Heterodyne or direct sampling?

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Heterodyne or direct sampling?

Post by g1hbe » Tue Dec 19, 2017 11:55 am

Here's one for those who know about such things. The two main types of SDR seem to be 'hybrid' and 'DDC'. The hybrid type uses familiar heterodyne techniques (a front-end tuner) to get the signals down to an IF where it is converted to digital format by an ADC. After that comes the DSP etc.
The DDC gets rid of the tuner and the antenna is connected (almost) directly to the ADC where the whole of the spectrum is converted to data.

My question is this: What effect(s) does each technique have on spurious responses? I'm familiar enough with the 'image' or 'second channel' response that we get with conventional superhet methods, and I know these can be banished by careful front-end filtering. But narrow filters work against the wide spectrum display favoured by SDR manufacturers and demanded by users.

DDC gets rid of the heterodyne idea and we might think that's the end of unwanted responses, but it surely isn't(?)
What are the spurii we can expect from a 0-30 MHz DDC receiver? Is it linked to sample rate, and how? Does decimation also introduce its own set of unwanted responses? Are we once again required to use narrow front end filtering to get rid?
I see the Perseus has a set of 10 filters across the range 0-30 MHz, none of them wider than about 2 MHz, so it looks as though there must be some spurious responses to watch out for. The ELAD has a set of optional plug-in filters. The Winradio G31 uses similar DDC, but AFAICS has minimal filtering but still gets good reviews. The new Airspy HF+ seems to promise clean responses but I cannot shake off the feeling of snake oil.
As a 64 year old radio enthusiast who has been buying, building and fiddling with radios since the early 60's, I suddenly find myself with so many questions!

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Re: Heterodyne or direct sampling?

Post by OH2BUA » Tue Dec 19, 2017 3:51 pm

I share Andy's curiosity with the same matters.

BTW... it think 2017 might be remembered as the year of really nice promiseware releases. Really high spec gear with easy pricing... most of them not yet shipping for some reason.

73, Jukka

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