Low-distortion Audio-range Oscillator - Page 14 - diyAudio
Go Back   Home > Forums > Design & Build > Equipment & Tools

Equipment & Tools From test equipment to hand tools

Please consider donating to help us continue to serve you.

Ads on/off / Custom Title / More PMs / More album space / Advanced printing & mass image saving
Reply
 
Thread Tools Search this Thread
Old 19th March 2012, 08:40 PM   #131
coluke is offline coluke  Italy
diyAudio Member
 
Join Date: Mar 2009
Quote:
Originally Posted by PChi View Post
Hello Coluke,
Thanks for pointing out my error and please forgive my ignorance. I admire anyone who has learnt a another language because I have tried in the past and failed.
Hi Pchi, no problem - didn't even notice it until Sonic's message

What about sidebands and noise performances of your generator? Level flatness and output THD tell us only half of the story. When I tried to build a low THD/THD+N APG generator I found especially difficult to keep THD+N low, even with a way better ALC loop than the one implemented in the generator this thread was started about. An APF ring can attain a very high Q (even higher than the respectable >60dB a SVF ring can operate at), but if I remember well operating Q is strongly dependent from the inverter gain, and in a completely non-linear way, so designing an ALC loop that fulfills all the requirements (high gain, fast response, high ripple rejection) can be very difficult. I abandoned the project, but maybe I was completely wrong (or my experience was too little at that time) and a very high performances APF generator is indeed feasible, although as far as I know this topology was never used in state-of-the-art low THD generators (AP, Tek, HP, Boonton, KH either use SVFs or Bridged-Ts).

The same holds for continous coverage via a dual-gang pot: after the very first tests I decided to get rid of pots - they are quite noisy, and you must pay a lot (way too much, imho) for a precision unit with reasonably matched elements, so I will definitely go for a discrete tuning mechanism - maybe something a-la HP239: two significant figures and a vernier control; or an even simpler selector of spot frequencies arranged in a 1-2-5 sequence - CW operation is something I don't really need.

IMHO doing a very low * THD * variable frequency generator is not that difficult, the real issue being to keep very low * THD+N * too.

Ciao,

L.

Last edited by coluke; 19th March 2012 at 09:09 PM.
  Reply With Quote
Old 19th March 2012, 09:30 PM   #132
Oddeo is offline Oddeo  United States
diyAudio Member
 
Join Date: Jan 2006
Location: Silicon Valley
Quote:
Originally Posted by PChi View Post
In my experience the Silonex LDR based Optocouplers aren't very linear, others may be better.
Paul -

Were you referring to the non-linearity in the LED drive current-to-LDR output resistance relationship, or did you have in mind something else?

In reading your comment I began to wonder if perhaps you had found an inherent non-linearity in the photocell element itself.

Specifically, I'm wondering if the resistance of the photocell changes at all (at a fixed illumination) as the voltage across the photocell varies. Sort of an analog to the Early effect, if you will.

If that is the case, then clearly an LDR-based gain control stage will introduce distortion even with a ripple-free ALC input voltage.

I suppose one could build a simple test circuit with a fixed-illumination LDR used as a feedback element around a low-noise op amp, then compare the distortion of that circuit to a version in which the LDR is replaced by a resistor of equivalent value.

Maybe I'll look into this a bit if no one has already done it.

--
Curt
  Reply With Quote
Old 19th March 2012, 09:54 PM   #133
diyAudio Member
 
Join Date: Jan 2008
Location: Virginia
The photo cell (LDR) has very low dependence of the resistivity with the current/voltage, that's why the AD note uses it.

Last edited by SoNic_real_one; 19th March 2012 at 10:13 PM.
  Reply With Quote
Old 19th March 2012, 10:55 PM   #134
PChi is offline PChi  United Kingdom
diyAudio Member
 
Join Date: Jan 2012
Location: Staines, UK
Quote:
Originally Posted by Oddeo View Post
Paul -

Were you referring to the non-linearity in the LED drive current-to-LDR output resistance relationship, or did you have in mind something else?

In reading your comment I began to wonder if perhaps you had found an inherent non-linearity in the photocell element itself.

