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Old 7th January 2013, 07:32 AM   #1481
1audio is offline 1audio  United States
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Quote:
Originally Posted by davada View Post
Hi Demian,

Dead link there.
The link is live again. It describes how the Agilent 3458 DVM gets exceptionally accurate AC voltage measurement (3 ppm) with undersampling. The max sample rate is 250 /second I think and it measures TRMS to 150 KHz. I think you could apply the same principle with a PIC (not get 3 ppm) but get good enough accuracy pretty easily with its internal adc. Add a mux and a good vref to self cal periodically to improve long term stability.

Another option if you are looking at pure sine waves is to borrow from an incremental voltmeter. You could amplifiy just the voltage range +/- 5% around the peak voltage to match the range of the ADC. If no samples are in range need more gain in the oscillator. If samples hit the positive limit reduce. The max voltage of any sample would be the peak value + noise + ADC errors. Collect and look at samples and you can determine the frequency as well as the max amplitude.

What frequency resolution and accuracy are you seeking? For that matter voltage accuracy and stability etc? SG's question is relevant since these all affect the device design and component performance.
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Old 7th January 2013, 08:19 AM   #1482
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Quote:
I don't need a fast detector for what I'm doing. These methods wouldn't work for a conventional multiplier like a Jfet type. Other detection methods are being discussed for an already stabilized loop that just needs to be brought to a reference level.
A slow loop still needs to meet stability constraints... And if the loop is slow you can much more easily apply lots of low-pass filtering, so why looking for anything more than a simple two-way rectifier? I think the problem you're facing has been solved already by many others, so don't try to re-invent the wheel!

There is an 1968 IEEE paper which shows exactly what you want to do (as far as I understand)--"fast" stabilization by a thermistor/lamp, and slow stabilization by a linear leveling loop:

Skehan, B.J.
Design of an amplitude-stable sine-wave oscillator
Solid-State Circuits, IEEE Journal of
Sept. 1968, Volume 3, Issue 3

Samuel
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Old 7th January 2013, 08:22 AM   #1483
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Another option if you are looking at pure sine waves is to borrow from an incremental voltmeter. You could amplifiy just the voltage range +/- 5% around the peak voltage to match the range of the ADC.
That's pretty much what's been done in the AP System One.

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Old 7th January 2013, 04:01 PM   #1484
1audio is offline 1audio  United States
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I thought it was a good idea.
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Old 7th January 2013, 04:51 PM   #1485
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It is, and a major contributor why the generator of this unit is as good as it is!

Samuel
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Old 7th January 2013, 05:20 PM   #1486
RNMarsh is online now RNMarsh  United States
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A little off course But -- why wouldnt a jFET be enclosed within an opamp feedback loop to linearize it for this app as being used in original 339A?
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Old 7th January 2013, 07:20 PM   #1487
davada is offline davada  Canada
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Quote:
Originally Posted by 1audio View Post
The link is live again. It describes how the Agilent 3458 DVM gets exceptionally accurate AC voltage measurement (3 ppm) with undersampling. The max sample rate is 250 /second I think and it measures TRMS to 150 KHz. I think you could apply the same principle with a PIC (not get 3 ppm) but get good enough accuracy pretty easily with its internal adc. Add a mux and a good vref to self cal periodically to improve long term stability.

Another option if you are looking at pure sine waves is to borrow from an incremental voltmeter. You could amplifiy just the voltage range +/- 5% around the peak voltage to match the range of the ADC. If no samples are in range need more gain in the oscillator. If samples hit the positive limit reduce. The max voltage of any sample would be the peak value + noise + ADC errors. Collect and look at samples and you can determine the frequency as well as the max amplitude.

What frequency resolution and accuracy are you seeking? For that matter voltage accuracy and stability etc? SG's question is relevant since these all affect the device design and component performance.
Hi Demian,

Thanks for the link.
I got a 401 a couple time. Dimitri sent me the pdf. I tried the link again and it worked the third time. My internet been acting weird lately.

The PIC has 13 analog IO two of which are reference inputs. Two comparators. Two event captures on either rising or falling edges. SPI, USB, 4 timers and more IO ports than you know what to do with. This is alll hardware modules separate from the processor. All in a 28 pin dip package. Quite handy little device. But still not suitable for real time work over 1KHz on the SPI side. I do have it setup for frequency counting 1Hz resolution 40MHz.

What your suggesting with the incremental voltmeter is technique that artificially increases
an ADC's resolution. This can also be helped along by narrowing the ADC input range by setting the reference higher and offsetting the (-) input pin. The PIC has pins for this option. If we can cram the 10bits into a 78.125mV span we have an equivalent resolution to 16bit over a 5V span. This can be set up with switches and a voltage diver network.
Sample once 10bit over 5V span then close in to 10 bit over a 78.125mV and a bit of math can make up a 16 bit word. Send that to a 16 bit DAC. If the PIC had two ADC this could be done simultaneously. I suppose the comparators could be used to find the coarse and then the ADC to do the fine.

It's a lot of fuss though and can make a guy's head hurt figuring it all out.

Cheers,
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Last edited by davada; 7th January 2013 at 07:31 PM.
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Old 7th January 2013, 09:10 PM   #1488
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Default LDR sub for the 339A's E1 or E2 Vactrol

I've been following this thread with some interest, especially as it dwelt on the HP 339A for a while. Coincidentally, my 339A's E2 has failed open circuit on the LED side. The design note and data sheet from PE/Vactec was useful but I'm still unsure if the part used by HP maps to one of those listed therein. Stabbing in the dark I've ordered one each of the VTL5C1 and VTL5C2 but wonder if any of you have determined a contemporary replacement. My 339A is stock.
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Old 7th January 2013, 11:23 PM   #1489
davada is offline davada  Canada
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Quote:
Originally Posted by johnfalc View Post
I've been following this thread with some interest, especially as it dwelt on the HP 339A for a while. Coincidentally, my 339A's E2 has failed open circuit on the LED side. The design note and data sheet from PE/Vactec was useful but I'm still unsure if the part used by HP maps to one of those listed therein. Stabbing in the dark I've ordered one each of the VTL5C1 and VTL5C2 but wonder if any of you have determined a contemporary replacement. My 339A is stock.
Hi John,

Not sure if those will work.

One thing you can try is to break the epoxy out on the LED side and find an LED to replace the one that's open. No doubt you can find some black epoxy at your local hardware store.

Cheers,
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Old 8th January 2013, 02:03 AM   #1490
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David, Thanks for the suggestion, for the sake of learning, I'll try the commercial devices and if they don't work I'll go the route you suggest ... if I do I'll have to decide on an LED color, I wonder if the VR part of the device is CdS. John
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