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QRP labs Arduino Shield Kit Schematic #shield


KG5ZID Hardy
 

Is there a better schematic than the hand-drawn one shown in the assembly instructions? I am attempting to use this shield with an AD9850 DDS module, and the LPF kit. In particular, the AD9850_reset pin is not shown on the hand drawn schematic. Using the numbering scheme used on the schematic, this would be pin 15, next to Data pin 14 on the module header. On my shield, this pin appears to be grounded. Furthermore, the header pin which would normally connect to the Arduino Uno D12 pin also appears to be grounded. This is shown on the schematic to be connected to Band 4. I don't see any solder bridges but that's always a possibility!
--KG5ZID Hardy


Ron Carr
 

Arduino pin 12 is not grounded on the etch.  You probably have a short somewhere.
You seem to be correct about DDS socket pin 15, it is grounded on the etch.  The Arduino reset line only goes to the button on the etch.   Do you need a reset for the DDS?  My memory is hazy but I seem to think that it enters serial mode depending upon which data bits are high and which are low when it powers up.


Bruce K1FFX
 

I wrote to Hans 2 or 3 years ago (I unfortunately didn't hold onto the email thread) about the
RESET signal on the Arduino shield.  Hans confirmed Ron's observation: the AD9850 is
initialized to an appropriate state at power-up and so he didn't see a need for supporting
RESET on the PCB.  I've used the AD9850 with the Arduino shield and it worked fine without
explicitly doing a RESET.

Hardy ... you mentioned building an LPF kit in your posting.  Since the AD9850 generates
a sine wave, depending on your application, the LPF may not be necessary.  Typical use,
as I understand it, is to use the LPF where a sine wave is needed, while the Si5351a generates
a square wave.

I'd be interested to hear how you plan to use an Arduino + Arduino shield.  I've used it as the
basis of both AD9850 and Si5351a based signal generators ... the latter used as LO for the
QRP Labs receiver module (and even, using an Arduino Ethernet shield, for a remote-controlled
LO) ... and as part of an Arduino-based scalar network analyzer.

- Bruce K1FFX


KG5ZID Hardy
 

Thanks Ron and Bruce. Turns out I was mis-counting Arduino pins. I was cstarting my count from the two unused pins! When I counted from the other end of the header and found the **real** pin 12, it was fine. I used my trusty pluggable microscope to take the following image of the AD9850 header pins and confirmed that yes indeed DDS_reset is grounded. Since all of the AD9850 sketches I looked at used reset in setup(), I did the same. Fortunately I checked things out ahead of time instead of plowing ahead with booting up the software! It would be nice if Hans would add a few words in the assembly instructions regarding the grounded reset pin. I noticed that pin is grounded on the Si5351a module. I included the LPF since I'm also using the PA which can introduce distortion in the sine wave. When I was bread boarding the gadget I noticed it was generating pretty good harmonics. My base was in the 80m band but I saw harmonics up through 20m at least. I have a few other Tx projects that could use a bit of flitering as well! My current use case is an RTTY beacon. I was "inspired" by F4GOH's article in Aug 2020 QST. I have been intending to tinker with DDS for some time and that was the kick I needed to get going. I did that sort of thing as an example in the computer engineering classes I taught years ago and the state-of-the-art has progressed a lot in 20 years! As far as the grounded reset pin is concerned I think I'll just ignore it for now. If I need it later, I can always hack that trace and rewire the board - won't be the first time hi hi.


--KG5ZID Hardy


KG5ZID Hardy
 

One more update on the shield schematic - Qout/Clk0 does NOT go to pin D5 on the Uno as shown in the schematic. There is another jumper, W6/W7 not shown on the schematic. Not a big deal unless you use D5 as a test point to check the output of the DDS module like I did! For what it's worth, my RTTY beacon actually works! It was decoded by fldigi 100%. Now to get an antenna attached and see who can hear it! Whoopee!