A DDS module I designed and built as a low phase noise RF source for my projects. For my dual conversion receiver I wanted sources with as little phase noise as possible. These AD9951 DDS chips are really nice. I’m clocking the chip with a 400 MHz LVPECL clock, which has reasonably small phase noise but I do think it sets the ultimate noise floor. (I don’t have anything to measure the module to make sure.)

I stuck two of the modules inside an aluminum box I bought off of Amazon, and I control the modules using an SPI port from my processor. The processor is an Atmel SAMS70 (an m7 mobile ARM processor), which is on a board my dad and I have been working on. There would be no problem using Arduino to do the same thing; in fact, I believe there are a couple Arduino libraries out there.

In the picture above you can see the box containing the microcontroller on the left and on the right you can see the DDS feeding an amplifier block. Those aluminum boxes are nice for RF projects.

I’m really happy with how these came out, they work great and are easy to use. They are expensive chips though. I think they were around $30 a peace, so my hand was shaking a little when I soldered them on.

As far as design details go, I mostly used the reference design from the datasheet. The LVPECL clock wasn’t in the datasheet, but the voltage was right for providing a strong clocking source (you just need to make sure you bias them properly). The reconstruction filter at the output is a 9th order Chebychev filter with a cutoff at 160 MHz. I probably could have pushed that up a little higher in frequency, but I really only want to use these modules as high as the 2m band so the 160 MHz cutoff is fine.

I was curious about the noise figure of a standard 2N3904 BJT used in an RF preamp, so I built the circuit shown above. Using my home built NF measurement kit, I found the circuit has a noise figure of about 7.5 dB.

My interest was sparked by a YouTuber I really like named Charlie Morris (ZL2CTM): https://www.youtube.com/channel/UCSNPW3_gzuMJcX_ErBZTv2g. He makes the coolest radios using the construction technique you see in the picture. This is one of his designs for a preamp that works in the HF bands. If you are interested, the design for the circuit can be found here: https://www.youtube.com/watch?v=iRABXFKScqk&t=653s

A couple things to note about the measured 7.5 dB noise figure:

• No attempt was made to maximize NF in the design. It was simply constructed as a gain block to provide 10 dB of gain and match 50 ohms input and output impedances. The design will work just fine for Charlie’s needs in the 80m and 40m bands.
• I found a data sheet saying the maximum NF of the 2N3904 is 5 dB. This is consistent with my result, I’m sure the feedback used in the design increases the NF a couple dB.
• I’m still working out the kinks in my home made noise figure kit. I’m pretty confident right now that my result is correct to within plus or minus 0.5 dB.

Fun afternoon project!