AGC for ZigBee, Wifi and Bluetooth in Commercial Cell Phones

Started by cogwsn 1 year ago7 replieslatest reply 1 year ago140 views

Hi, 

In the commercial cell phones mostly we have WiFi, ZigBee and Bluetooth working. I was wondering if they use the same RF front end for sampling the signals as all the mentioned three standards reside on the same band (2.45 GHz). If yes, how the AGC(saturation of ADC) is managed knowing that the power level of all the three standards are very different. 

Regards

Sumit 

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Reply by jimelectrApril 6, 2017

Up until late last year, I worked at Broadcom Ltd, so I ran into this issue.  Most of the time separate front ends.  I'm less familiar with ZigBee than BT and WiFi, though.  A few times I did see BT and WiFi sharing an LNA but never a PA.  BT uses FSK, while WiFi uses OFDM, and the channel BW's are 1 MHz and 20/40/80 MHz, respectively.  BT tends to use low IF transceiver architectures, while WiFi uses direct conversion.  Tradeoffs among cost, complexity, and performance (as usual).

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Reply by artmezApril 4, 2017

Not exactly my field, but...

An RF front-end, regardless of the bandwidth of its input, has only finite signal input level capacity. Some designs are better than others, but all are finite. To avoid out-of-band signals from reducing the amplitude available (and hence SNR) to the band(s) of interest, the "raw" input is typically pre-tuned via one or more bandpass sections.

For all input signals, a peak detector can be used to adjust the gain of the front-end to reduce limiting/clipping, because it's the peaks that cause clipping or compression, which cause the distortion that robs any signal of its full dynamic range capability.

The average level could be used, but it's less effective at reducing clipping/compression for composite signals with large peak-to-average ratios (and it's the clipping and compression that creates a lot of nonlinearity distortion).

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Reply by dima2882April 4, 2017

The systems I have worked on used completely different front ends for high-speed/high-power interfaces like WiFi aka IEEE802.11xx vs low-speed/low-power interfaces based on IEEE802.15.4. Different chips, modulation schemes, etc.

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Reply by motilitoApril 5, 2017

Your question intrigued me so I just googled it:

http://www.ti.com/general/docs/datasheetdiagram.ts...

This example from TI shows that the front ends are different. I guess that other vendors may use a single port for the chip but will split/switch it internally due to the different requirements for each standard.

At the end of the day it mostly depend on the system guy who designed it.

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Reply by fabateraApril 11, 2017

Do you have an example of cell phone with zigbee?

Wifi and BT ok, but I never saw one with zigbee.

For the circuits you would need aelectronics capable of working with different modulations and power.

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Reply by motilitoApril 11, 2017

I think that zigbee is designed for low data rate, low power sensors so I do not see it integrated into a cellphone.

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Reply by cogwsnApril 11, 2017

Its not very common, but yes some smart phones are integrating zigbee modules.