electro-music.com   Dedicated to experimental electro-acoustic
and electronic music
 
    Front Page  |  Articles  |  Radio
 |  Media  |  Forum  |  Wiki  |  Links  |  Store
Forum with support of Syndicator RSS
 FAQFAQ   CalendarCalendar   SearchSearch   MemberlistMemberlist   UsergroupsUsergroups   LinksLinks
 RegisterRegister   ProfileProfile   Log in to check your private messagesLog in to check your private messages   Log inLog in  Chat RoomChat Room 
Live streaming at radio.electro-music.com

  host / artist show at your time
  Faux Pas Quartet and friends Music From Last Thursday
Please visit the chat
 Forum index » DIY Hardware and Software » Developers' Corner
My current VCO - comments/help appreciated
Post new topic   Reply to topic Moderators: DrJustice
Page 1 of 1 [11 Posts]
View unread posts
View new posts in the last week
Mark the topic unread :: View previous topic :: View next topic
Author Message
m.o



Joined: Jul 05, 2014
Posts: 27
Location: Sweden

PostPosted: Sun Aug 03, 2014 8:40 am    Post subject: My current VCO - comments/help appreciated Reply with quote  Mark this post and the followings unread

Hi guys,

This is what I currently have for VCO in my "poly project".
This is not a unique design in any way, I have been reading schematics and experimenting trying to understand how a basic VCO works, so I have borrowed from multiple sources (MFOS, Yusynth, etc etc).
It works mostly fine over about 4 (low) octaves as far as I've tested it.

* During breadboarding I had lots of trouble getting a FET to work for resetting the integrator capacitor, that's why I ended up using a 2n3904 - it works but the reset-part of the waveform is a bit "bouncy".

* The Cap in the reset-path from the comparator was something that came from experimenting with the circuit on breadboard (LTSpice doesn't seem to need it really).

* The V5 source was used for testing/simulating sync input.

* The relationship between the current through R15 and the size of the C2 cap seems to work, but I'm wondering if I should lower the current (and decrease the cap value) as I see many designs using a much higher value for R15 (470k -1M ?).

* The D2 + U6 path (disconnected in this pic) was supposed to be the "HF compensation" (again I started by copying/looking at existing circuits) but with current values it seems to give way too much current - I have to got to several Mega-Ohms for it to calm down - this is why I'm wondering if I should change the R15 - C2...

Any thoughts/comments?


my vco 2.1.gif
 Description:
 Filesize:  410.61 KB
 Viewed:  73 Time(s)
This image has been reduced to fit the page. Click on it to enlarge.

my vco 2.1.gif


Back to top
View user's profile Send private message
JovianPyx



Joined: Nov 20, 2007
Posts: 1247
Location: West Red Spot, Jupiter
Audio files: 161

PostPosted: Sun Aug 03, 2014 12:29 pm    Post subject: Reply with quote  Mark this post and the followings unread

A 2N3904 needs current to turn it on, whereas an FET (Or MOSFET) needs only a voltage. The bounciness is probably because of the extra current needed to turn the 2N3904 on during the discharge pulse.

Looking on the web, I see a lot of schematics for VCOs that use a FET for discharging the cap so it is something that has been done before and is known to work.

IMO, FET should work better than BJT mainly because it doesn't draw current through it's gate. (well, there is leakage, but the current is far smaller than the BJT base current).

_________________
FPGA, dsPIC and Fatman Synth Stuff

Time flies like a banana.
Fruit flies when you're having fun.
BTW, Do these genes make my ass look fat?
corruptio optimi pessima
Back to top
View user's profile Send private message Visit poster's website
m.o



Joined: Jul 05, 2014
Posts: 27
Location: Sweden

PostPosted: Sun Aug 03, 2014 3:42 pm    Post subject: Reply with quote  Mark this post and the followings unread

JovianPyx wrote:
A 2N3904 needs current to turn it on, whereas an FET (Or MOSFET) needs only a voltage. The bounciness is probably because of the extra current needed to turn the 2N3904 on during the discharge pulse.

Looking on the web, I see a lot of schematics for VCOs that use a FET for discharging the cap so it is something that has been done before and is known to work.

IMO, FET should work better than BJT mainly because it doesn't draw current through it's gate. (well, there is leakage, but the current is far smaller than the BJT base current).


You are probably right, I'll just have to keep at it (I know this should be basic knowledge) but I've had some difficulty finding explanations about FETs that go beyond "they turn things on with a voltage", for example the relationship between the gate voltage and the voltage/polarity across the drain-source...
Back to top
View user's profile Send private message
m.o



Joined: Jul 05, 2014
Posts: 27
Location: Sweden

PostPosted: Mon Aug 04, 2014 4:24 pm    Post subject: Reply with quote  Mark this post and the followings unread

I tested swapping R15 to 1MOhm and C2 to 270pF.
With this configuration it didn't quite work until I removed C3 (from the reset path), but when I did I got quite a nice looking saw (I'm thinking that if it needs to be there, it should probably be the same order of magnitude as C2...).
The range of the oscillator dropped down to something like 6Hz - 1kHz (about), which I didn't quite expect (I reduced the reference current to 1/10th and the cap size to less than 1/10th so I expected a higher frequency).

