4 ms·
That's mostly right, the theoretical limit to the amount of information you can carry over a channel is set by the bandwidth and the channel signal to noise rat
by baobrien 9y ago
That's mostly right, the theoretical limit to the amount of information you can carry over a channel is set by the bandwidth and the channel signal to noise ratio. See the Shannon Limit [1]. I should add, however, the lower frequency you build your radio to operate on, the narrower your channel usually is. Antennas and filters tend to only be well matched over one or a few narrow frequency bands. These bands of good impedance get smaller as you go down in frequency, logarithmically.
[1] https://en.wikipedia.org/wiki/Noisy-channel_coding_theorem https://en.wikipedia.org/wiki/Noisy-channel_coding_theorem
- pdelbarba 9y agoThere's also the limitation of oscillator precision IIRC. 1ppm deviation is 1hz at 1mhz and 1khz at 1ghz
- baobrien 9y agoModern frequency synthesizers can easily do a few Hz step size up in the gigahertz range and be within a few ppm. That doesn't really matter that much though, since modern radios (at least these digital ones) are built as SDRs. They can shift the signal around and select whatever band they feel like.
- pdelbarba 9y agoSure, but that's on the receiver side for carrier recovery. Wouldn't it be problematic if you mashed a bunch of channels/allocations sufficiently close together that they started wandering on top of each other?
- PoachedSausage 9y agoModern DAB/DVB receivers are usually zero-IF, the wanted band of frequencies is mixed down to baseband and filtered with a sharp cutoff Surface Acoustic Wave filter then sampled with a fast ADC. Selectivity is then dependent on DSP. On the transmit side, you have to carefully combine the outputs of your various transmitters with a complex filter network, which at high powers looks something like: http://admin.mb21.co.uk/tx/userimages/8448proc20120403120238.jpg http://admin.mb21.co.uk/tx/userimages/8448proc20120403120238...