Why is DC balance needed in RF communications ?

[Bean]

Hi all,
· I'm playing with the Parallax 433Mhz transmitter and receiver. And discover (as I thought) that after a long period of "0" or "1", the next state change get extended quite a bit.

· I figured that I'd need to keep a DC balanced signal, but I must admit I'm don't really understand WHY it is neccessary. Can anyone shed some light on why you need to do this ?

· I'm working on a write-up on how to get the best performance with them using SX/B.

Bean.


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[pjv]

Hi Bean;

Most simple detectors are configured as level-slicers. Since most RF circuitry operated on single sided supplies, these levels are DC bias sensitive, and hence work better (or only at all) if the DC component is removed, or at least held stable.

When transmitting a string of data with more lo's than hi's, or the reverse, the DC component shifts, and effectively, the slicer reponds at a different level relative to the data average.

One good way to counter this is to transmit Manchester coded data which has a high and a low for every bit, keeping the whole stream balanced regardless of data content. Unfortunately this also ups the bandwidth requirement because data is transmitted at only half the rate.

Seems it's tough to get something for nothing.

Cheers,

Peter (pjv)

[johnnyairplane]

Bean,
Maybe a 'little off' here, but if the transmitter is FM or perhaps even PM - a DC signal will modulate the carrier by some small amount.
PJV I believe is hitting the problem right on the head,

In the FM broadcast world, if you've got DC bias, you've got a modulation problem....and then a channel balancing problem...
and then a 'sounds terrible' problem.....

If you 'play' with a transmitter, you can modulate the carrier with a small DC signal...
This builds up over time... the 'center' of your channel becomes shifted and then...
then, it appears that you've lost data.... It's reference is just off - a bit (ha ha ha)..

Thems my experiences...
a capacitor? - it usually helps.....

John