What is Software Defined Radio? Part 1 of 2
If one does a Google search on the question above, you’ll get many different answers.? Nearly all of them true, but none of which really lead me to a proper understanding.
For example…..
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·???????? It’s a radio where the computer does the ‘heavy lifting’.? True but what does that mean?
·???????? It’s a radio that doesn’t have coils and other analogue components that can drift.? True but what’s it really mean?
·???????? Without analogue components, it’s a radio that can tune huge swathes of frequency.? Also true, but why and how?
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Let’s start and the very beginning, a very good place to start
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Probably the first thing my father taught me about radio and electronics was ‘beat frequency’.? No, not how often he hit me (never) but that when two sounds play at once, a third and fourth signal is produced.?
The sum of the frequencies (A + B) and the difference in the frequencies (A-B).
When you look for it, you’ll hear it in everyday life.? An aircraft with two propellers has tones that rise and fall as they mix. If two machines are making a noise that are close, you’ll hear a third (and possibly fourth) sound as they add and subtract from each other.
And in harmony and disharmony.? Search YouTube for ‘barbershop quartets’ (yes, I know how cool it’s not.? You’ll hear four people singing, but if one listens carefully, there appears to be a fifth; the so called ‘magic fifth’.? By carefully pitching, the sum and difference of tones creates a new ‘voice’ completely.
Hetrodyne
No, heterodyne is not a CIS guy who accelerates one gram by a velocity of one centimetre per second.? ?(Sounded funnier in my head.? A ‘Dyne was a cgs unit of force, and cis means…Oh never mind. Sigh)
Anyway, what a heterodyne really is, is that new frequency produced by the addition of two tones.? If I feel motivated, I’ll simulate this and post it here xxxxxxxxxxxx.
So another thing my father taught me, and the second electronic diagram I learned to draw, was the superheterodyne receiver block diagram, and later I was able o plug in some3 numbers.? Keep in mind this is a simplification made for an 11-year old!
·???????? The radio signal is converted to an electrical signal by the antenna.? (A very small one!? A few microvolts).?
·???????? It is mixed with the signal from the local oscillator
·???????? Out of the mixer comes the sum and difference
·???????? One of those is filtered out and amplified by the 1st Intermediate Frequency (1st IF) amplifier
·???????? This signal is fed into a second mixer. And again we get sum and difference.
·???????? One of these two is filtered out and amplified by the 2nd IF amplifier
·???????? The voice/music is de-modulated from the radio wave
·???????? Amplified
·???????? And fed into a loudspeaker
And here is the key concept.? The heterodyne receiver is all about moving frequencies, from 90 MHz down to audio frequencies e.g. 40 Hz to 20 KHz.
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Imagine if we could take the 90MHz radio signal and heterodyne it straight down to audio frequencies! (40 Hz to 15 KHz)
Well, we kind of can!? There are technical reasons why this isn’t ideal, but this is the goal!
A crystal set you may have built in your youth (if you’re old like me!) is effectively doing just this!
Imagine now we have an analogue to digital converter that will process fast enough to convert the 90.0MHz signal into digital bits your PC can process!
Grasp this, and you’ve basically understood the 35,000 foot version of SDR!
A crystal set. 1503 KHz in, 40 Hz to 15 KHZ out!