First comparison between AD9xxx DDS and Si5351 clock chip

In the meanwhile I changed the VFO section my current project, which is a “high performance” TRX for 40 meters, from an AD9834 module to the Adafruit Si5351 breakout board.

My first impression (just from the listerning to the receiver) are as follows:

  • The number of audible spurs has decreased significantly with the “Si”. There are some, but very much less than with the AD9834 DDS module. They completely disappear when the receiver is connected to the antenna because band noise is stronger in this case.
  • The number of strong broadcast stations that I receive is zero. The receiver did not have many problems at all before with the AD9834 because the front end consists of a three-pole 7MHz filter, a Dual Gate MOSFET serving as a regulated rf preamplifier and a doubly balanced diode ring mixer. But some broadcasters were discernable as images from the spurs the AD9834 DDS generated. This problem has been solved completely with the “Si”.

Another advantage of the Si5351 is that it has more output channels that you can use. Or to say more exactly: The “A” version of the chip has 3 outputs (CLK0, CLK1, and CLK2). So, you can easily implement split mode or RIT functionality¬† just by e. g. connecting CLK0 to the RX mixer and CLK1 to the TX mixer and adjusting the frequencies as needed in the respective situation.You also don’t have to decouple the rf lines coming from the module because they are decoupled internally in the Si5351 chip.

The frequency span of the Si5351 is much bigger than that of the ADs. I could get out up to 120 MHz with my simple code. But minimum frequency is 2.5 kHz according to the datasheet. With the ADs you can get near DC in this case. The waveform of the “Si” on the other hand turns to something like a sinewave when you go above 80 MHz but that doesn’t matter that much.

With the “bigger” ADs (AD9951 etc.) you can go up to 120 Mhz provided you have a 400MHz clock generator. But you need 2 or 3 different voltages (1.8, 3.3 and maybe 5 Volts if you microcontroller runs on 5V). The Si5351 only needs 3.3 V (or 5V if you use the Adafruit breakout board or a similar one)

But one shortcoming of the Si5351 should also be mentioned: The software coding for the Si5351 is a little bit more complex than the AD9xxx. I worked two days because I wanted to write my own functions based on the data sheet which turned out a little bit complicated.

For the AD9xxx when economically coded you only need 2 to 3 dozens of lines of C source code. When checking thru my “minimal code” you will find much more programming effort for the “Si”. But this is not a problem at all.

Conclusion: The Si5351 is a great alternative to the AD9xxx chips made by AD. I will do further experiments and keep you informed.

73 de Peter


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