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Old 5th January 2013, 08:13 PM   #1461
1audio is offline 1audio  United States
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Here is a really good approach to measuring the voltage using sampling techniques. http://www.home.agilent.com/upload/c...?&cc=US&lc=eng I don't think you need to go to the 3 ppm capability of the HP3458 to get value from this and if you had both level and frequency you could steer both to the desired values with a PIC or Arduino. (Linuxworks, where are you?)
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Last edited by 1audio; 5th January 2013 at 08:16 PM.
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Old 5th January 2013, 08:13 PM   #1462
davada is offline davada  Canada
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Quote:
Originally Posted by RNMarsh View Post
It doesnt look too unwieldly and a pcb is already made to try it with. Now... what is the goal, again? The cause of thd that this is going to affect a reduction? Just checking... sort of a resync operation. -RNM
Hi Rick,

The FFT calculates and holds the peak. The peak is compared to a reference level and then
output to an integrator. The integrator's job is to find the DC value output to the multiplier which satisfies the loop requirements for the reference output level. The PIC would output a value to a DAC connected to the lamp drivers.

We only need a single frequency output from the FFT. Only the bins closet in to the FR are needed and anything that spills into adjacent bins ,AKA buckets, can be added back the FR peak. Since there is only one frequency of interest we can allow the FFT to wrap around the nyquist. It will show up as a lower alias frequency peak. Therefore sample rate limit of the PIC's ADC is not really a problem.

The question is how low a sample rate can we use?

The number of samples sets the number of bins. The fewer bins means more bandwidth is crammed into one bin including noise.

The second question is how few number of samples can we get away with and still have a reasonable accuracy. There is also the question of bit depth. Is 10 bits enough.
The smaller the FFT is the faster it computates.


The project in the link is a toy not an instrument.

I,m no expert on FFT so if anyone is more knowledgeable please comment.

Microchip has a white paper on using FFT as a peak detector.

I already have a development board for the PIC18F2455 which is the same as the PIC18F4550 just fewer IO ports.
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Old 5th January 2013, 09:02 PM   #1463
davada is offline davada  Canada
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Quote:
Originally Posted by 1audio View Post
Here is a really good approach to measuring the voltage using sampling techniques. http://www.home.agilent.com/upload/c...?&cc=US&lc=eng I don't think you need to go to the 3 ppm capability of the HP3458 to get value from this and if you had both level and frequency you could steer both to the desired values with a PIC or Arduino. (Linuxworks, where are you?)
Hi Demian,

Dead link there.
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Old 5th January 2013, 09:05 PM   #1464
dimitri is offline dimitri  United States
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David, check your e-mail
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Old 6th January 2013, 12:16 AM   #1465
RNMarsh is offline RNMarsh  United States
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Quote:
Originally Posted by davada View Post
Hi Rick,

The FFT calculates and holds the peak. The peak is compared to a reference level and then
output to an integrator. The integrator's job is to find the DC value output to the multiplier which satisfies the loop requirements for the reference output level. The PIC would output a value to a DAC connected to the lamp drivers.
Something similar to this I figure ShibaSoku harmonic distortion analyzer is doing... displays the individual harmonics peaks from an FFT was my guess. -RNM
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Old 6th January 2013, 12:38 AM   #1466
davada is offline davada  Canada
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Originally Posted by RNMarsh View Post
Something similar to this I figure ShibaSoku harmonic distortion analyzer is doing... displays the individual harmonics peaks from an FFT was my guess. -RNM
From it's description I think the Shibasoku is a heterodyne process. It's a very sophisticated selective voltmeter.

But since no one has a service manual for it who knows.

Cheers,
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Old 6th January 2013, 05:11 AM   #1467
RNMarsh is offline RNMarsh  United States
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It appears to be treated as a top secret national treasure.
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Old 6th January 2013, 06:57 AM   #1468
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For $149 you can get a Texas Instruments evaluation module with FFT software. These are wonderfully designed and built kits.
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Old 6th January 2013, 07:05 AM   #1469
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Quote:
Originally Posted by davada View Post
From it's description I think the Shibasoku is a heterodyne process. It's a very sophisticated selective voltmeter.

But since no one has a service manual for it who knows.

Cheers,
You could do this, then, with an HP 3586 Selective Level Receiver programmable via the HP-IB.
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Old 6th January 2013, 07:34 AM   #1470
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Quote:
Originally Posted by davada View Post
From it's description I think the Shibasoku is a heterodyne process. It's a very sophisticated selective voltmeter.

But since no one has a service manual for it who knows.

Cheers,
Lock-in amplifier principle? I've been looking at some PAR units, but not sure if it is worth the hassle.

jan
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