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 29th October 2012, 09:51 AM   #811
diyAudio Member
 
Join Date: Jun 2005
Location: Zürich
Quote:
Those seem to be exceptional results.
You can get every sensible variable gain element to show essentially arbitrary low distortion by reducing the signal level across it--at the cost of higher noise. As Bob mentioned, we need a figure of merit for the multiplier which catches this tradeoff (I'm thinking about it).

Samuel
  Reply With Quote
Old 29th October 2012, 10:53 AM   #812
KSTR is offline KSTR  Germany
diyAudio Member
 
KSTR's Avatar
 
Join Date: Jul 2007
Location: Central Berlin, Germany
Wouldn't using two JFETs in a differential fashion be a way to reduce remaining nonlinearity of Rds vs Vds? I mean, one JFET as is on the input side of the summing junction and another matched one coming from the output, each with their series resistors. Then feeding the second one a copy of the control voltage (with additional DC bias) so that the Rds changes do work in "push-pull". Compressing/Expanding distortion vs signal would pretty much cancel out, only the difference of the Rds vs Vds nonlinearities would be effective. Also, for freqs above tuning the CV could be injected in opposite polarity (with an 1st order allpass phase shifter) so that high frequency CV ripple and noise would change both Rds' almost in phase, producing little effect on the total gain.

Just a few thoughts... well, another one : Would an expansion of the amplitude detector as shown in the EDN article to four quadrants (with 90deg phase shifters) produce any benefits? I could see faster settling as well as less and higher freq ripple of the detector output. I simmed this already and it's looking good and has not too much circuit overhead.

Last edited by KSTR; 29th October 2012 at 10:55 AM.
  Reply With Quote
Old 29th October 2012, 02:24 PM   #813
vicnic is offline vicnic  Latvia
diyAudio Member
 
Join Date: Oct 2004
Location: Riga
Quote:
Originally Posted by KSTR View Post
Wouldn't using two JFETs in a differential fashion be a way to reduce remaining nonlinearity of Rds vs Vds?
Properly built JFET VCR can be reduced to simplified schematic and it is absolutely symmetrical:
Click the image to open in full size.
In my opinion, differential fashion is not needed, cause this fashion reduce only nonsymmetry. In the other hand, when we use two or more JFET VCRs in series, AC gain drops in every JFET and this is a way for to reduce distortions. But, I do not know at this time, which way is better: more than one JFET in series, or simple signal amplification from one JFET. This situation must be analyzed from different courses: noise, easy to bulid, cost...

Victor
  Reply With Quote
Old 29th October 2012, 03:42 PM   #814
diyAudio Member
 
Join Date: Sep 2006
Quote:
Originally Posted by vicnic View Post
Properly built JFET VCR can be reduced to simplified schematic and it is absolutely symmetrical:
Click the image to open in full size.
In my opinion, differential fashion is not needed, cause this fashion reduce only nonsymmetry. In the other hand, when we use two or more JFET VCRs in series, AC gain drops in every JFET and this is a way for to reduce distortions. But, I do not know at this time, which way is better: more than one JFET in series, or simple signal amplification from one JFET. This situation must be analyzed from different courses: noise, easy to bulid, cost...

Victor
I chose the 4091 JFET for my original oscillator based on high threshold and low Vds_on. This seems to keep the FOM high.

If one is using optimized drain feedback to the gate, this tends to kill the second harmonic distortion, leaving only 3rd, which would seem to be about what using two JFETs differentially would do.

BTW, I believe I originally saw the JFET AGC element with gate feedback on an HP oscillator. I don't remember the model or when it was designed, but I saw the schematic around 1970.

I used to devour schematics in the user manuals of HP test equipment at the time where I worked (BTL).

Cheers,
Bob
  Reply With Quote
Old 29th October 2012, 06:08 PM   #815
diyAudio Member
 
Join Date: Jun 2005
Location: Zürich
Figure with noise vs. Vc of post #797 fixed.

