Low-distortion Audio-range Oscillator - Page 143 - diyAudio
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Old 2nd January 2013, 03:24 AM   #1421
davada is offline davada  Canada
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Quote:
Originally Posted by scott wurcer View Post
You don't want Ohms you want current.
Yes I know I don't want ohms.
I was just testing it with an ohm meter.
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Old 2nd January 2013, 03:38 AM   #1422
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Its an interesting approach and could be quit linear over the range of use. -RNM
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Old 2nd January 2013, 04:22 AM   #1423
davada is offline davada  Canada
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This is the circuit used and spectrum of the output.

For 6Vrms input to the lamp there is a 5.56Vdc output.
At 3Vrms the output drops to 0.823Vdc.

That's not very linear.

I'm concerned about the level of distortion as well.
This is with a 0.1uF feedback cap on the output amplifier.
I's much worse with out the cap.
R is 10M.
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Old 2nd January 2013, 04:39 AM   #1424
1audio is offline 1audio  United States
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Here is another possibility-
Using a small micro (Arduino?) with an internal ADC or external ADC to detect the level of the oscillator output and then an internal DAC to send a voltage to the optical link that does the slow control loop for overall leveling. I'm sure linuxworks could tell us if the code is difficult and it could implement whatever PID is necessary to settle as fast as the system can.

One big reason that different frequencies will settle differently is the matching and balance of the R/C networks (and the GBW of the amplifiers). A really interesting possibility once you have a micro is electronic tuning. Using the micro for frequency detection and another 2 quadrant multiplier you could tune the system very precisely.
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Old 2nd January 2013, 05:02 AM   #1425
davada is offline davada  Canada
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Quote:
Originally Posted by 1audio View Post
Here is another possibility-
Using a small micro (Arduino?) with an internal ADC or external ADC to detect the level of the oscillator output and then an internal DAC to send a voltage to the optical link that does the slow control loop for overall leveling. I'm sure linuxworks could tell us if the code is difficult and it could implement whatever PID is necessary to settle as fast as the system can.

One big reason that different frequencies will settle differently is the matching and balance of the R/C networks (and the GBW of the amplifiers). A really interesting possibility once you have a micro is electronic tuning. Using the micro for frequency detection and another 2 quadrant multiplier you could tune the system very precisely.
I tried this with a PIC controller and found that they don't work very well for real time control. They work well for motor control but that is very low frequency.
The module timing for SPI transfer takes a while to set up and so I couldn't get a continuous stream of transfer. The processor is running at 48MHz. The SPI at 12MHz.

DSP I think would be a better approach.

Lt has a couple of high speed ADC which handle the timing for SPI transfer. So it's possible
to do a ADC to MDAC transfer with out a processor and there is no overhead in the timing.

I am planning MDAC tuning for this oscillator and that part will be controlled by a PIC micro.
The PIC is USB and there will be a Windows interface with a frequency counter, tuning and level controls. A cheesy Windows interface but functional. I'm not really a programmer but can do embedded in C.

The preliminary work with the PIC is done already. I just need to get the oscillator going
and that needs an AGC.

I'll try again with an LDR tomorrow and see how it goes. Failing that then I'll get some boards made up at a board house and get on with it.

Cheers,
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Old 2nd January 2013, 05:20 AM   #1426
1audio is offline 1audio  United States
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My Boonton does the frequency trim through an optoisolated link. I get very good phase noise plots with it. If you have the local loop for stabilizing the oscillator then the control loop doesn't need to be very quick at all. For frequency they use JFET switches for both resistors and caps. Today I would use a controlled resistor array and switch the caps with JFETS. Then the final steering would be using a 4 Q multiplier. The PLL code would be the most difficult to get right.
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Old 2nd January 2013, 05:35 AM   #1427
1audio is offline 1audio  United States
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Here is a digital volume control chip that could be used in a control loop for AGC (and frequency trim) http://semicon.njr.co.jp/eng/PDF/MUSES72320_E.pdf The opamps are external so its just resistors and jfets when used that way. Two to a chip so both functions could be managed. There are probably others that could work as well (and lots cheaper, these guys have an opamp thats $50).
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Old 2nd January 2013, 05:40 AM   #1428
davada is offline davada  Canada
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Originally Posted by 1audio View Post
My Boonton does the frequency trim through an optoisolated link. I get very good phase noise plots with it. If you have the local loop for stabilizing the oscillator then the control loop doesn't need to be very quick at all. For frequency they use JFET switches for both resistors and caps. Today I would use a controlled resistor array and switch the caps with JFETS. Then the final steering would be using a 4 Q multiplier. The PLL code would be the most difficult to get right.

The PIC has a 10 bit ADC and some are available with 12bit. This might be enough to set the level. 10 bit on 5V is 5mV resolution and 12bit is 1.3mV. The oscillator doesn't need to be sampled ever cycle to do this but we still need a fast sampler to capture the peak.

If the oscillator is running around 3Vrms then it fits into 12 bit over 10V for 2,5mV resolution.
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Old 2nd January 2013, 05:49 AM   #1429
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Here is a cheaper solution: http://datasheets.maximintegrated.com/en/ds/DS1882.pdf These are non-volatile so the oscillator could start from where it last was set.
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Old 2nd January 2013, 06:07 AM   #1430
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That would translate into .1% accuracy. It should be good enough as long as a local loop handles he primary stabilization.
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