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Old 14th December 2012, 07:41 PM   #1141
dimitri is offline dimitri  United States
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10kOhm in 22kHz bandwidth has -114dbV noise -you will get this with 100kohm 10:1 input attenuator

for the power amp you can put 100 Ohm 10:1 input attenuator and obtain -136dbV noise if your front end has 1nv/sqrtHz

Samuel is right, having -130/-140 dB THD+N and input signal from mV to 100V in mind you can think only about modular design, with separate input modules optimized for each input voltage/impedance range

Last edited by dimitri; 14th December 2012 at 07:45 PM.
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Old 14th December 2012, 07:45 PM   #1142
davada is offline davada  Canada
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Quote:
Originally Posted by RNMarsh View Post
Right.
Hi Rick,

You haven't a hope of ever getting that from 339a or anything like it.
You would start with with a noise floor of -160db at least. The distortion doesn't add nearly
as much to that as the wide band noise. I have de-tune the notch filter in the 339a enough to raise the fundamental 15db before it start to show up on the meter with a measure floor of 0.0012%. Once this is done the disto measurement is right on. The limitation of the 339a is the noise. Otherwise we would have to add an decade range to the measurement.

We are better off finding an instrument that can measure the THD+N with out amplification rather than having to add 80dB of gain so a meter can display it. Today's ADC's are getting close but have a ways to go. These cheap little audio ADC's haven't a chance but are good enough for GP work. After all there isn't a lot that do what they can. I think we need to look at the industrial stuff.

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Old 14th December 2012, 09:42 PM   #1143
davada is offline davada  Canada
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Does anyone know of a complete digital control of amplitude and phase for a notch filter?
This includes amplitude and phase measurements.

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Old 14th December 2012, 10:11 PM   #1144
1audio is offline 1audio  United States
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Originally Posted by davada View Post
Does anyone know of a complete digital control of amplitude and phase for a notch filter?
This includes amplitude and phase measurements.

Cheers,
Digital control "servo" for an analog notch filter? Or a digital filter? I think all the current solutions, including the AP, are analog. The SRI has analog preprocessing and digital filtering. The R&S is similar.

AKM has a scheme for "bonding" multiple ADC's to get greater dynamic range. I think the app note got to -126 dB wideband using 4 ADC's. I can post it if I can find it.
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Old 14th December 2012, 10:30 PM   #1145
davada is offline davada  Canada
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Quote:
Originally Posted by 1audio View Post
Digital control "servo" for an analog notch filter? Or a digital filter? I think all the current solutions, including the AP, are analog. The SRI has analog preprocessing and digital filtering. The R&S is similar.

AKM has a scheme for "bonding" multiple ADC's to get greater dynamic range. I think the app note got to -126 dB wideband using 4 ADC's. I can post it if I can find it.
Hi Demian,

I mean digital control of an analog notch filter.
The AP is SVF I think. I'd like to see something using other filter types but the measurement is all essentially the same.

It would be ideal to do this using a set of SVF and SVO because the quadrature is available off the oscillator. However it would be limited to the set unless the an external oscillator was synchronized to the internal SVO of the set.

Yes please post anything you have.

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Old 14th December 2012, 11:10 PM   #1146
RNMarsh is offline RNMarsh  United States
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let's narrow the issue down to: 1volt and 10Vac input; 1K and 100 Ohm Z input. If just one -- 1v/1k. How close can we get to -130+N ?

I dont expect the 339A to get us there. the idea of redistributing the gains will do wonders, i'm sure. And, that would be about IT for the 339A. Pretty good upgrade results for low bucks and one doesnt have to build the box, power supply and all the guts. If I had to do all that, I'd rather go hang myself in the morning.

The 339A is about the cheapest instrument that DIY'ers will ever be able to do simple modes to (parts, value changes and IC replacements) and get excellent results. However, next up the ladder of affordability might be the popular HP 8903_ or others? If the full potential can be realized in that one or other readily available used gear, what rewards will they bring? This little 339A excercise shows us there is a lot that can be had in refurbishing them for improved performance if the fundamental design is sound (?). :-) Thx-RNMarsh

Last edited by RNMarsh; 14th December 2012 at 11:16 PM.
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Old 14th December 2012, 11:21 PM   #1147
RNMarsh is offline RNMarsh  United States
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Quote:
Originally Posted by 1audio View Post
AKM has a scheme for "bonding" multiple ADC's to get greater dynamic range. I think the app note got to -126 dB wideband using 4 ADC's. I can post it if I can find it.

That is interesting. Could also apply to an existing, older design?.... stack new, multiple ADC's.
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Old 15th December 2012, 01:11 AM   #1148
richiem is offline richiem  United States
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Richard -- RE the HP 8903 -- I think it is a poor choice all around. The chain of amps is even longer than in the 339 and the gain structure varies with auto operation in some odd ways, making the monitor output reference level hard to figure out. Plus the filters are absolutely in all the wrong places. I *think* the oscillator may be very good, but I doubt it's better than the 339. But the notch system is certainly no better than the 339's. Not to say that it isn't moddable, but the plug-in card system makes some things more difficult as breaking up and rea-arranging a sequence of amps is quite a challenge -- and then the microprocessor control system goes nuts....
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Old 15th December 2012, 01:19 AM   #1149
RNMarsh is offline RNMarsh  United States
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Richard -- RE the HP 8903 -- I think it is a poor choice all around. The chain of amps is even longer than in the 339 and the gain structure varies with auto operation in some odd ways, making the monitor output reference level hard to figure out. Plus the filters are absolutely in all the wrong places. I *think* the oscillator may be very good, but I doubt it's better than the 339. But the notch system is certainly no better than the 339's. Not to say that it isn't moddable, but the plug-in card system makes some things more difficult as breaking up and rea-arranging a sequence of amps is quite a challenge -- and then the microprocessor control system goes nuts....
OK. I believe you. Sounds like a bad example to use. Just thinking outloud - a thought about if there is something better and ready made to upgrade, what might it be? Thx for info -RNMarsh
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Old 15th December 2012, 01:53 AM   #1150
richiem is offline richiem  United States
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I'm with you about starting from scratch these days -- time is a bit short. So I cannibalize other stuff. I am going to try to get the active twin-t working with auto-tuning -- I suspect the pull-in range will be small, but I don't care about that -- I'm willing to turn some knobs to get it in range. As I've said, I'm not sure how good the active twin-t is in either total noise or in distortion -- that's the trouble here all the way round -- everything we're concerned about is lower than whale manure....
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