VCO with triangle help. - Page 1
Forum section: electronics / beginners

author: krolu
author_rank: Contributor
timestamp: August 20, 2016, 09:27:46 am
content: Hello,From couple of days now I'm trying to make a VCO with triangle output for audio frequencies. I tried couple of schematics that I found on the internet but there is always something that I don't like in it. I want the simplest approach that I can get that will produce nice triangle. I like op amps so this is what I'm looking for. I have found a very simple schematic that is giving me super results but the problem is that this is regulated by resistor value and I want to control the frequency with input voltage. Here it is:Why i like it? Because its minimum number of parts, controlling R1 is regulating frequency very nicely, changing frequency do not change voltage levels on the output, and most of all using TL074 is producing very nice and stable wave.There should be a possibility to connect it with a current source as usually this is the way, but I don't know how. Maybe somebody would know how could I modify this circuit to make it voltage controllable, please?

author: Zero999
author_rank: Super Contributor
timestamp: August 20, 2016, 09:34:47 am
content: See page 1 of the LM358 data sheet. It will work with the TL074, except it won't work with the control voltage near -V, as the TL074's common mode range doesn't include the negative supply.http://www.ti.com/lit/ds/symlink/lm158-n.pdf

author: kripton2035
author_rank: Super Contributor
timestamp: August 20, 2016, 09:50:08 am
content: if you control the frequency with the resistor, it's not a VCO but a RCO ...

author: krolu
author_rank: Contributor
timestamp: August 20, 2016, 09:55:23 am
content: Yes i know that one, andeven tried to build it. It is the most popular schematic when you look for a VCO but I didn't like it because from what i have seen it was not as stable in wide frequency range without changing to different cap values and it has pretty big DC constant at the output. The one I showed earlier is zero centerd without using a cap at the output.

author: krolu
author_rank: Contributor
timestamp: August 20, 2016, 10:11:06 am
content: Quote from: kripton2035 on August 20, 2016, 09:50:08 amAs I wrote in the first post, I want to control it with a voltage! If I wanted RCO I already have it so why would I write for help.

author: Zero999
author_rank: Super Contributor
timestamp: August 20, 2016, 10:27:56 am
content: Firstly you need to start with a specification.What's the frequency range? Output voltage? Does it matter if the frequency changes as the supply voltage changes?Did you click on the link in my previous post?Check out the following thread:https://www.eevblog.com/forum/beginners/spikes-on-triangle-wave-in-vco/

author: danadak
author_rank: Super Contributor
timestamp: August 20, 2016, 12:00:00 pm
content: You could always consider this, the ICL8038. Even thoughobsolete still available on ebay. Gives you sine/tri/square.Ramp easy to achieve with additional comparator.https://www.google.com/url?sa=t&rct=j&q=&esrc=s&source=web&cd=6&cad=rja&uact=8&ved=0ahUKEwiKityV-M_OAhUJrB4KHbMABkcQFgg3MAU&url=https%3A%2F%2Fwww.intersil.com%2Fcontent%2Fdam%2FIntersil%2Fdocuments%2Ficl8%2Ficl8038.pdf&usg=AFQjCNGATe_GMj8vKnVTuETXAEj4-n_JRw&sig2=ja1F7AUyoxlR9p0s2-SCxgRegards, Dana.

author: krolu
author_rank: Contributor
timestamp: August 20, 2016, 01:50:01 pm
content: I already thought about ICL8038 but that is obsolete don't give me good vibes, I even have one in my parts bin.

author: crazyguy
author_rank: Regular Contributor
timestamp: August 20, 2016, 02:31:53 pm
content: How about Exar XR2206 ?

author: Lee Leduc
author_rank: Regular Contributor
timestamp: August 20, 2016, 04:28:00 pm
content: Have you considered the LM13700 OTA? It's often used as the basis for a VCO in analog music synthesizers.Here's the datasheet link. See Figure 33.www.ti.com/lit/ds/symlink/lm13700.pdf

author: kripton2035
author_rank: Super Contributor
timestamp: August 20, 2016, 05:55:53 pm
content: texas instrument application notewith the values here, you get 20Hz to 200Hz change in output frequencychange C to change the base frequencychange the 2 R to change the symetry of the waveform.

author: Zero999
author_rank: Super Contributor
timestamp: August 20, 2016, 07:18:16 pm
content: Quote from: kripton2035 on August 20, 2016, 05:55:53 pmI posted that before but he didn't bother to click on the link.There's a minor error in that version of the schematic: the BJT should have a base resistor. Without it, the square wave will only be 0.7V, as it will be clipped by the base-emitter junction of the transistor and the op-amp will be current limiting which could slow it down. Another issue is the TL082's output won't go down to the negative rail, which will mean the transistor will take awhile to turn off.Here's it fixed. I've added a base resistor and speed-up capacitor to reduce the storage time of the BJT and cleaned the image up.

author: kripton2035
author_rank: Super Contributor
timestamp: August 20, 2016, 09:15:08 pm
content: Quote from: Hero999 on August 20, 2016, 07:18:16 pmthanks for that. I did not clic on your link eitherI scanned this from one of my book searching for something else...

author: Zero999
author_rank: Super Contributor
timestamp: August 20, 2016, 09:57:20 pm
content: The BJT could also be replaced with a MOSFET but a potential divider would need to be added, if the total supply voltage exceeds the maximum gate voltage.Quote from: krolu on August 20, 2016, 09:55:23 amWhat sort of frequency range do you need?To centre the voltage around 0V, a bipolar power supply is required.Have you looked at the schematic on the datasheet for the LM339? It's very similar to the oscillator circuit posted previously.http://www.ti.com/lit/ds/symlink/lm339-n.pdfIf you want to run it off a dual supply, so the output is centred around 0V, then connect where it says V+/2 to 0V and 0V to -V.With a supply voltage of 30V and a control voltage ranging from 250mV to 50V the frequency will sweep from 700Hz to 100kHz. If having the control voltage higher than +V is a problem, then some of the resistor values can be changed so the maximum control voltage becomes near +V.

author: Zero999
author_rank: Super Contributor
timestamp: August 20, 2016, 11:32:32 pm
content: And here it is. The circuit runs of +/-15V, the output is centred around 0V and the control voltage can range from nearly -15V to +15V.

author: gerry
author_rank: Newbie
timestamp: August 06, 2020, 07:06:26 pm
content: Quote from: Zero999 on August 20, 2016, 07:18:16 pmhi this can give 20-20khz???

author: Zero999
author_rank: Super Contributor
timestamp: August 06, 2020, 10:04:22 pm
content: Quote from: gerry on August 06, 2020, 07:06:26 pmYes, but not with the component values shown.In your private message, you said you've done an LTSpice simulation, but were having difficulty getting it to work. Please post the .asc file here, as an attachment.

author: gerry
author_rank: Newbie
timestamp: July 17, 2021, 01:46:29 am
content: hi thanks for reply..so what values need to change to make it 20-20khz

author: Zero999
author_rank: Super Contributor
timestamp: July 17, 2021, 07:08:55 pm
content: Quote from: gerry on July 17, 2021, 01:46:29 amWhilst this circuit can work over that frequency range. It's not practical to do it without having more than one range. You'll need a switch to change the value of C to two or more values, to convert 20Hz to 20kHz.