building a function generator - Page 1
Forum section: electronics / projects

author: Deathwish
author_rank: Unknown
timestamp: June 04, 2015, 07:51:05 am
content: Ok, so I have NO design qualifications except for having built my own eprom and pic programmers some years back, so I am probably talking out of my rectal cavity here.I have managed to get my hands on 2 ICL8038 and 1 Exar 2206 IC's.I am unsure if I want to build a single unit using all 3 of these each doing one specific waveform, or 3 different units each for a differing purpose. I want in one respect a FG set up to check my scope out and set it up for calibration purposes if that is the right term. I also want if at all possible to have the unit or units capable of auto adjusting itself [read self calibrate] via a gprs system onboard. Is this possible, and how expensive would it get roughly.Maybe range selectable by pic chip or if not so reliable and clear on outputs old style rotary or push buttons.The 2 ICL chips appear to have a max output of 1Mhz [square wave only] and the 2206 of 200Khz but I want more than that so ....ideas, or comments welcome. Just remember that it will take me some time to build anything because of finances and I want the plans etc available to the open community for free if it works out. I see too many commercial ones people can not afford.

author: smjcuk
author_rank: Frequent Contributor
timestamp: June 04, 2015, 08:26:10 am
content: Both the chips you have are obsolete so producing a reference design with these in is going to be problematic in the future unfortunately. Just about every nice IC disappears from the market so I've gone back to using generic parts where possible for analogue stuff.Also they're basically RC-based analogue function generators which means they're not particularly accurate or frequency stable without lots of work. You would probably need to use a DDS chip to get decent frequency stability for scope calibration without spending too much. However if you want a basic function generator up to a couple of MHz they're fine. Datasheets have all the info you need to accomplish that. Above that frequency, you're better served with an LC oscillator or a DDS or something else.You won't be able to calibrate an analogue function generator against GPS time signal easily. You're better with a TCXO (temperature compensated crystal oscillator) as a local frequency standard inside a simple frequency counter and then set the output of your function generator based on that as a standard. You can build a frequency counter with discrete logic easy enough with a time commitment or use a PIC and a prescaler.

author: Rerouter
author_rank: Super Contributor
timestamp: June 04, 2015, 08:26:40 am
content: As you are familiar with a pic, unless you really desire a triangle / sawtooth wave, i would recomend going with a ebay DDS module instead (e.g. AD9850), much better granularity, and a much wider frequency range,You can then swap out the original crystal for a better one if you so desire, but the one that came on my module was already rated to 10ppm, which is better than anything i cared to do,

author: tron9000
author_rank: Frequent Contributor
timestamp: June 04, 2015, 08:27:56 am
content: I have about 5 of the XR-2206 - am planning on doing the same thing at some point, but in the middle of 3projects + work + daddy duties+moving house!I'm looking at the datasheet for theXR-2206(link). Have you got the specs mixed up cos the max frequency of this IC says 1MHz?sure if you had it pic micro controlled and an analogue switch, sayDG408/409, it could work. You could dabble in getting a frequency display, might be fun to learn, or a pain!

author: Deathwish
author_rank: Unknown
timestamp: June 04, 2015, 08:40:45 am
content: Trust me to buy obsolete IC's , they were cheap and seemed to do what I wanted, mind you I cant find any BC547 trannies about here either, so it all comes of ebay for me. But thanks for the info and , hang on be back in a sec, ok , I'm back. Tron9000, I have the data sheets for both, you are correct, it does go to 1Mhz in the specs, I could of sworn I read it only went to 200Khz but I was obviously wrong.DDS is something I haven't given any thought too. Mainly because I have these IC's and a couple of pics and my brain just thought, hmm, it could be cheap and easy to do. I will certainly have a look at that avenue.

author: smjcuk
author_rank: Frequent Contributor
timestamp: June 04, 2015, 08:46:07 am
content: Quote from: Deathwish on June 04, 2015, 08:40:45 amThey're all obsolete the moment you buy them so don't worry. They still work. BC547s can be substituted with virtually any generic NPN such as BC337.You can probably push them past 1MHz a bit. There may be characteristic curves or slew times listed on the datasheet that should give you a clue as to when they start bending.

