DDS generator problems

• DDS generator problems
  Posted by Deathwish on 14 Aug, 2015 09:26
• I purchased one of these a while back. http://www.ebay.co.uk/itm/291399023398?_trksid=p2057872.m2749.l2649&ssPageName=STRK%3AMEBIDX%3AIT
  
  I attached it to an atx psu, and then a psu I just finished making last night. All I get out of it is freakiness. I cant give you a pic as my phone camera is broken, but if you draw a sine wave and then along its entire length place masses of white dots that is what my scope shows. if you then wait a few minutes the waveform starts to look like someone has put a sandcastle on top of another one on each cycle.
  
  It is doing my head in. Any ideas
• #1 Reply
  Posted by electros6 on 14 Aug, 2015 10:08
• Does the lcd power up. what is the scope you are using , please provide some details
  
• #2 Reply
  Posted by Deathwish on 14 Aug, 2015 10:12
• lcd is fine, scopes are a tektronix 2225 and a rigol ds1054z
• #3 Reply
  Posted by mij59 on 14 Aug, 2015 10:26
• Could be a problem with the output filter of the generator, whats output level and frequency of the generator ?
  With the Rigol you should be able to save wave forms to usb
• #4 Reply
  Posted by Deathwish on 14 Aug, 2015 10:38
• Sorry was busy trying to play with it. It is now showing me a double sine wave ..... then it turns off, so I think it is kind of naffed, but the psu is fine so thats good..
• #5 Reply
  Posted by Farley on 14 Aug, 2015 15:05
• This looks like the same device in another auction:
  http://www.ebay.co.uk/itm/DDS-Function-Signal-Generator-Module-Sine-Square-Sawtooth-Triangle-Wave-Digital/400971779496?_trksid=p2047675.c100005.m1851&_trkparms=aid%3D222007%26algo%3DSIC.MBE%26ao%3D1%26asc%3D20140122125356%26meid%3D7a90dc301d8646c0864eda7b437897df%26pid%3D100005%26rk%3D2%26rkt%3D6%26sd%3D291399023398
  
  In the schematic shown in that listing it doesn't appear that there is a decoupling capacitor on the AVR power pin. Assuming the module you have has the same processor, you might try adding a 100nF cap directly across pins 10 and 11 of the AVR. Keep the leads a short as possible. Also, it doesn't appear there is any decoupling on the LM358 power pins.
  
  Another thing to try would be to re-seat anything in a socket or connecter.
  
  After that, if still no joy, look for poor solder joints and/or retouch/reflow all the solder connections on the PCB.

  ---

  
• #6 Reply
  Posted by Deathwish on 14 Aug, 2015 18:09
• yes thats the pos, it is now on LSD, the screen is doing funky things and displaying random char$ and then it turns itself off at random. It is in the cupboard till it learns to behave.
• #7 Reply
  Posted by Deathwish on 14 Aug, 2015 18:33
• the user manual etc
  
  https://www.dropbox.com/s/z0ktc666sct1gj1/H11887install.pdf?dl=0
• #8 Reply
  Posted by tom66 on 14 Aug, 2015 22:13
• Bear in mind the output filter these use is very basic, essentially nonexistant, so don't expect a clean wave at a high frequency unless you're prepared to add an external filter.
• #9 Reply
  Posted by Deathwish on 14 Aug, 2015 22:55
• I can do that I suppose if I can find one
• #10 Reply
  Posted by jimdeane on 16 Aug, 2015 23:53
• Does the PSU need a minimum load, and do you have a load resistor in place?
• #11 Reply
  Posted by Deathwish on 17 Aug, 2015 00:00
• No its not the psu, I tried a atx one then built my own , it does the same on both. for some reason i purchased an ad9850 module and a ublox lea 4t gps receiver too, I cant remember exactly why , I think i was going to make a sig gen of my own but I lost all train of thought on it a while ago.