Quote:
Originally Posted by Radio Wrangler
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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This is a brilliant and funny analogy
! I and Q are clown plots!
Once a upon a time, I knew a bloke and a girl who are both Manx. That was 30 years ago and all i can remember they always complained about the weather and there is nothing to do in the island. .
W2AEW produced a series of excellent videos about Q and I that he demonstrated the ideas using a scope:
https://www.youtube.com/watch?v=h_7d-m1ehoY
Mmmm i have spent all afternoon scratching my head, looking at the datasheets and examples applications using 6BA6 and 6AU6. One of the trickest part is to get the right plate and screen voltages for the IF amps and limiters. Just to have to calculate the optimal grid/cathode bias and dropper resistors for the plate...too low plate voltage for the limiter resulting in clipping for weak AM noises but too low gain...I can see very flat response curve on the frequency sweep.