Quote:
BTW, Bob, could you post a graph of multiplier control voltage vs. frequency for your oscillator? This would be interesting to determine the necessary multiplier authority.
I've just quickly checked this with my current prototype oscillator, and the control voltage from 10 Hz to 100 kHz was below +-200 mV (multiplier scaling factor 0.05, multiplier decoupling 10x). So it looks like that, for sufficient leveling authority alone, a multiplier authority of 5m should be well sufficient for a carefully design SVF oscillator. The tradeoff is mostly with respect to settling time.

Would still be interested to see this data for the Cordell oscillator, if anyone has time!

Samuel
  Reply With Quote
Old 29th October 2012, 07:24 PM   #816
diyAudio Member
 
scott wurcer's Avatar
 
Join Date: Jan 2004
Location: cambridge ma
Quote:
Originally Posted by KSTR View Post
Wouldn't using two JFETs in a differential fashion be a way to reduce remaining nonlinearity of Rds vs Vds? .
I've tried this and there was no improvement over the single FET. Paralleling FET's to have lower Rdon for the same Vp might help. If you read part two of my Linear Audio article you will find a hint on how to make Victor's idealized VCR exactly, floating and completely isolated from the control voltage. I will draw it up tonight.
__________________
“The earth's rotation will slow within days and stop for several days just prior to the pole shift. This is when you and your loved ones should be situated at your safe location.”
  Reply With Quote
Old 30th October 2012, 03:57 PM   #817
RNMarsh is offline RNMarsh  United States
diyAudio Member
 
RNMarsh's Avatar
 
Join Date: May 2012
Location: 2457 Cascade Trail; Cool, CA. 95614
SW -- Where are you getting all this free time... retired, yet? is every day, Saturday yet?
  Reply With Quote
Old 30th October 2012, 08:10 PM   #818
diyAudio Member
 
scott wurcer's Avatar
 
Join Date: Jan 2004
Location: cambridge ma
Quote:
Originally Posted by RNMarsh View Post
SW -- Where are you getting all this free time... retired, yet? is every day, Saturday yet?
I took vacation for my niece's wedding and the storm intervened. So I have some peace to work on things that would not get done while I watch all 10 Jason movies.
__________________
“The earth's rotation will slow within days and stop for several days just prior to the pole shift. This is when you and your loved ones should be situated at your safe location.”
  Reply With Quote
Old 30th October 2012, 08:19 PM   #819
diyAudio Member
 
scott wurcer's Avatar
 
Join Date: Jan 2004
Location: cambridge ma
Here's my idea. The photovoltaic MOSFET driver generates a small isolated current with up to 8V compliance. You just force this current into a large resistor to control the gate voltage of the VCR. The RC can be at a very low frequency rather easily. I envision a second control channel could be used to cancel the thirds, thinking out loud right now.
Attached Images
File Type: jpg vcr.JPG (58.0 KB, 278 views)
__________________
“The earth's rotation will slow within days and stop for several days just prior to the pole shift. This is when you and your loved ones should be situated at your safe location.”
  Reply With Quote
Old 1st November 2012, 07:47 PM   #820
PChi is offline PChi  United Kingdom
diyAudio Member
 
Join Date: Jan 2012
Location: Staines, UK
That floating VCR controlled by a Photovoltaic MOSFET driver is rather clever. The isolated control opens up some possibilties.
I think that it is similar in function to the Photo FET Optocouplers like the Fairchild H11F1M but lower distortion.

I am messing around with ideas to meet Samual Groner's reasonable and realistic design goal for a very good DIY oscillator. Progress is slow though because I am back at work unfortunately (bad for my sanity but good for the bank balance).
There are some other parameters to consider.
Output impedance. I prefer relatively low, < 100 ohms, it's always possible to add resistance externally.
Single ended or differential output, semi or true differential. I think that floating single ended is good enough for normal use.
DC or AC coupled. I prefer AC coupling.
  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 01:53 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