Cheap Function Generator Limitation? - Page 1
Forum section: electronics / testgear

author: joseph nicholas
author_rank: Frequent Contributor
timestamp: September 24, 2016, 12:06:03 pm
content: Hi guys, I have a ICL8038 on a cheap chinese dyi circuit board. Here is the schematic below:It works, but the circuit severely limits its use vis a vi the data sheet.In addition I cant adjust the amplitude at all.The chip can use a split + - power supply but I don't know if this is implemented in the circuit design.Does anyone have any suggestions as to improving the values etc or advice about using the + - chip ability vs just a +, ground scheme?Thanks in advance.nico

author: markce
author_rank: Regular Contributor
timestamp: September 24, 2016, 03:00:31 pm
content: This extension wlll be more usefull:http://www.zen22142.zen.co.uk/Circuits/Testgear/funcgen.htm

author: Fungus
author_rank: Super Contributor
timestamp: September 24, 2016, 03:12:06 pm
content: Quote from: joseph nicholas on September 24, 2016, 12:06:03 pmUse a potentiometer...

author: joseph nicholas
author_rank: Frequent Contributor
timestamp: September 24, 2016, 03:41:32 pm
content: Quote from: Fungus on September 24, 2016, 03:12:06 pmThanks for your response. I you look at the schematic it is connected to a potentiometer it just doesn't effect the wave form much.This kit is widely available so your responses might be of value to others since I think this was bought by a lot of others.nico

author: Fungus
author_rank: Super Contributor
timestamp: September 24, 2016, 03:50:40 pm
content: Quote from: joseph nicholas on September 24, 2016, 03:41:32 pmI mean put a potentiometer across the output. Use the center pin of the pot for the signal.

author: rstofer
author_rank: Super Contributor
timestamp: September 24, 2016, 03:53:54 pm
content: One potentiometer in your circuit controls frequency and the other attempts to control distortion of the sine wave. Neither have anything to do with amplitude.https://www.intersil.com/content/dam/Intersil/documents/icl8/icl8038.pdfSee page 6 top right paragraph where it states that the output is fixed by the power supply.See also figure 6 where they add an external op amp buffer with gain. They also mention that the sine wave has a relatively high output impedance so you can't load it down very much.

author: joseph nicholas
author_rank: Frequent Contributor
timestamp: September 24, 2016, 05:03:36 pm
content: Quote from: rstofer on September 24, 2016, 03:53:54 pmHi, There are 3 potentiometers in the schematic. One controls the frequence, (It does the job), one controls the duty cycle (works but barely, and not up to specs), the third controls the modulation (this does nothing). The amplitude is controlled by the Vcc on the chip?If were to power the circuit with a minus 12 and plus 12 would this release the magic smoke?

author: rstofer
author_rank: Super Contributor
timestamp: September 24, 2016, 07:30:08 pm
content: Quote from: joseph nicholas on September 24, 2016, 05:03:36 pmIt doesn't look like the unit implements frequency modulation. They're just holding the input at some value.QuoteYou can look at Figure 1 in the datasheet and see where some pins should be ground referenced, some connect to V+ and some to V-. Figures 3A and 3B show alternatives for the timing components.Clearly, you will get a larger voltage swing with +-12V but why do you need so much? The other concern is the square wave output. It should probably be ground-referenced if you want it to work with logic circuits.The op amp solution is far better than trying to redesign the circuit.

author: markce
author_rank: Regular Contributor
timestamp: September 25, 2016, 11:52:56 am
content: In my previous response I linked to a good complete design with symmetrical stabilized supply and output buffering.Your current design could be easily exteded to this.There is also an "Everything You Always Wanted to Know About the ICL8038" (AN013.1) from Intersil:http://www.intersil.com/content/dam/Intersil/documents/an01/an013.pdf

author: joseph nicholas
author_rank: Frequent Contributor
timestamp: September 25, 2016, 02:15:34 pm
content: Thank, I've seen these data sheets already. The is a polish web site with a dyi function gen that uses this chip.I'm cometed to use the circuit board that I posted.Thanks anyway.nico

author: med6753
author_rank: Super Contributor
timestamp: September 26, 2016, 10:44:26 am
content: Many years ago I built up a circuit using the ICL8038 as a quick "scope checker" for items such as amplitude, timebase, and linearity. The frequency is fixed at approximately 5KHz and I used trim pots to fix the amplitude at 0.5v p-p for the square wave, sine wave, and triangle wave. Schematic attached. You can use this as guide for your own design.And here's an example on the Tek 2465:

author: joseph nicholas
author_rank: Frequent Contributor
timestamp: September 26, 2016, 10:56:32 am
content: Yes the chip works good, its the circuit kit that I bought that is problematic.