What exactly is the base band? I always thought that it is a synonym to intermediate frequency but it appears not to be the case.

For example I have Bluetooth signals in the range of 2.4 to 2.48 GHz. What does it mean to mix it down to the base band?

  • \$\begingroup\$ Hint : baseband has a centre frequency (= carrier frequency) of 0 Hz. Mixing down to baseband is sometimes called "direct conversion" because there is no intermediate frequency. \$\endgroup\$ – Brian Drummond Nov 6 '17 at 12:58
  • \$\begingroup\$ @BrianDrummond What to do, if there are multiple channels (like in Bluetooth, in this case 40 with 2 MHz BW each) and you want to mix all of them down to baseband. Do I have to take the frequency at 2.44 to the whole BW is covered? Or do I need an LO Freq for each channel? \$\endgroup\$ – OcK Nov 6 '17 at 13:47
  • \$\begingroup\$ @OcK typically you only need to access one of those channels at a time, so the LO is rapidly retuned. If you need many, you may end up with multiple signal chains in parallel - perhaps in hardware, perhaps today in software. \$\endgroup\$ – Chris Stratton Nov 6 '17 at 13:52
  • \$\begingroup\$ @ChrisStratton thats correct, the tuning of the LO would be no problem. The problem is, that I don't know on which Channel I have to recieve at a specific moment, therefore I don't know which frequency to tune to. But atleast I do know now, how it works! \$\endgroup\$ – OcK Nov 6 '17 at 13:55
  • \$\begingroup\$ @OcK - part of what a frequency agile scheme like the various Bluetooth standards does is specify how devices synchronize their frequency selection. Generally you wouldn't implement this from scratch unless your goal is to explore the implementation rather than use the result; if you're trying to build intercepting diagnostic gear, first realize that the frequency selection isn't designed to benefit eavesdroppers though a lot can be done without receiving it all, next look at existing implementations like the ubertooth dongle/software. Spend enough and you can also process the whole band. \$\endgroup\$ – Chris Stratton Nov 6 '17 at 14:01

Baseband is low frequency signal that is to be sent across (eg: human voice in telephone). The usual procedure to send a baseband signal is to modulate it using a high frequency carrier in this case Bluetooth (or you can say mix it) i.e. in the range of 2.4 to 2.48 GHz.

Baseband definition (also includes a part of modulation) : https://en.wikipedia.org/wiki/Baseband

This would further help you understand : enter image description here

  • \$\begingroup\$ So, if I should mix my received BL signal down to base band, it means, that I have to take a relatively high local oscillator frequency , lets say 2.39GHz ? \$\endgroup\$ – OcK Nov 6 '17 at 10:33
  • \$\begingroup\$ Yes thats how modulation works :) reference for modulation : en.wikipedia.org/wiki/Modulation. I edited my answer to give a visual understanding \$\endgroup\$ – rsg1710 Nov 6 '17 at 10:34
  • \$\begingroup\$ Thanks for doing that.I already know what modulation is, but baseband does not directly have to be associated with modulation. \$\endgroup\$ – OcK Nov 6 '17 at 10:41
  • \$\begingroup\$ But thanks for your link to wikipedia, there it says: "Baseband has a low-frequency—contained within the bandwidth frequency close to 0 hertz up to a higher cut-off frequency." So the difference between baseband and intermediate frequency is, that the baseband goes from 0 to an upper cut-off frequency. \$\endgroup\$ – OcK Nov 6 '17 at 10:42
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    \$\begingroup\$ With digital modulation, baseband will often be complex, that is, making a distinction between +ve and -ve frequency, or using I and Q modulation channels. \$\endgroup\$ – Neil_UK Nov 6 '17 at 10:55

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