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Old 28th October 2009, 06:49 PM   #1981
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Glancing over the Toshiba notes again, I found a serious oversight that I made. You have to subtract .7-.8V from the Vgs (off) and use that voltage to bias the device This makes it MORE difficult to calculate. I will have to work on it, but it may be difficult to do with just any jfet, and get something useful. For example, a 2SK170 can never be TC zero, because its pinch off voltage is too low. Interesting problem.
Therefore for Zero TC, stick with a Zener equivalent reference. Everybody else, it doesn't matter that much, anyway.
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Old 28th October 2009, 06:50 PM   #1982
syn08 is offline syn08  Canada
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Originally Posted by john curl View Post
Glancing over the Toshiba notes again, I found a serious oversight that I made. You have to subtract .7-.8V from the Vgs (off) and use that voltage to bias the device This makes it MORE difficult to calculate. I will have to work on it, but it may be difficult to do with just any jfet, and get something useful. For example, a 2SK170 can never be TC zero, because its pinch off voltage is too low. Interesting problem.
Therefore for Zero TC, stick with a Zener equivalent reference. Everybody else, it doesn't matter that much, anyway.
And what did I say about this in the last few pages?

BTW, that's a simple quadratic equation that I routinely had to solve by hand, 30 years ago when I was a student.
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Old 28th October 2009, 07:02 PM   #1983
dimitri is offline dimitri  United States
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Quote:
Anatech, why not just use the good bench meter, like an ST or HP, and just filter it at 10KHz with a 6 dB/octave rolloff filter?
10kHz 6 dB/octave rolloff filter will have noise bandwidth 12.6kHz

it is good to have also 100Hz high-pass filter and 60dB of amplification

spectrum analyzer is absolutely required to understand what are you measuring - noise or hum harmonics or anything else
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Old 28th October 2009, 07:02 PM   #1984
anatech is offline anatech  Canada
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Hi John,
I completely agree, and that is how I normally approach doing this if I wanted an average of the entire band. Those older (solid state!) HP AC voltmeters perform far better than most people might think. Both of mine reside with the other equipment that went with my shop when I sold it. Sadly, I don't think the new owners have any clue what they have there.

Why did I suggest using an averaging DVM with fast response? Your question did concern using the 3581A with a tight bandpass filter of 1 Hz. The output of which will obviously be all over the face of the meter. Using a wide band meter would average the entire band in with the frequency you wanted to investigate. That seems to be completely out of the parameters of your question. Anyway, allowing the newer meter to take several peak readings and average those might give you a higher average reading - or not. You'd have to run the experiment and see. Not "you" as in John Curl, the anyone who is interested "you". I'm not really sure how a filter might actually indicate compared to a proper detect and average method.

I didn't think the method I suggested confused anything. It seemed to be straight forward to me. Building a filter to stick in front of the meter actually may be less certain than simply allowing the 3581A's filters (a known quantity and ready to go) to do the work. Either way, you have another box between the meter and circuit. Using the HP box rather than building another one would be my direction if we needed to filter anything. Then there is the question of input impedance and frequency response to deal with. Again, the 3581A is a known quantity. A constructed circuit has theoretical performance that may not be reality. It hasn't been characterized yet. No thanks!

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B&K Graph recorder doesn't have the paper width to make it as readable as the old method
Just as well, because I really hated calibrating chart recorders. What a pain! From a calibration technician's viewpoint. At least you end up with a hard copy to back up your numbers though.

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Old 28th October 2009, 07:06 PM   #1985
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Well, I thought that it would be easier, but it is still not impossible. Toshiba has shown examples of Zero TC operation, with the 2SK30 for example that look good and easily doable. I haven't tried every device yet, to find an optimum, but it is a moot point anyway, because the follower buffer would have to be compensated at well. However, with a bipolar buffer follower, the Vbe would drop, and the beta would increase over a reasonable temperature range, so maybe a TC on the jfet is not such a bad idea, after all.
As far as low impedance drive, the LM329 is ONLY good for that, IF you don't add an RC noise reducing filter, so therefore you would have to live with 7-100uV from your source. I can't do that, myself. Perhaps others can, with their circuits. IC's sure are great for power supply rejection.
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Old 28th October 2009, 07:23 PM   #1986
anatech is offline anatech  Canada
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Hi dimitri,
Quote:
spectrum analyzer is absolutely required to understand what are you measuring - noise or hum harmonics or anything else
Absolutely! And that is the reason why I put the plastic down for a 3585A. I'm still paying it off as a matter of fact, but as a tool - it's wonderful! Later on, a 3562A or equiv. would be a welcome addition.