author: Deathwish
author_rank: Unknown
timestamp: June 04, 2015, 08:51:27 am
content: I ordered some 2N2222's of ebay, still not turned up yet. much like my new freq counter that was meant to be on a 48hr RM recorded delivery a week ago. I swear they use sheep for delivery vehicles in wales.

author: smjcuk
author_rank: Frequent Contributor
timestamp: June 04, 2015, 09:04:48 am
content: Quote from: Deathwish on June 04, 2015, 08:51:27 amWatch out for ebay 2n2222s. I bought a batch which had a stupidly low hFE and got rather hot. Made by some Indian semiconductor fab I've never heard of.If it's any consolation, it's not much better here in London. I caught our RM delivery guy jamming a bag full of components through my letterbox with his foot a couple of months back and they managed to break (yes break into two distinct parts) a book in a sealed package.

author: dom0
author_rank: Super Contributor
timestamp: June 04, 2015, 09:14:48 am
content: Quote from: smjcuk on June 04, 2015, 09:04:48 amCDIL?

author: Deathwish
author_rank: Unknown
timestamp: June 04, 2015, 09:19:17 am
content: Mine are meant to be coming from this seller.http://www.ebay.co.uk/itm/291210981527?_trksid=p2057872.m2749.l2649&var=590364892156&ssPageName=STRK%3AMEBIDX%3AITHe is only a few miles away so I cant see why it is taking RM this long to get them here, unless they are coming via china first, that annoys me when they say it is in the UK but comes via china to a friend who then repacks them and forwards them on.

author: smjcuk
author_rank: Frequent Contributor
timestamp: June 04, 2015, 09:23:12 am
content: Quote from: dom0 on June 04, 2015, 09:14:48 amThat's the ones.

author: Deathwish
author_rank: Unknown
timestamp: June 04, 2015, 09:42:34 am
content: I only want a couple to build a colpitts crystal osc for testing crystals. I have NO spares at all at present apart from the IC's etc I got, been to busy buying the hardware to do the jobs I want too do first.Thanks to our host I have spent over £800 of my savings [ almost all I had ] on items because its so addictive and I think I remember why I stopped years ago.

author: tautech
author_rank: Super Contributor
timestamp: June 04, 2015, 09:45:51 am
content: Quote from: Deathwish on June 04, 2015, 09:42:34 amThis is a FREE forum, Dave says so.

author: Deathwish
author_rank: Unknown
timestamp: June 04, 2015, 09:54:08 am
content: Hi Tautech.It's only free in his mind, he has no idea what he has unleashed on the world in here, then again maybe he does and it is all some despicable dastardly plan for world domination.As your a Siglent fan take a look at this auction and spot the problem.http://www.ebay.co.uk/itm/171814669984?_trksid=p2055119.m1438.l2649&ssPageName=STRK%3AMEBIDX%3AIT

author: tautech
author_rank: Super Contributor
timestamp: June 04, 2015, 10:18:29 am
content: Quote from: Deathwish on June 04, 2015, 09:54:08 am32 Kbts memory not 40.Description lists NO cables but further down...........

author: Deathwish
author_rank: Unknown
timestamp: June 04, 2015, 10:31:17 am
content: I msg'd him as he was selling it as a 1102 and the pic is of a 1052, he has now changed it and thanked me for pointing it out. But yes the no probes got me too considering he said it is all working etc etc

author: Muttley Snickers
author_rank: Unknown
timestamp: June 04, 2015, 10:32:57 am
content: It's Wireless, a new model perhaps !Did somebody mention a dastardly plan.....I was on another thread and heard it like a dog whistle......Muttley

author: Deathwish
author_rank: Unknown
timestamp: June 04, 2015, 10:38:11 am
content: Dastardly plan .. trust you Muttley to be up for that one. I suppose you want to have a pitstop along the way as well .

