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Old 13th December 2012, 05:22 AM   #1121
1audio is offline 1audio  United States
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Default Input protection

I have attached clips from three different approaches to input protection.

The Fluke 8922 is a TRMS voltmeter. They use high input Z and diode connected transistors to internal rails to limit the max voltage at the sensitive circuit. it doesn't degrade the input enough to affect its specs (.5% 50 Hz to 200 KHz) and a min input of .18 mV

The Amber 3501 has a series resistance (500 Ohms) and light bulbs plus a shunt diode to a current source to limit the max input to the sensitive stage.

The Boonton 1121 uses small fuses (1/16 A) in series with a min R of 10K and diodes tied to the internal rails to protect the input. That 10K resistor probably limits the minimum noise of the unit. it still will get to .0007 % THD with a 30 KHz bandwidth so its not a real limitation. (I can't get the oscillator down that far yet. Work to do.) It does have autoranging so the input attenuator will switch up pretty quickly.

My current take would be series fuses at a high Z node with diodes. I would much rather replace fuses than an expensive opamp.
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File Type: zip input protection.zip (193.0 KB, 27 views)
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Old 13th December 2012, 05:28 AM   #1122
1audio is offline 1audio  United States
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Quote:
Originally Posted by RNMarsh View Post
that one and several others as well. Whats up with these companies not being SOTA in lowest harmonic distortion attainable... and at their prices, too?? [no need to answere] -RNM
When I used the thd+noise instrument from Demian, it was reading on my little amp... .0005% . Too close to the test equip residual? The FFT output was uncalibrated so that didnt help. can it be easily/accurately calibrated? Looking for a test system to easily and accurately measure harmonics in the .00001 - .0001% area. Thx-RNMarsh
The closed loop THD only with that box (Shibasoku 725) plus the generator is around .00005 (-130 dB) or less. I have verified that its accurate. You use the individual harmonic measurement to calibrate the FFT output to known levels.

One of the big challenges, and Scott touched on it, is settling time/ frequency response from the AGC. If you can allow 30 seconds for settling it may be possible to use a lower distortion solution. All of the commercial products today are expected to sweep pretty quickly with infinitely low distortion. Something must give.
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Old 13th December 2012, 05:34 AM   #1123
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Quote:
Originally Posted by davada View Post
Smoking.

What version of software do you have?
.964 It thinks its in a 32 bit system. . .
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Old 13th December 2012, 05:37 AM   #1124
davada is offline davada  Canada
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.964 It thinks its in a 32 bit system. . .
It's probably compiled in win32.

Do you want to try a previous version?
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Old 13th December 2012, 05:39 AM   #1125
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I'll wait for a response from the mothership before I do much more. I'm too busy to put much time in right now anyway.
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Old 13th December 2012, 06:54 AM   #1126
RNMarsh is offline RNMarsh  United States
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Quote:
Originally Posted by 1audio View Post
The closed loop THD only with that box (Shibasoku 725) plus the generator is around .00005 (-130 dB) or less. I have verified that its accurate. You use the individual harmonic measurement to calibrate the FFT output to known levels.

One of the big challenges, and Scott touched on it, is settling time/ frequency response from the AGC. If you can allow 30 seconds for settling it may be possible to use a lower distortion solution. All of the commercial products today are expected to sweep pretty quickly with infinitely low distortion. Something must give.
I have played - decades ago - with such long time constant oscillators.... but 30 seconds is way too long if I want to make null and balancing adjustments. The Shibasku 725 is 5-10 times better than i need so that should be accurate enough for now. Need to find another.. cant get to you often enough for any long term developemt work... just a spot measurement now and then. I'll be looking for one. Thx-RNMarsh
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Old 13th December 2012, 07:09 AM   #1127
RNMarsh is offline RNMarsh  United States
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Default Shibasoku AD725D - LG

hah!! I no sooner say I'll look for one and the next second there it is on eBAY ! A nice looking AD725D. And, it ships from Calif.
L.G. My present to myself. Well... it tiz the season. Now for that great FFT software I am looking for. -RNM

I'll be able to check that 339A out real good now and then some.... and get back to amps et al.

Last edited by RNMarsh; 13th December 2012 at 07:17 AM. Reason: L.G.
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Old 13th December 2012, 01:38 PM   #1128
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Quote:
The AP 2700 is spec'ed at -112 to -115dB. I want better, unfortunately. Thx-RNMarsh
AP internal oscillator
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Old 13th December 2012, 02:14 PM   #1129
davada is offline davada  Canada
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AP internal oscillator


Got to love that notch depth. Dual filters?
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Old 13th December 2012, 02:18 PM   #1130
davada is offline davada  Canada
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AP internal oscillator
This looks like an LP tuning on the notch filter or an elliptic on the oscillator filter???
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