A Simple Si5351 Signal Generator
Lately I’ve been working on some simple receiver designs and decided it’s time to put together a real signal generator. I like the smell of melted solder as much as anyone else but I’m getting tired of rebuilding the wheel for each receiver design I want to try.
With that in mind, I decided to expand upon the Si5351 USB controller I described previously. The original is a useful circuit but if I planned to use this on a regular basis there were a couple of improvements I wanted to make.
First, USB communications using the 3.3V Trinket weren’t as reliable as I would like so I switched to the Trinket 5V version. This meant adding a 3.3V regulator to power the Si5351. In addition, when using the original circuit I usually ended up feeding the clock signals through 5V level shifters so I decided to incorporate them directly.
The resulting schematic is shown in the figure below. The regulator is simply a 78L33 taking its input from the Trinket 5V pin and supplying the Si5351 VDD. The level shifters are CMOS inverters, one for each clock output. The particular chip I used has a single gate per chip but these could just as easily be replaced with a quad gate chip. I just happened to have the single gate chips on hand.
You may notice there are no I2C level shifters between the Trinket and Si5351, despite their operating at different voltage levels. In previous projects I’ve found level shifters aren’t necessary going between 5V and 3.3V chips so long as the bus is powered from the 3.3V supply and the 5V side reads 3.3V as a logic high. It’s not strictly according to specs but it works and saves a few components.
The circuit is powered from the USB bus and uses the same Python library developed for the original circuit. This library can be pulled down from its GitHub repository.
A photo of the completed circuit in its Altoids Tin case is shown below. Like most of the one-off circuits I’ve done lately it’s built on stripboard using adapters for the surface mount parts. Later I may put together a printed circuit board and, if so, will provide an update.