author: Deathwish
author_rank: Unknown
timestamp: June 04, 2015, 10:46:44 am
content: I swear my postman has one of those plans, I reckon he deliberately knows when i am waiting for something exciting to turn up and hides it till the end of the week just to keep me wound up. He hates climbing the stairs to my flat and this is his way of getting revenge. I wont say the "D" word anymore because I know it will get your ears perked up

author: Kleinstein
author_rank: Super Contributor
timestamp: June 04, 2015, 04:52:22 pm
content: The 8038 and 2206 are in many respects replaced by DDS chips. In most respects a DDS chip like AD9833 or AD9850 is better than the old analog style generator, especially if you want a stable frequency or digital control. The only thing DDS Chips are not good at (and many don't even support it), is generating triangle or sawtooth signals.So the old analog chips may be useful for a simple generator with a pot for control. There may be a µC to display the frequency via a simple counter. A PLL like control is possible, but not that easy.

author: Deathwish
author_rank: Unknown
timestamp: June 05, 2015, 07:59:22 am
content: Well I just bought both DDS types 9850/9851. bleh. why is it the Chinese can knock out a complete unit for less than the cost of the IC's themselves [rhetorical whine and moan].

author: Kleinstein
author_rank: Super Contributor
timestamp: June 05, 2015, 08:15:35 am
content: It's a good question, why these Chinese modules are so cheap.As far as I read, It started with some modules with poor output filters (wrong filter frequency with AD9850). It's perfectly OK to sell these at a low price. However these first series should have been sold. Maybe they found out that even these modules sold so good, that they made a larger series of new one. Buying in larger quantities chip prices can go down - they might even settle for low grade (or less tested) chips to bring the price down. Except for the DDS chip, there is nothing really expensive on these boards.I have not heard of real fakes with these modules. Only some where an AD9850 is uses instead of an expected (though not promised AD9850 - e.g. calling a modul with AD9850 something like GEN9851 with details only in the fine print).

author: tautech
author_rank: Super Contributor
timestamp: June 05, 2015, 08:20:58 am
content: Quote from: Deathwish on June 05, 2015, 07:59:22 amWelcome to the world we live in.So you know about eBay, what about Aliexpress.....While you're in the spending mood, you'll need one of these:http://www.aliexpress.com/item/High-quality-Smart-SMD-Tester-Capacitance-Meter-Multimeter-MS8910-3000-counts-LCD-display-Auto-Scanning-Auto/32316959233.html?spm=2114.32010308.4.126.pRE4HNOhh shit what have I done.....let Deathwish loose on the world.

author: Deathwish
author_rank: Unknown
timestamp: June 05, 2015, 08:32:36 am
content: Behave will you [ Hmm thats like asking a fox in the hen house not to chase the hens isn't it]. Why do I need one of those ?. I have tweezers that connect to my dmm's already. Honestly your just building me up to bigger purchases then your going to bend my arm to buy a siglent scope or such arent you, I have seen this D******dly deed done before ya know.Anyway, it seems my need to design or try to make a Freq gen is moot as this kind person has already done so, it would be like trying to reinvent the wheel when someone more able than I has done so.http://www.vk5tm.com/homebrew/dds/dds.phpLikewise I found this as well on another site. why is this so familiar. Probably as Ihave read the thread in here...http://www.g3src.org.uk/page36.htmlAs for aliexpress, I have seen it before but I am unsure about protection on purchases and speed of delivery amongst other things like import tax.

author: tautech
author_rank: Super Contributor
timestamp: June 05, 2015, 08:38:17 am
content: Quote from: Deathwish on June 05, 2015, 08:32:36 amPM me and I'll give the run down.

author: Deathwish
author_rank: Unknown
timestamp: June 05, 2015, 09:01:40 am
content: I am resisting that one but losing the fight I think...... PM'ing you is gonna lead to Garry spending more money. I will later if thats ok but I am in hunt the internet mode for ideas and things to use here, then I am going to tidy up a bit and post a pic of mywork areajunk table.