Specifically, I'm wondering if the resistance of the photocell changes at all (at a fixed illumination) as the voltage across the photocell varies. Sort of an analog to the Early effect, if you will.

If that is the case, then clearly an LDR-based gain control stage will introduce distortion even with a ripple-free ALC input voltage.

I suppose one could build a simple test circuit with a fixed-illumination LDR used as a feedback element around a low-noise op amp, then compare the distortion of that circuit to a version in which the LDR is replaced by a resistor of equivalent value.

Maybe I'll look into this a bit if no one has already done it.

--
Curt
Hello Curt,

I was refering to the non-linearity of the resistance versus voltage of the photocell element. I wasn't concerned about the LED drive current to LDR resistance transfer function. I used the Silonex parts to fine tune the notch of a distortion analyzer and found that they added some distortion. The web site Silonex Inc.: Products: Audiohm Optocouplers: Audio Characteristics has some information on linearity.
I experimented with the parts some years ago and only have limited data available.
With (I think an NSL-37V51) connected in series with 6.2 kohm with a 20 kohm in parallel with the photocell. 12.5 V peak to peak across the series combination. LED driven so that the Photocell = 9 kohm the distortion was mainly third harmonic at 0.2 %, measurement frequency = 1 kHz.
With 1 V across the combination the distortion was 0.05 %.
Some measurements with the intended operating conditions would be good.

Regards
Paul
  Reply With Quote
Old 20th March 2012, 03:08 PM   #135
Oddeo is offline Oddeo  United States
diyAudio Member
 
Join Date: Jan 2006
Location: Silicon Valley
Thanks for the Silonix Audio Characteristics link, Paul. That's very informative.

--
Curt
  Reply With Quote
Old 20th March 2012, 05:15 PM   #136
PChi is offline PChi  United Kingdom
diyAudio Member
 
Join Date: Jan 2012
Location: Staines, UK
Hi L.

I don't know what the sidebands are like unfortunately. I am measuring the THD+N without limiting the bandwidth beyond what the Analog Devices AD536A RMS to DC Converter does at about 0.4 V rms with it's level dependent bandwidth. It's typical - 3dB Bandwidth at 100 mV rms is 450 kHz so it should be a little better than that at 400 mV but I am unable to verify it. The rms value looks about right when viewing the peak to peak residual on an Oscilloscope.
The noise is less than 0.004 % of 3 V rms as measured by the 'Distortion Analyzer' so is less than 120 uV. I would like to be able to make better measurements but haven't got the equipment.
The oscillator amplitude of 5 V rms and low impedances has kept the noise low.

The ALC loop has high DC gain but low AC gain, It takes many seconds to stabilise when the Oscillator Frequency is set to 10 Hz. I wasn't worried about fast stabilisation.
I tried to calculate the change in amplitude with loop gain but my differential equation skills aren't good enough. I have simulated the response.
The ALC error amplifier doesn't have high ripple rejection because I am relying on low ripple from the Sample and Hold.
I am using standard dual gang potentiometers (which are expensive). I have found variable frequency useful for general purpose but I agree that for measuring distortion some spot frequencies are all that is required.

I am going to attempt to attach a pdf of the Schematic as it is now. There is another page containing the output attenuator and buffers and also a 5 V regulator for the monostable. It isn't complete yet. I intend to change the reference voltage and am still investigatingh distortion at 25 kHz.
I had to export a .dxf file from KiCAD then use Open Office to create the pdf. I guess that there is a better way.
If anyone wants the KiCAD files I can attach them.

Regards
Paul
Attached Files
File Type: pdf Oscillator section 0_1 modified.pdf (116.8 KB, 216 views)
  Reply With Quote
Old 22nd March 2012, 02:02 AM   #137
davada is online now davada  Canada
diyAudio Member
 
Join Date: Jul 2004
Location: Fort St John, BC Canada
Hi Coluke,

Just out of curiosity,

I am wondering what is the measured distortion level out of the multiplier of your SVO?
At what operating level of the oscillator and input level to the multiplier?

