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Old 1st Aug 2021, 1:11 pm   #43
Radio Wrangler
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Join Date: Mar 2012
Location: Fife, Scotland, UK.
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Default Re: DIY FM tuner with 6CW4 nuvistor and ECC85

My usual explanation is that a man cycles past on an ordinary bicycle and all you get is a graph of pedal height versus time. You can tell how fast he is pedalling but not whether he is driving his bicycle onwards or whether he is back-pedalling. This is because both pedals are in the same plane and the ambiguity is unresolved.

Now, someone from the Isle of man cycles past (they all have three legs, look at their flag) on his '3-phase' bicycle with three pedals ar 120 degree intervals. Again you get plots of the heights of each pedal, and so long as you know which pedal is which plot you now CAN tell between forwards and backwards pedalling.

Finally, a clown cycles past, erratically, on a bicycle with only two pedals, but fitted at 90 degrees to each other on their shaft. You get two plots, and again you can tell the direction because the pedals are out of the same plane.

With receivers converting to IFs centred on zero (or close tozero) then stuff above the tuned frequency becomes a positive frequency in the IF, stuff below becomes a negative frequency. You need the perspective view of two phases of channels to discriminate between them. On a single channel, negative and positive frequencies are indistinguishable.

David
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