Low-distortion Audio-range Oscillator - Page 13 - 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 17th March 2012, 09:51 PM   #121
davada is offline davada  Canada
diyAudio Member
 
Join Date: Jul 2004
Location: Fort St John, BC Canada
Hi PChi,

Lt has some excellent rms to dc convertors that extend up to 500KHz.
Take a look at the LTC1968. I have some of these and they are very good.

Cheers,

David.
__________________
David.
  Reply With Quote
Old 17th March 2012, 10:00 PM   #122
coluke is offline coluke  Italy
diyAudio Member
 
Join Date: Mar 2009
Hi Davada,

ARTA's dual-channel FR mode (driving an EMU0202/0204) is indeed one of my favourite tools, and I've been using it to test my home-made notches for quite a long time. The problem with this direct method is that even at its best resolution (which is 44.1kHz SR, 131072 acquisition length - about 0.3Hz), I did never manage to get the actual depth of a high-rejection notch, the only way to reliably do this being fixing input frequency and then fine-tuning the notch itself. The -3db bandwitdth of a >60db Q bandpass filter is about 1Hz or less @ 1kHz, so I never even tried this method for determining the running Q of a generator- with barely 3 experimental points in range of interest you're likely going to underestimate the actual output level at the fundamental...; nevertheless it's an estimate I'm interested in, so I've decided to give this method a try - it actually seems to work fine: I've got a Q value around 63dB @ 1kHz, 20Vpp out, quite load independent up to 1.5kohm load, which seems actually in very good agreement with my previous - indirect - estimates.

As far as the ARTA's frequency compensation feature, I did have the same idea , and made a few tests a few months ago or so, either with ARTA itself, or importing data in MATLAB and letting it do all the math. This method actually works pretty well, failing only to provide accurate results in the vicinity of the frequency the notch is tuned to, but it doesn't seem to be a real issue since we are mainly interested in the level of the harmonics. One major advantage of this technique is that you can drive a simple passive twin-t notch at a pretty high level to extend the dynamic range of your spectrum analyzer (ie your soundcard) and let ARTA (or Matlab) do the boring task of automatically compensate for its far-from-flat response - a 20Vpp-0.0001% THD sine has harmonics in the -100dBv range, something an EMU0204 will catch smoothly.

If you don't want to play with active notches and the issues related to achieve high selectivity and good flatness and high acceptance and low THD and low noise and ..., well, this could be a nice alternative.

Ciao,

L.
  Reply With Quote
Old 17th March 2012, 10:13 PM   #123
coluke is offline coluke  Italy
diyAudio Member
 
Join Date: Mar 2009
Quote:
Originally Posted by PChi View Post
I really need to produce a Rev 0.2 PCB to tidy up the changes and remove some of the excess features to make the PCB smaller.
It looks very nice nevertheless - is it your APF generator with a T&H/S&H peak detector? Can you tell us some more about its architecture?

Ciao,

L.
  Reply With Quote
Old 18th March 2012, 11:27 AM   #124
PChi is offline PChi  United Kingdom
diyAudio Member
 
Join Date: Jan 2012
Location: Staines, UK
Quote:
Originally Posted by coluke View Post
It looks very nice nevertheless - is it your APF generator with a T&H/S&H peak detector? Can you tell us some more about its architecture?

Ciao,

L.
Hello Ciao,
Thanks for the complements. Yes it is based on an APF (All Pass Filter) generator with an (S&H) Sample and Hold controlling a multiplier.
I'm still playing with the circuit. I will attach the schematic and board layout when I can work out a way of attaching a KiCAD schematic.

I have just bought 'The Design Of Active Crossovers' by Douglas Self which includes some interesting measurement results on the LME49990 which shows that the distortion in the voltage-follower mode (gain, Av = 1) with 2 kohm source impedance is poor beyond about 10 kHz. I have used the LME49990 based on the data sheet claims and need to investigate further.

It's never going to be as low distortion as a fixed frequency Oscillator.

Hello David,
Thanks for the suggestion to look at the Linear Technology rms to DC converters. I think that the choice between the Analog Devices parts and the LT parts isn't clear. If I get around to designing a new Notch Filter board I will look closely at the compromises between the two types.
  Reply With Quote
Old 18th March 2012, 09:27 PM   #125
davada is offline davada  Canada
diyAudio Member
 
Join Date: Jul 2004
Location: Fort St John, BC Canada
Quote:
Originally Posted by PChi View Post
Hello Ciao,
Thanks for the complements. Yes it is based on an APF (All Pass Filter) generator with an (S&H) Sample and Hold controlling a multiplier.
I'm still playing with the circuit. I will attach the schematic and board layout when I can work out a way of attaching a KiCAD schematic.

