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I think you are misunderstanding what I am talking about. I would like to
know what's the reason that Bluetooth is often implmented by Low IF architecture
, but the most popular architecture of 11b/g is Zero IF? What are the
reasons behind this selection?
Both Low IF and Zero IF architecture can save the external IF filter, but that's
not my question.
The fundamental difference in LAN networks is, depending on what network you hook up to, you can have different bandwidths. Do you want a LAN receiver with 3 different bandwidth IF filters and two SP3T switches, or do you want to take a performance hit, use higher speed DSP processors, and do your filtering digitally?
In bluetooth, where battery power is an issue, a fixed bandwidth IF filter cuts out a lot of DSP processing.
Thanks for reply, but
I don't really understand your statement. Even zero IF is adopted, the Channel select Filter is still required. And, once the channel bandwidth is different between different standard, the Bandwidth of Filter should be cahnged. So, the requirement
of tunable filter bandwidth should not be the reason.
What's the effort of DSP once the bandwidth of IF filter is not fixed?
The Polyphase Band Pass Filter used by Low-IF architecture is harder to be implemented in a wider bandwidth (the Q-factor is already very low).
Not to the last Low-IF has better immunity to interferers compare to ZIF, and Bluetooth needs this because is using narrower carrier bandwidth (~1MHz compares to ~22MHz in WLAN).
Ya, I agree that it's not easy to implement a low Q polyphase filter, this may
increase the order of filter and make higher noise to the receiver.
But why Low IF can immunize the interference than ZIF? the interference
here means the image? or adjacent channel? or other jammings?
Is it right in wlan high pass of the mixed down signal will not corrupt the signal since wlan siganl does not have too much information carried in DC while for bluetooth which has different modulation method and carries critical information in DC.
The channel bandwidth play a key role in determining receiver architectures.
Bluetooth has a bandwidth of 1MHz. This is not wide enough for direct down-conversion (zero-IF) because DC offset and flicker noise appear at Mixer outputs. These noise-like source will occupy in the most of the bluetooth bandwidth. Further more, Bluetooth employs GFSK modulation method that has significant amount of energy at DC once the signal of interested is directly down-converted. So the interested signal energy is mixed with the noise-like energy. If a HPF is used to eliminate the DC offset and flicker noise, Bluetooth will lose great portion of signal of interest. Thus, for bluetooth, the low-IF structure instead of the zero-IF can be one of the solutions to avoid DC offset and flicker noise.
For WLAN, zero-IF structure will work because the channel bandwidth of 22MHz is wide enough and the DC offset and flicker noise will occupy only a little portion of the bandwidth. In other words, SNR degradation caused by DC offset and flicker noise is small enough. Regardless, DC offset and flicker noise will degrade the SNR. So flicker noise should be as small as possible and DC offset cancellation is needed after a frequency down-converter.
For Bluetooth, low-IF architecture is preferred rather than zero-IF architecture. This
is because the modulation scheme of Bluetooth is gaussian frequency shift keying
(GFSK). Since the spectrum of GFSK has considerable energy spectrum at zero
frequency, a high-pass filter cannot be used to remove the dc offset with minimum
distortion of the received signal.
In IEEE 802.11, the center subchannel is unused , which means that there is an empty spectrum of 312.5KHz in the middle of the bandwidth, and the empty center sub-channel can be used for dc-offset cancellation. Thus, if the corner frequency of the high-pass filter is below 156.25KHz, the neighboring subchannels carrying the information data are not affected.