Hi John,
Quote:
For example, a 2SK170 can never be TC zero, because its pinch off voltage is too low. Interesting problem.
It's been explained to me that a long channel device works better as a CCS. The noise created by a J-Fet will be attenuated anyway by your Vref. Assuming your CCS looks like a 350 K resistance (not unreasonable), the other part of the AC noise divider will be the dynamic impedance of the reference. A few ohms for an LED, 5 ~ 12 ohms for a zener depending on where it's operating at. That's a lot of noise attenuation. If you are looking at an active load, as in a load for a transistor collector, you get less attenuation unless your CCS is cascoded in order to get up into the several meg. ohm region.

I don't think going to a 2SK117 or JAN # is going to ruin the quality of a CCS that much. It might be good to find some acceptable JAN parts to get away from dependency on Japanese suppliers. Normal experimenters have too much trouble getting these, and now I do too. My normal guy who bought directly from Japan retired several years ago, choking my supply of real devices off.

-Chris

EDIT:
Hi John,
Quote:
IC's sure are great for power supply rejection.
At low frequencies only. Everything goes to pot past 10 KHz typically.
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Last edited by anatech; 28th October 2009 at 07:26 PM. Reason: Responding to post above, added comment
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Old 28th October 2009, 07:26 PM   #1987
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Anatech, remember 'Bounce,bounce, bounce'? If I am not mistaken, you are stating what I was concerned about when using a wave analyzer with narrow pickup. Thanks for stating:"Your question did concern using the 3581A with a tight bandpass filter of 1 Hz. The output will obviously be all over the face of the meter." Please remember, I said it first, and this was my intent when I said: 'Bounce, bounce, bounce' because that what it can look like. Please understand that I was recommending wide band settings, not narrow, in order to reduce the measurement inaccuracy. I regret adding a passing comment to the person who recommended narrow bandwidth measurement.
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Old 28th October 2009, 07:27 PM   #1988
syn08 is offline syn08  Canada
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Quote:
Originally Posted by dimitri View Post
10kHz 6 dB/octave rolloff filter will have noise bandwidth 12.6kHz

it is good to have also 100Hz high-pass filter and 60dB of amplification

spectrum analyzer is absolutely required to understand what are you measuring - noise or hum harmonics or anything else
True, and the 60dB gain has to be low noise. A low noise opamp (nomina odiosa here, I guess) will do, but the entire setup becomes complicated.
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Old 28th October 2009, 07:28 PM   #1989
Salas is offline Salas  Greece
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Quote:
Originally Posted by john curl View Post
As far as low impedance drive, the LM329 is ONLY good for that, IF you don't...
Your argument for erratic LM samples for noise, when combining a low impedance goal strengthens my curiosity about a string of 4 generic 1.8Vf Red or Yellow or Green color Leds well bypassed only with a high value low ESR cap, fed by low noise Jfet CCS, being the two trick pony in the end. We will know if syn08 will run a test for them too.

Edit: Make that a 3 trick pony. Resistant to feeder Jfet's Idss Tc related wandering VS a relatively high value resistor, too.
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Old 28th October 2009, 07:53 PM   #1990
anatech is offline anatech  Canada
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Hi John,
Not wanting to "get into it", but the 'Bounce,bounce, bounce' was not what I had a problem with. In fact, it should be obvious that I agree with you from my recent posts, and the fact that what you said was a truthful reality. One thing I am familiar with is using instrumentation to some degree.

Quote:
Please understand that I was recommending wide band settings, not narrow, in order to reduce the measurement inaccuracy.
I agree with that as well, but the bandwidth setting will be forced by what the focus of the measurement is. I'm not making a statement on the suitability of instrument settings. I'm only commenting that if that setting is required, there is a way to get meaningful numbers from it.

Quote:
I regret adding a passing comment to the person who recommended narrow bandwidth measurement.
We both do, and that was the reason for my post.

I have respect for most everyone here, which includes both you and syn08. But everyone is different and personalities can clash. This is more likely behind a monitor and keyboard than in person.

I hope to met you in person one day John. At the very least, you would be interesting to talk to in person I think.

The line of discussion is fruitful right now, let's continue. There are no lasting issues worth worrying about.

BTW, 2SK30A might work well for CCS duty. They also have a 50 VDC breakdown, 10 V more than the 2SK170. I'd like to find a number commonly available from US inventory to use. Most have a breakdown in the 25 VDC region, some up to 40 or 50 VDC.

I wonder how noisy / quiet the current regulator diodes are. Too bad they are so expensive to use. Can't get much simpler though, and some of those have 100 VDC breakdown voltages. An example would be http://www.vishay.com/docs/70195/70195.pdf
The prices shown at Mouser will scare any adult!

-Chris
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