpretation of that, you are correct, of course, since the world in which we live is indeed 'analogue'! However, you and I well know that in the world of electronics, it is almost universally accepted that the word 'analogue' is restricted to refer to continuous sine waves, even if they contain harmonics; 'pulse' waveforms are similarly usually limited to refer to waveforms that are distinctly non-sinusoidal and usually have a distinct 'pulse' nature: rising edge; flat top; falling edge. Of course, a 'pulse' type waveform can have rising and falling edges that are so slow and a decidedly non-flat top that the waveform is 'halfway' between 'pulse' and 'analogue', (e.g. line flyback pulse in a T.V.), but that point is not relevant to this thread.

Al.

It's certainly not my experience that "the word 'analogue' is restricted to refer to continuous sine waves". I don't ever think I've seen that particular view stated before.

Thinking hard about why you say that, I'm wondering if that's perhaps because of the amount of RF work that you do - and you certainly have a lot more experience in that area than me - but most of what I do is very definitely analogue electronics, and sine waves are only a sub-set of the types of signals that I encounter

[Argus25]

Originally Posted by dave cox

Just doing 'levelled sine wave' testing will miss any nastiness in the phase response.

Exactly right, if you look at my post above you will see that I say the leveled sine wave generator is for checking the bandwidth. And you will see in my article that in adjusting the vertical amplifier, there is no role for the sine wave generator (except for setting one bias adjustment in the Tek scope) all the setting up is done with the PG506 square wave generator and the tunnel diode pulser, which makes sure the vertical amplifier is free from both amplitude & phase distortion. The Tek Time Marker generator is useful for the timebase calibration.

If one was just stuck with the leveled generator alone, it would not be possible to accurately set the frequency response and the transient response presets properly thoughout the vertical amplifiers. So the answer is, both tools (SG503 leveled generator & the PG506 + tunnel diode pulser, or equivalent) and a time maker generator are required and both of course are recommended scope calibration tools by Tek for the task at hand. I also included in the article Tek's tunnel diode pulser circuit that works driven by a function generator, in case someone wants to make their own. They are expensive otherwise, but some have or can afford them.

Also the required 50R in line ladder attenuators for scope calibration work are easy to make, how to make these is in this 2465B article pages 24 to 27, to see the Tek calibration generators and how to make the attenuators & tunnel diode pulsers for scope calibration:

http://worldphaco.com/uploads/TEKTRO...THE_DS1225.pdf

[Skywave]

From Wikipedia: definition of a pulse wave and definition of an analogue signal.

https://en.wikipedia.org/wiki/Pulse_wave
https://en.wikipedia.org/wiki/Analog_signal

Al.

[Skywave]

Originally Posted by mhennessy

Thinking hard about why you say that, I'm wondering if that's perhaps because of the amount of RF work that you do - and you certainly have a lot more experience in that area than me - but most of what I do is very definitely analogue electronics, and sine waves are only a sub-set of the types of signals that I encounter

Maybe - but thinking about this, I suspect that our two different angles on this have a good deal to do with the difference in ages between us. In my formative years, digital electronics didn't exist: everything was 'analogue'. When, later, RTL came along, at first, it was quite a struggle trying to think in terms of 1's and 0's !
Sometimes in the later stages of one's life, it gets difficult to set aside the foundations on which one has built a lifetime of understanding.

Al.

[Herald1360]

Hasn't digital electronics been around since the 1940s? Certainly since the earliest electronic computers.

[Skywave]

Perhaps I should have been a bit more precise with my choice of words when I said:
"In my formative years, digital electronics didn't exist: everything was 'analogue' ".
So I'll re-phrase that:
In my formative years and in my world, digital electronics didn't exist: everything was 'analogue'.

O.K. now?

Al.

[Radio Wrangler]

When doing the occasional lecture, I quite like to leap around like a loony (quite an easy act for me...) and cup both hands together during a jump. I peer intently into a gap between fingers and announce that, good, I've caught an electron!

I then choose a victim in the audience to show it to, and then ask, very innocently, if it's an analogue one? or a digital one?

There are electrons and voltages and currents, they simply exist but this digital and analogue business lies in the assignment of meanings to those voltages and currents.

Does 3.7 volts mean a logical '1' or does it mean just 3.7 volts? or 200rpm into a tachogenerator?

Certainly we use different sorts of circuitry depending on the intended meaning of voltage signals, but the assignment of meaning is the key.

And an analogue measurement of a logic signal has importance when you want to check whether the logic levels are in spec.

A digital voltmeter is measuring an analogue aspect by digital means, and giving the result numerically. So is this a digital measurement or an analogue one? The parameter being evaluated is analogue and the result is more important than the means. I could argue that a logic probe is a true digital voltmeter. It expresses the logical state of the place being probed.

David

David

[mhennessy]

What separates analogue from digital is quantisation.

But having quantised a signal, you then have to get those binary representations from place to place - whether inside a bit of gear, or between bits of gear. In either case, that transmission of the symbols becomes an analogue problem again, and issues like level, characteristic impedance and termination raise their head - things that an RF person will know and love already. I could give lots of examples (I did, but deleted them for brevity), but suffice to say, analogue problems - often called "physical layer" problems - are what we mostly have to contend with where I work. If you take care of the analogue aspects, the complicated digital stuff sitting on top generally works just fine.

Very happy to say (lots!) more, but worried about taking this more OT

[TonyDuell]

I think I've mentioned these two quotes before :

'Digital circuits are made from analogue parts' (Don Vonada at Digital Equipment Corporation)

'A digital circuit is like a tame animal, an analogue circuit a wild animal. Every so often the digital circuit goes back to the wild' (Prof M. V. Wilkes, probably misquoted as he said it to me over dinner one evening).

I wonder if the term 'analogue signal' is a back-formation from 'analogue computer' An analogue computer was named because you made an analogue of the system you wanted to investigate. In other words you had a system that you could describe by (differential) equations, you then made a (often electrical) system using adders, integrators, etc that followed the same equations and you did tests on that. The signals in such machine were, of course, continuously variable.

I would also argue that digital (2 state) electrical systems pre-date analogue (continuous) ones in that the telegraph pre-dates the telephone. Relays (effectively digital amplifiers) are older than analogue signal amplifiers.

[Argus25]

Originally Posted by TonyDuell

I think I've mentioned these two quotes before :

'Digital circuits are made from analogue parts' (Don Vonada at Digital Equipment Corporation)

Some of this sort of talk reminds me of a remark made by a Lawyer:

"There is no truth and there are no lies, only data to be manipulated"

So an equivalent remark could be:

There is no digital and there is no analog, only voltages and currents to be interpreted"

[Skywave]

To me, last few posts contain many perceptive truths. I have now come to realise that trying to draw a distinct line between 'analogue' and 'pulse' (or 'digital') is not realistic.
So, before this thread gets closed on account of 'topic drift', can we return to this thread's subject, please: Tektronix 465 restoration?

Al.

[Herald1360]

Originally Posted by Skywave

Perhaps I should have been a bit more precise with my choice of words when I said:
"In my formative years, digital electronics didn't exist: everything was 'analogue' ".
So I'll re-phrase that:
In my formative years and in my world, digital electronics didn't exist: everything was 'analogue'.

O.K. now?

Al.

Making absolute statements to a bunch of "on the spectrum" (pun intended) characters is a fraught exercise.