David.
__________________
David.
  Reply With Quote
Old 22nd March 2012, 09:31 AM   #138
coluke is offline coluke  Italy
diyAudio Member
 
Join Date: Mar 2009
Hi Davada,

I was looking for very low THD+N, so the ALC multipliers are directly connected to ring's output: @1kHz, 20Vpp out on 10k load, AD633 output sits at about 130mVpp - only 2nd and 3rd harmonic are visible, both in the -100dBv range, which seems indeed to account for a very low ALC-injected THD at generator output.

L.
  Reply With Quote
Old 22nd March 2012, 11:32 AM   #139
coluke is offline coluke  Italy
diyAudio Member
 
Join Date: Mar 2009
Quote:
Originally Posted by PChi View Post
I am going to attempt to attach a pdf of the Schematic as it is now.
Hi PChi - thanks for the schematics. Had a look at it: it seems you're actually discharging the hold capacitor somewhere during the cycle (had no time to check all the timings) - I may be wrong, but it's pretty unlikely that you're going to keep CV ripple low this way. Did you manage to check CV ripple at the S&H output?

L.
  Reply With Quote
Old 22nd March 2012, 12:44 PM   #140
PChi is offline PChi  United Kingdom
diyAudio Member
 
Join Date: Jan 2012
Location: Staines, UK
Hi L,
Thanks for looking at the schematics and the comments.
I am not discharing the hold capacitor during the cycle. When prototyping the circuit I found that if the Oscillator wasn't running (depending on leakage currents) the sample capacitor could have a high voltage which sets the loop gain below 1 hence no oscillation so no zero crossing to trigger the monostable so no discharge of the hold capacitor. The extra monostable section 'MONO 2' is a watchdog to ensure that if the oscillator isn't running the hold capacitor gets discharged.
I used a low speed comparator on the prototype because it was all I had but the AD790 likes to oscillate even with a ground plane and surface mount decoupling capacitors so the 'watchdog' is probably unecessary.

The zero crossing detector IC4 triggers the Monostable IC5 on only the positive going transistions (when it's working properly). I had a little trouble that I need to investigate further with the AD790 which has a little internal hysteresis occasionally producing glitches on the negative going input transitions which are slow when the Oscillator was set to 10 Hz. The Monostable output turns on the analog switch IC7 for about 500 ns to sample the signal from the output of IC6 which is shifted by about 90 degrees relative to the zero crossing detector input. So it's sampling the peak voltage approximately.
The ripple is low at the Sample and Hold output until the Oscillator frequency is increased. At 100 kHz there is about 30 mV of ripple mainly caused by the input signal changing during the sampling period. I believe (though I haven't checked fully) that it is responsible for the increase in distortion that I am measuring at 25 kHz.

I would prefer to use an integrated sample and hold but couldn't find one that was good enough. It needs very low droop so that the hold capacitor doesn't discharge when the Oscillator is set to 10 Hz and reasonably fast acquistion at 100 kHz. I used a low charge injection analogue switch to minimise sample and hold voltage steps.
I would prefer a better comparator than the AD790 with less delay and a little more hysteresis.
Paul
  Reply With Quote

Reply


Hide this!Advertise here!
Thread Tools Search this Thread
Search this Thread:

Advanced Search

Posting Rules
You may not post new threads
You may not post replies
You may not post attachments
You may not edit your posts

BB code is On
Smilies are On
[IMG] code is On
HTML code is Off
Trackbacks are Off
Pingbacks are Off
Refbacks are Off


Similar Threads
Thread Thread Starter Forum Replies Last Post
Radford Low Distortion Oscillator Series 2 audiomik Equipment & Tools 21 19th February 2014 10:46 AM
ultra-low distortion audio oscillator geekysuavo Analog Line Level 16 26th March 2013 03:04 PM
Low distortion oscillator? rjm Equipment & Tools 30 4th May 2011 10:45 PM
Can we improve this low distortion sine oscillator ? gaetan8888 Solid State 22 29th March 2009 12:30 PM
Simple, low distortion 1kHz oscillator jackinnj Solid State 4 6th October 2003 03:58 AM


New To Site? Need Help?

All times are GMT. The time now is 03:44 AM.


vBulletin Optimisation provided by vB Optimise (Pro) - vBulletin Mods & Addons Copyright © 2014 DragonByte Technologies Ltd.
Copyright 1999-2014 diyAudio

Content Relevant URLs by vBSEO 3.3.2