John Curl's Blowtorch preamplifier part II

[anatech]

Hi John,
I guess I can try to generate other types of modulation, but here you will see a couple that will be very familiar to anyone working on the IF strip of an FM tuner. These happen to have been taken from a Marantz 2385 I was working on last year.

Because the spectrum analyzer I'm using is a swept type (HP 3585A), it "sees" the results of sweeping the fundamental frequency back and forth at 1 KHz (in this case). The only way you could get something that looks like AM modulation is to capture the fundamental, and the high and low limits only. Otherwise, any sample or sweep will show many of the discrete frequencies as it does here.

I'm not really sure what you did John, can you post what you are seeing?

I downloaded and read the paper on TIM that pooge kindly posted a link to. It explained most of the questions I had with regard to the display under discussion.

One thing I really wonder about in figure 5 where you have compared several op amps looking for dynamic intermodulation distortion. The one part that is missing that would seem to be a poster child for your beliefs. The LM318, speedster of the group. You could have really illustrated your preference for wide band open loop performance by including it in a 20 dB, feed forward configuration. I only ask because it is mentioned just before this comparison was made. Any reason to exclude this part?

-Chris

[Bob Cordell]

Originally Posted by john curl

Point well taken, but look at the levels, are they not a little too much to ignore? I would have to do the entire test over, just to be sure, because the actual waveform frequencies are guessed at (by Janneman, I presume), and not measured. Also, we don't have the spectrum changes with level, that are not put on the paper. This is unfortunate, because I did many of the tests, myself, and this would bring out any real deviations.
However, I give it over to you, that the added IM byproducts 'could' be accounted for, perhaps. I still am not sure about this, because FM modulation of the the fundamental test frequency would create the same sort of symmetrical sidebands. Perhaps, my other associate will resolve this dilemma in a few months (that is what he told me, last week) when he decides to show me how to measure PIM by HIS method.

Hi John,

I'm guessing you are responding to my post. I agree, the levels of those blips are way too much to ignore, and should indeed be added into the TIM calculation. I'm just saying that they are not inharmonic and that they are not unexpected when hitting a 741 that hard.

Cheers,
Bob

[john curl]

Actually, Janneman, you DOCTORED with your own (unproven) input. But I won't complain at the moment, unless you are off by a few Hz, and even you are guessing. I have at the moment a FM modulated square wave on my test bench that makes virtually the same levels and frequencies of the glitches that you have named, yourself. I can't tell the difference. So, is it AM or FM that makes up the unmarked frequencies?

[anatech]

Hi John,
Same conditions as in your paper?

-Chris

[john curl]

It is difficult to separate AM from FM in the graph by anyone, that can be absolutely proven, without completely emulating the test. I am not in a position to completely emulate the test at this time.

[janneman]

Originally Posted by john curl

Actually, Janneman, you DOCTORED with your own (unproven) input. But I won't complain at the moment, unless you are off by a few Hz, and even you are guessing. I have at the moment a FM modulated square wave on my test bench that makes virtually the same levels and frequencies of the glitches that you have named, yourself. I can't tell the difference. So, is it AM or FM that makes up the unmarked frequencies?

Not quite. I didn't measure them, true. But the reference to the 2F2-xF1 is in the paper, and when I calculated those, they fit perfect. It doesn't proof conclusively they actually are that, but it does explain what's on the graph.

But with that stuff you have on your bench you could have a strong argument for the FM case. If only you would get over the point of saying 'I have it!' and actually post some graphs, outline the measurement setup, levels, settings etc! We could talk about contents for a change. You know how that is done, you used to write AES papers, remember?

(BTW If you need some help with the mechanics of posting those graphs or labeling them, just let me know. Only too happy to help out there. Really).

jd

[john curl]

If I were you, I would check your assumptions more closely. I already have.
At this time, I maintain it is FM modulation.

[janneman]

Originally Posted by john curl

If I were you, I would check your assumptions more closely. I already have.
At this time, I maintain it is FM modulation.

Assumptions? I only took what you guys said in the paper and took it one step further. Are you saying you and Matti and Leinonen were wrong at the time?
Anyway, does that mean you have no intention to provide anything tangible on this? Too bad.

jd

[scott wurcer]

Originally Posted by john curl

If I were you, I would check your assumptions more closely. I already have.
At this time, I maintain it is FM modulation.

Another question to ask is where are all the missing intermods if these are at different frequencies? Suddenly numerous expected tones are absent with others at "almost" all the same frequencies. Pretty far fetched. BTW it is clear from your graph that evens are present so all those even order intermods are expected.

As an asside I would experiment with a low frequency tri-wave of different slopes and high frequency tone. Since the tri-wave is constant slope it's "quadrature" is a DC level. This would allow a better exploration of the shape of the input non-linearity. Also the error signal in the presence of feedback is more seperable in the low vs high OLBW cases.

[janneman]

Originally Posted by john curl

If I were you, I would check your assumptions more closely. I already have.
At this time, I maintain it is FM modulation.

Let me help you once more John, because clearly you still haven't re-read your own paper. This is what YOU said in YOUR and Matti's and Eero's paper. Do you now say this was **? Are you now saying that it was wrong, that since then you have found The Truth (tm), which however you seem unable or unwilling to back up in any way? Did you tell Matti and Eero that they too, had it all wrong in '76?

jd