I have just bought 'The Design Of Active Crossovers' by Douglas Self which includes some interesting measurement results on the LME49990 which shows that the distortion in the voltage-follower mode (gain, Av = 1) with 2 kohm source impedance is poor beyond about 10 kHz. I have used the LME49990 based on the data sheet claims and need to investigate further.

It's never going to be as low distortion as a fixed frequency Oscillator.

Hello David,
Thanks for the suggestion to look at the Linear Technology rms to DC converters. I think that the choice between the Analog Devices parts and the LT parts isn't clear. If I get around to designing a new Notch Filter board I will look closely at the compromises between the two types.
I thought you were complaining about your RMS detector.

David.
__________________
David.
  Reply With Quote
Old 18th March 2012, 10:49 PM   #126
diyAudio Member
 
Join Date: Jan 2008
Location: Virginia
Quote:
Originally Posted by PChi View Post
Hello Ciao...
That's not his name, it is an italian salute
His nick is "coluke", signed "L" - probably from Luca (italian/romanian for name Luke).

Last edited by SoNic_real_one; 18th March 2012 at 10:52 PM.
  Reply With Quote
Old 18th March 2012, 11:31 PM   #127
coluke is offline coluke  Italy
diyAudio Member
 
Join Date: Mar 2009
Quote:
Originally Posted by SoNic_real_one View Post
That's not his name, it is an italian salute
His nick is "coluke", signed "L" - probably from Luca (italian/romanian for name Luke).
Yeah Sonic is right - didn't notice that PChi was referring to me as 'Ciao'.

L.
  Reply With Quote
Old 19th March 2012, 12:08 AM   #128
diyAudio Member
 
Join Date: Jan 2008
Location: Virginia
I was looking into this thread for some ideas for a generator.
My options are either the wien bridge like in the Linear App Note43 (fig48) or the schematic from the first post with a proper integrator in the detector stage (easy to fit).

Low noise OpAmps are easyer to find this days...
  Reply With Quote
Old 19th March 2012, 09:59 AM   #129
coluke is offline coluke  Italy
diyAudio Member
 
Join Date: Mar 2009
Sonic, I'd go for a SVF topology; if you want to try something as simple as a Wien Bridge but with far better out-of-the-box performances you can go for a HP239A clone - even in its original form (2 opamps in the Bridged-t generator plus an output buffer) it is capable of quite amazing THD figures. At present I'm playing with a slightly more complex experimental version of it employing a full-wave peak detector and OPA2134 in place of the orginal Harris opamp, and it's impressive to see how well it works.

L.
  Reply With Quote
Old 19th March 2012, 11:03 AM   #130
PChi is offline PChi  United Kingdom
diyAudio Member
 
Join Date: Jan 2012
Location: Staines, UK
Quote:
Originally Posted by coluke View Post
Yeah Sonic is right - didn't notice that PChi was referring to me as 'Ciao'.

L.
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.

You are right David I was complaining about the RMS detector.
I listed the rms to DC converter that I used because that influences the measurement bandwidth and so effects the thd result.
Just for information for other readers the AD536A bandwith for 1 % error is specified as 120 kHz with an input signal of 1 V rms. It drops to 45 kHz with 100 mV. The LTC1968 has superior bandwith especially at lower signal levels but being digital goes a bit funny at low signal levels.

I chose the All Pass Filter Phase Shift circuit becaue the gain doesn't change much with frequency adjustment when using an un-matched dual gang potentiometer. Less that 1 % with a bit of tweeking from 10 Hz to 100 kHz. Also there is an almost 90 degree phase shifted (depending on component match) signal to drive a zero crossing comparator to control a Sample and Hold. Giving a low ripple control voltage.

I think that the issue with a Wien bridge is that the gain does vary with frequency because of component mis-match which the variable gain element has to compensate for. The Linear Technology AN43 circuit uses an LDR. In my experience the Silonex LDR based Optocouplers aren't very linear, others may be better. As the frequency is varied the Distortion trim needs to be adjusted to cope with the mismatch of R1 and R2 which isn't practical. It's also difficult to stabilise the amplitude control loop with the filtering causing problems. If the frequency is fixed then the circuit or the later configuration AN132 is good.

I think that the State Variable Filter topology is good for fixed frequency but I think that gain varies with oscillation frequency due to dual gang potentiometer mis-match so I believe that it's dificult to come up with a low distortion variable frequency version.

I guess that all this proves that it's suprisingly difficult to make a variable frequency low distortion oscillator.

Regards 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 07:23 PM.


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