37.5 MHz synthesiser and diode-ring mixer

Many USB software defined radios cannot sample signals less than about 24 MHz. I wanted to listen to some of the HF radio bands, and to do that I decided to use a mixer and an oscillator to shift the HF frequency band (~3-30 MHz) up into the SDR’s range. There were only two problems: I didn’t have a mixer, and I didn’t have an oscillator that could generate frequencies greater than 24 MHz. So I decided this would be a nice opportunity to build them.

I decided to use the common diode-ring mixer. This is useful as it doesn’t have to be powered (like a Gilbert-cell) and has excellent isolation between the RF and LO ports. I made a prototype on veroboard, before making one on a PCB. Although the veroboard version would likely have worked quite well, I used the PCB version to try out a new PCB tool (DipTrace – which I highly recommend). Photos of the veroboard and PCB version are shown below. (And yes, I realise SMA connectors aren’t necessary at frequencies of interest, but they are so cheap in bulk).

The diode-ring mixer on a PCB.
The diode ring mixer prototype on veroboard.

Next I needed to make the synthesiser. I decided to use a DDS chip, and chose Analog Devices’ AD9834. With a 75 MHz reference clock the synthesiser can generate frequencies up to 37.5 MHz. The board  is shown in the pictures below.

The PCB after soldering was finished.
Testing the synthesiser.
The final product (without its case). Buttons are used to increase or decrease the frequency of interest and change the frequency step.

It was able to successfully generate frequencies up to 37.5 MHz as required! The diode-ring mixer and synthesiser work nicely, and I was able to up-convert radio signals. In the picture below you can see the AM radio band has been up-converted by 29.7 MHz. The signal on the far left is the LO frequency. The red line is tuned to the AM radio station 4KQ (693 kHz, showing up at a frequency of 30.416 MHz).

AM radio spectrum as received by the SDR after being up-converted by 29.7 MHz. The centre grey line is at 30.368 MHz, the red line is tuned to AM radio station 4KQ at 693 kHz, now showing up at ~30.4 MHz