27 November 2016

Si5351 output waveforms

I finally managed to have myself some shack time this weekend and one of the curiosities I had was to check the output waveform of the Silicon Labs Si5351 I2C programmable VCXO. The chip is quite popular among homebrewers thanks to its low price, ease of use, the fact that it generates 3 independent outputs across a very wide bandwidth (8kHz to 160MHz) and that it offers lower phase noise than the cheap & cheerful AD9850/1 DDS modules.

Adafruit Si5351 module

By the way, there are also some versions of the Si5351 that have 8 independent outputs instead of 3, if you really need that many.

The test setup included an Arduino Duemilanove with the NT7S library, an Adafruit Si5351 module and a Tektronix 2225 oscilloscope. I used the middle output for the test (CLK1), which was terminated with a 75ohm load. Oscilloscope vertical scale is 0.1V/div and horizontal scale is adjusted depending on frequency.

First test, 100kHz output:

Si5351 output at 100kHz

Output looks like a clean square wave, with a nice rise time. Peak to peak amplitude is about 650mV into 75ohm, which puts the power output at 0.7mW (-1.5dBm).