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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?

Harry Svensson
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OcK
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  • 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. –  Nov 06 '17 at 12:58
  • @BrianDrummond What to do, if there are multiple channels (like in [Bluetooth](https://goo.gl/images/L9AU8d), 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? – OcK Nov 06 '17 at 13:47
  • @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. – Chris Stratton Nov 06 '17 at 13:52
  • @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! – OcK Nov 06 '17 at 13:55
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    @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. – Chris Stratton Nov 06 '17 at 14:01
  • @ChrisStratton yes that's true. It is a random frequency hopping and I think you could get the sequences. But herefore there has to be a connection which I don't have. But that's too much into the detail, I will figure this out. Really helpful conversation here, I got a little bit smarter :D edit: in this case both (receiving and transmitting BLE) devices are mine. The other reveiver with the (baseband mixer) could maybe get the sequence from the receiving BLE device. – OcK Nov 06 '17 at 14:11

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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

rsg1710
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  • 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 ? – OcK Nov 06 '17 at 10:33
  • Yes thats how modulation works :) reference for modulation : https://en.wikipedia.org/wiki/Modulation. I edited my answer to give a visual understanding – rsg1710 Nov 06 '17 at 10:34
  • Thanks for doing that.I already know what modulation is, but baseband does not directly have to be associated with modulation. – OcK Nov 06 '17 at 10:41
  • 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. – OcK Nov 06 '17 at 10:42
  • Agreed, as I said before any signal which has very small bandwidth can be defined as baseband signal. Bluetooth however is a high frequency carrier wave the mixing is done for modulation purposes. – rsg1710 Nov 06 '17 at 10:46
  • Yes that is true. – rsg1710 Nov 06 '17 at 10:47
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    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. – Neil_UK Nov 06 '17 at 10:55
  • @Neil_UK What Im trying to achieve is, mixing the signals down to baseband then do an IQ demodulation and sample with an adc – OcK Nov 06 '17 at 11:10
  • No, mixing to a real baseband will destroy the IQ modulation, that is, permanently confuse the +ve and -ve frequencies. You need to mix to either a complex baseband in one go, with I and Q local oscillators, or (much more commonly these days) mix to a low IF, sample with fast enough ADC, then digitally demodulate down to a complex baseband. – Neil_UK Nov 06 '17 at 11:17
  • @Neil_UK Sorry, you are right. That's actually what I'm doing. Herefore I have an IC. The datasheet says: "The ... is a direct I/Q demodulator usable in digital wireless communication systems including cellular. An RF signal in the frequency range of 800 MHz to 2,700 MHz is directly downconverted to the I and Q components at baseband using a local oscillator (LO) signal at the same frequency as the RF signal. " So if ble signals are between 2.4 to 2.48 GHz an LO with 2.4GHz should be the right choice. Am I right? – OcK Nov 06 '17 at 11:37
  • It depends what 'between 2.4 and 2.48', I know nothing about BLE. If there's one wide channel centred on 2.44, then 2.44 is the LO you need. If a narrow channel can be centred on any LO between 2.4 and 2.48, then your LO needs to be similarly agile to demodulate that specific channel. I can be more specific if you link to the actual IC or data sheet. – Neil_UK Nov 06 '17 at 12:12
  • @Neil_UK oh im sorry. BLE has 40 channels between 2.4 and 2.48 GHz with a channel bandwidth of 2 MHz. I would like to 'cover' the whole bandwidth. The IC I mentioned has a Pin for external LO, so it should be no problem for the IC. So for the channels: does the LO frequency have to be the center frequency of each channel? [BLE Channels:](https://goo.gl/images/L9AU8d) – OcK Nov 06 '17 at 12:22
  • So far you've quoted 'direct downconversion', which means same LO as channel centre you're receiving, and 'using a LO at the same frequency as the RF signal', which means the same LO as channel centre you're receiving. So it sounds like you'll need an LO which can be stepped in 2MHz steps between 2.4 and 2.48GHz, though whether the centres are on or off the 2MHz raster is not clear, even from your BLE channels link. If you want more certainty than that, then you will need to link to the IC or its data sheet, or the BLE PHY documentation – Neil_UK Nov 06 '17 at 14:13
  • @Neil_UK This is exactly what I needed to know! Thanks alot. The centers are on 2402+k*2MHz, k=0,...,39 – OcK Nov 06 '17 at 14:20