I still need to calibrate this properly to see if the HF trim network works...
Back to top
View user's profile Send private message
m.o



Joined: Jul 05, 2014
Posts: 27
Location: Sweden

PostPosted: Sun Aug 10, 2014 4:08 pm    Post subject: Reply with quote  Mark this post and the followings unread

I went back and tried using a JFET (BF245B, what I had lying around) for the reset as advised (There are definitely reasons why this is what is generally used).

Any recommendations for a commonly available FET to use (IIRC the BF245 are discontinued), 2N7000 ?

This worked great for the reset with just a 100k resistor in the path from the comparator, it also removed my problem with the oscillator going drastically flat after 4 octaves or so.
I would like to understand what was actually happening with the bipolar transistor - I suppose the reason it (kinda) worked with the diode and cap was that the "shape" of the reset-pulse/current is more important...

My concerns with the reference current and the integrator cap size came from how I had my CV summing set up (not shown in the SPICE -screenshot, I had quite a bit of bias towards V-) , so I think most major problems are sorted out now.

I still need to establish the HF-compensation part though, so I'll probably go back to that.
Back to top
View user's profile Send private message
JovianPyx



Joined: Nov 20, 2007
Posts: 1247
Location: West Red Spot, Jupiter
Audio files: 161

PostPosted: Sun Aug 10, 2014 6:23 pm    Post subject: Reply with quote  Mark this post and the followings unread

For JFETs, 2N3819 or MPF-102 might work.

I would think that 2N7000 works too, it's a MOSFET so has even lower gate current.

_________________
FPGA, dsPIC and Fatman Synth Stuff

Time flies like a banana.
Fruit flies when you're having fun.
BTW, Do these genes make my ass look fat?
corruptio optimi pessima
Back to top
View user's profile Send private message Visit poster's website
m.o



Joined: Jul 05, 2014
Posts: 27
Location: Sweden

PostPosted: Wed Sep 03, 2014 12:39 pm    Post subject: Reply with quote  Mark this post and the followings unread

Just posting the updated schematic here.


vco-basics 2.pdf
 Description:

Download
 Filename:  vco-basics 2.pdf
 Filesize:  36.3 KB
 Downloaded:  40 Time(s)

Back to top
View user's profile Send private message
okelk



Joined: May 08, 2014
Posts: 29
Location: Vienna

PostPosted: Thu Sep 04, 2014 2:43 am    Post subject: Reply with quote  Mark this post and the followings unread

Thanks for the update.
I'm very interested in basic vco designs at the moment as I'm working on one too (basic tricore... more to fully understand it than to do anything better than others...)
Back to top
View user's profile Send private message
m.o



Joined: Jul 05, 2014
Posts: 27
Location: Sweden

PostPosted: Fri Sep 05, 2014 12:26 pm    Post subject: Reply with quote  Mark this post and the followings unread

okelk wrote:
Thanks for the update.
I'm very interested in basic vco designs at the moment as I'm working on one too (basic tricore... more to fully understand it than to do anything better than others...)


So kind of the same reason as me then.
I haven't looked too hard at tri-core oscillators, what is your approach to "reversing current" (is that how they work generally?) - I have seen OTAs and I think some ones using some kind of analog switch...
Back to top
View user's profile Send private message
Baloo



Joined: Jul 09, 2013
Posts: 13
Location: France

PostPosted: Tue Sep 30, 2014 5:24 pm    Post subject: Reply with quote  Mark this post and the followings unread

Did you get a fine v/oct tracking ?
Back to top
View user's profile Send private message
m.o



Joined: Jul 05, 2014
Posts: 27
Location: Sweden

PostPosted: Wed Oct 01, 2014 2:10 pm    Post subject: Reply with quote  Mark this post and the followings unread

They track pretty well over 5 octaves.
(I have tested/calibrated them using a Doepfer A-190-2 Midi to CV which has 0-5V output which is why I haven't tested larger range.)

I haven't gone into the high-frequency compensation yet (hasn't needed to in the range I've used it, < C5).
The simulation with the 2N7000 gives much longer reset time than the actual MPF102 / BF245 I have used, so there's something to look into further.
Also choosing a lower value cap and/or lower trigger level for the comparator (less total charge to dump) would probably also help improve the reset time.
Back to top
View user's profile Send private message
Display posts from previous:   
Post new topic   Reply to topic Moderators: DrJustice
Page 1 of 1 [11 Posts]
View unread posts
View new posts in the last week
Mark the topic unread :: View previous topic :: View next topic
 Forum index » DIY Hardware and Software » Developers' Corner
Jump to:  

You cannot post new topics in this forum
You cannot reply to topics in this forum
You cannot edit your posts in this forum
You cannot delete your posts in this forum
You cannot vote in polls in this forum
You cannot attach files in this forum
You can download files in this forum
e-m mkii

Please support our site. If you click through and buy from
our affiliate partners, we earn a small commission.


Forum with support of Syndicator RSS
Powered by phpBB © 2001, 2005 phpBB Group
Copyright © 2003 through 2009 by electro-music.com - Conditions Of Use