Topband: SDR Mythbusters - ADC Overload myths debunked...

[James Rodenkirch]

>From Stu, K6TU
 
As with any technological change, there are many myths, past truths or part truths that get repeated endlessly and out of context by those frightened or challenged by the change.
 
 Software defined radios are no exception to this in the world of Ham Radio.
 
 Steve Hicks, N5AC and the VP of Engineering at FlexRadio has posted an excellent explanation and bust of the ADC overload myth on the FlexRadio community.  You don’t need to be registered on the community to read this excellent write up:
 
 https://community.flexradio.com/flexradio/topics/adc-overload-myths-debunked?utm_source=notification&utm_medium=email&utm_campaign=new_topic&utm_content=topic_link
 
 
 Here is an extract of the first few paragraphs to whet your appetite - well worth a read as extending education on the world of Software Defined Radios…
 
 Stu K6TU
  
 ADC overload myths debunked
 
 I've received some feedback that there is some confusion circulating on other ham radio reflectors regarding how analog to digital converters (ADCs) work in radio applications.  Specifically, some of the comments tend to say that direct sampling ADCs just won't work in strong signal environments so I'd like to explain why this is not factual for those who are interested. I have a few points to illustrate this.
 
 As hams we tend to think of strong signals in terms of their total power, how many total Watts they are.  When you think of signals in this way, you can add their power in your head and think: two -10dBm signals add to -7dBm total power (3dB increase).  In fact, you can take multiple signals and add them together in a power meter and the power meter will show the total power of all signals.  But this is the average and not instantaneous power.
 
 An ADC, on the other hand, is really a discrete signal device.  All of the signals get chopped into samples and so the real question is: how do the signals add together in the discrete time domain?  To answer this, we have to look at the signals and how they interact.  An RF carrier is like any AC signal -- it is a sine wave that varies from negative to positive voltage along the curve of a sine wave.  If we add two sine waves of exactly the same amplitude, frequency and phase, the peak voltage will be doubled (6 dB).
 
 But two signals of the same amplitude and phase on the same frequency isn't reality.  Reality is signals all across the bands that are totally unrelated (uncorrelated)...
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