LF: AD9850 controlled by RS-232

[Markus Vester <markusvester@aol.com>]

I have recently aquired a couple of cheap AD9850 DDS boards, and found that it is possible to control them directly from the PC through RS-232 handshake lines (RTS, DTR, TXD) without any additional hardware. This can be used as a simple means to generate WSPR RF signals without a linear SSB exciter.

 

Details regarding the boards and interfacing format were found on 
http://www.theladderline.com/inexpensive-ad9850-dds-boards-ebay#comment-195

 

Five short wires connect the 9-pin RS232 to the "lower" row on the AD9850 board, with the 5V supply tapped directly off the USB device.
http://df6nm.bplaced.net/wspr/ad9850/ad9850_at_rs232.jpg

 

 (*)  5V = Vcc   (1)
 (4) rts = clock (2)
 (3) txd = latch (3)
 (7) dtr = data  (4)
 (5) gnd = reset, gnd (5,6)

 

As the USB-to-Serial converter has only low current outputs, the internal clamping diodes  provide adequate voltage limiting at the digital inputs. To configure the AD9850 for serial protocol, additionally pins D0 (2) and D1 (3) on the "upper" row have to be be connected to Vdd (1), and D2 (4) to gnd (10).

 

I have uploaded software and sourcecode to
http://df6nm.bplaced.net/wspr/ad9850/ddswspr.zip (77k)

In addition to a basic frequency setting utility (serdds.exe), there is also a WSPR transmit program (ddswspr.exe) which allows you to generate WSPR at different speeds. BTW The latter can alternatively be used to generate wspr with a FE-5680A Rubidium synthesizer (/rb option).

 

As the onboard 125 MHz sources seem to deviate by several kHz it is useful to calibrate the frequency and edit ddsref.txt. Frequency stability is adequate but not very good, my units exhibited a warmup drift of about 2 ppm.

 

Hope this may be useful.

 

Best 73,
Markus (DF6NM)