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A few years back I wrote this article on my personal website, about the pro's and con's of a DDS vs Si570? and a lot of it applies also to the Si5351A - which is just a cousin of the Si570 after all (with its own pro's and con's, another comparison!).

Yes a DDS is easier to program for - but there are a lot of established code examples around for the Si5351A now, such as QRP Labs simple examples on sub-pages here - so I think code complexity is not such an issue as it was.?

The main difference performance-wise is that DDS generate spurious outputs due to the discrete nature of the accumulator/DAC synthesis. These spurious outputs are NOT filterable, because they can be very close in to the desired carrier frequency. A PLL such as the Si5351A doesn't suffer this problem, but since it is a PLL with a VCO internally, it has lower phase noise performance than a DDS. The DDS has a sinewave output which is important for some applications - though as soon as that passes through an amplifier stage you've probably lost that sinewave anyway.

I won't repeat the whole article here. My favourite at the end of it was the DDS - but that is an extremist elitist view, on making a perfect-as-possible HF-only oscillator using a top-of-the-range (and very expensive, complex etc) AD9910 or AD9912.?

For PRACTICAL purposes, such as in a kit like the BITX40 or like the QRP Labs kits the Si5351A is a very nice device, because of its low cost, and very high performance:cost ratio. The phase noise isn't as good as an Si570 but it has the benefit of low cost, and multiple outputs which is also a useful feature. The phase noise is plenty good enough for 99% of applications.?

For PRACTICAL purposes, for the majority of amateur radio applications, whether you are using an AD9850 DDS or Si5351A (lower members of their respective families than the AD9910 or the Si570), all the discussion of phase noise and spurs etc isn't very important. Both these devices have very much better phase noise, stability, range, etc performance than L-C VFOs which we were perfectly happy with until a few years ago!?

73 Hans G0UPL