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Old 19th September 2011, 03:23 AM   #11
richiem is offline richiem  United States
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Hi Ken -- 1) Just floats. 2) 1Meg
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Old 23rd September 2011, 06:30 PM   #12
klewis is offline klewis  United States
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Dick,

I fired it up this morning and was able to get -60db null without much fiddling. I will work on getting it to -80 this weekend. With the -60db my HP3562A spectrum analyzer showed a noise floor below -120db. Pretty darn good! Photos of the finished box attached. I think I will find a way to cement the trimmers and maybe turn a few of them around so that they can be accessed from the top of the box.

Thanks again for a great design / tool.

Ken
Attached Images
File Type: jpg 2011-09-23_06-25-47_962.jpg (498.3 KB, 545 views)
File Type: jpg 2011-09-23_06-26-12_780.jpg (557.9 KB, 531 views)
File Type: jpg 2011-09-23_06-26-29_484.jpg (780.5 KB, 415 views)
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Old 24th September 2011, 03:39 AM   #13
richiem is offline richiem  United States
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Great battery clip!!!

BTW, doncha luv them switches?

Last edited by richiem; 24th September 2011 at 04:00 AM.
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Old 24th September 2011, 04:51 AM   #14
klewis is offline klewis  United States
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Yeah, I got a bit carried away. The switches are great.
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Old 26th September 2011, 03:18 PM   #15
klewis is offline klewis  United States
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Here are screen shots from my HP3562A spectrum analyzer. The analyzer can resolve 60 to 80 db of signal.

The first photo is with the signal from HP8903A 10kHz 0.5Vrms straight into the analyzer. The signal floor is -90db due to the fundamental being at about -5 db (85 db of resolution). The 2nd, 3rd, etc. harmonics are showing, probably because I'm feeding them directly in the analyzer without a load.

The second photo is the same signal through the Twin -T. You'll see a round circle on the fundamental, it's the marker that displays in the upper left corner - in this instance the null was -77.393db at 10kHz. The horizontal line at approx. -125db (-124.97) was placed by me to represent the noise floor of the analyzer. The second harmonic is at about -103db, the third at about -118db. The spikes at 45kHz are either from the florescent lights or my smart electric meter. I'm pretty confident that this image represents the harmonic profile of the HP8903A signal.

I also tested the slope of the notch by tuning the filter to -80db at 10kHz then feeding it an 11kHz, through 20kHz signal in 1k increments. By 16kHz the notch was not changing the value of the fundamental.
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File Type: jpg ana 10k without null.jpg (453.6 KB, 380 views)
File Type: jpg ana 10k with null.jpg (505.3 KB, 352 views)
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Old 27th September 2011, 03:59 AM   #16
richiem is offline richiem  United States
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Nice work, Ken -- the Twin-T is working great!

Now, about the 8903 -- can you run these tests with a 1kHz fundamental? I'm curious about the oscillator in the 8903. There don't seem to be any THD tweaks for minimizing the 2nd H, and it looks a bit high to me for a 10kHz fundamental. Just curious -- I have an 8903E -- no oscillator -- so I can't check it myself.
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Old 28th September 2011, 02:37 PM   #17
klewis is offline klewis  United States
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Quote:
Originally Posted by richiem View Post
Nice work, Ken -- the Twin-T is working great!

Now, about the 8903 -- can you run these tests with a 1kHz fundamental? I'm curious about the oscillator in the 8903. There don't seem to be any THD tweaks for minimizing the 2nd H, and it looks a bit high to me for a 10kHz fundamental. Just curious -- I have an 8903E -- no oscillator -- so I can't check it myself.
Dick,

I ran a series of tests, seems the 2nd H varies with the output voltage of the 8903A.

Note that the device is set on auto scale, so, the images jumps around a bit. There is a way to set the upper/lower range, but, I made these images before work today and more importantly, before coffee.

First, I re-ran the 10kHz tests. All voltages are rms. Images as follows:
1. 0.5v 2nd H -103db
2. 0.1v 2nd H -123 (bouncing in the noise floor)
3. 0.2v 2nd H -121
4. 0.3v 2nd H -102
5. 0.4v 2nd H -106
6. 0.6v 2nd H -116
7. 0.7v 2nd H -107

I'll post the 1k results in the next.

Ken
Attached Images
File Type: jpg IMG_20110928_055955.jpg (410.3 KB, 160 views)
File Type: jpg IMG_20110928_060106.jpg (417.6 KB, 73 views)
File Type: jpg IMG_20110928_060136.jpg (425.7 KB, 53 views)
File Type: jpg IMG_20110928_060153.jpg (433.7 KB, 48 views)
File Type: jpg IMG_20110928_060208.jpg (463.6 KB, 42 views)
File Type: jpg IMG_20110928_060227.jpg (448.0 KB, 38 views)
File Type: jpg IMG_20110928_060249.jpg (479.4 KB, 38 views)
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Old 28th September 2011, 02:47 PM   #18
AndrewT is offline AndrewT  Scotland
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Is this technique/equipment a close relation to Cordell's distortion magnifier?
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regards Andrew T.
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Old 28th September 2011, 02:49 PM   #19
klewis is offline klewis  United States
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I couldn't get as deep of a null, but, it didn't effect the results. I think that I am going to move the three trimmers to the top of the box for external adjustment as well.

Here are the 1kHz results:

Image:
1. 0.5v 2nd H -100db
2. 0.1v 2nd H -118
3. 0.2v 2nd H -112
4. 0.3v 2nd H -108
5. 0.4v 2nd H -104
6. 0.6v 2nd H -116
7. 0.7v 2nd H -108
8. 1.0v 2nd H -105

I found a Krohn Hite 4402b Oscillator on ebay last week. Hopefully it will arrive before the weekend. I'll post results for it as well.

Ken
Attached Images
File Type: jpg IMG_20110928_060953-0-5v.jpg (495.7 KB, 57 views)
File Type: jpg IMG_20110928_061020-0-1v.jpg (528.1 KB, 58 views)
File Type: jpg IMG_20110928_061051-0-2v.jpg (520.1 KB, 43 views)
File Type: jpg IMG_20110928_061107-0-3.jpg (538.3 KB, 33 views)
File Type: jpg IMG_20110928_061123-0-4.jpg (533.7 KB, 34 views)
File Type: jpg IMG_20110928_061135-0-6.jpg (543.3 KB, 34 views)
File Type: jpg IMG_20110928_061158-1-0.jpg (535.0 KB, 38 views)
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Old 28th September 2011, 03:06 PM   #20
klewis is offline klewis  United States
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Quote:
Originally Posted by AndrewT View Post
Is this technique/equipment a close relation to Cordell's distortion magnifier?
Andrew, good question, I'm not sure but I think it is close. Dick is more qualified to answer. I think I can explain what this technique does. The specturm analyzer has a resolution of 80 db or so. When you null the fundamental frequency, you are reducing the db range of the signal by the null value. So, with the fundamental at -60 instead of 0 db the analyzer can now theoretcally resolve down to -140 db. The noise floor of the analyzer kicks in at -125. This is all done with the gain setting of the twin t at 0 db. When switched to +20 db the twin t amplifies the fundamental and the harmonics up out of the noise floor of the analyzer, in effect magnifying the distortion.

My tests were with the twin t set at 0db. Mostly because the 2nd H was already above the noise floor. I'll give it a try at +20 and see if the 3rd H is visible.

Ken
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