Devlog #2 – Finishing the Schematic

I finally reached a pretty big milestone: the entire schematic is finished. I still need to add footprints for all the parts and start PCB layout, but the entire radio is now connected together electrically.

Getting Too Focused on RF

One funny thing that happened while designing this was that I got so focused on the RF side of the project that I basically forgot about something kinda important: power.

I was spending so much time looking at filters, amplifiers, mixers, and oscillators that I completely forgot that none of those parts work without proper power rails.

When I went back through everything, I also realized I had made another beginner mistake. Earlier in the project I was labeling rails like VDD and VDDA, but at this level that isn’t really enough anymore.

Different chips need different voltages, and some have separate analog and digital supplies, so I need to clearly label exactly what voltage each rail is and what it powers.

Simplifying the Local Oscillator

The last part of the schematic that gave me trouble was the IF local oscillator for the internal mixer in the CMX992.

At first I thought I would have to design a bunch of extra filtering and signal conditioning around it, which was making that section way more complicated than I wanted for a V1.

After reading through the datasheet more, I realized I could just use another CMOS oscillator to generate the signal I needed.

Changing the RF Design

I also made a few big changes to the RF front end.

Originally the plan was for the SDR to cover 50 MHz–450 MHz, but I expanded that to 50 MHz–850 MHz.

This lets the radio cover a much larger range of signals, including:

• FM radio
• Aircraft communications
• NOAA weather satellites

I also changed the filtering strategy:

• The pre-LNA filters are now 3rd-order Butterworth filters. Since these are before the LNA, keeping insertion loss low is extremely important because any loss here directly hurts sensitivity.
• The post-LNA filters are now 5th-order Elliptic filters. Since the signal has already been amplified, I can focus more on getting stronger rejection of unwanted signals.

Basically, the first filters are focused on keeping weak signals alive, while the second filters are focused on cleaning up the signal afterward.

No USB Data

Another design decision I made was removing USB data from the USB-C connector.

The board is not meant to directly communicate with a computer. The SDR will communicate through an FPGA and STM32, which handle the digital processing and control.

Because of that, the USB-C port is only there for power. This simplifies the board and removes unnecessary routing.

What’s Next?

• Add footprints to all components
• Verify every footprint against the datasheets
• Start PCB layout
• Begin designing the physical board